CONTENTS
Main Page Dynamic Development
The Foundations of Developmental
Biology
Gametogenesis
From Sperm and Egg to Embryo
Genetic Regulation of Development
Organizing the Multicellular
Embryo
Generating Cell Diversity
Dynamic Development at a
Glance
Learning Resources
Research Resources
The Developmental Biology Journal
Club
Developmental Biology Tutorial |
Establishment of Spatial Patterns of Gene Expression During Early Vertebrate
Development: Hox Genes
by Dr. Derrick Rancourt
Department of Medical Biochemistry
University of Calgary
We shall now continue from the discussion of homeotic genes in Drosophila and
discuss parallels that have been observed in higher eukaryotes, namely vertebrates.
Much of the material that I shall be discussing is new and not in the text
because this particular field has advanced considerably in the past five
or so years.
Take Home Lesson:
The homeotic (Hox) complex governs both the overt
and non-overt segmentation that occurs in vertebrates.
Key Features of this lecture are :
1) The vertebrate homeotic complex comprises four
distinct Hox gene clusters (Hox A, B, C, D) that are organized into thirteen
homology (or paralogue) groups.
2) The chromosomal organization of the genes in each Hox cluster reflects
its anterior-posterior expression in the body plan (spatial colinearity).
Unlike in Drosophila, vertebrate Hox genes are also temporally colinear
and are expressed in an anterior to posterior direction. In general, members
of the same paralogue group are expressed at the same time and have the
same anterior boundary of expression.
3) Homeotic genes are expressed within segmented and unsegmented structures
within the body plan. Hox gene expression in some unsegmented structures
arise from segmented precursors. The specification of segmented structures
may be due to a specific combination of homeotic genes (or Hox Code).
4) Modern genetic methods such as targeted mutagenesis in mouse have begun
to reveal the function of homeotic genes in vertebrates. In general, homeotic
phenotypes observed in specific hox gene mutations are restricted to the
anterior boundary of expression.
5) The genetic redundancy of the vertebrate homeotic complex may have enabled
the rapid evolution of vertebrates.
Evolution of the Homeotic Complex
Homeosis was first defined by Bateson (1894) to
describe natural variants of a species where certain parts of the body plan
displayed morphological features typical of other regions. This idea was
reiterated by the work of Lewis, who found several homeotic mutants affecting
segment identity in Drosophila. After the cloning of the Drosophila
homeotic complex in Drosophila, these genes were found to be transcription
factors and coined to be homeotic selector genes, meaning that the
fate of a given segment was selected by the expression of specific homeotic
genes. Of course, it was immediately important to see if mouse and other
vertebrates had a similar set of genes - especially because vertebrates
only appear to be partially segmented animals.
Once this was confirmed, a mad race began in the mid eighties to clone all
of the homeotic genes, which only became complete about two years ago. Now,
in the mouse and human (and presumably other vertebrates including chicken
and Xenopus), there are thirty nine genes that are organized on four
different Hox clusters (A, B, C, D) found at four distinct chromosomal loci.
Based on homologies, between themselves and their Drosophila counterparts,
these genes have been organized into thirteen homology groups called paralogues.
There are several interesting points that can be made simply by comparing
the Drosophila and vertebrate complexes. First, paralogue groups
9-13 all appear to derived from Abd-A through some gene duplication
event, while similarly paralogue groups 2 and 3 are derived from proboscapedia.
Unlike in Drosophila, where the Antennapedia and Bithorax complexes
were separated in evolution, each vertebrate homeotic complex is intact.
However, within each cluster, there are genes missing, which is thought
to be an important feature in our evolution. (We shall return to this point.)
In Amphioxus, a near vertebrate ancestor, there is only one complex
that resembles a complete Hox A cluster. That means, for instance, that
hoxa-8 is present this animal and that it has since disappeared. Similarly,
that ancestral homeotic cluster first quadruplicated itself four times,
after which individual genes were lost in evolution. As we shall discuss
later, there is considerable genetic redundancy within the homeotic gene
complex, and this redundancy made it relatively easy to lose specific homeotic
genes in evolution without drastically affecting the body plan.
Hox Gene Expression and Colinearity
Whereas the colinearity that occurs between the
homeotic complex and anterior-posterior expression was first recognized
by Lewis in his genetic experiments, it was formally demonstrated in mouse
using in situ hybridization. Here, members
of the hoxB cluster have different anterior boundaries of expression in
the CNS and prevertebra, which reflected their relative chromosomal organization.
In 1989, hox genes were given names according to the order in which they
were discovered. In 1997, we know that the anteriorly expressed genes reside
at the 3' end of the complex and that unlike in Drosophila (whose
homeotic genes are all expressed at the same time), the mouse homeotic complex
is also temporally colinear, meaning that as the body plan develops
in an anterior-posterior direction during gastulation, there is a sequential
expression of the homeotic complex. We also know that the paralogue groups
are expressed with roughly the same anterior boundaries in segmented tissues
and that the next more posterior group is expressed one or two segments
more posterior.
(See Fig. 14.39, Browder et al., 1991;
Gilbert, 1997, Fig. 16.3; Kalthoff, 1996, Fig. 22.6; Wolpert et al.,
1998, Fig. 4.8)
There are some interesting differences
in the regulation between vertebrate and Drosophila homeotic genes.
- First, each vertebrate hox cluster is the size
of the Ubx gene in Drosophila. This observation has been
used to argue that the regulation of the Drosophila complex is more
complex. Indeed, whereas we have observed shared regulatory elements in
the vertebrate genes, in Drosophila each homeotic gene appears to
function more or less independantly.
- Second unlike in Drosophila, there are
no equivalent pair rule and segment polarity genes setting up segmentation
for the homeotic genes. While many homologues of these genes exist, such
as even skipped and paired, these homologues are often expressed after
the homeotic genes are expressed. Thus, it is still a mystery how vertebrate
hox genes are turned on in embryogenesis.
In vertebrates, the main evidence of segmentation
is in the vertebral column. The head, the fore- and hindlimbs are later
adaptations onto the early vertebrate skeleton. The earliest expression
studies focused on the expression of hox genes in the prevertebra of mid-gestation
embryos. In these experiments, it was realized that specific paralogue groups
had anterior boundaries within prevertebra and that each vertebra had its
own hox address or code. Whereas the anterior boundaries are well defined,
the posterior boundaries of hox genes are not; suffice it to say that many
more homeotic genes are expressed within posterior segments than in anterior
segments.
Hox Gene Expression and Embryogenesis
Recall that during neurulation, the vertebral precursors,
somites, condense out of axial medoderm as the neural folds emerge
from Hensen's node. Whereas the somites are the earliest and most overt
evidence of the segmentation at this time, there is a theory that the nervous
system is segmented as well. As the neural folds are emerging and before
we begin to see somites, electron microscopists have observed transient
stuctures within the neural folds called somitomeres. Although these structures
will disappear at somitogenesis, it is believed that the basic neural tube
is segmented, although there is little overt evidence to demonstrate this.
However, the existence of somitomeres helps to explain the boundaries of
hox gene expression within the midgestation CNS.
However, if we look at the CNS earlier in embryogenesis, shortly after neurulation,
and during somitogenesis, segmentation is obvious in the rhombencephalon,
which is the CNS precursor of the hindbrain. In this structure and at this
time there is a segmental periodicity to the expression of the most anteriorly
expressed genes. So, each rhombomere has its own hox code as well (i.e.,
except R1 and R2 ,which do not express hox genes.
(Browder et al., 1991, Fig. 14.40; Gilbert,
1997, Fig. 16.4; Kalthoff, 1996, Figs. 22.7-22.8; Wolpert et al.,
1998, Figs. 4.23-4.24)
Hox gene are not expressed anterior to
the hindbrain. The forebrain and midbrain are more advanced structures in
vertebrate and although there is evidence that these structures are also
segmented, the homeotic gene family had nothing to do with the elaboration
of these structures. An important feature of the rhombencephalon is that
cranial neural crest cells delaminate from this structure as the neural
tube is closing. These specialized neuroectodermal cells are a more recent
adaptation of jawed vertebrates and will give rise to, or contribute to,
the formation of the jaw, the cranial nerves and soft tissues of the neck
and throat, including the thyroid and thymus. Anterior hox genes that are
expressed in rhombencephalon are also expressed in their derivative neural
crest cells. These neural crest cells will migrate to the branchial arches
and give rise to these specialized structures. Each branchial arch therefore
has its own hox code, and - as we shall see - mutations within the anterior
genes can affect the emergence of these specialized tissues.
Whereas hox gene expression has been closely studied in cranial neural crest,
little is known about the role of hox genes in trunk neural crest. Although
various hox gene have been found to be expressed in soft tissues such as
the lung, gut, etc., little is known about whether this expression is the
result of segmentation (i.e., being derived from trunk neural crest migration)
or whether this expression is de novo.
The Hox Code
The hox code model predicts that combinations of
hox genes specify the development of the anterior-posterior axis. Evidence
for the code comes from three lines of experimentation:
- Comparative Anatomy
indicates that the appearance of specific vertebrae such as the cervico-thoracic
junction is correlated with the expression of specific hox genes. HoxC6
expression is invariantly in the cervicothoracic prevertebra in mouse,
chicken, goose, Xenopus and zebrafish. Interestingly, the length
of the neck is different in each species. Similarly, the positioning of
the lumbral-sacral junction may be a function of members of the hox10 paralogous
family.
- Retinoic acid induced homeosis during mouse
embryogenesis. Retinoic acid, a developmental
steroid and teratogenenic agent, has been found to alter the axial expression
of homeotic genes when administered during somitogenesis. Each hox cluster
has cis elements that respond to retinoic acid and thus normal developmental
regulation is perturbed. The anterior expression boundaries of several
anterior hox genes were shifted posteriorly, resulting in the posteriorization
of vertebrae. Similarly, posterior gene expression was shifted anteriorly,
resulting in an anteriorization of fate. Similar results were observed
in cranial neural crest derivatives of the hindbrain: Expression boundaries
shifted anteriorly, and R2 and R3 resembled the characteristics of R4 and
R5.
(See Gilbert, 1997, Fig. 16.5; Kalthoff, 1966, Fig. 22.18; )
- Transgenic induced homeosis in mouse.
Gain of function mutations, in which a posterior gene is expressed more
anteriorly, resulted in a posteriorization of anterior vertebrae. This
obsevation led to the suggestion of posterior prevalence, meaning
that when moved to an anterior location, a posterior gene had priority
over its anterior collegues. Loss of function mutations, in which a gene
is removed by targeted mutagenesis, resulted in an anteriorization of vertebrae.
In this situation, the hox code of the mutant prevertebra or rhombomere
would now partially resemble its near-anterior neighbour. Following the
posterior prevalence model, the respecified vertebra no longer expresses
the posterior hox gene, which made this segment different than its anterior
neighbour.
Targeted Mutagenesis in Mice
More recently, other types of manipulations have
been possible. Reporter genes such as lacZ can be integrated into a locus
so that it is under the control of native elements (see Targeted Modifications in Mice).
Gene swaps can be performed so that when applied to the homeotic complex,
a posterior gene can be made more anterior and visa versa. Very subtle mutations
can be made such as point mutations. In this case, the neo cassette is placed
within an intron so that expression of the gene in not perturbed. We can
also make small and large deletions within the complex to destroy regulatory
elements. Although people have been able to delete an entire hox complex
in ES cells, they have thus far been unable to generate these mice.
I was a postdoctoral fellow in the laboratory where gene targeting was developed.
Part of our interest was to apply this new technology to understanding the
homeotic complex. There were approximately 30 genes known in the complex
at the time, and we undertook the insurmountable task of knocking out each
and every gene in the complex. By the time I left, this work was about three
quarters complete. I want to highlight a few interesting experiments that
shed light on the complexity of this multigene family.
The first gene that we disrupted was hoxA3,and the phenotype that was observed
was soft tissue defects in the neck and upper chest. For example, the glottis
and epiglottis were aberrant, the thymus and thyroid were reduced and there
were significant alterations of the aortic arteries. Because all of these
structures were derived from the neural crest, which invaded the fourth
branchial arch, these results were the first to demonstrate that hox gene
expression was important for specifying the fate of neural crest cells.
This result has been confirmed with mutations in other anterior genes such
as hoxB2 ,which is expressed up to the R1 boundary. Interestingly in this
mutation, bones of the jaw, which arise from neural crest in the second
branchial arch, are duplicated posteriorly as a result of an anteriorizing
homeotic mutation. This is "heady" stuff here, but the interesting
part of this mutation, is that the jaw structure of this mutant resembles
that of reptiles and thus this hoxB2 mutation is considered to be atavistic
- meaning that it is found in ancestral animals.
An interesting observation that was made following the A3 mutation was that
mutations in the paralogue D3 did not result in a soft tissue defect but
resulted in a skeletal defect, suggesting that - whereas paralogues appeared
to have similar expression boundaries - their roles were distinct in different
tissues. In this case, the first cervical vertebra was fused to the occipital
bone, which was considered to be an anteriorizing homeotic mutation.
At this time, of course, we wondered what the A3, D3 double mutant might
look at. What was emerging in the gene targeting field was the idea of genetic
redundancy; i.e., that phenotypes in mice could be made less severe because
they were masked by the function of related genes. Genetic redundancy was
confirmed when A3, D3 double mutants were generated. Interestingly, one
or two mutant copies of the D3 gene were found to exacerbate the A3 soft
tissue phenotype, whereas one or two mutant copies of the A3 were found
to exacerbate the D3 skeletal phenotype. Thus, it appears that - whereas
both hox genes have a primary responsibility in soft tissue or skeletal
development - they appear to be able to contribute independantly to each
function.
Genetic redundancy was not restricted to the paralogous member, however,
because in experiments that I performed, we observed similar genetic interactions
between neighbouring hox genes. Now we know that a complex web of interactions
govern the homeotic complex and that neighbouring paralogues and even next
to neighbouring paralogues can augment each others' phenotypes just like
in the case of A3, D3.
As a result, a considerable amount of genetics will still be required to
understand the complexity of the homeotic complex.
Genetic Redundancy of the Homeotic Complex Permits
Evolution of the Body Plan
With the exception of some of the more anterior
genes in the complex, mutations in many hox genes resulted in only minor
phenotypes that did not affect morphology greatly. Often, in mutations affecting
vertebral structure, only one side of the animal was affected, suggesting
that there is some plasticity in the response hox code. Hox genes have long
thought to play an important role in limb development. However, when limb
genes such as A11 or D11 were made, the phenotypes were hardly detectable.
On the other hand, once two mutations were put together, the effects were
dramatic. For example, the absence of both A11 and D11 results in a severe
and life threatening reduction in the radius and ulna. What this result
tells us is that the redundancy of the Hox complex permits some flexibility
in the response to mutational change.
Redundancy permits a rich potential to allow for evolutionary alterations
of the body plan, through the following mutational changes:
- variations in the number of homeotic genes, by
deletion and or duplication;
- increases in the number of hox complexes through
whole complex duplication events;
- mutations affecting the timing, position or level
of homeotic gene activation, which may bemost relevant to generate small
adptive changes;
- aterations in the regulatory interactions between
Hox proteins and their targets, through muations of the coding sequenc
of Hox genes.
Recently, heterochronic mutations have been made
within the hox gene complex by changing the position of a given gene within
the complex. When hoxD11 is targeted into D13, a significant limb phenotype
results - not from the inactivation of hoxD13 itself but from the inappropriate
activation of D11 at the wrong time.
Digging Deeper:
Recent Literature
Mann, R.S. 1997. Why are Hox genes clustered? BioEssays 19: 661-664.
Morrison, A., Ariza-McNaughton, L., Gould, A., Featherstone, M. and Krumlauf,
R. 1997. HOXD4 and regulation of the group 4 paralog genes. Development
124: 3135-3146.
References
Bateson, W. 1894. Materials for the Study of Variation. Macmillan.
London.
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology.
Third edition. Saunders College Pub. Philadelphia.
Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer.
Sunderland, MA.
Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill.
New York.
Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and
Meyerowitz, E. 1998. Principles of Development. Current Biology.
London. |
