Download posterior fossa anomalies

Neural Tube Defects
Lecture 1
REVIEW OF THE EMBRYOLOGY OF NEURALATION
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Approx. 16 days post-ovulation the notochord, derived from mesoderm, induces the development of the
overlying ectoderm through a process mediated by the signaling molecule Shh (sonic hedgehog). The newly
formed neuroectoderm forms into the neural plate, followed by the development of the neural groove.
Neural folds develop on the lateral sides of the neural plate, and neural crest cells, precursors of spinal and
cranial nerves, leptomeninges, Schwann cells, and melanocytes, arise along the apex of the neural folds.
The neural folds close at 2 locations along the neural groove.
o 2 Closures:
 First, closure occurs at the cervical-occipital boundary
 Followed by a second closure at the extreme rostral end of the neural plate.
o Fusion of the neural plates proceeds bidirectionally from the cervical-occipital boundary and the rostral
end of the neural plate.
o After neural tube formation, the axial skeleton begins to develop and eventually encases the maturing
central nervous system.
o Neural tube defects (NTD) result from abnormal neural tube closure during the 3rd and 4th weeks of
gestation.
Scanning electron microscopic images and diagrams demonstrating neuralation.
Shaping Folding  Elevation  Convergence  Closure
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Neural tube defects account for most of the central nervous system malformations. Neural tube closure abnormalities
are second only to congenital heart defects in frequency and affect 1 in 1000 births in American Caucasians.
Neural Tube
 The formation and closure of the neural tube occurs between the 18th and 26th days post-conception.
 Formation of the neural tube occurs in both primary and secondary phases. Primary Neurulation, the process by
which the brain and spinal cord are developed, neural folds form and converge towards the midline at a single
point followed by fusion to initiate the formation of the neural tube. Fusion of the neural fold proceeds from
this point both rostrally and caudally. The ends of the neural tube remain open. These open areas, both located
rostrally and caudally, are referred to as rostral and caudal neuropores. Closure of these neuropores occurs at
the end of this developmental process around day 26. An encephalocele develops with failure of the rostral
neural tube to close and a myelomeningocele forms as a result of failure of the caudal tube to close.
Neural tube defects result from:
1. A failure of a portion of the neural tube to close completely, or
2. The process of reopening of the region of the neural tube after successful closure.
All neural tube defects are characterized by abnormalities involving some combination of meninges, neural tissue and
overlying soft tissue or bone. Prenatal ultrasonography is very useful in the detection of neural tube defects.
More info from Lecture 2 Notes**
Caudal neural tube closure abnormalities
1. Myelomeningocele (spina bifida cystica)
 Myelomeningocele the most commonly encountered neural tube closure abnormality and the
predominant form of spina bifida cystica.
 See above notes**
2. Spinal meningocele
 See above notes**
** These entities are part of an over-arching category referred to as spinal dysraphism.
Most of these lesions are first detected by screening maternal serum alpha-fetoprotein and prenatal ultrasound
diagnosis. Diagnosis is typically confirmed with amniocentesis combined with detailed ultrasound examination. In
addition, prenatal magnetic resonance imaging with T2 weighted sequences can provide detailed structural information
that may be helpful in evaluating these patients.
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DEFECTS OF NEURAL TUBE CLOSURE
Anencephaly:
 Characterized by replacement of most of intracranial contents by a vascular mass known as the area
cerebrovasculosa. Anencephalics may also have associated ganglia, portions of cranial nerves, and a variable
amount of medullary and cerebellar tissue.
 Anencephaly is incompatible with independent existence and is usually detected early in gestation by prenatal
ultrasonography.
 Elevated maternal serum alpha fetoprotein (AFP) levels
 Females >Males
 Occasionally Familial
 Spinal involvement may range from failure of the upper cervical vertebrae to fuse to complete exposure of the
spinal cord, or craniorachischisis.
o Craniorachischisis- a condition characterized by complete exposure of the spinal cord as a result of
failure of neural tube closure. Bottom Left Image
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Anencephaly is demonstrated in the image in the middle. Note that the cranium has failed to form and
cerebral hemispheres with overlying meninges are exposed. Dorsal view of large neural tube defect is
seen on the right. The mass like lesion seen in the image on the left is the area cerebrovasculosa.
Myelomeningocele:
 Defined as herniation of spinal cord and meningeal tissue through a vertebral defect.
 Myelomeningocele may occur at any level along the path of neural tube closure, but the most commonly
affected area is within the lumbosacral region.
 Myelomeningocele the most commonly encountered neural tube closure abnormality and the predominant
form of spina bifida cystica.
 All of these lesions are characterized by the failure of the ectoderm to separate properly from the neural
ectoderm and the persistence of the neural placode, a flat plate of un-neurulated tissue.
 Slight female>male
 This entity may also be associated with Arnold-Chiari malformations (see CNS malformations SDL) and
hydrocephalus.
 Myelomeningoceles located above the 12th thoracic vertebra:
o Females>males
o associated with developmental abnormalities in other organ systems
 Myelomeningoceles located below the level of the 12th thoracic vertebra
o more commonly solitary
o equally in males and females
o associated with less severe neurologic impairment
 Grossly: may be a cystic mass covered by a thin membrane of skin with spinal cord floating within, or a flat, open
lesion (myelocele) with a mass of vascular connective tissue and neural tissue in disarray.
 Microscopically: myelomeningoceles demonstrate an atrophic epidermis lacking rete pegs and skin appendage
tissue. This lesion is not infrequently ulcerated. Beneath the epidermis, there is a conglomeration of fibrotic
connective tissue, thin walled and dilated blood vessels, and islands of glial tissue.
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3 established risk factors for the development of myelomeningoceles:
1. Maternal pre-gestational diabetes
2. Inadequate maternal intake of folic acid
3. In utero exposure to anti-epileptic drugs.
** It is estimated that adequate peri-conceptual folic acid supplementation alone could produce up to a 70%
reduction in the risk of neural tube defects.
Immediate clinical consequence of myelomeningocele: an increased susceptibility to CNS infections due to
poorly formed and frequently ulcerated skin or cystic coverings at the site of the myelomeningocele.
Other clinical consequences:
o Chronic urinary tract infections and pyelonephritis are major sources of morbidity and mortality in
myelomeningocele patients.
o Other symptoms encountered in up one third of myelomeningocele patients include apnea, swallowing
difficulties and impaired head control.
o Hydrocephalus is also a complication and can lead to some impairment.
Lower extremity sensory and motor function can range from normal to near normal and low sacral lesions to
paraplegia for lesions above the level of L3 (lumbar 3).
Image of a myelomeningocele containing a protruding sac with malformed spinal cord tissue.
TAILBUD DEFECTS
Spina Bifida Occulta:
 also referred to as occult spina bifida
 mildest form of neural tube defect
 probably reflects failure of tail bud development or secondary neurulation
 The cord may appear grossly normal, but the central canal may be distended (hydromyelia).
 Other associated findings include:
o diastematomyelia (splitting of the cord into 2 hemicords separated by a septum of fibrous meninges)
o tethered cord (characterized by a low conus medullaris and thickening of the filum terminale, often
associated with lipoma) This abnormality affects the lower lumbar and sacral levels of the spinal cord.
 a closed lesion (unlike the other NTD above)
 The defect is often indicated by:
o overlying hairy skin
o dimpling or cutaneous hyperpigmentation (see photograph below) .
o may also be associated with sacral, anorectal and urogenital defects.
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HERNIATION OF NEURAL TUBE THROUGH AXIAL MESODERMAL DEFECTS
Meningocele:
 Congenital spinal meningoceles are characterized by a vertebral defect with an associated cystic lesion containing
herniated dura and arachnoid; the underlying spinal cord remains normally positioned. Skin in meningocele is intact.
 These lesions are rare, occurring in 1 in 10,000 live births and without any gender predilection.
 Patients with meningocele are neurologically normal, in contrast to patients with myelomeningocele.
 Additionally, meningoceles are not typically associated with hydrocephalus. Because of the intact skin covering, the
instance of CNS infections in meningoceles is not increased.
Encephalocele:
 An encephalocele is a diverticulum of malformed central nervous system tissue extending through a defect in
the cranium.
 This most commonly occurs within the occipital region or the posterior fossa (up to 75% of cases). More
uncommonly, encephaloceles may involve the parietal and fronto-ethmoidal regions. Encephaloceles range in
size.
 Smaller encephaloceles may contain jumbled fragments of CNS tisse.
 Larger encephaloceles can contain large portions of the cerebral hemispheres and sometimes hindbrain.
 Occipital encephalocele may be associated with Meckel-Gruber syndrome, a syndrome inherited in an
autosomal recessive pattern. This syndrome is characterized by:
o occipital encephalocele
o polycystic kidneys
o hepatic fibrosis
o bile duct proliferation
o Affected children may also have polydactyly (more fingers/toes), cleft palate, and microcephaly.
 Most encephaloceles are sporadic and not part of an associated inherited syndrome.
 Anterior encephaloceles
o generally sphenoidal or frontal in location.
o The instance of encephaloceles is approximately 1 in 5000 live births.
o Males>Females
o often associated with visible facial deformities such as hypertelorism
o more frequently associated with agenesis of the corpus callosum.
 Posterior encephaloceles
o Generally within the occipital region
o Females >Males
o Occipital encephaloceles may be associated with Chiari II and Dandy-Walker malformations.
o Image of a posterior encephalocele with a portion of the brain protruding through an occipital defect in
the cranium.
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CNS Malformations
Lecture 2
**See lecture 1 for Neural Tube Defects Notes
FOREBRAIN ANOMALIES
Definition: Forebrain anomalies chiefly refer to abnormalities of brain volume.
 Cortical development malformations and defects:
o Megalencephaly: increased brain volume
o Microencephaly: decreased brain volume
 most common of the two
 can occur in a wide range of settings including those associated with:
 chromosomal abnormalities
 fetal alcohol syndrome
 as well as human immunodeficiency virus infection acquired in utero
o Neurons and glial cells of the cerebral cortex are generated around the ventricles of the brain and
migrate to the cortex through adhesion molecules that are present on their membranes. Cortical
development entails the generation of stem cells and their differentiation into neurons and glia,
migration to the cortex, and organization into functional layers. Impairment of these processes can lead
to a variety of malformations known as neuronal migration defects.
1. Neurons fail to migrate at all from ventricles (seen in periventricular heterotopias) or neurons
migrate only half way (subcortical band heterotopia)
2. Neurons reach the cortex but large numbers do not. No normal cortical layers leading to
formation of lissencephaly (sulci are absent except usually for sylvian fissure, cortex is thick and
consists of molecular and 3 neural layers. Related to loss of LIS1 gene on chromo 17) and
cobblestone cortex
3. Neurons “overshoot” the cortex and end up in the subarachnoid space
4. Late stage of migration and cortical migration disrupted (polymicrogyri)
o Abnormal migration leads to abnormal gyral pattern. The nomenclatures of neuronal migration defects
reflect the naked eye appearance of the cortex. In addition to abnormal folding the cortex can be thick
and disorganized. Severe neuronal migration defects such a lissencephaly, cobblestone cortex and
polymicrogyri are associated with psychomotor retardation and intractable seizures.
o Images of neuronal migration abnormalities. Polymicrogyri (left) is characterized by more numerous yet
smaller than normal gyri. Lissencephaly is characterized by an abnormally smooth cortex (right) There
is a notable absence of gyri and sulci.
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Most neuronal migration defects have some genetic basis. Genotypes and phenotypes may overlap.
The same gene mutation may cause a variety of different phenotypic findings.
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In lissencephaly, sulci are absent except usually for the sylvian fissure. The cortex is thick and consists of
molecular and three neuronal layers.
 This abnormality is thought to be related to complete loss of the LIS1 gene located on
chromosome 17p13. If there is complete loss of the LISS1 gene, the condition may be fatal.
Deletion of one copy causes lissencephaly. The LIS1 protein forms a complex with other
proteins that are crucial for cell migration, cell division and intracellular transport.
 Lesions of the LIS1 gene and other contiguous genes cause the Miller-Dieker syndrome: which is
a combination of lissencephaly with:
 dysmorphic facial features
 visceral abnormalities
 polydactyly
POSTERIOR FOSSA ANOMALIES
Aqueductal atresia and aqueductal stenosis are common causes of congenital hydrocephalus with the Chiari type
malformation being a close second.
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Aqueductal atresia
o A disruption that occurs either in utero or postnatally
o It may be caused by thrombus formation from interventricular bleeding, infection and other pathologic
abnormalities that may create gliosis and obliterate the aqueduct.
o In some instances, a few rudimentary ependymal lined tubules are seen in place of the aqueduct but
these small channels are not sufficient to convey cerebrospinal fluid from the third to the fourth
ventricle.
o **Aqueductal atresia cannot be distinguished from aqueductal stenosis on imaging studies.
Aqueductal stenosis
o May be inherited in the complex fashion, either autosomal recessive or X-linked patterns
 The best understood form of aqueductal stenosis is an X-linked aqueductal stenosis, a disorder
caused by mutations in the L1CAM gene on chromosome Xq28. The gene encodes a cell
adhesion molecule and patients with this syndrome may have associated mental retardation,
absence of cortical spinal tracts as well as agenesis of the corpus callosum among others.
o Sagittal section of fetal brain which was affected by aqueductal stensosis. Note the marked distention of
the lateral ventricle caused by aqueductal stensosis related hydrocephalus.
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Chiari Malformations
o Chiari type II malformation is a syndrome or an association of anomalies which are characterized by:
 neural tube defects (usually lumbosacral meningomyelocele)
 abnormalities of the posterior fossa and craniocervical junction
 hydrocephalus
 In this abnormality of the posterior fossa, posterior fossa contents reside in a large foramen
magnum associated with low insertion of the tentorium and a shallow posterior fossa. As a
result of these abnormalities, the cerebellum and brainstem are crowded and are displaced into
the upper cervical canal. Medulla is elongated and may be folded dorsally whereas the
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aqueduct and the fourth ventricle are collapsed. There is often associated aqueductal atresia.
Blockage of cerebrospinal fluid flow caused by these abnormalities may lead to hydrocephalus.
Patients who have Chiari type II malformations also have hydromyelia or syringomyelia.
Chairi type II malformation demonstrating elongation of the brainstem. A portion of the
cerebellum, medulla oblongata, and 4th ventricle reside below the level of the foramen
magnum
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Chiari type I malformations
 mild variant of Chiari type II malformations
 the volume of the posterior fossa is reduced leading to overcrowding and herniation of the
cerebellar tonsils and dorsal cerebellum into the spinal canal
 Similar to the Chiari type II malformations, Chiari type I patients may also have syringomyelia
and some may have hydrocephalus.
 There is no associated neural tube defect.
 Many patients are asymptomatic but others may complain of dizziness, cranial nerve
abnormalities and headache.
Dandy-Walker Malformation
o The Dandy-Walker malformation is a posterior fossa abnormality where the primary feature is complete
or partial agenesis of the cerebellar vermis.
o The hemispheres of the cerebellum are maintained and are connected by a thin membrane of neural
tissue which forms a fourth ventricle roof.
o There is associated obstruction of cerebrovascular fluid from the fourth ventricle associated with this
abnormality. Because of obstruction the fourth ventricle enlarges and the membrane that forms its roof
balloons creating a large posterior fossa cyst. This cyst then pushes the tentorium superiorly.
o Obstruction of cerebrovascular fluid causes hydrocephalus.
o The clinical profile, etiology and genetics of Dandy-Walker malformation are complex and heterogenous.
 Some of these patients have severe neurological deficits and additional developmental
problems including agenesis of the corpus callosum and neuronal migration abnormalities.
 These patients maintain apparent normal intelligence, especially after shunting of cerebrospinal
fluid.
 Most of the cases of Dandy-Walker malformation are sporadic although there are rare familial
cases associated with genetic abnormalities including trisomies of chromosomes 3, 9, 13 and 18.
o Dandy-Walker malformation demonstrating absence of the cerebellar vermis (left). CT image
demonstrating a cystic appearing 4th ventricle.
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Syringomyelia
o Word derived from the term syrinx, a Greek term for tubular cavity, is a tubular formed cavity of the
spinal cord which can affect the cervical and upper thoracic segments.
 The cavity is located within the central gray matter of the spinal cord. Over time the cavity
enlarges.
 The syrinx is lined by a glial tissue and contains a cerebrospinal fluid like fluid which
accumulates and progressively grows under pressure causing associated atrophy of the gray and
white matter of the spinal cord.
 Symptoms of compression and atrophy of the spinal cord oftentimes show up within the second
and third decade of life. Early on, patients may demonstrate dissociated anesthesia
characterized by segmental loss of pain and temperature sensation corresponding to the
distribution of the syrinx. Other findings may include denervation, atrophy of muscle and
kyphoscoliosis.
 Pressure within the syrinx may be relieved by shunting of the syrinx fluid or by laminectomy.
o Syringomyelia is often associated with type I Chiari malformations.
o The pathogenesis of syringomyelia is essentially unknown and is probably multifactorial.
o Syringomyelia may also be seen both superiorly and inferior to the location of spinal cord tumors such
as:
 Ependymoma
 pyelocytic astrocytomas
 hemangioblastomas
o Histologic section of spinal cord demonstrating syringomyelia. Note the marked dilatation of the spinal
cord central canal with associated destruction of central gray matter.
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VASCULAR MALFORMATIONS
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occur in 0.1 to 4.0% of the general population with 4 subtypes of congenital vascular abnormalities
1. Developmental venous abnormalities
o are the most common in autopsy series
o incidence of 2%
o usually benign, although they may uncommonly present with seizures, progressive neurologic
deficits and hemorrhage
o these anomalies are known as venous angiomas
 are composed of radially arranged configuration of medullary veins (caput medusae)
separated by normal brain parenchyma
 these lesions are usually supratentorial with a frontal lobe predominance
 lesions may be solitary although multiple lesions have been reported associated with other
clinical syndromes
o Headache is the most common presenting complaint.
o Dx: cerebral angiography and is considered the gold standard for diagnosis of venous abnormalities
 Other imaging modalities including computed tomography (CT) and magnetic resonance
imaging (MRI) may also be helpful.
o Tx: conservatively in the majority of patients and headaches and seizures may be managed
medically
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2. Capillary telangiectasias
o usually benign
o small lesions most commonly found in the:
 pons
 middle cerebellar peduncles
 dentate nuclei
o Multiple lesions are common
o These lesions are composed of small dilated capillaries containing no smooth muscle or elastic
fibers. The intervening brain tissue is oftentimes normal. Process may be associated with
microhemorrhage and gliosis. Some of these abnormalities are associated with angiomatous
phacomatoses such as Osler-Weber-Rendu (hereditary hemorrhagic telangiectasia) syndrome.
o Dx: MRI helpful; Clinically these lesions are usually silent found incidentally on neuroimaging studies
or at autopsy.
o nonoperable lesions
3. Cavernous malformations
o least common (0.4%)
o greater tendency toward neurologic sequelae.
o referred to as cavernous angiomas, cavernous hemangiomas, or cavernomas
o These lesions may be sporadic or be inherited in a familial pattern.
o Cavernous malformations have a characteristic mulberry appearance grossly with engorged purplish
clusters of vessels. Tissue immediately surrounding these abnormalities may demonstrate gliosis
and hemosiderin-laden macrophages due to previous hemorrhages.
o mean age of 30-40 years
o in men and woman
o Patients may present with a symptomatic hemorrhage though seizures and progressive neurologic
deficits may be associated with the process.
o Blood flow in cavernous malformations is limited making diagnosis by angiography difficult. Other
imaging studies such as magnetic resonance imaging (MRI) may be more helpful. Asymptomatic
cavernous malformations are typically observed irrespective of their location.
o Indications for surgical removal of the area include:
 progressive neurologic deficit
 intractable epilepsy
 recurrent hemorrhage
4. Arteriovenous malformations
o greater tendency toward neurologic sequelae.
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Metabolic Diseases of the Nervous System
Lecture 3
NEURONAL STORAGE DISEASES
NEURONAL CEROID LIPOFUSCINOSES
 Characterized by a form of lisosomal storage disease grouped because of their accumulation of lipofuscin, an
autofluorescent substance seen with a variety of ultrastructural appearances within neurons.
 Neuronal dysfunction is typically associated with mental and motor deterioration with seizures as well as
blindness.
 Research efforts have identified up to 8 causative genetic loci for these disease entities.
 Abnormalities in protein modification appear to lead to accumulation of lipofuscin or the neuronal dysfunction
is not understood.
 Tay-Sach’s disease is considered disease form identified in early infancy with associated developmental delay.
These patients eventually demonstrate prominent paralysis and loss of neurologic function with death in a short
term period.
LEUKODYSTROPHIES
KRABBE DISEASE
 autosomal recessive
 resulting from a deficiency of galactocerebroside beta-galactosidaseo This enzyme is required for the catabolism of galactocerebroside  galactose & ceramide.
o Numerous mutations are associated with the gene which is located on chromosome 14 q31. The end
result of the disease process appears to be a cytotoxic compound that may cause oligodendrocyte
injury.
 disease is progressive; with onset symptoms between the age of 3- 6 months. Long term survival past the age of
2 years is infrequent.
 disease is characterized by loss of motor function including stiffness and weakness and gradual worsening
difficulties in feeding.
 Pathologically, the brain shows loss of myelin and oligodendrocytes. Neurons and axons appear to be spared.
 Microscopically globoid cells are present and are diagnosed as macrophages in the parenchyma around blood
vessels.
 Histologic section from a patient with Krabbe disease showing characteristic globoid cells containing eosinophilic
cytoplasm with accumulation of galactocerebroside.
o
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Metachromatic leukodystrophy
 autosomal recessive
 progressive storage disorder of myelin-producing cells
 caused by a deficiency of the lysosomal enzyme arylsulfatase A, located on chromosome 22q13. and the
consequent accumulation of sulfatide.
 Infantile forms of disease present with ataxia, dysarthria, dysphagia, and vision or hearing abnormalities – death
usually occurs within only a few years in this form of disease.
 Adult 20s or 30s forms of disease are associated with behavior changes, inappropriate behavior, emotional
lability and dementia in end stages.
 MRI usually demonstrates non-enhancing lesions, particularly within the deep white matter.
 Pathologically, cerebral atrophy with associated hydrocephalus ex vacuo may be seen.
 Metachromatic material accumulates on toluidine blue staining in CNS macrophages, leading to the
designation of metachromatic leukodystrophy.
Adrenoleukodystrophy
 X-linked
 progressive peroxisomal disorder of myelin/myelin-producing cells caused by a deficiency of
adrenoleukodystophy protein and accumulation of very long chain fatty acids.
 Various clinical findings including:
o sphincter disturbances
o cognitive deficits
o vision and hearing difficulties
 In childhood, death can occur in 2+ years.
 Pathologically, confluent symmetric lesions in parieto-occipital lobes that advance in a caudal-rostral
progression. Perivascular lymphocyte cuffing is also seen in the childhood cerebral form of the disease.
MITOCHONDRIAL ENCEPHALOMYOPATHIES
Mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS):
 the most common neurologic syndrome caused by mitochondrial abnormalities
 The syndrome is defined by:
o recurrent periods of acute neurologic dysfunction
o cognitive changes
o muscle weakness
o lactic acidosis
 Stroke-like symptoms are reversible and do not correspond well to specific vascular territories.
 Pathologically, areas of infarction may be seen, some associated with tissue calcification.
 The most common mutations seen in MELAS are in transfer RNA (tRNA).
Myoclonic epilepsy and ragged red fibers (MERRF):
 maternally transmitted disease
 Affected patients have:
o seizure disorders
o Myoclonus
o features of myopathy.
o Ataxia may also be seen.
 Most cases of MERRF are associated with tRNA mutations that are distinct from MELAS.
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Leigh syndrome (sub acute necrotizing encephalomyopathy):
 disease of early childhood.
 The disease is characterized by:
o lactic academia
o psychomotor development arrest
o feeding abnormalities
o seizure disorders
o weakness
o hypotonia
 Death from the disease usually occurs in 1-2 years.
 Pathologically, there are multifocal, moderately symmetrical regions of brain destruction with associated blood
vessel proliferation providing a spongiform appearance to tissue.
 Numerous mutations in mitochondrial genome encoded components of the oxidative phosphorylation
complexes have been described.
TOXIC AND ACQUIRED METABOLIC DISEASES
Vitamin deficiencies:
Thiamine (Vitamin B1) deficiency
 Thiamine deficiency results in beriberi which is associated with cardiac failure. Thiamine deficiency may also be
associated with psychotic symptomatology and ophthalmoplegia as well as Wernicke encephalopathy.
Korsakoff’s syndrome is characterized clinically by memory disturbances and confabulation. Because
Wernicke’s encephalopathy and Korsakoff’s syndrome are often seen together, Wernicke-Korsakoff syndrome
is oftentimes a term used to apply to these patients.
 Morphologically, Wernicke’s encephalopathy is characterized by hemorrhage and necrosis in the mamillary
bodies in the walls of the third and fourth ventricles. Microscopically, there is over time infiltration of
macrophages and development of cystic spaces with hemosiderin laden macrophages. Lesions within the
dorsomedial nucleus of the thalamus seem to correlate with memory disturbance and confabulation.
 Cross section of brain demonstrating mamillary body hemorrhages (left) in a chronic alcoholic with thiamine
deficiency. Mamillary body atrophy is seen on the right in a chronic alcoholic.
Vitamin B 12 deficiency
 Deficiency in vitamin B12 obviously causes a macroscopic anemia but also causes severe effects to the nervous
system in deficiency state. Neurologic symptoms include numbness and tingling in the extremities early in the
course of deficiency and slight ataxia in the lower extremities. Over time this progresses to include spastic
weaknesses of the lower extremity. Complete paraplegia may also occur later in the course. Prompt vitamin
replacement therapy improves symptomatology although complete recovery is generally poor.
 Microscopic examination of vitamin B12 deficiency tissues demonstrate swollen myelin layers with associated
vacuoles. This pathology begins segmentally within the mid thoracic level of the spinal cord. Over time axons
in the ascending tracts of the posterior columns and descending pyramidal tracts degenerate.
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NEUROLOGIC SEQUELAE OF METABOLIC DISTURBANCES AND TOXIC DISORDERS
Hypoglycemia
 The brain is very sensitive to low serum glucose levels. Lower glucose levels present with symptoms much the
same as those seen in hypoxic states. Lower glucose levels result in selective injury to pyramidal neurons of the
cerebral cortex and, if severe, result in cortical pseudolaminar necrosis. The hippocampus and cerebellum are
also sensitive to low glucose levels.
Hyperglycemia
 Hyperglycemia is most commonly associated in uncontrolled or poorly controlled patients with diabetes
mellitus and associated with ketoacidosis and hyperosmolar coma. Affected patients are dehydrated and
develop confusion, stupor, and ultimately coma.
Carbon monoxide
 Brain injury is the result of hypoxia in acute carbon monoxide exposure. Carbon monoxide interferes with the
oxygen carrying ability of hemoglobin. Selective injury to cortical neuron layers III and V as well as hippocampal
neurons and cerebellar Purkinje cells are usually demonstrated in carbon monoxide injuries. In addition,
bilateral necrosis of the globus pallidus may be demonstrated, a finding that tends to correlate more than other
forms of hypoxia, with carbon monoxide exposure. Later abnormalities include demyelination of white matter
tracts.
Methanol
 Acute methanol toxicity usually occurs when one is exposed to it as a contaminant in imbided alcoholic
beverages (e.g. moonshine). Methanol toxicity preferentially affects the retina, resulting in blindness due to
degeneration of retinal ganglion cells. Severe bilateral necrosis of the putamen and focal white matter necrosis
may also be seen. Formate (formic acid), a metabolite of methanol, likely has direct retinal toxicity.
Ethanol
 In most instances, unless severe, acute ethanol intoxication is reversible. Chronic alcohol abuse could potentially
lead to nutritional deficiency associated abnormalities in the brain, or to direct toxic effects of alcohol.
 Cerebellar dysfunction occurs in about 1% of chronic alcoholics and is associated with ataxia, gait
abnormalities and nystagmus.
 Histologically, there is atrophy and loss of the granule cell layer in the anterior cerebellar vermis.
 Bergmann gliosis is defined as loss of Purkinje cells and proliferation of adjacent astrocytes between the
granular and molecular layers of the cerebellum in advanced cases of chronic alcohol abuse.
 Note atrophy of the superior aspect of the cerebellar vermis taken from a patient with a history of chronic
alcohol abuse.
Radiation
 High doses of radiation (greater than 1000 rems) can lead to coma followed by death. Delayed effects of
radiation exposure include headaches, nausea, vomiting and papilledema. Patholgically, large areas of
coagulative necrosis and adjacent edema may be seen in those exposed to high levels of irradiation. Areas of
radiation induced injury are generally restricted to the white matter, and all cells within the area undergo
necrosis. Radiation can also induce neoplasia, usually developing many years after radiation therapy for other
tumors such as meningioma, glioma and sarcoma.
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Lysosomal Storage Disorders
Lecture 4
INTRODUCTION.
Lysosomal storage disorders (LSDs), of which greater than 50 are known, are caused by defective activity of lysosomal
proteins, resulting in accumulation of unmetabolized substrates. LSDs result from the inherited deficiency of one or
more of the many catabolic enzymes that are located within the lysosome, each characterized by the accumulation of
specific substrates. The majority of the LSDs are inherited in an autosomal recessive manner, except three: the X-linked
disorders Fabry disease, Hunter syndrome, and Danon disease. Ethnicity can also be an important when considering the
diagnosis of LSDs. Certain groups have an increased carrier frequency for specific disorders: Gaucher disease, is the most
common genetic disorder in Ashkenazi Jews, with a frequency of 1 in 855 live births. An increased incidence of
galactosialidosis is found among individuals of Japanese ancestry. Pompe disease is reported to have an increased
frequency in subjects of African or Chinese ancestry.
LSDs occur at a collective frequency of approximately 1:5000 live births. They can be caused by defects in soluble
lysosomal enzymes, in non-enzymatic lysosomal proteins (soluble or membrane-bound), or in non-lysosomal proteins
that impinge upon lysosomal function. The degree of residual function of the defective protein influences the age of
symptom onset. Patients without, or almost without, a given protein present symptoms in-utero or in early infancy,
whereas milder mutations lead to juvenile or adult onset disease. The majority of LSDs involve storage in both the
central nervous system (CNS) and visceral tissues. CNS pathology is a common hallmark of LSDs, and LSDs are the most
common cause of pediatric neurodegenerative disease. Cell death is well documented in parts of the brain, and in other
cells of LSD patients.
LYSOSOMES.
Nearly every eukaryotic cell, except the erythrocyte, contains lysosomes, and many lysosomal substrates have key roles
in cell structure and function. Consequently, the effects of lysosomal malfunction are widespread.
ENDOSOMAL-LYSOSOMAL SYSTEM.
Eukaryotic cells have developed an internal membrane system that allows them to endocytose molecules, via
endosomes (a large endosome, seen with professional phagocytes such as macrophages, containing large
macromolecules or microorganisms is called a phagosome), and deliver them to intracellular digestive enzymes stored in
organelles called lysosomes. The interior of endosomes is mildly acidic (pH approximately 6), and is thought to be the
site where the hydrolytic digestion of endocytosed molecules begins. Under certain conditions organelles or portions of
cytoplasm may become enclosed in membrane derived from the endoplasmic reticulum. These are termed
autophagosomes, This process is used in all cell types for disposal of obsolete parts of the cell itself. The normal liver
cell’s mitochondria, for example, have an average life span of 10 days. The mitochondria are then digested in
autophagosomes. The autophagosomes fuse with the lysosome for digestion. Metabolites from lysosomal digestion are
delivered to the cytosol. Also, some proteins are sorted in the trans-Golgi network, delivered to the lumen of lysosomes,
and then release their contents by exocytosis into the extracellular space (ex. collagenase released by osteoclasts).
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The lysosome is just one component of connected intracellular organelles, collectively known as the endosomal–
lysosomal system . It is now generally accepted that its principal components are the early endosome, situated at the
cell periphery, the late endosome, which tends to be perinuclear, and the lysosome. They form a chain that is
responsible for the trafficking and digestion of endocytosed molecules. The endosomes also participate actively in
sorting and recycling. The final compartment of this system is the lysosome. It is characterized by the presence of a
membrane, a low internal pH, and many hydrolytic enzymes.
Lysosomes are formed in the rough endoplasmic reticulum and transferred to the Golgi complex, where enzymes are
modified and packaged for lysosomes. A mannose-6-phosphate (M6P) group are added to the N-linked oligosaccharide
of soluble lysosomal enzymes within the lumen of the cis-Golgi network that are recognized by M6P receptors in the
trans-Golgi network and packaged into vesicles that will bud-off and fuse with vesicles to become lysosomes.
Lysosomes are part of the "intracellular digestive tract." They contain special enzymes that are primarily destined for
intracellular organelles, rather than extracellularly, and they function in the acidic environment of the lysosomes. This
requires special processing within the Golgi apparatus.
LYSOSOMAL ENZYME PROCESSING.
As with all other secretory proteins, lysosomal enzymes (acid hydrolases) are synthesized in the endoplasmic reticulum
and transported to the Golgi apparatus. Within the Golgi complex they undergo a variety of post-translational
modifications. A special modification involves the attachment of terminal mannose-6-phosphate groups to some of the
oligosaccharide side chains. The phosphorylated mannose residues are recognized by specific receptors found on the
inner surface of the Golgi membrane. Lysosomal enzymes bind these receptors and are segregated from other secretory
proteins within the Golgi. Then, small transport vesicles containing the receptor-bound enzymes bud-off from the Golgi,
and proceed to fuse with the lysosomes, and the vesicles are shuttled back to the Golgi.
Lysosomal enzyme synthesis is summarized in the figure below. These enzymes are glycoproteins that are synthesized in
the rough endoplasmic reticulum (ER). At this early stage they are inactive. They translocate through the ER membrane
with the help of N-terminal signal sequences. Once in the lumen of the ER, they undergo N-glycosylation and lose the
signal sequence. Then they move to the Golgi compartment through vesicles, and they acquire a mannose 6-phosphate
(M6-P) ligand. This process requires the sequential action of two enzymes, a phosphotransferase and a diesterase.
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The acquisition of the M6-P marker separates glycoproteins that are destined for the lysosome, from secretory
glycoproteins. Failure of acquisition of this marker results in mistargeting of lysosomal enzymes. They will not enter the
lysosome and substrate breakdown will not occur. This is what happens in I-cell disease. Glucocerebrosidase, which is
associated with the lysosomal membrane (and Gaucher disease), does not acquire the M6-P residue, although it does
undergo N-glycosylation and is targeted. The precise mechanism by which this occurs is unknown. The receptor–protein
complex then moves to the late endosome, where the low pH causes it to dissociate. The hydrolase moves into the
lysosome, and the receptor is recycled either to the Golgi to acquire another ligand, or to the plasma membrane. The
final steps for the lysosomal enzyme include proteolysis, folding and aggregation.
Synthesis and intracellular transport of lysosomal enzymes.
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ENDOCYTIC-PHAGOCYTIC-AUTOPHAGIC PATHWAYS.
The basic steps of the endocytic pathway are summarized in the figure below. The material to be broken down in
lysosomes may be extracellular or intracellular. Extracellular materials enter the cell either by endocytosis or
phagocytosis, depending on the nature of the molecule. Receptor-mediated endocytosis is the process by which most
biologically important extracellular substances are internalized. This occurs by binding to specific cell surface receptors.
Ligands are first delivered to early endosomes, and then transported to late endosomes. Finally they are delivered to the
lysosomes.
Phagocytosis is the route of entry into the cell for microorganisms and cellular debris. Such particles are incorporated
into phagosomes, which fuse with primary lysosomes to form secondary lysosomes. Finally, intracellular materials also
undergo autophagy.
Although a small amount of hydrolysis takes place in endosomes, the bulk of it takes place in the lysosome. This is
because it is only in the acid milieu of the lysosome that hydrolases are active. The low pH of the lysosome is maintained
by the vacuolar proton pump.
Lysosomes are intracellular organelles distinguished by their high concentration of hydrolytic enzymes, and are the
principle site of controlled intracellular digestion of macromolecules. They are lipid bilayer membrane limited vesicles
that contain many different hydrolytic enzymes (all acid hyrolases) including proteases, nucleases, glycosidases, lipases,
phospholipases, and sulfatases. The enveloping membrane separates lytic enzymes from the cytosol. All enzymes
require an acid environment for optimal activity, and within the lysosome the pH is maintained at about 5 in the interior.
Note the lysosome membrane protects the cell cytosol from the enzymes, and even if they should leak out the pH of the
cytosol is about 7.2. A proton pump in the lysosomal membrane pumps protons, using ATP hydrolysis, into the lysosome
lumen to maintain the acidic environment.
The lysosomal enzymes catalyze the breakdown of a variety of complex macromolecules. Molecules may be derived
from the metabolic turnover of intracellular organelles (autophagy), or they may be acquired from outside the cells by
phagocytosis (heterophagy).
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LYSOSOME MICROANATOMY.
Lysosomes are present in nearly all cells, but most abundant in cells with phagocytotic activity, such as macrophages and
neutrophils. Lysosomes are diverse in shape and size, which depends on their function with the cell. They are generally
spherical in shape with a size of roughly 0.05 to 1.2 micrometers in diameter, with a uniformly granular electron-dense
appearance in electron microscopy. The activity of the enzymes very depending on the cell type and may be identified
by enzyme histochemistry, but the most common enzymes are acid phosphatase, ribonuclease, deoxyribonuclease,
cathepsins, sulfatases, lipases, and beta-glucuronidase.
Lysosomes that have not entered into a digestive event are primary lysosomes, which can be very small and exist in
most cells (0.05 micrometers in diameter), and impossible to identify in the absence of histochemical tests of their
contents, but a few, such as macrophages and neutrophils have larger primary lysosomes up to 0.5 micrometers in
diameter that may be visible by light microscopy. Environmental material taken into a phagosome is digested by the
fusion of the phagosome with the lysosome (phagolysosome) upon release of the lysosomal enzymes into the
phagolysosome. This structure is called a secondary lysosome, in which digestion occurs. Secondary lysosomes can be
larger (up to 2 micrometers in diameter) and have a heterogeneous appearance microscopically, depending on the
material digested. After digestion of the contents of the secondary lysosome, nutrients diffuse through the lysosomal
membrane and enter the cytoplasm. Undigestible compounds are retained in the vacuole and are called residual bodies.
In long-lived cells, such as neurons, myocardial cells, and hepatocytes, a large quantity of material may accumulate and
are referred to as lipofuscin (age or wear and tear pigment), that does not seem to harm the function of the cells.
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H & E stained tissue sections.
LYSOSOMES AND PHAGOSOMES.
Lysosomes are capable of secreting their contents after fusion with the plasma membrane. Phagosomes, on the
contrary, are formed by the phagocytosis of bacteria and cellular debris; they eventually transform into
phagolysosomes.
Lysosomal Constituents of Leukocytes
Neutrophils and monocytes contain lysosomal granules, which, when released, may contribute to the inflammatory response.
Neutrophils have two main types of granules. Both types of granules can fuse with phagocytic vacuoles containing engulfed
material, or the granule contents can be released into the extracellular space.
Release of Leukocyte Products and Leukocyte-Mediated Tissue Injury
Leukocytes are important causes of injury to normal cells and tissues under several circumstances: As part of a
normal defense reaction against infectious microbes, when adjacent tissues suffer "collateral damage", when the
inflammatory response is inappropriately directed against host tissues, as in certain autoimmune diseases, or
when the host reacts excessively against usually harmless environmental substances, as in allergic diseases.
In all these situations, the mechanisms by which leukocytes damage normal tissues are the same as the
mechanisms involved in antimicrobial defense, because once the leukocytes are activated, their effector
mechanisms do not distinguish between offender and host. During activation and phagocytosis, neutrophils and
macrophages release microbicidal and other products not only within the phagolysosome, but also into the
extracellular space. The most important of these substances are lysosomal enzymes, present in the granules, and
reactive oxygen and nitrogen species.
LYSOSOMAL STORAGE DISORDERS.
With a deficiency of a functional lysosomal enzyme, catabolism of its substrate is incomplete. This leads to the
accumulation of the undegraded, or partially degraded insoluble metabolite within the lysosomes. These organelles
become stuffed with undigested material and enlarge, and numerous enough to interfere with normal cell functions,
and this produces lysosomal storage disorders.
In addition to nonfunctioning or missing enzymes, lysosomal storage disorders can result from lack of any protein
essential for normal function of lysosomes; such as, a lack of an enzyme activator or protector protein, lack of a
substrate activator protein, or a lack of a transport protein required for removal of the digested material from the
lysosomes.
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In general, the distribution of the stored material in lysosomes, and organs affected, is determined by the tissue where
most of the material to be degraded is found, and the location where most of the degradation normally occurs. For
example, brain is rich in gangliosides, and defective hydrolysis of gangliosides, as occurs in GM1 and GM2
gangliosidoses, results primarily in accumulation within neurons and consequent neurologic symptoms. Defects in
degradation of mucopolysaccharides affect virtually every organ, because mucopolysaccharides are widely distributed in
the body. Because cells of the mononuclear phagocyte system are especially rich in lysosomes and are involved in the
degradation of a variety of substrates, organs rich in phagocytic cells, such as the spleen and liver, are frequently
enlarged in several forms of lysosomal storage disorders.
PATHOGENESIS OF LYSOSOMAL STORAGE DISORDERS.
Given the many steps in the synthesis and processing of lysosomal hydrolases, it is not surprising that there are many
ways in which they can become dysfunctional. There may be inherent defects of enzyme synthesis, folding, activation,
targeting, or membrane protein defects for example.
Lysosomal storage disorders are divided into categories based on the biochemical nature of the accumulated
metabolite, creating subgroups such as the glycogenoses, sphingolipidoses (lipidoses), mucopolysaccharidoses (MPSs),
and mucolipidoses.
Pathogenesis of lysosomal storage diseases. In the illustration a complex substrate is normally degraded by a series of
lysosomal enzymes (A, B, and C) into soluble end products. If there is a deficiency or malfunction of one of the enzymes
(e.g., B), catabolism is incomplete and insoluble intermediates accumulate in the lysosomes.
MICROSTRUCTURAL EFFECTS OF LYSOSOMAL STORAGE DISORDERS.
The buildup of undigested material secondary to lysosomal enzyme dysfunction results in the formation of typical
histochemical and ultrastructural changes. Light microscopy often reveals engorged macrophages with a characteristic
appearance, such as that of ‘crumpled tissue paper’ in macrophages in Gaucher disease, or ‘sea-blue histiocytes’ in
Niemann–Pick disease. Characteristic ultrastructural changes have also been described based on the appearance of
residual bodies. These are vacuoles containing undigested material, and are the hallmark of primary storage in these
disorders. Residual bodies are described in Tay–Sachs disease and Niemann-Pick disease.
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HISTORICAL PROGRESSION IN UNDERSTANDING.
The first description of a lysosomal storage disorder was that of Tay-Sachs disease in 1881, but the lysosome was not
discovered until 1955, by Christian De Duve. Pompe's disease was the first demonstrating a link, by Hers in 1963,
between an enzyme deficiency and a storage disorder. This culminated in the successful treatment of Gaucher disease
with β-glucosidase in the early 1990s. It is now recognized that these disorders are not simply a consequence of pure
storage, but result from perturbation of complex cell signalling mechanisms. These in turn give rise to secondary
structural and biochemical changes.
The majority of lysosomal storage disorders (LSDs) result from defective lysosomal hydrolysis of endogenous
macromolecules and their accumulation. They tend to be multisystemic and are always progressive, although the rate of
progression may vary.
As a result, a variety of pathogenic cascades are activated such as altered calcium homeostasis, oxidative stress,
inflammation, altered lipid trafficking, autophagy, endoplasmic reticulum stress, and autoimmune responses. It has
become clear that simple storage does not satisfactorily explain the organ enlargement or pathology that is seen in
storage disorders.
For some diseases, wide ranging and systematic studies have been performed, whereas for other diseases, scant data
are available. This reflects to some extent the relatively low frequency of each individual disease in the population.
PATHOLOGIC LSD DERANGEMENTS.
LSD classification is usually based on the biochemical nature of the accumulating substrate; However, in many LSDs,
there is accumulation of secondary metabolites, such as cholesterol, that are unrelated to the primary genetic defect.
Cholesterol storage in the sphingolipidoses is probably related to the membrane association, and modes of regulation of
these lipids, although a precise mechanism is lacking to explain the concomitant changes in sphingolipid and cholesterol
levels in the LSDs.
Defective intracellular calcium signaling is a key common pathway in LSDs with impaired calcium homeostasis leading to
endoplasmic reticulum (ER) stress, oxidative stress, and cell death. However, the mechanism leading to impaired calcium
homeostasis is different in each LSD. Altered calcium homeostasis can be classified according to the organelle in which
defective calcium signaling is observed:
 For example, increased calcium release from the ER occurs in models of neuronal forms of Gaucher disease due
to overactivation of the ER calcium channel, the ryanodine receptor; glucosylceramide (GlcCer), the lipid that
accumulates in Gaucher disease, directly modulates the ryanodine receptor. In Niemann-Pick type A disease,
elevated cytosolic calcium is due to a reduction in the rate of calcium uptake by the sarco/endoplasmic
reticulum Ca2+-ATPase (SERCA).
 Mitochondrial calcium homeostasis is altered in at least two LSDs. In one neuronal model of GM1 gangliosidosis,
GM1 accumulation in a glycosphingolipid-enriched fraction of mitochondria-associated ER membranes results in
elevated mitochondrial membrane permeabilization, opening of the permeability transition pore, and activation
of the mitochondrial apoptotic pathway.
 Niemann-Pick type C cells display a major reduction in lysosomal calcium stores as a result of sphingosine
storage, resulting in defective endocytic fusion and trafficking, which subsequently induces cholesterol,
sphingomyelin, and glycosphingolipid storage.
Accumulating evidence suggests that reactive oxygen species (ROS) are important, and are perhaps common mediators
of cell death in many LSDs. The central role that oxidative stress plays in integrating other cellular pathways and stresses
suggests activation in LSDs as a secondary biochemical pathway, rather than as a direct result of accumulation of the
primary substrate.
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Macrophage activation following storage is seen in many LSD. For example, raised concentrations of cytokines or
chemokines have been found in patients with Gaucher disease and have been postulated to play a role in the
pathogenesis, especially that of bone disease.
Although the LSDs involve storage of self-components, a common host response is the inappropriate activation of the
immune system, resulting in chronic inflammation. The exact mechanisms leading to immune activation are unknown,
but probably reflect altered signaling pathways in response to storage. In Gaucher disease, the main storage cell types
are macrophages, which are found throughout the body, but are particularly prevalent in the liver, spleen, and bone
marrow. Substrate storage in macrophages leads to macrophage activation and release of multiple cytokines and to the
release of the chitinase, chitotriosidase, which serves as a useful clinical biomarker for this disease.
In LSDs with CNS pathology, brain inflammation is a common feature. In the brain, macrophage lineage cells are the
microglia, which respond to trauma and disease by activating the inflammatory response. Upon substrate accumulation
in LSDs, an inflammatory response is triggered that is not self-limiting, and once triggered; it progressively increases in
parallel with the storage burden. Numerous studies in multiple LSDs indicate that the inflammatory process contributes
to pathogenesis. One example is the GM1 and GM2 gangliosidoses, where activation of both CNS and peripheral
inflammation predates the onset of clinical signs and involves elevation of multiple proinflammatory cytokines. Despite
the fact that inflammation is a downstream event in the pathogenic cascade, it may nevertheless be a target for
adjunctive therapy in multiple LSDs. Thus, when a mouse model of Sandhoff disease was treated with non-steroidal antiinflammatory drugs to prevent peripheral immune cell recruitment to the brain, clinical benefit resulted.
Autophagy is a vacuolar, self-digesting mechanism responsible for removal of long lived proteins and damaged
organelles. Autophagosomes fuse with lysosomes for degradation of their cargo by lysosomal hydrolases. Autophagy can
also serve as a programmed cell death mechanism, with impairment of autophagosome-lysosome fusion or
overinduction of autophagy leading to autophagosome accumulation and cell death. Activation of autophagy has also
been observed in Pompe disease. Studies demonstrated that autophagic buildup has a profound effect on the endocytic
pathway.
In the ER, secretory and transmembrane proteins fold into their native conformations and undergo post-translational
modifications. When these functions are impaired, misfolded proteins accumulate in the ER lumen and activate the
unfolded protein reaction (UPR), which can initiate apoptosis. Unfolded protein accumulation can occur in response to
changes in the ER environment, including glucose starvation, reducing agents, and depletion of ER calcium stores.
Because calcium homeostasis is altered in LSDs, this pathway could also be potentially involved in LSD pathology.
LYSOSOMAL FUNCTION AND STORAGE IN THE CNS, AND THE ROLE OF MICROGLIA.
The LSDs affecting the central nervous system (CNS) pose the greatest challenges in treatment. Microglia are the
resident macrophage population of the brain. Microglia have many characteristics of macrophages, including the
presence of hydrolases. They are derived from circulating blood monocytes that invade the brain early in postnatal life
and become amoeboid microglia, which then differentiate into ramified microglia. Postnatally, they are continuously
replaced by blood-borne monocytes that cross the blood–brain barrier into the brain parenchyma. The BBB effectively
prevents most polar blood-borne solutes from crossing. Yet monocytes cross the BBB and differentiate into microglia.
Monocyte–endothelial cell interactions are somehow responsible for monocyte migration across the BBB.
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ORIGIN OF LYSOSOMAL SUBSTRATES IN THE CNS.
The origin of the substrate may differ inside and outside the CNS. Gaucher disease is a good example. There are three
broad clinical phenotypes. Patients with type I Gaucher's disease do not have neurological involvement (nonneuronopathic), while patients with types II and III do (neuronopathic). Patients with the neuronopathic forms of
Gaucher's disease have increased levels of the substrate glucosylceramide (GCS) in the brain. Glucosylceramide is
derived from two sources: Inside the CNS it is derived predominantly from gangliosides, while elsewhere it is derived
predominantly from the breakdown of blood cells. In type I patients, the degradation of blood cell derived
glucosylceramide is blocked, but there is sufficient enzyme activity in the CNS to break down ganglioside-derived
glucosylceramide, thus preventing its accumulation in the brain. In types II and III, however, there is less residual enzyme
activity, insufficient to degrade even ganglioside-derived GCS in the CNS.
SECRETION-RECAPTURE PATHWAY.
A significant proportion of newly synthesized enzyme is not bound to the M6-P receptor in the Golgi, but instead is
secreted and then endocytosed into neighboring cells via M6-P receptors on their plasma membranes. Understanding
this secretion–recapture pathway helps with the understanding of mucolipidoses types II. This disorder is characterized
by grossly elevated extra-lysosomal (plasma and cytosol) levels of a large number of lysosomal enzymes that require the
M6-P recognition marker for receptor-mediated uptake. It was later proven that they were characterized by failure of
enzymes to acquire this recognition marker.
CELL TYPE CLASSIFICATION.
It is useful to separate the various disorders according to the predominant cell type involved, as this has the most
important implications for therapy:



Neurological involvement: Most LSDs have neurological involvement.
Mesenchymal involvement: This group comprises essentially all the mucopolysaccharidoses, in whom
mesenchymal involvement is universal.
Reticuloendothelial involvement: This group comprises many of the sphingolipidoses. Reticuloendothelial cells
are usually far more accessible to therapy than mesenchymal cells and neurons. Hence this group of disorders
tends to respond the best to therapy, especially in those disorders in which the CNS is not involved.
RELATIONSHIP TO RESIDUAL ENZYME ACTIVITY.
The severity of the phenotype is closely related to the residual enzyme activity. There is a ‘critical threshold’ of enzyme
activity where the enzyme activity can deal with substrate influx. Below this, it cannot and there is accumulation of
substrate. In general, the lower the residual activity, the earlier the age of onset and the more severe the disease. For
those diseases for which enzyme-based therapies are available, residual enzyme activity is important in determining
response to treatment. The lower the residual activity, the less satisfactory the response.
CLINICAL FEATURES.
A greater awareness of clinical features may help to reduce misdiagnosis and promote the early detection of lysosomal
storage disorders. Implementing therapy at the earliest stage possible is crucial for several of the lysosomal storage
disorders.
Molecular genetics is a potentially useful tool, but in practice it has relatively few clinical applications. Virtually all known
LSDs are characterized by considerable phenotypic heterogeneity and assays are not routinely available. Therefore the
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distinction often has to be made clinically, which is prone to error. This has resulted in difficulty when considering
treatment.
Generally, clinicians are taught that newborns with LSDs appear normal at birth and that the symptoms develop
progressively over the first few months of life or even after many years. However, a portion of these patients can be
mildly symptomatic as early as the first few days of life or even before birth, or they may have transient symptoms in the
newborn period. Nonimmune hydrops fetalis (NIHF) can be the initial presentation, indicating prenatal involvement.
NEONATAL AND PEDIATRIC PRESENTATIONS.
Most newborns with LSDs appear normal at birth, because many of the toxic metabolites
cross the placenta during pregnancy and are cleared by the mother during gestation. The interval between birth and the
onset of clinical symptoms can range from hours
to months.
Symptoms Encountered in Newborns with LSDs
System
Neurologic
Manifestations
Hypotonia
Floppy-infant syndrome
Trismus
Strabismus
Opisthotonus
Spasticity
Seizures
Peripheral neuropathy
Developmental delay
Irritability
Extrapyramidal movement
disorder
Hydrocephalus
Respiratory
Congenital lobar emphysema
Impaired cough
Recurrent respiratory infections
Hoarseness
Endocrine
Osteopenia
Metabolic bone disease
Secondary hyperparathyroidism
Congenital adrenal hyperplasia
Cardiovascular Cardiomegaly
Congenital heart failure
Arrhythmias
Wolff-Parkinson-White
syndrome
Cardiomyopathy
Dysmorphology
Head and neck
Microcephaly
Enlarged nuchal translucency
Microstomia
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Micrognathia/microretrognathi
a
Long philtrums
Limbs
Bilateral broad thumbs and toes
Bilateral club feet
Eversed lips
Flattened nasal bridge
Short nasal columella
Oral
Macroglossia
Molar hypoplasia
Hypertrophic gums
Facial
Absent nasal septum
Coarse facies
Low-set ears
Gastrointestinal
Hepatosplenomegaly
Neonatal cholestasis
Bones and joints
Lytic bone lesions
Joint contractures
Dysostosis multiplex
Hyperphosphatasemia
Vertebral breaking
Broadening of tubular bones
Punctuate epiphysis
Craniosynostosis
Painful joint swelling
Skin
Congenital ichthyosis
Collodion infant
Hypopigmentation
Telangiectasias
Extended Mongolian spots
Ocular
Corneal clouding
Megalocornea
Glaucoma
Cherry-red spots
Fundi hypopigmentation
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Bilateral cataracts
Bilateral epicanthal inferior
orbital creases
Palpebral edema
Hypertelorism
Hematologic
Anemia
Thrombocytopenia
Hydrops fetalis NIHF
Congenital ascites
Recurrent fetal losses
DIAGNOSIS.
The recent development and availability of enzyme-replacement therapy (ERT) for several of the LSDs makes diagnosis
early in the clinical course particularly important. Early diagnosis and intervention is essential for maximizing the
potential benefit from some of these therapies and may prevent irreversible organ damage.
Because there is an overlap of clinical features in many of the LSDs, it is difficult to establish a diagnosis solely on the
basis of clinical presentation. Fortunately, different accurate laboratory assays based on detection of the storage
product, enzymatic assays, and DNA diagnostics
have been developed. There are also biomarkers such as chitotriosidase that, although not optimally specific, can help
monitor disease load. For example, in Gaucher disease, chitotriosidase levels decrease after ERT.
Urine screens that test for elevated levels of secreted substrate material are used routinely to examine the pattern of
glycosaminoglycans and oligosaccharides. After determining that the level of glycosaminoglycans is elevated,
electrophoresis can further support the diagnosis of the MPSs, although the definitive diagnosis is made by enzyme
analysis. Generally, panels of enzyme activity assays are performed on a combination of leukocytes and plasma and
predominantly include enzymes involved in the digestion of glycosphingolipids and oligosaccharides. Diseases tested for
in these panels include Gaucher disease, Niemann-Pick disease types A and B, acid lipase deficiency, GM1 and GM2
gangliosidosis, Krabbe disease, metachromatic leukodystrophy, mucolipidosis type 2 and 3, fucosidosis, alphamannosidosis, MPS type VII, and Schindler disease. There are a few methods for the determination of enzymatic
activities that have been developed recently on the basis of elution of the enzyme from a dried blood spot.
Molecular analysis is rarely used as the primary screening tool for the diagnosis of LSDs. However, molecular analysis
plays an important role with respect to carrier and prenatal testing for a variety of LSDs. In cases when there is a strong
diagnostic suspicion, sequencing of the relevant genes can be used to detect mutations. However, establishing that the
nucleotide change identified is pathologic, rather than a mere polymorphism, can be challenging.
Population screening for the LSDs is not performed routinely except for high-risk ethnic groups, for which screening for
specific disorders may be appropriate, such as Tay-Sachs disease in the Ashkenazi Jewish population.
PRINCIPLES OF THERAPY.
A variety of non-specific therapeutic measures are available for most disorders and not discussed. Specific therapy can
be broadly divided into enzyme-based therapies and non-enzyme based therapies. The enzyme-based therapies
currently available are Bone Marrow Transplant (BMT) and Enzyme Replacement Therapy (ERT).
Following engraftment, donor bone marrow provides a constant and permanent endogenous supply of enzymeproducing cells. Transfer of enzyme to surrounding cells takes place and clears accumulated substrate. Importantly, cells
of donor origin have been demonstrated in the brain following human BMT. Experimental evidence has demonstrated
enzyme transfer to surrounding neurons. However, this mechanism is unlikely to operate in Gaucher disease, in which
the enzyme is membrane-bound. While BMT does result in enzyme expression and substrate clearance, this is
incomplete, possibly due to the slow rate of microglial engraftment. Experiments on transgenic mice suggest that, while
there is quick and complete engraftment in bone marrow and spleen, donor microglial engraftment is much slower.
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The addition of enzymes derived from other sources to cultures of deficient fibroblasts resulted in clearance of the
storage products, thereby providing a rational basis for exogenous enzyme replacement (ERT). The results of ERT vary
considerably from disease to disease.
Important considerations are the age of onset, rapidity of progression and the presence or absence of neurological
involvement. Within each disease there is considerable variation. Mildly affected patients are the most likely to respond.
Because the replacement enzymes do not cross the blood-brain barrier, ERT does not correct central nervous system
(CNS) manifestations.
Substrate reduction therapy (SRT) is a non-enzyme-based therapy. First applied to the GSL group of disorders, the imino
sugar N-butyldeoxynojirimycin (NB-DNJ) had been shown to inhibit ceramide-specific glucosyltransferase, which
catalyses the first step in GSL biosynthesis. This results in the inhibition of biosynthesis of all glucosylceramide-based
glycosphingolipids. Studies of Tay-Sachs disease demonstrated reduction in the CNS substrate load and amelioration of
the clinical symptoms.
Chaperone therapy: Lysosomal enzymes may undergo misfolding as a result of mutations in the encoding gene.
Misfolded proteins are capable of aggregation and accumulation in cells and this may lead to cell death. They are
normally eliminated by the endoplasmic reticulum-associated degradation pathway with the help of naturally occurring
molecular ‘chaperones’, small molecules that ensure their safe degradation via this pathway. In some cases, the active
site may fold normally, which means that they are capable of hydrolysis. Such proteins are candidates for salvage by
pharmacological ‘chaperones’. These are specific, small molecular weight ligands that reversibly bind to such proteins,
stabilize them and ensure their correct targeting to the lysosome. A prerequisite is that the binding is reversible. That is
to say, having safely ‘chaperoned’ the mutant enzyme to the lysosome, the ligand–protein complex must then dissociate
so that the enzyme is free to bind to its substrate. This approach is also known as enzyme enhancement therapy and is
attracting clinical interest, especially as it has been shown that chaperones are capable of crossing the BBB and may
therefore have therapeutic potential for the CNS. The disadvantage of chaperone-mediated therapy is that it is likely to
be effective only in those patients in whom the mutation does not inactivate the catalytic site.
A Short List of Major Lysosomal Storage Disorders (The highlighted representative disorders will be
discussed)
Major Accumulating
Metabolites
Disease
GLYCOGENOSIS
Enzyme Deficiency
Type 2-Pompe disease
α-1,4-Glucosidase (the only glycogen storage
disease also a lysosomal storage disease)
Glycogen
GM1 ganglioside β-galactosidase
GM1 ganglioside, galactosecontaining oligosaccharides
Tay-Sachs disease
Hexosaminidase, α subunit
GM2 ganglioside
Sandhoff disease
Hexosaminidase, β subunit
GM2 ganglioside, globoside
GM2 gangliosidosis variant AB
Ganglioside activator protein
GM2 ganglioside
SPHINGOLIPIDOSES
GM1 gangliosidosis
Type 1-infantile, generalized
Type 2-juvenile
GM2 gangliosidosis
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SULFATIDOSES
Metachromatic leukodystrophy
Arylsulfatase A
Sulfatide
Multiple sulfatase deficiency
Arylsulfatase A, B, C; steroid sulfatase; iduronate
sulfatase; heparan N-sulfatase
Sulfatide, steroid sulfate,
heparan sulfate, dermatan
sulfate
Krabbe disease
Galactosylceramidase
Galactocerebroside
Fabry disease
α-Galactosidase A
Ceramide trihexoside
Gaucher disease
Glucocerebrosidase
Glucocerebroside
Niemann-Pick disease: types A
and B
Sphingomyelinase
Sphingomyelin
MUCOPOLYSACCHARIDOSES (MPSs)
MPS I H (Hurler)
α-l-Iduronidase
MPS II (Hunter)
l-Iduronosulfate sulfatase
Dermatan sulfate, heparan
sulfate
MUCOLIPIDOSES (MLs)
I-cell disease (ML II) and pseudoHurler polydystrophy
Deficiency of phosphorylating enzymes essential for Mucopolysaccharide,
the formation of mannose-6-phosphate recognition glycolipid
marker; acid hydrolases lacking the recognition
marker cannot be targeted to the lysosomes but are
secreted extracellularly
OTHER DISEASES OF COMPLEX CARBOHYDRATES
Fucosidosis
α-Fucosidase
Fucose-containing
sphingolipids and glycoprotein
fragments
Mannosidosis
α-Mannosidase
Mannose-containing
oligosaccharides
Aspartylglycosaminuria
Aspartylglycosamine amide hydrolase
Aspartyl-2-deoxy-2acetamido-glycosylamine
OTHER LYSOSOMAL STORAGE DISORDERS
Wolman disease
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Acid lipase
Cholesterol esters,
triglycerides
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Acid phosphate deficiency
Lysosomal acid phosphatase
Phosphate esters
GM2 GANGLIOSIDOSES.
GM2 gangliosidoses are three lysosomal storage disorders caused by an inability to catabolize GM2 gangliosides.
Degradation of GM2 gangliosides requires three polypeptides encoded by three diifferent genes. The phenotypic effects
of mutations affecting these genes are fairly similar, because they result from accumulation of GM2 gangliosides. The
underlying enzyme defect, however, is different for each.
TAY-SACHS DISEASE.
Tay-Sachs disease is the most common form of GM2 gangliosidosis, and results from mutations in the α-subunit locus on
chromosome 15 that cause a severe deficiency of hexosaminidase A. This disease is especially prevalent among Jews,
particularly among those of Eastern European (Ashkenazic) origin, in whom a carrier rate of 1 in 30 has been reported. `
Morphology
Hexosaminidase A is absent from virtually all tissues, so GM2 ganglioside accumulates in many tissues (e.g., heart, liver,
spleen), but the involvement of neurons in the central and autonomic nervous systems, and retina dominates the clinical
picture.
On histologic examination, the neurons are ballooned with cytoplasmic vacuoles, each representing a markedly
distended lysosomes filled with gangliosides. Fat stains, such as oil red O, are positive. With the electron microscope,
cytoplasmic inclusions can be seen; the most prominent is whorled configurations within lysosomes composed of onionskin layers of lipid membranes.
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Ganglion cells in Tay-Sachs disease. A, Under the light microscope a large neuron has obvious lipid vacuolation. B, A
portion of a neuron under the electron microscope shows prominent lysosomes with whorled configurations. Part of the
nucleus is shown above. (A, courtesy of Dr. Arthur Weinberg, Department of Pathology, University of Texas
Southwestern Medical Center, Dallas, TX; B, electron micrograph courtesy of Dr. Joe Rutledge, University of Texas
Southwestern Medical Center, Dallas, TX.)
In time there is progressive destruction of neurons, proliferation of microglia, and accumulation of complex lipids in
phagocytes within the brain substance. The ganglion cells in the retina are swollen with GM2 ganglioside, particularly at
the margins of the macula. A cherry-red spot thus appears in the macula, representing accentuation of the normal color
of the macular choroid contrasted with the pallor produced by the swollen ganglion cells in the remainder of the retina.
This finding is characteristic of Tay-Sachs disease, and other storage disorders affecting the neurons.
Clinical features
Infants appear normal at birth, but begin to show signs and symptoms at about age 6 months. There is relentless motor
and mental deterioration, beginning with motor incoordination, mental obtundation leading to muscular flaccidity,
blindness, and increasing dementia. Early in the course of the disease, the characteristic, but not pathognomonic,
cherry-red spot appears in the macula of the eye in almost all patients. Over the span of 1 or 2 years a complete
vegetative state is reached, followed by death at age 2 to 3 years.
More than 100 mutations have been described in the α-subunit gene; most affect protein folding. Such misfolded
proteins trigger the "unfolded protein" response leading to apoptosis. Substrate reduction therapy has been used
effectively, but these findings have given rise to the possibility of chaperone therapy of Tay-Sachs disease.
NIEMANN-PICK DISEASE, Types A and B.
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Niemann-Pick disease types A and B refers to two related disorders that are characterized by lysosomal accumulation of
sphingomyelin due to an inherited deficiency of sphingomyelinase. Type A is a severe infantile form with extensive
neurologic involvement, marked visceral accumulations of sphingomyelin, progressive wasting and early death within
the first 3 years of life. Type B disease patients have organomegaly, but generally no central nervous system
involvement, and they usually survive into adulthood.
As with Tay-Sachs disease, Niemann-Pick disease types A and B are common in Ashkenazi Jews. The acid
sphingomyelinase gene maps to chromosome 11p15.4. More than 100 mutations have been found in the
sphingomyelinase gene and there seems to be a correlation between the type of mutation, the severity of enzyme
deficiency, and the phenotype.
Morphology
In the classic infantile type A variant, a missense mutation causes almost complete deficiency of sphingomyelinase.
Sphingomyelin is a ubiquitous component of cellular membranes (including organelles), and so the enzyme deficiency
blocks degradation of the lipid, resulting in its progressive accumulation within lysosomes, particularly within cells of the
mononuclear phagocyte system. Affected cells become enlarged, sometimes to 90 μm in diameter, due to the distention
of lysosomes with sphingomyelin and cholesterol. In frozen sections of fresh tissue, the vacuoles stain for fat. Electron
microscopy confirms that the vacuoles are engorged secondary lysosomes that often contain membranous cytoplasmic
bodies resembling concentric lamellated myelin figures, sometimes called "zebra" bodies.
The lipid-laden phagocytic foam cells are widely distributed in the spleen, liver, lymph nodes, bone marrow, tonsils,
gastrointestinal tract, and lungs. The involvement of the spleen generally produces massive enlargement, sometimes to
ten times its normal weight, but the hepatomegaly is usually not quite so striking. The lymph nodes are generally
moderately to markedly enlarged throughout the body. Sea-blue histiocytes are ceroid-laden macrophages detectable
by May-Giemsa staining.
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In the brain the gyri are shrunken and the sulci widened. The neuronal involvement is diffuse, affecting all parts of the
nervous system. Vacuolation and ballooning of neurons constitute the dominant histologic change, which in time leads
to cell death and loss of brain substance. A macular cherry-red spot similar to that seen in Tay-Sachs disease is present in
some individuals.
Niemann-Pick disease in liver. The hepatocytes and Kupffer cells have a foamy, vacuolated appearance due to deposition
of lipids. (Courtesy of Dr. Arthur Weinberg, Department of Pathology, University of Texas Southwestern Medical Center,
Dallas, TX.)
Clinical manifestations
Manifestations in type A disease may be present at birth, and almost invariably become evident by age 6 months.
Infants typically have a protuberant abdomen because of the hepatosplenomegaly. Once the manifestations appear,
they are followed by progressive failure to thrive, vomiting, fever, and generalized lymphadenopathy as well as
progressive deterioration of psychomotor function. Death usually within the first or second year of life.
Diagnosis
The diagnosis is established by biochemical assays for sphingomyelinase activity in liver or bone marrow biopsy.
Individuals affected with types A and B as well as carriers can be detected by DNA analysis.
GAUCHER DISEASE.
A cluster of autosomal recessive disorders resulting from mutations in the gene encoding glucocerebrosidase. This
disease is the most common lysosomal storage disorder.
Glucocerebrosidase, which is associated with the lysosomal membrane, does not acquire the M6P residue, although it
does undergo N-glycosylation and is targeted. The precise mechanism by which this occurs is unknown.
Glucocerebrosidase is an enzyme that normally cleaves the glucose residue from ceramide. As a result of the enzyme
defect, glucocerebroside accumulates principally in phagocytes, but in some also in the central nervous system.
Glucocerebrosides are continually formed from the catabolism of glycolipids derived mainly from the cell membranes of
senescent leukocytes and erythrocytes. The pathologic changes in Gaucher disease are caused not just by the burden of
storage material, but also by activation of macrophages and the consequent secretion of cytokines such as IL-1, IL-6, and
tumor necrosis factor (TNF).
Morphology
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Glucocerebrosides accumulate in massive amounts within phagocytic cells throughout the body in all forms of Gaucher
disease. The distended phagocytic cells, are known as Gaucher cells, and are found in the spleen, liver, bone marrow,
lymph nodes, tonsils, thymus, and Peyer's patches. Similar cells may be found in both the alveolar septa and the air
spaces in the lung. In contrast to other lipid storage diseases, Gaucher cells rarely appear vacuolated, but instead have a
fibrillary type of cytoplasm that looks like crumpled tissue paper. Gaucher cells are often enlarged, sometimes up to 100
μm in diameter, and have one or more dark, eccentrically placed nuclei. Periodic acid-Schiff staining is usually intensely
positive. With the electron microscope the fibrillary cytoplasm can be resolved as elongated, distended lysosomes,
containing the stored lipid in stacks of bilayers.
Image: Gaucher disease involving the bone marrow. Gaucher cells (A, H&E; B, Wright stain) are plump macrophages that
characteristically have the appearance in the cytoplasm of crumpled tissue paper (B), due to accumulation of
glucocerebroside. (Courtesy of Dr. John Anastasi, Department of Pathology, University of Chicago, Chicago, IL.)
Clinical Features
The clinical course of Gaucher disease depends on the subtype. In type I, symptoms and signs first appear in adult life
and are related to splenomegaly or bone involvement. Most commonly there is pancytopenia or thrombocytopenia
secondary to hypersplenism. Pathologic fractures and bone pain occur if there has been extensive expansion of the
marrow space. Although the disease is progressive in the adult, it is compatible with long life. In types II and III, central
nervous system dysfunction, seizures, and progressive mental deterioration dominate, although organs such as the liver,
spleen, and lymph nodes are also affected.
Diagnosis
The diagnosis of homozygotes can be made by measurement of glucocerebrosidase activity in peripheral blood
leukocytes or in extracts of cultured skin fibroblasts. Because more than 150 mutations in the glucocerebroside gene can
cause Gaucher disease, it is not possible to use a single genetic test.
Therapy
Replacement therapy with recombinant enzymes is the mainstay for treatment of Gaucher disease; it is effective, and
those with type I disease can expect normal life expectancy with this form of treatment.
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MUCOPOLYSACCHARIDOSES.
MPSs are related syndromes that result from genetic deficiencies of lysosomal enzymes involved in the degradation of
mucopolysaccharides (glycosaminoglycans). Chemically, mucopolysaccharides are long-chain complex carbohydrates
that are linked with proteins to form proteoglycans. They are abundant in the ground substance of connective tissue.
The glycosaminoglycans that accumulate in MPSs are dermatan sulfate, heparan sulfate, keratan sulfate, and
chondroitin sulfate. The enzymes involved in the degradation of these molecules cleave terminal sugars from the
polysaccharide chains along a polypeptide or core protein. In the absence of enzymes, these chains accumulate within
lysosomes in various tissues and organs of the body.
Several clinical variants of MPS, classified from MPS I to MPS VII, have been described, each resulting from the
deficiency of one specific enzyme. All of the MPSs, except one (Hunter syndrome is X-linked), are inherited as autosomal
recessive traits. Within a given group, subgroups exist that result from different mutant alleles at the same genetic locus.
Thus, the severity of enzyme deficiency and the clinical picture even within subgroups are often different.
In general, MPSs are progressive disorders, characterized by coarse facial features, clouding of the cornea, joint stiffness,
and mental retardation. Urinary excretion of the accumulated mucopolysaccharides is often increased.
Morphology
The accumulated mucopolysaccharides are generally found in mononuclear phagocytic cells, endothelial cells, intimal
smooth muscle cells, and fibroblasts throughout the body. Common sites of involvement are the spleen, liver, bone
marrow, lymph nodes, blood vessels, and heart.
Microscopically
Affected cells are distended and have apparent clearing of the cytoplasm to create so-called balloon cells. Under the
electron microscope, the clear cytoplasm can be resolved as numerous minute vacuoles. These are swollen lysosomes
containing a finely granular periodic acid-Schiff-positive material that can be identified biochemically as
mucopolysaccharide. Hepatosplenomegaly, skeletal deformities, valvular lesions, and subendothelial arterial deposits,
particularly in the coronary arteries, and lesions in the brain, are common through all of the MPSs. In many of the more
protracted syndromes, coronary subendothelial lesions lead to myocardial ischemia. Thus, myocardial infarction and
cardiac decompensation are important causes of death.
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Balloon cells within the connective tissue of the wrist of a patient
Clinical Features
Two well-characterized syndrome’s features are described: Hurler syndrome, also called MPS I-H, results from a
deficiency of α-l-iduronidase. It is one of the most severe forms of MPS. Affected children appear normal at birth but
develop hepatosplenomegaly by age 6 to 24 months. Their growth is retarded, and, as in other forms of MPS, they
develop coarse facial features and skeletal deformities. Death occurs by age 6 to 10 years and is often due to
cardiovascular complications. Hunter syndrome, also called MPS II, differs from Hurler syndrome in mode of inheritance
(X-linked), absence of corneal clouding, and milder clinical course.
I-CELL DISEASE.
I-cell disease (mucolipidosis type 2, ML-2) is a rare lysosomal disorder that presents at birth or in the first few months of
life with profound developmental delay and microcephaly. The acquisition of the M6-P marker separates glycoproteins
that are destined for the lysosome, from secretory glycoproteins. Failure of acquisition of this marker results in
mistargeting of lysosomal enzymes. They will not enter the lysosome and substrate breakdown will not occur. This is
what happens in I-cell disease. These patients lack the enzyme responsible for the first step in the Golgi apparatus, i.e.
the phosphotransferase. Without this marker, the proteins are instead excreted outside the cell -- the default pathway
for proteins moving through the Golgi apparatus. Consequently, all enzymes requiring the M6-P marker fail to enter the
lysosome; these patients have very high plasma levels of all of these enzymes.
Lysosomes cannot function without these proteins, which function as catabolic enzymes for the normal breakdown of
substances throughout the body. As a result, a buildup of these substances occurs within lysosomes because they
cannot be degraded, resulting in the characteristic "I cells," or "inclusion cells." These cells can be identified under the
microscope. In addition, the defective lysosomal enzymes normally found only within lysosomes are instead found in
high concentrations in the blood.
Electron microscopy of the blood cells revealed a large number (up to 20) of cytoplasmic vacuoles, some of which have
no visible content, although most have an aggregation of small globular or possible tubular structures. In addition, a
round osmiophilic structure is found in most cells.
I-cell disease is an autosomal recessive disorder caused by a deficiency of GlcNAc phosphotransferase, which
phosphorylates mannose residues to mannose-6-phosphate on N-linked glycoproteins in the Golgi apparatus within the
cell. Without mannose-6-phosphate to target them to the lysosomes, the enzymes are transported from the Golgi to the
extracellular space, resulting in large intracellular inclusions of molecules requiring lysosomal degradation in patients
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with the disease. Hydrolases secreted into the blood stream cause little problem as they are deactivated in the neutral
pH of the blood.
Clinical features
Neonatal patients with I-cell disease have presented in the neonatal period with features of “metabolic bone disease”
accompanied by increased serum parathyroid hormone and alkaline phosphatase activity, but normal calcium
concentrations. Secondary hyperparathyroidism is a rare diagnosis and is probably a result of impaired transplacental
calcium transport due to the underlying lysosomal disorder. Cardiomyopathy may be associated with I-cell (and other
LSDs) disease, although generally not in the newborn. I-cell disease is also characterized by coarse facial features and
hypertrophic gums, which are unique to this disease at this age.
I-cell disease can present with neonatal hepatosplenomegaly. Neonatal jaundice may be the result of a bile duct injury
as part of I-cell disease. I-cell disease should be in the differential diagnosis of neonatal cholestasis. I-cell disease should
be part of the differential diagnosis of significant craniosynostosis in the neonate or prenatal diagnosis of short femurs,
especially if coarse facies is seen postnatally. Early eye involvement has also been described with I-cell disease; the most
frequent ocular finding is corneal clouding.
Typically, by the age of 6 months, failure to thrive and developmental delays are obvious symptoms of this disorder.
Some physical signs, such as abnormal skeletal development, coarse facial features, and restricted joint movement, may
be present at birth. Children with ML II usually have enlargement of certain organs, such as the liver (hepatomegaly) or
spleen (splenomegaly), and sometimes even the heart valves. Affected children often have stiff claw-shaped hands and
fail to grow and develop in the first months of life. Because of their lack of growth, they develop short-trunk dwarfism.
These young patients are often plagued by recurrent respiratory tract infections, and carpal tunnel syndrome. Children
with ML II generally die before their seventh year of life, often as a result of congestive heart failure or recurrent
respiratory tract infections.
Treatment
There is no cure yet for I-Cell disease/Mucolipidosis II disease. Treatment is limited to controlling or reducing the
symptoms that are associated with this disorder.
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Histology of the Nervous System
Lecture 5
Lecture 6- The Lab is in PowerPoint form**
Overview of the nervous system (Figure 9-1)
 main divisions
o central nervous system - composed of brain and spinal cord >> residence of the vast
majority of nerve cell bodies
o peripheral nervous system - composed of ganglia and nerves >> components found in
virtually all organs of the body
 cell types
o neurons - highly specialized to receive, process, store and transmit information
o glial cells - various support roles
General histological characteristics of nervous tissue
 neurons
o cell body or soma (Figure 9-3)
 nucleus contains prominent nucleolus >> transcriptionally very active
 Nissl bodies - darkly staining RER >> not present in axon or axon hillock
 large number of golgi complexes, mitochondria and lysosomes
 often sequester substances in storage vesicles over time (ex. aluminum, iron, lipofuscin,
melanin, etc.)
o cytoplasmic extensions - shape maintained by neurofilaments >> visible with silver or gold stains (Figure
9-9b)
 dendrites - transmit impulses towards cell body >> typically several and
highly branched
 axon - long, typically nonbranching processes that transmit impulses away
from cell body
 synaptic button - enlarged terminal end of an axon >> site of neurotransmitter release
 allow structural classification as bipolar, multipolar or pseudounipolar (Figure 9-4)
Nucleus with euchromatin- because the cell is active
These cells are large in length compared to other cells in the body
Myelination of the peripheral nerves via Schwann cells- wrapped with plasma membrane
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Above is a: Stylized image of gray matter- cell bodies of neurons
The interlacing of the dendrites and axons is called neuropil

axon myelination - allows impulse to travels up to 150X faster along an axon >> formed via wrapping of support
cells (Figures 9-3 and 9-22)
o myelinating cells
 CNS - oligodendrocytes >> can myelinate several axons (Figure 9-10a)
 PNS - Schwann cells >> can myelinate only one axon, but can ensheath
several unmyelinated axons (Figures 9-21 and 9-25)
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o
nodes of Ranvier - myelinating cells approximately 1-2 mm in length >> leave gaps
between myelin sheaths (Figure 9-23)
Above: The lipid stains with the heavy metal so the myelin stains darker. The higher power image shows
how the schwann cells wrap around it.
Above: Is in the CNS and provides support to axons and some support to axons
Have a fried egg appearance- central dark nucleus with clearing area around it- there are tumors oligodendrogliomas and
they have a very effective treatment.
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
synapse - a specialized cellular junction that allows communication between cells even though the
participating cells are not in direct physical contact >> visible with EM (Ancillary Figure)
 A. neurosecretory granules - contain neurotransmitter plus packaging and transport proteins >>
contents released into synaptic cleft upon stimulation of action potential
 structurally classified according to components involved >> axo-dendritic, axo- somatic or axoaxonal (Figure 9-7)
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Histological features of the CNS
 glial cells
o astrocytes - large star shaped cells (processes not typically evident) >> several types functionally but
only two types histologically >> fibrous and protoplasmic (Figures 9-10b and 9-11) >> heterochromatic
nuclei surrounded by neuropil
 form framework guide so that axons and dendrites migrate to correct locations during fetal
development >> maintained in certain brain regions in the adult
 move fluid from extracellular spaces to blood vessels
 may play role in blood-brain barrier - form end feet around blood vessels >>
create a highly selective permeability
 gliosis - form scar when neurons lost
Above: This is another type of glia in the CNS
Stained with gold or silver because it’s the only way to see the processes of the astrocytes
Glioblastoma- usually in temporal or parietal lobe- most common and malignant brain tumor
Astrocytoma
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Above: Can’t make out the processes of the astrocyte as much as you can with gold/ silver stains
Can you separate a astrocyte from a microglia? A microglia is a macrophage and its hard to differentiate
the two on an H&E stain.
o
o
oligodendrocytes - myelinate axons within CNS (ancillary figure)
 Appear as heterochromatic nuclei surrounded by translucent region >> may or may not be
adjacent to somas
ependyma - appear as ciliated simple cuboidal epithelial cells that line the ventricles and the central
canal of the spinal cord (Figures 9-10c and 9-12)
 glial limitans - unique basement membrane via intertwining of ependymal cells and underlying
astrocytes
These cells line the ventricals in the CNS and the spinal cord. They are characterisitically cuboidal
with some cilia. These cells are good at making CSF.
Pathology Lectures 1-5 (6 is in PowerPoint form)
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Marcia Reeves
Pathology Exam 1 Page 42 of 47
o
microglial cell - specialized immune cells of the CNS that demonstrate a low level of phagocytosis >> will
increase in number if pathogen present within CNS (Figures 9-10d and 9-13)
Above: Microglia has a phagocytic function in the CNS- you can use Ab to highlight the processes that are only on a
macrophage using immunocytochemistry. These are derived from mesodermal derived, whereas other glia are
ectodermal.

gray matter and nuclei - composed of unmyelinated fibers and nerve cell bodies (Figure 9-14 )
o neuropil – material consisting of entanglements of nerve cell processes in the CNS (Figure 9-9)
Above: Gray matter- neuropil plus the corresponding cell bodies
White matter is myelin
Swiss cheese is where the myelin is
Area with nuclei is the gray matter
The deposition of the intracellular pigment has a goldish pigment from lipid peroxification- lipofuscin


white matter - composed of myelinated and unmyelinated tracts (Figure 9-17)
choroid plexus - lining of ventricles that produces cerebral spinal fluid (CSF) via selective
supplementation of plasma
o composed of simple cuboidal epithelium overlying a highly vascularized connective
tissue (Figure 9-20)
Pathology Lectures 1-5 (6 is in PowerPoint form)
filtration and
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Marcia Reeves
Pathology Exam 1 Page 43 of 47
Choroid plexus makes CSF- housed in ventricular system of CNS
It has capillary morphology and the pathways are lined by ependymal cells
Capillary is separated by thin layer of pia mater
If you make too much CSF can have hydrocephalus- choroid plexus papillomas.
Histological features of the PNS
 nerve - a bundle of axons and dendrites bound together by connective tissue (Figures 9-26)
o connective tissue component >> easily visualized in cross section
 endoneurium - surrounds individual axons/dendrites and any associated
Schwann calls
 perineurium - groups axons/dendrites into small bundles called fascicles
 epineurium - tough outer sheath that binds fascicles into a nerve
o cross section of a nerve - differ in terms of fascicle size and arrangement plus ratio of
myelinated and unmyelinated axons
o nerves look very different in longitudinal section (Figure 9-28)
Pathology Lectures 1-5 (6 is in PowerPoint form)
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Marcia Reeves
Pathology Exam 1 Page 44 of 47
Looks wavy on longitudinal section
Patients with perineural invasion do not typically respond well to cancer treatment

ganglia - only sites where neuron cell bodies are located outside of the CNS
o sensory (cranial and dorsal root) ganglia - recognized by pseudounipolar neurons surrounded by a
capsule composed of satellite cells (ancillary figure)
o autonomic ganglia
 sympathetic ganglia – recognized by multipolar neurons that lack capsule of satellite cells
(ancillary figure)
 cell bodies accumulate lipofuschin with age
 parasympathic ganglia – quite small and embedded within wall of organ to be innervated
(ancillary figure)
Above: This is a ganglion node in the PNS- ganglion has large cell bodies inside a fibrous capsule
These have a lot of ER and golgi because they are so active.
The ganglia have satellite (supporting) cells
If a child has chronic constipation and megacolon  Hirschsprung's disease
Pathology Lectures 1-5 (6 is in PowerPoint form)
Block 3 - 2012-13
Marcia Reeves
Pathology Exam 1 Page 45 of 47
Pathology Lectures 1-5 (6 is in PowerPoint form)
Block 3 - 2012-13
Marcia Reeves
Pathology Exam 1 Page 46 of 47
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Pathology Lectures 1-5 (6 is in PowerPoint form)
Block 3 - 2012-13
Marcia Reeves
Pathology Exam 1 Page 47 of 47