Download Chapter 10 Genetics

Genetics
Chapter 10 – Developmental Genetics
Development – Basic Concepts

In the US, 2-3% of all live-born children have a major birth defect (one that substantially impacts
health)

Body plan – The process of a single fertilized egg dividing, growing, forming different cell types,
tissues, and organs all in a species-specific arrangement (patterning of body parts).
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Ectopic Expression – Expression of the gene produc in an abnormal location
o

Example:

PAX6 gene causes defect of the eye such as cataracts and aniridia (absence of the iris) in
humans

PAX6 in humans is analogous to pax6 gene in mice (produces abnormally small eyes),
and eyeless gene in drosophilia (produces well-formed eye but misplaced on the body of
the fly)

Amazing because this gene pathway has been conserved through 60 to 500 million years
of lineage from drosophilia to humans. Many genes work this way which is why we can
study non-human genetics and apply results to humans.
Overview of Development Processes (Different proteins that form structures and provide signals
to coordinate development occur in each step described below)
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Axis Specification
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Defines the major axes of the body
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Ventral/Dorsal
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A/P
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Medial/Lateral
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Left/Right
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Specification of polarity, which direction things should move, is very important.
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Once axis is defined, organogenesis occurs
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Pattern Formation

Series of steps in which differentiated cells are arranged spatially to form tissues and
organs.
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Includes processes such as induction which occurs when the cells of one embryonic
region influence the organization and differentiation of cells in a second region
Organogenesis
Genetic Mediators of Development
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Paracrine Signaling Molecules
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Called paracrine factors because they are secreted into the space surrounding cells (not like
hormones (blood)).
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Four major families of paracrine Signaling molecules: Fibroblast growth factor (FGF),
Hedgehog family, Wingless (Wnt), and Transforming Growth Factor B (TGF-B)
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Fibroblast Growth Factor family (FGF)

There are at least 22 fibroblast growth factors that participate in cell migration,
growth, and differentiation.

FGFs interact with FGFRs (receptors). FGFRs are composed of a signal peptide, three
immunoglobulin-like domains, a membrane-spanning segment, and an intracellular
tyrosine kinase domain.

FGFs binding to FGFRs leads to Phosphorylation, and activation of tyrosine kinase
domain.

FGFRs are expressed widely in developing bone. Mutations in these receptors lead
to disorders of generalized bone growth (skeletal dysplasias).

Most prevalent disorder is achondroplasia (ACH). More than 250,000 people worldwide. Characterized by disproportionate short stature (limbs are disproportionately
shorter than the trunk) and macrocephaly. ACH is characterized almost always by a
glycine to arginine substitution in transmembrane domain of FGFR3, resulting in
FGFR3 activation.
o


FGFR is normally expressed in resting chondrocytes and restrains chondrocyte
proliferation and differentiation. Therefore overactivation causes further
inhibition of growth.
Degrees of long bone shortening depend on the altered domain and the degree of
FGFR3 that is activated.
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Lesser degree of FGFR3 activation produces Hypochondroplasia.
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Greater degree of FGFR3 activation can produce Thanatophoric dysplasia
(virtually lethal).

FGFR3 inactivation will cause bone-lengthening therefore some amount of activation
is needed.

Group of autosomal dominant disorders are characterized by premature fusion of
the cranial sutures (craniosynostosis syndromes).
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Most common is Apert Syndrome
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Caused by mutations in FGFR1, FGFR2, or FGFR3
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Cysteine to tyrosine substitution in FGFR2 can cause either Pleiffer or Crouzon
syndrome
Hedgehog family (Shh)
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First isolated in drosophila
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Sonic hedgehog (Shh) is human homolog

Shh participates in axis formation, induction of motor neurons within neural plate,
and patterning of the limbs
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Primary receptor of Shh is transmembrane protein encoded by patched gene.
Normal patched action is to inhibit the function of another membrane protein
smoothened. Smoothened is encoded by gene Smo.

Shh binding to patched receptor results in disinhibition of smoothened and
activation of an intracellular signaling cascade that targets GLI family of transcription
factors

Mutations of PATCHED (PTC) causes Gorlin syndrome characterized by rib anomalies,
cysts of the jaw and basal cell carcinomas. (Germline mutations)

Somatic mutations of PTC produce only basal cell carcinoma. (Somatic mutations)

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Wingless family (Wnt)
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Named after drosophila gene wingless. Vertebrate homolog is integrated gene.

Establishes polarity during Drosophila limb formation. Genes encode secreted
glycoproteins that bind to members of the frizzled and low-density lipoprotein
receptor-related protein families.

19 different Wnt genes have been identified in humans and participate in
specification of dorsal/ventral axis and formation of the brain, muscles, gonads, and
kidney.
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Homozygosity for mutations in WNT3 causes tetra-amelia (absence of all four limbs)
in humans.

WNT signaling has been associated with the formation of tumors also.
Transforming Growth Factor β (TGF-β)

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Supergene family composed of TGF-Beta family, the bone morphogenetic protein
(BMP) family, the activin family and the Vg1 family.
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BMPs are not limited to bone formation and development but were named bc of
their ability to induce bone formation.
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Mutations in the BMP family, cartilage-derived morphogenetic protein 1
(CDMP1) cause skeletal abnormalities.
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Nonsense mutation in CDMP1 causes dominantly inherited brachydactyly
(short digits)

Homozygous for a duplication in CDMP1 have brachydactyly and shortening
of the long bones in disorder called acromesomelic dysplasia.

Homozygous missense mutation produces autosomal recessive disorder
Grebe chondrodysplasia – extreme shortening of the long bones and digits.
Extracellular Signals and their Responses
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Signal transduction systems

RTK/Ras GTPase/MAPK (RTK-MAPK) signaling pathway

Regulates various cellular functions such as gene expression, division, differentiation,
and death.

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Mutations in genes that encode several components of this pathway cause human
malformation syndromes
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Noonan syndrome is characterized by short stature, characteristic facial features,
webbing of the neck, and congenital heart disease (stenosis of the pulmonary
outflow tract). Usually caused by gain-of-function mutations in the protein
tyrosine phosphatase, non-receptor-type, 11 gene (PTPN11)
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Other similar clinical characteristics appear in Costello syndrome and
cardiofaciocutaneous syndrome (CFC).
Secreted proteins inhibit function of certain BMPs.
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DNA Transcription Factors
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Transcription factors are genes encoding proteins that activate or repress other genes.
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Many families of transcription factors: HOX, PAX, EMX, MSX, SOX, and T-box families
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SOX Family of proteins activates transcription indirectly by bending DNA so that other factors
can make contact with promoter regions of genes
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Prototypic SOX gene is the SRY (Sex-determining Region of the Y Chromosome), and this
encodes the mammalian testis-determining factor.

Sox9 is expressed in genital ridges of humans, and is up-regulated in males and downregulated in females before gonad differentiation. Sox9 also regulates chondrogenesis
and expression of the collagen gene.
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Mutations in the Noggin gene, encodes one of these inhibitors, and causes
fusion of the bones in various joints. Over time, cartilage forms in excess and
ultimately fuses the bones of the joint together (synostosis). Usually effects the
spine, middle ear bones, and limbs (particularly hands and feet).
Mutations in SOX9 cause disorders characterized by skeletal defects (campomelic
dysplasia) and sex reversal that produces XY females.
Mutations in SOX10 results in a syndrome characterized by Hirschsprung disease
(hypomotility of the bowel dues to reduce enteric nerve cells), pigmentary disturbances,
and deafness.
Extracellular Matrix Proteins
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Macromolecules such as fibrillins, collagens, proteoglycans, large glycoproteins, etc. which
serve as the structural basis and also are active mediators of development by allowing for
movement of cells along their structural matrices.
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Fibrillin-1 and elastin coordinate microfibril and extracellular matrix formation.
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Mutations in these genes result in Marfan syndrome (Fibrillin-1) and supravalvular
aortic stenosis (elastin)
Integrins and glycosyltransferases are cell surface receptors that help EMPs adhere to a
cell’s surface. Laminins is a molecule that makes this attachment more permanent.

Mutations in LAMC2, gene that encodes a subunit of laminin, causes autosomal
recessive junctional epidermolysis bullosa (JEB). Epithelial cells cannot properly
anchor themselves and the result is spontaneously large blisters of the skin.
Pattern Formation
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The process by which ordered spatial arrangements of differentiated cells create tissues and
organs is called pattern formation. Regional specification takes place in several steps: definition
of the cells of a region, establishment of signaling centers that provide positional information,
and differentiation of cells within a region in response to additional cues.
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Examples include Shh protein involved in patterning of the vertebrate neural tube, somites, and
limbs, and also distinguishes left from right.
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Mutations in the human Shh gene, SHH, can cause abnormal midline brain development
(holoprosencephaly).
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Attachment of the SHH protein to cholesterol appears to be necessary for proper patterning
of hedgehog signaling.
Gastrulation
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Occurs between days 14 and 28
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Embryo is transformed into three-layer structure composed of ectroderm, endoderm and
mesoderm
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Occurs by invagination of the epiblast in the primitive streak to form the layers and the
process is dominated by cell migration.
Neurulation and Ectoderm
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Neurulation is the formation of the neural tube
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Neural tube forms from the dorsal mesoderm and the overlying ectoderm
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Initiates organogenesis
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Divides ectoderm into three different cell populations: Neural tube (brain and spinal cord),
epidermis of the skin, and the neural crest cells
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Neural tube closure begins at five separate sits and some correspond to locations of
common neural tube defects
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Anencephaly (absence of brain)
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Occipital encephalocele
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Lumbar spina bifida
Disorders, Defects and Mutations of Neural Crest Development
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Hirschsprund Disease (HSCR)
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Reduced or absent migration of neural crest cells in the enteric tract of the digestive
system which alters normal bowel control and movement and digestion.
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Sex bias, with males affected four times as often as females
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Hypomotility of the bowel, severe constipation
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Most commonly caused by mutations that inactivate the RET (rearranged during
transfection) gene which encodes a receptor tyrosine kinase.
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Mutations of endothelin-B (EDNRB) or its ligand (EDN3) also cause HSCR but this
may also lead to melanocyte abnormalities that produce hypopigmented patches of
skin and hearing loss. – Called Waardenburg-Shah syndrome
Mesoderm and Endoderm
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Mesoderm can be divided into five components:
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Notochord
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Dorsal mesoderm
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Forms axial skeleton, skeletal muscles, and connective tissue of the skin.
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Lies adjacent to the notochord
Intermediate mesoderm
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Midline structure that induces neural tube formation, and body axis
Forms the kidneys and genitourinary system.
Lateral mesoderm
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Head mesenchyme
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Muscles of the eyes and head
Endoderm is to form the linings of the digestive tract and the respiratory tree.
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Outgrowths of the intestinal tract form the pancreas, gallbladder and liver.
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Bifurcation of the respiratory tree forms the right and left lungs.
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Produces pharyngeal pouches (and with cells derived from neural crest) give rise to the
endodermal structures such as the middle ear, thymus, parathyroids, and thyroid.
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Budding and branching is common to endoderm-derived structures
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FGFs and BMPs appear to control this process.
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(Correlate how FGF mutations can relate to this budding and branching)
Axis Specification
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Differentiates into the heart, appendicular skeleton, connective tissue of the viscera
and body wall, and connective tissues of the amnion and chorion.
All chordates have three axes
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Anterior/Posterior
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Dorsal/Ventral
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Left/Right
Formation of the A/P Axis
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Defined by notochord.
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Anterior end is distinguished by the node
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Gene nodal is responsible for initiating and maintaining the primitive streak and also
plays a role in right/left differentiation
The Hox family of genes are responsible for A/P axis in humans.
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Four clusters (Hoxa, Hoxb, Hoxc, and Hoxd)
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39 Hox genes divided among clusters (Hoxa1-13, etc.)
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The anterior/posterior axis of a developing mammalian embryo is defined by the
primitive streak and patterned by combinations of Hox genes. Collectively, these
combinations identify various regions along the a/p axis of the body and limbs.
Disrution of Hox genes produces defects in body, limb and organ patterning.
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Formation of the Dorsal/Ventral Axis
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Noggin and chordin encode secreted proteins that are capable of dorsalizing ventral
mesoderm and restoring dorsal structures that have been ventralized.
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Bmp-4 is expressed ventrally and induces ventral fates, patterning the dorsal/ventral axis.
Noggin and chordin bind the Bmp-4 to prevent it from activating its receptor and ventralizing
the organism.
Formation of the Left/Right Axis
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At least three mechanisms contribute to the left/right asymmetry seen in many organisms
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Unpaired organs begin development in the midline and then lateralize to one side or the
other (heart, liver)
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Mirror image of a paired structure can regress, leaving a lateralized, unpaired structure
(blood vessels)
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Organs begin as asymmetrical outgrowths from a midline structure (lungs)
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Certain cilia tend to play an important role. Their function depends of two proteins, leftright dynein (lrd) and polycystin-2. Ciliary dyskinesias are a result of dynein abnormalities
and PKD1 mutations which leads to polycystin-2 defects show autosomal dominant
polycystic kidney disease in humans.
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Mutations in zinc-finger protein of the cerebellum (ZIC3) of the Gli transcription factor family
located on the X chromosome are the most common known genetic cause of human
laterality defects.
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Males = randomization
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Females = reversal
After L/R asymmetry has been established, the left and right sides of the organs must also be
patterned
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Two factors, dHAND and eHAND play roles in patterning the right and left ventricles of
the heart
Many conjoined twins display L/R defects (usually the right twin)
Formation of Organs and Appendages
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After gastrulation
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Many of the genes and factors begin to come into play at this point.
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Many of the human birth defects have prominent roles in this phase of development.
Craniofacial Development
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Neural crest cells from the forebrain and midbrain contribute to the nasal processes, palate,
and mesenchyme of the first pharyngeal pouch.

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This mesenchyme forms the maxilla, mandible, incus and malleus.
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Neural crest cells of the anterior hindbrain migrate and differentiate to become the
mesenchyme of the second pharyngeal pouch and the stapes and facial cartilage.
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Cervical neural crest cells produce the mesenchyme of the third, fourth and sixth pharyngeal
arches which becomes the muscles and bones of the neck.
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All neural crest cells of these groups are specified by the Hox genes.
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Mutations in FGFR genes of the cranial mesenchymal fusion will lead to craniosynostosis
(early closure of the bones of the skull).
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Craniosynostosis also can be caused by mutations in MSX2
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Mutations in GLI3 cause disorders such as Greig cephalopolydactyly and Pallister-Hall
syndrome.
Limb Development
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Prevalence of limb defects is second only to that of congenital heart defects.
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Derived from lateral plate mesoderm (bone, cartilage, and tendons) and somatic mesoderm
(muscle, nerve, and vasculature).
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Proximal/distal growth of limb is dependent on a region of ectoderm called the apical
ectodermal ridge (AER).
o
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Two genes, radical fringe (r-Fng) and Wnt7a, are expressed in the dorsal ectoderm which
instructs the mesoderm to dorsalize. R-Fng and Wnt7a are inhibited on the ventral side
of the limb by Engrailed-1. Ventralization occurs after dorsalization.
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FGFs stimulate proliferation of mesodermal cells in the progress zone (PZ).
Defects of the anterior and posterior elements of the upper limb occur in the Holt-Oram
syndrome and ulnar-mammary syndrome.
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Holt-Oram is caused by mutations in the gene TBX5 and is usually characterized by heart
defects also (ASD) because of TBX5’s close relationship with TBX2.5 in heart
development (leading to ASDs)
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Ulnar-mammary syndrome is caused by mutations in the gene TBX3.
Organ Formation
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Once a specialized cell within an organ is terminally differentiated, various proteins turn on
its molecular machinery so that it may perform its intended function.