Download Genetics

Genetics
Dr. Joseph de Nanassy
Associate Professor, uOttawa
Chief of Pathology, CHEO
[email protected]
737-7600 x 2897
Objectives
☺ Develop a basic understanding of the
genetic apparatus
☺ Comprehend definitions of major genetic
abnormalities
☺ Correlate molecular abnormalities and
genetic defects
Outline
I. Definitions
Genetic code
Chromosomes, Genes, Cell Division
Molecular mechanisms
II. Abnormal fetal development
Malformations, deformations, dysplasias,
disruptions
III. Perinatal pathology
Birth defects
Metabolic disorders
The Cell
Nucleus
☺ DNA: arranged in chromosomes
(network of granules = nuclear chromatin)
☺ RNA: spherical intranuclear structure(s)
- nucleolus / nucleoli
Genetic Code
☺ A series of messages contained in the
chromosomes
☺ This code regulates cell functions by way
of directing the synthesis of cell proteins
☺ The code corresponds to the structure of
the DNA
☺ The code is transmitted to new cells
during cell division
DNA structure
DNA replication
mRNA and tRNA
Chromosomes
☺ Exist in pairs – homologous: 22a + 1s
☺ Composed of double coils of DNA
☺ Basic unit: nucleotide
phosphate group
deoxyribose sugar
base: purine (A, G)
pyrimidine (T, C)
Genes
☺ Basic units of inheritance
☺ Segments of the DNA chain
☺ Beads on a (chromosome) string
☺ Determine cell properties, both structure
and functions unique to the cell
Genome
☺ Sum total of all genes contained in a
cell’s chromosomes
☺ Identical in all cells
☺ Not all genes are expressed in all cells
☺ Not all genes are active all the time
☺ May code for enzymes or other
functional proteins, structural proteins,
regulators of other genes
Gene Product
☺ A protein specified by a gene
☺ Transcribed into mRNA
☺ Translated through tRNA and
cytoplasmic ribosomes into protein
Human Genome
☺ 3 billion pairs of DNA nucleotides
☺ 50,000 – 100,000 genes
☺ Genes = 10% of human genome
☺ Exons: parts of the DNA chain that code for
specific proteins
☺ Introns: the parts in-between the exons
☺ Both exons and introns are transcribed but only
the exons are translated (introns are removed
from mRNA before leaving nucleus)
Sex chromosomes
☺ Genetic sex = composition of X and Y
☺ Large X: many genes, many activities
☺ Small Y: almost entirely male sexual diff.
☺ Female: XX, male XY
☺ One X randomly inactivated and
nonfunctional after first week of embryonic
development
☺ Same inactivated X in descendant cells
Lyon hypothesis
Barr body
Y chromosome
☺ Stains with some fluorescent dyes
- bright fluorescent spot in the nucleus
☺ Normal female: sex chromatin body
but no fluorescent spot
☺ Normal male: fluorescent spot
but no sex chromatin body
Cell Division
☺ Mitosis: somatic cells (PMAT)
Daughter cells have the same number of
chromosomes as the parent cell.
☺ Meiosis: gametogenesis (1st and 2nd div)
Number of chromosomes reduced by half.
Chromatids
☺ Paired chromosomes
Before mitosis, the DNA chains duplicate
to form new chromosome material.
The duplicated chromosomes lie side by
side = chromatid.
Mitosis = the process by which chromatids
separate into chromosomes.
Mitosis
☺ Interphase: DNA duplication to form
chromatids just before mitosis
☺ Prophase: centriole migration, mitotic spindle
☺ Metaphase: chromosomes line up in centre,
chromatids still joined at centromere
☺ Anaphase: chromatids separate into
chromosomes
☺ Telophase: new nuclear membranes form,
cytoplasm divides
Mitosis
Meiosis
☺ First meiotic division: duplication of
chromosomes to form chromatids
☺ Prophase of meiosis: homologous
chromosomes lie side by side over entire
length = synapse.
Interchange of segments of homologous
chromosomes = crossover.
2 Xs side by side just like the autosomes.
X and Y end to end – no crossover.
Meiosis
☺ Metaphase: paired chromosomes arrange in
middle of cell
☺ Anaphase: homologous chromosomes migrate
to opposite poles of the cell;
each chromosome is composed of two
chromatids, the chromatids are not separated
☺ Telophase: two new daughter cells form;
each contains half the chromosome number =
reduction of chromosomes by half; interchange
of genetic material occurred during synapse
Meiosis
☺ Second meiotic division = mitotic division
2 chromatids separate, 2 new daughter
cells are formed with half the normal
number of chromosomes
Meiosis
Gametogenesis
☺ Gonads: testes, ovaries; contain
☺ Precursor cells or germ cells; mature into
☺ Gametes: sperm, ova; in gametogenesis
☺ Spermatogenesis, oogenesis
Gametogenesis
Primary follicles
Oogenesis vs. spermatogenesis
☺ One ovum (+ 3 polar bodies) vs. four
spermatozoa
☺ Oocytes formed before birth vs. continuous
spermatogenesis (‘fresh’ sperm)
Prolonged prophase of first meiotic division until
ovulation – more frequent congenital
abnormalities in ova of older women (longer
exposure to potentially harmful environmental
influences until meiotic division resumes at
ovulation)
Chromosome Analysis
Karyotype
Genes and Inheritance
☺ Locus: specific site of a gene on the
chromosome. Since the chromosomes
exist in pairs, genes are also paired.
☺ Alleles: alternate forms of a gene can
occupy the same locus (homo, hetero)
☺ Recessive gene: expressed only when
homozygous
☺ Dominant gene: homo or hetero or co☺ Sex-linked gene: X, recessive, hemi
Gene Imprinting
☺ Genes occur in pairs on homologous
chromosomes, one from each parent
☺ Different effects of gene whether ♀ or ♂
☺ Genes modified during gametogenesis
☺ Gene imprinting: additional methyl
groups added to DNA molecules
☺ Basic structure identical;
in some diseases different expression
(behaviour) depending on parent of origin:
hereditary disease as a result of imprinting
Genetic Engineering
☺ Insertion of a gene encoding a desired
product (e.g. insulin) into a bacterium
☺ Bacterial gene spliced enzymatically,
recombinant DNA inserted into plasmid
(circular DNA segment in bacterium),
dividing bacterial population produces
desired protein
Gene Therapy
☺ Normal gene inserted into defective cell
☺ Compensates for the missing or
dysfunctional gene, in somatic cells only
☺ Can be inserted into mature cell (ly)
☺ Can be inserted into stem cell (bone
marrow)
☺ Used to treat e.g. ADA deficiency, CF, …
Congenital / Hereditary Diseases
☺ Congenital: present at birth
☺ Hereditary (genetic): result of
chromosome abnormality or
defective gene
Causes of malformations
1. Chromosomal abnormalities
2. Gene abnormalities
3. Intrauterine injury (e.g. drugs, radiation,
infection, environmental, etc)
4. Environmental effect on genetically
predisposed embryo
Chromosomal abnormalities
☺ Nondisjunction: failure of homologous
chromosomes in germ cells to separate
from one another during 1st or 2nd meiotic
division
☺ Sex chromosomes or autosomes
☺ Extra chromosome: trisomy (24 or 47)
Absent chromosome: monosomy (22 or
45)
Nondisjunction in meiosis
☺ Chromosome Deletion: Broken piece of
chromosome is lost from cell
☺ Translocation: Not lost, just misplaced
and attached to another chromosome
- reciprocal: between two nonhomologous
chromosomes (no loss or gain of genetic
material - no loss of cell function)
- in germ cells: deficient or excess
chromosome material – abnormal zygote
Translocation in gametes
Sex chromosome abnormalities
Turner syndrome
Klinefelter syndrome
Autosomal abnormalities
☺ Loss: aborted embryo
☺ Deletion: congenital anomalies
☺ Trisomy: syndromic, e.g. 21, 13, 18
Trisomy 21 (Down)
T21 causes
1. Nondisjunction during gametogenesis
(95%)
2. Translocation (few)
3. Nondisjunction in zygote (rare)
Translocation T21
Zygote nondisjunction T21- Mosaic
Abnormal gene diseases
☺ Individual gene abnormalities
☺ Hereditary diseases transmitted mostly
on autosomes, only a few on sex
chromosomes.
☺ Gene mutation: spontaneous
environmental
☺ Minor structural change may result in
major functional abnormality (e.g. SSD)
Modes of Inheritance
☺ Autosomal dominant (a dominant gene
expressed in the heterozygous state)
☺ Autosomal recessive (expressed only in
homozygous individual, disease only if both
alleles are abnormal)
☺ Codominant (full expression of both alleles in
heterozygous state)
☺ X-linked (usually affects male offspring; the
abnormal X-linked gene acts as dominant gene
when paired with the Y chromosome)
Intrauterine Injury
1. Drugs: thalidomide (phocomelia), DES
(cervical cancer), street drugs (IUFD),
smoking (IUGR), alcohol (FAS), etc
2. Radiation: x-rays
3. Maternal infections:
- Rubella virus (CVS, CNS, chr. infection)
- CMV (microcephaly, chronic infection)
- Toxoplasma gondii (hydrocephalus,
systemic infection)
Thalidomide baby
Prenatal CMV infection
Multifactorial Inheritance
☺ Combined effect of multiple genes
interacting with environmental agents,
e.g. cleft palate, cardiac malformations,
club foot, hip dislocation, spina bifida, etc
☺ Cause: developmental sequence fails to
reach a certain point at an appropriate
time (threshold)
Genetically determined variation
in rate of development
Effect of harmful environmental agents
on susceptibility to
congenital malformations
Interaction of genetic predisposition
and environmental factors
in cleft palate
Prenatal Diagnosis
of Congenital Abnormalities
1. Examination of fetal cells for
chromosomal, genetic or biochemical
abnormalities
2. Examination of amniotic fluid for products
secreted by the fetus
3. Ultrasound of the fetus to detect
malformations (NTD, hydrocephalus, …)
Prenatal Diagnosis
of Congenital Abnormalities
Main indications for amniocentesis
1. Maternal age (>35)
2. Previous infant with T21 or other
chromosomal abnormality
3. Known translocation T21 carrier
4. Other chromosomal abnormality in either
parent, e.g. t(7;21)
5. Risk of genetic disease in the fetus that
can be detected prenatally (thalassemia)
6. Previous infant born with neural tube
defect (multifactorial inheritance, ~5%)
Methods of fetal DNA analysis
1. Enzyme analysis of DNA: resultant
DNA fragments different in health and
disease, e.g. sickle cell anemia
2. DNA probes: same complementary
nucleotide arrangement as in defective
DNA gene – binds to mutant gene
Molecular Genetics of
Solid Pediatric Tumors
☺ Mechanisms for tumor development
1. Creation of novel fusion proteins
2. Loss of tumor suppressor genes
3. Activation of proto-oncogenes
Translocations, Oncogenes, Tumor
suppressor genes
NB: MYCN amplification and
1p deletion by FISH
NB: Double-minute chromosomes
by FISH
RB: MYCN probe to detect
homogeneously staining region in
metaphase spread and interphase nuclei
Ewing sarcoma: t(11;22)
EWS green, FLI-1 pink, t yellow
E-RMS: Spectral karyotype
t(1;3), t(1;15), t(1;21)
Abnormal Fetal Development
☺
☺
☺
☺
Malformation
Deformation
Dysplasia
Disruption
Prenatal development, pre-embryonic
Prenatal development, early embryonic
Prenatal development, late embryonic
Fetal development
Normal gametogenesis
Meiosis
Abnormal gametogenesis
♂ & ♀ gametes
Sperm penetrating oocyte
Fertilization
Causes of human congenital anomalies
Malformations
☺ Intrinsic abnormalities of blastogenesis
and organogenesis affecting the
morphogenetically reactive fields of the
embryo = developmental field defects
☺ Occur alone or in combination
(syndromes or associations)
☺ Severe (spina bifida aperta) or
mild (spina bifida occulta)
Malformations
☺ Causally heterogeneous
☺ Intrinsic causes: mendelian mutations,
chromosome abnormalities,
environmental interactions (multifactorial),
mitochondrial mutations
Disruptions
☺ Environmental (exogenous) causes
producing abnormalities of morphogenetic
field dynamics
☺ E.g. rubella, thalidomide, isotretinoin,
alcohol, etc
Rubella embryopathy
Diabetic embryopathy
Dysplasias
☺ Disturbances of histogenesis, occurring
later and somewhat independently of
morphogenesis
☺ Morphogenesis is prenatal,
histogenesis continues postnatally in all
tissues that have not undergone
end differentiation
☺ Dysplasias may predispose to cancer
Neurofibromatosis
Tuberous sclerosis
Deformities
☺ Secondary changes in form or shape of
previously normally formed organs or body
parts
☺ Caused by extrinsic forces (e.g. Potter
syndrome) or intrinsic defects (e.g.
fetal akinesia syndrome with congenital
arthrogryposis)
Oligohydramnios (Potter) sequence
Arthrogryposis
Sequences
☺ Secondary consequences of
malformations, disruptions, dysplasias, or
deformities
☺ E.g. renal adysplasia leads to Potter
oligohydramnios sequence
DiGeorge anomaly leads to tetany,
hypoparathyroidism, heart failure,
conotruncal congenital heart defect
Minor Anomalies
☺ Disturbance of phenogenesis in fetal life
☺ Phenogenesis: the process of attaining
final quantitative anthropometric traits of
the race and family (variant familial
developmental pattern)
☺ Causes:
intrinsic (chromosome imbalance)
extrinsic (teratogens)
Syndromes
☺ Patterns of anomalies proven or
presumed causally related
☺ Causes:
- chromosome mutations
- imprinting defects
- aneuploidy
- multifactorial disorders
- teratogenic sequences
Treacher-Collins syndrome
(mandibulofacial dysostosis) AD
Leprechaunism
(defective insulin binding) AR
Associations
☺ Idiopathic multiple congenital anomalies of
blastogenesis
Vertebral anomalies
V
Anorectal anomalies
A
TracheoEsophageal defects
TE
Radial and Renal defects
R
☺ Single hit during gastrulation affecting multiple,
morphogenetically closely related structural
primordia
Metabolic Disorders
☺ Most are inherited as AR, some are
X-linked, a few are AD.
☺ Great variability in presentation
☺ Some present with dysmorphic features
☺ Storage material in RES and other
tissues
Storage Diseases
☺ Lysosomal Lipid Storage Diseases
Nieman-Pick: sphyngomyelin
Gaucher disease: glucocerebrosidase
Tay-Sachs disease: Gangliosidoses
Metachromatic leukodystrophy
☺ Mucopolysaccharidoses (I, II, III, VII)
Hurler syndrome
COH Disorders
☺ Glycogen Storage Diseases
☺ Galactosemia
Glycogen storage disease type II
Amino Acid Disorders
Misc.
☺ Fatty Acid Beta-Oxidation Defects
(LCAD, MCAD, SCAD)
☺ Organic Acidemias
☺ Defects in Purine Metabolism
☺ Carnitine Deficiency
☺ Peroxisomal Disorders
☺ Disorders in Metal Metabolism
☺ Defects in Copper Metabolism
References
☺ Wigglesworth: Textbook of Fetal and
Neonatal Pathology
☺ Moore, Persaud: The Developing Human
☺ Perspectives in Pediatric Pathology,
Volume 21, Society for Pediatric Pathology
☺ Gilbert-Barness: Potter’s Atlas of Fetal
and Infant Pathology
☺ Crowley: An Introduction to Human
Disease, Pathology and Pathophysiology
Thank you