Dr.Nüket Yürür Kutlay

Dr.Nüket Yürür Kutlay

Fertilization, to form zygote (single cell)

10.000 trillion cells 200 distinct cell types

This is fully formed human being with complex biocemistry, physiology...

Many mysteries remain

 Analysis of developmental mutants of the fruit fly

embryos has provided insight into the genetic regulation of develomental processes.

 Early developmental phases in embryos of different

organisms are regulated by similar genes.

 Development of a fruit fly from the fertilized egg cell to adult organism takes 9 days.

 Phenotype of a fruit fly is known very well.

 The head of the adult has three segments, thorax has three whereas abdomen has eigth segments

 A fruit fly has 14 parasegments

 The segmental

organization is discernible in the larva

 Many developmental mutations are known in

Drosophila melanogaster. They can be classified into different hierarchical gene classes.

 A mutation for anterior

maternal effect, bicoid, leads to a larva without head or

thorax

 A mutation for posterior

maternal effect, nanos, effects the end of the larva

 Gap (Krüppel, Knirps,

Huncback) genes establish the basic patern of segmental

organizations.

 The genes for pair-rule determine the organization and

developmental fate of the 14 parasegment

 Mutation of ftz; leads to fewer than normal segments.

 Segment polarity genes (>10 genes) determine the polarity of each segment.

 Homeotic selector genes

determine the ultimate fate of each segment.

 Antp is normaly expressed in the second thorasic segment. If it is incorrectly expressed in the

head, antennae will transform into legs

 The embryonic development is

determined by genes that are active only during specific phases

 Maternal effect genes code for early gene products that determine the polarity of the embryo

 Bicoid gene determinates anterior region

 Nanos gene determinates posterior region

 The genes for embryonic development are organized in functional hierarchy.

 Gap genes; transcription factors

 Pair rule genes are induced

 Segment polarity genes are expressed

 Selector genes(Homeotic genes)

 Antennapedia is expressed in parasegments 5 and 6.

 It contains a segment of highly conserved DNA sequences that is identical in a wide variety of organisms from drosophila to

mammals.

 It codes for about 60 aa(homeodomain). It contains four domains of helical protein.

Mutation for the bithorax complex (BX-C) causes an aditional thoracic segment

 There is a same anterior posterior orientation of series of genes from drosophila to human.

 There are four groups of homeo genes in human and mouse different from drosophila



Vertebrates;

 Mouse

 Chick

 Zebra fish

We appreciate all these experimental animals because of their contribution to our

knowledge about developmental processes

Human

During the embryonic stage,

 craniocaudal(rostrocaudal / anterior posterior),

 dorsoventral,

 proximodistal axes established,

as cellular aggregation and differentiation lead to tissue and organ formation

Polarity

 A key concept in

development at all stages is emergence of polarity

 In the fertilized egg, the point of entry the sperm determines the plane

through which the first cell cleavage occurs.

 In blastocyst; an inner cell mass or embryoblast(to form embryo) and outer cell mass or trophoblast ( to give rise to placenta)

The gene families in vertebrates usually show strong homology with drodophila

 Many development genes produce transcription

factors

 Switch gene on and off

 Control many other genes in coordinated sequential cascades

 Regulation of fundamental embryological processes

 Induction

 Segmentation

 Migration

 Differentiation

 Apoptosis

 All these processes are mediated by GF, cell

receptors and chemicals known as morphogens.

 Across species the

signaling molecules are very similar, which tend to be members of

 TFG β family

 WNT(wingless) family

 HH(hedgehog) family

TFG β family

There are 33 members of cytokine (signaling molecules that enable cell to communicate) family. They can be divided in to two gruops:

 BMP’s ( bone morphogenic proteins)  TGFβ

 Nodal

 Activins

 Myostatin

Acting through various SMAD proteins

Very broad range of cellular and developmental processes

Embryo

Lefty 1

Mid-line

Left hand Right hand

Nodal

Lefty2

Left hand specificTF(Pitx2,

etc) is activeted Rigth hand specificTF is

inactivated Nodal

Lefty2

 In very early development,

integrity of many gene families (Nodal, SHH, Notch) is essential to the establishment of left-right body axis. (situs solitus)

 LEFTA Situs inversusSitus inversus

 LEFTB Situs ambiguousSitus ambiguous

 NODAL IsomerismIsomerism

WNT pathway

Two main branches: βcatenin dependent(canonical)

βcatenin independent

SonicHedgehog-Patched-GLI Pathway



SHH is expressed in:

 Notochord

 Brain

 ZPA(the zona of

polarizating activity of developing limbs)



PTCH(patched)



SMO(smoothened)

Holoprosencephaly(in which the primary defect is

incomplete cleavage of the developing brain into seperate hemispheres and ventricles)

Cyclopia( severe form, a single central eye)

SLOS: Holoprosencephaly+genital anomalies+syndactyly

due to defect in cholesterol biosynthesis(7-dehydrocholesterol reductase)

Gorlin syndrome

Mutation in PTCH

Multipl basal cell carsinoma Odontogenic keratocysts Bifid ribs

Calsification of the falx cerebri Overian fibromatoma

Macrocephaly

Rubenstain-Taybi Syndrome

Characteristic facial features Angulated thumbs

Postaxial polydactyly

Rostro- caudal polarity

 Development of presomitic mesoderm (PSM):

 Wnt

 FGF

 The somits form from the PSM in a rostrocaudal direction;

 The key pathway is notch-delta signaling

 Notch receptor

 Its ligads delta-like1, delta like 3 to gether with presenil 1, mezoderm posterior2

 Spondylocostal dysostosis tip 1

(abnormal development of bones in the spine and ribs)

 Delta like 3

 Mesoderm posterior2

HOX (homeobox) genes

 Conserved 180 bp sequence-Homeobox

 TF,

 There are four HOX gene clusters in human

 Downstream targets (at least 35)

 Morphogen signals

 Operating a lot of genes that mediate

 Cell adhesion

 Cell division

 Cell death

 Cell movement

Development of Central nervous systems

Axial skeleton and limbs

Gastrointestinal tract Urogenital tract

External genitalia

Hand-Foot-Genital Syndrome

 Mutation in HOXA13

 This shows autosomal dominant inheritance

 Shortening 1. and 5.

digits

 Hypospadias (the

opening of the urethra is on the underside of the penis instead of at the tip)

 Bicornuate uterus

Mutation in HOXD13



Synpolydactyly



Autosomal dominant inheritance



Additional digit between 3. and 4. finger



Phenotype in homozygotes is more severe



There are a few syndromes which have been attributed to HOX gene mutation.



What could be the possible reasons?

 One possible explanation is that most HOX mutations are so devastating that embryo cannot survive

 The degree of homology between HOX genes in different clusters could lead to functional redundancy

Paired –Box(PAX) Genes



Highly conserved



A 130aa DNA binding

transcription

regulator domain



9 genes in human and mice

GENE Location Development Abnormality PAX2 10q24 Renal-coloboma

Sendrome

PAX3 2q35 Waardenburg Send. Type1

PAX6 11p13 Aniridia

PAX8 2q12 Absent or ectopic thyroid gland

PAX9 14q12 Oligodontia

Waardenburg Send. Type 1

 Autosomal dominant inheritance

 Sensorineural hearing loss,

 Depigmentation in hair/skin

 Abnormal pigmentation in iris

 Widely spaced inner canthi – absent in type 2



PAX6

 The importance of expression of the PAX gene family is on an eye development

SRY type HMG(high mobility group) Box (SOX) genes

 Homology with SRY, by sharing a 79 aa domain HMG

bending DNA  Other TF can bind

 Expressed in specific tissues

 Mutation in SOX10

 Chromosome 22

 Waardenburg

Syndrome(rare form) High incidence of

Hirschsprung

Mutation in SOX2

 Chromosome 3q26

 AEG(Anoftalmia- esophageal atresia- genital hypoplasia in male )Syndrome



Mutation in SOX9( on

chromosome 17)

campomelic displasia



Regulates TypeII collagen

expression



Bowing of long bones



Abnormal

vertebra costa



Very poor long- term survival



Sex reversal in

chromosomal

male(46;XY)

T-BOX(TBX) GENES

 TF (T domain)

 On chro 12 TBX3 TBX5

 Loss of function mutations;



TBX3 Ulnar-

mamary Syndrome



TBX5Hold-Oram Synd.(Conj. Heart abnormalities,

upper limbs radial ray defects( from mild hypoplasia of thumbs to

complete absence of the forearm)



TBX1  _DGS

Zing Finger genes

Gene location

GLI3 7 Greig Syndrome

PallisterHall Syndrome

WT1 11 Denys-Drash

ZIC2 13 Holoprosencephal

y

ZIC3 X Laterality defect

GLI3

Greig Syndrome.

Large deletion or translocation

Pallister-Hall

Syndrome: Frameshift mutation

Postaxial polydactyly Hypotalamic

hamartoma

Imperforate anus

WT1

 Densy-Drash sendromu

 Ambiguous genitalia(external)

 Progressive renal failure

Signal transduction (signaling)genes

 RET proto-onkogene

 A cell surface tyrosine kinase

 Gain of function—

Thyroid Ca

 Loss of function—

Hirschsprung disease (failure of migration of ganglionic cells to the submucosal and

myestenic plexuses ofthe large bowel)

 FGFR

FGFR

Craniosynostos is

FGFR1 Pfeiffer FGFR2 Apert

Crouzon

Jacson-Weiss Pfeiffer

FGFR3 Crouzon Skeletal

dysplasia

FGFR3 Akondroplasia Hipokondropla zi

Thanatophoric dysplasia

 Apert Syndrome

 Craniosynostosis

 Hand and food abnormalities

 FGFR2

Pfeiffer Syndrome:

Addition to these

features, the thumbs and big toes are board

Crouzon syndrome:

In which the limbs are normal



Acondroplasia

 Tm domain

 Rhizomelic(proximal shortening)

 Head is enlarged with frontal bossing

 Intelligence and life expectancy are

normal

Limb development

Four main phases



Initiation(HOXFG F)



Specification



Tissue

differentiation



Growth

Developmental Genes and Cancer

 Multiple endocrine neoplasia, type 2A (MEN 2A) is a hereditary syndrome characterized by medullary carcinoma of the thyroid, pheochromocytoma, parathyroid hyperplasia or adenomas (causing hyperparathyroidism)

 Multiple endocrine neoplasia, type 2B (MEN 2B) is an autosomal dominant syndrome

characterized by medullary thyroid carcinoma, pheochromocytoma, multiple mucosal neuromas and intestinal ganglioneuromas, and often a marfanoid habitus and other skeletal abnormalities

PAX3 Waardenburg

Syndrome Type1

Alveolar

rabdomyosarcoma

KIT Piebaldism Mast cell leukemia

PTCH Gorlin Syndrome Basal cell carsinoma

RET Hirschsprung disease MEN2A, MEN2B,

Tyroid CA

WT1 Denys-Drash Wilms tümörü

Further reading;

Emery’s elements of medical genetics

Color Atlas of Genetics, Passarge