Download Homeotic genes

Hox Gene
Presented by:
Asmaa Hassan
Azza Elshafie
Amna A/bagi
Introduction and Discovery
• How the undifferentiated cells in a fertilized embryo
separate into a head and a tail end, and how the eyes,
legs, antennae, and other organs all form in their
correct positions?
• which genes?
• How many are they?
• How do they work?
• How the genes causing them cooperate during body
segmentation?
Ed Lewis, discoverer of homeotic genes
and founder of developmental genetics
Homeotic genes (Selector genes)
Genes that encode proteins that bind to specific
sequences of DNA and thus influence the
organism’s subsequent development.
or Are genes that determine which parts of the
body form what body parts.
• Homeotic proteins can activate one gene but repress
another producing effects that complementary and
necessary for the ordered development of an
organisms.
Homeotic genes specify body segment identity in Drosophila.
Fig. 14
The bithorax pseudo-alleles
normal
wing
T2
T3
post-bithorax
haltere
normal
haltere
one gene, or several genes???
The homeotic genes ANT-C and BX-C encode
products are involved in controlling the
normal development of the relevant adult fly
structures.
The Nobel Prize in Physiology or Medicine 1995
Edward B. Lewis
Christiane -Volhard and
Eric Wieschaus
In 1983, he succeeded in
cloning Antennapedia, one of
the homeotic genes,which
determines the characteristics
of the body segments of
Drosophila, and elucidated its
structure. Furthermore, in the
Walter Jakob Gehring
Switzerland / March 20, 1939
Developmental Biologist; Professor,
University of Basel
Antennapedia gene, he found
a specific base sequence
common to all homeotic genes
and called it "homeobox."
Walter Gehring cloned Antennapedia
(here, with Ed Lewis)
and searched for homologies with the genes of the bithorax
complex
• A homeobox is a conserved* DNA sequence found within
genes that are involved in the regulation of patterns of
development (morphogenesis) in animals, fungi and plants.
•
Bind to another gene in turn, thus triggering a cascade of
gene expressions resulting in the segmentation of the embryo.
*Whether from flies, worms, mice, or humans, all homeotic
genes contain a segment called a homeobox, made up of 180
base pairs of DNA, which codes for an important domain of a
transcription factorÑa protein that regulates the activity of
other genes.
A homeobox is about 180 base pair long. It encodes protein
domain (the homeodomain) 60 aa-long homeodomain
which when expressed (e.g. as protein) can bind DNA.
The homeodomain bound to DNA
Function:
 Homeobox genes encode transcription factors
which typically switch on cascades of other genes.
• Recognize its desired target genes*.
• Act in the promoter region of their target genes* as
complexes with other transcription factors, often also
homeodomain proteins.
* The target genes of Hox genes promote cell division,
cell adhesion, apoptosis, and cell migration.
• Control genes are important elements in building
complicated organisms like flies.
• Help lay out the basic body forms of many animals,
including humans, flies, and worms. They set up the
head-to-tail organization “directing instructions “
Function of the Hox genes in vertebrates?
CLASSIFICATION
Homeotic Loci
• HL = Genes whose products provide positional
information in a multicellular embryo.
• HOM-generally invertebrates
• HOX-vertebrates
• MADS-plants
• Classification of homeobox genes into the
following classes:
• Hlx, NK-2, TCL, NEC, ems, Dll, msh, NK-1,
en, Abd-b, Antp, lab, cad, eve, prd, prd-like,
cut, POU, LIM, ZF.
In Drosophila
• In Drosophila hox cluster consists of eight genes
arranged in two groups referred as Antennapedia
complex and Bithorax complex on third chromosome
named HOM-C (homeotic complex).
Bithorax
complex
AbdominalA(abd-A)
ultrabithorax(U
bx)
Antennapedia
complex
sex combs
reduced(scr)
deformed(dfd),
Proboscipedia(p
b),
AbdominalB(Abd-B)
Labial(lab),
Antennapedia(a
ntp) .
Eight Genes Regulate the Identity of Within
the Adult and Embryo
labial (lab)
proboscipedia (pb)
Deformed (Dfd)
Sex combs reduced (Scr)
Antennapedia (Antp)
Ultrabithorax (Ubx)
abdominal A (abd-A)
Abdominal B (Abd-B)
.
In human
• Human HOX Genes (HOX clusters)
• 39 Hox genes (i.e., they must be important)
distributed in 4 linkage groups
• Contain homeobox domain (highly conserved)
• Genes are expressed in sequences that correlate with
development of specific regions
• ParaHox paralogon (4q, 5q, 13q and Xq) The NKL
(NK-Like or NK-Linked) genes or MetaHox
paralogon (2p/8p, 4p, 5q and 10q) Somewhat
aberrantly, the Dlx and En gene families group with
the NKL subclass in phylogenetic analyses.
Chromosomal distribution of human homeobox
genes
Holland et al. BMC Biology 2007
Charactistics
• Hox genes exhibit some important properties like;
1) Found in clusters: Hox genes are physically linked
on the chromosome.Eight genes are organized in one
cluster in flies (Drosophila) and four clusters
harboring 39 hox genes in Humans.
2) Colinearity: The gene order in the cluster mimics the
order of expression of genes and their function along
the anterior–posterior (A–P) body axis: genes at the 5′
end of the cluster are expressed in, and pattern, the
posterior part of the body, whereas genes at the 3′ end
pattern the anterior end of the body.
Colinearity
They display colinearity:
a) Spatial colinearity: the more anteriorly expressed
genes are in one end(at the 3′ end ), the more posterior
ones at the other end of the gene complex (at the 5′ end
of the cluster).
b)Temporal colinearity: genes on one end of the
complex (anterior) are expressed first, those on the
other (posterior) end are turned on last (In some
species mainly vertebrates ).
3) Homeotic transformations: When mutant one
segment gets transformed into likeness of another.
For example Ultrabithorax (ubx) which is normally
expresses in T3 segment of Drosophila ,plays a vital
role is formation of Haltere (modified wing) by
repressing various wing patterning genes. In flies
mutant for ubx halteres are modified into wings and
you have a four winged fruit fly.
Developmental function of Ultrabithorax - in T3: reduce wings to halteres?
4) Functional hierarchies among genes in the cluster, called
“phenotypic suppression” in Drosophila and “posterior
prevalence” in vertebrates. Normally genes present in posterior
region are dominant over anterior expressing ones.
(Abdominal B in drosophila) is dominant over all others in
complex).
• When cells mutant for particular hox gene ,then the hox gene
anterior to the mutant one will now starts expressing in those
cells.
Regulation
Hierarchy of genes in Drosophila development
Maternal factor
Gap genes
Development
of the
number of
segments
Pair rule genes
Selector genes (Hox genes)
Realisator genes
Development
of the features
of the
segments
Before
• The initial genes critical to this process are often collectively
referred to as “maternal effect” genes based on the fact that
they are transcriptionally derived from the maternal genome.
• Of this group, the genes bicoid and nanos play critical roles in
creating the initial polarity along the AP axis.
• The designation of “anterior” is driven by the activities of
Bicoid, where as posterior” specification relies on the activity
of Nanos.
• Bicoid and Nanos proteins regulate the activity of
other maternal genes, including hunchback and
caudal, respectively.
• The interactions among these maternal factors
ultimately create gradients within the embryo that
will drive gene expression patterns during later stages
of development.
•
Protein gradiants as follow:
1. From anterior
posterior
Bicoid and hunchback
2. From posterior
anterior
Nanos and caudal
• Maternal factors stimulation of expression of gap
genes(Kruppel, knirps ,giant, huckebein, tailless,
orthodenticle, and button head) in overlapping
domains along the AP axis.
• As transcription factors, the products of these genes
in turn regulate the expression of the pair-rule genes,
which have expression domains that reveal a
parasegmental organization within the embryo.
Hierarchy of genes in Drosophila
development
giant (gap gene)
hunchback (gap gene)
bicoid (maternal factor)
Krüppel (gap gene)
Regulation
• Gap and pair-rule proteins act through cis-acting regulatory
regions referred to as initiator enhancer elements, to either
activate or repress homeotic gene expression.
• For example hunchback and Kruppel negatively regulate the
homeotic genes
Target
The hox genes regulate what are called realisator genes or
effectors genes that act :
• at the bottom of such hierarchies to ultimately form the tissues,
structures, and organs of each segment.
• Segmentation involves such processes as morphogenesis
(differentiation of precursor cells into their terminal
specialized cells).
• programmed cell death.
• the movement of cells from where they are first born to where
they will ultimately function.
• it is not surprising that the target genes of Hox genes promote
cell division, cell adhesion, apoptosis, and cell migration.
After
• In general, homeotic genes and their counterparts in
other species require the activity of cofactor proteins
to modulate their activity in a context-dependent
manner
• For example, one known target of homeotic selector
proteins is the transcription factor Distal-less (Dll).
• Dll is required for the formation of legs in thoracic
segments, and its expression is negatively regulated
in abdominal segments by the actions of Ubx and
AbdA they bind to the enhancer and suppress it.
• Another two cofactors Extradenticle (Exd) and
Homothorax which act to improve the selectivity of DNA
binding.
• Exd binds different Hox proteins forming different
heterodimers that bind selectively to specific target sequences
• Exd may also contribute to specificity of Hox function by
converting bound Hox proteins from repressors to activators.
• The segmentation genes Sloppy-paired and
Engrailed also contribute to the contextual activity of
homeotic selector proteins These two cofactors do not
influence DNA binding, but they appear to play a role
in mediating the transcriptional repression by the
homeotic selector proteins at the Dll repressor
element.
• One target gene is decapentaplegic (dpp), which is expressed
in an A–P domain of VISCERAL MESO DERM in D
melanogaster.
• This dpp expression pattern is provided, in part, by the Hox
proteins Ultrabithorax (UBX) and Abdominal-A (ABD-A),
which activate and repress dpp transcription, respectively.
Fixation
• How, then, do fly embryonic cells “fix” the state of
homeotic gene expression to ensure continuity during
later stages?
• The modification of chromatin structure appears to
play a key role in the maintenance of homeotic
selector gene expression.
• Two classes of genes—the Polycomb Group and the
Trithorax Group—play opposing roles with regard to
stabilizing homeotic gene expression.
• By remodeling chromatin structure into more compact states,
the proteins of the Polycomb Group inactivate the cisregulatory regions that control homeotic genes, thereby
inhibiting their transcription during subsequent stages
• By contrast, the Trithorax Group proteins appear to
act collectively to keep chromatin in a state that
favors transcriptional activation
Pattern of Binding of Polycomb Protein
The normal pattern of binding of Polycomb protein to Drosophila giant chromosomes, visualized with an antibody against Polycomb. The
protein is bound to the Antennapedia complex (ANT-C) and the bithorax complex (BX-C) as well as about 60 other sites. (A, from G. Struhl,
Nature 293:36-41, 1981. ©1981 Macmillan Journals Ltd.; B, courtesy of B. Zink and R. Paro, from R. Paro, Trends Genet. 6:416-421, 1990.)
Regulation in gene expression
Evolution
Evolution
• Gene duplication is believed to play a major role in
evolution.
• The two genes that exist after a gene duplication
event are:
Paralogs (code for proteins with a different function
and/or structure).
Orthologous (code for proteins with similar
functions but exist in different species)
Susumu Ohno, 1970
HOX genes are evolutionarily highly conserved and control
similar phenotypic characters among
Distantly related organisms.
Relatively closely related species.
The formation of similar phenotypic characters can be explained
by the conservation of shared hox genes. By contrast, different
phenotypic characters are believed to be generated by
duplication of hox genes and their functional differentiation.
It has also been hypothesized that the loss of some hox genes
are responsible for morphological differentiation.
Short
indels
Duplications occur at all genomic scales!
Domain
(exon)
Gene
Gene cluster
Segment
Chromosome Genome
Homeobox genes are present in the genomes of all
animals which have so far been mapped as well as in
the genomes of plants and fungi, indicating that the
origins are ancient and precede the divergence of
these kingdoms.
The changes in Hox gene number and genomic
organization played an important role in metazoan
body-plan evolution
Evolution of Hox gene cluster
Hypothetical
common
ancestor
Amphioxus
Evolution of Hox gene cluster
• Vertebrata genome contains 4 Hox clusters,
Drosophyla genome contains only one.
• In general: we can see one-to-four relationship
between invertebrate and vertebrate gene number.
• Two genome duplications happened.
The Cambrian explosion
• Nearly all the extant phyla of kingdom Animalia
emerged within few tens of million years
• This evolutionary explosion started about 530 million
years ago
• The assumed cause of it the genome duplication
The genome duplications
Larger Hox gene number
More complicated body pattern
Mutations
Background
• Among the most fascinating kinds of abnormalities in animals
are those in which one normal body part is replaced by another
(Homeosis).
•
Calvin Bridges (1915) isolated a spontaneous mutant of
Drosophila in which part of the haltere was transformed into
wing tissue. The mutant was called bithorax (duplication of a
thoracic segment).
•
In the following decades other mutations affecting segment
identity were identified e.g. certain Antennapedia mutations
causing transformation of antennae into legs.
Homeotic Mutations
• The direction of homeotic transformations depends on whether
the mutation causes Loss of homeotic gene function where the
gene normally acts or Gain of function where the gene
normally does not act.
•
Ultrabithorax (Ubx) acts in the haltere to promote haltere
development and repress wing development. Loss of function
mutations in Ubx transform the haltere into a wing.
•
In antenna-to-leg transformations of Antennapedia the
mutants reflect a dominant gain of Antennapedia gene
function in the antennae.
Examples of Homeotic Mutations
Normal adult fly
Antennapedia
mutant
Bithorax mutant
Antennapedia Mutations
Wild-type
Mutants
Mammalian Hox mutation
HOX genes play a fundamental role in the
development of the vertebrate central nervous
system, axial skeleton, limbs, gut, urogenital
tract and external genitalia
Hox Gene Expression in the Mammalian
Limb
HoxD gene complex is
expressed in a specific
pattern in the developing
mouse forelimb. The
pattern of gene expression
correlates with the linear
arrangement of the genes
in the genome.
Hox Gene Knockouts in Mice Lead to Limb
Defects
• Hoxa11 or Hoxd11 lead to the loss of the radius and
ulna
Cell Death & Digit Formation
Some humans are born
with webbed fingers Webs
removed just after birth In
normal development: Cell
Death (Apoptosis) Occurs
between Digits Large
amount of lysosomal
enzyme activity
Syndactyly
Hand foot genital syndrome
HOXA13 mutation
• There are several human syndromes which involve
defects of the limbs and the Müllerian ducts or its
derivatives. The hand-foot-genital (HFG) syndrome is
an autosomal dominant (fully penetrant).
• HOXA13 nonsense mutation
(tryptophan → stop codon) .
HOX GENES AND LUNG DEVELOPMENT
Abnormal expression of HOX genes is associated with
several congenital lung abnormalities
e.g. HOXB5 is over-expressed in both
Bronchopulmonary sequestration
Congenital cystic adenomatoid malformation.
HOX GENES AND LEUKAEMIA
In leukaemia there is chromosomal translocations.
Such translocations lead to the creation of fusion genes,
and may involve individual HOX genes or regulators
of HOX gene activity (HOXA9 or HOXD13 genes).
HOX GENES AND CANCER
Neoplastic growth in mammary epithelial cells is
associated with increased expression of human
growth hormone resulting in overexpression of
HOXA1 in mammary carcinoma cells resulted in upregulation of Bcl-2, an antiapoptotic factor, and
increased total cell numbers.
Retinoic acid
• RA probably plays a role in axis specification during normal
development & appears to be critical for the initiation of limb
bud outgrowth, also has been shown to activate the expression
of more posterior Hox genes.
• RA teratogenesis, in which mouse embryos exposed to retinoic
acid show a different pattern of Hox gene expression along the
anterior-posterior axis and abnormal differentiation of their
axial structures.
• Exogenous RA given to mouse embryos in utero can cause
certain Hox genes to become expressed in groups of cells that
usually do not express them.