HOX genes are homeotic selector genes meaning they are genes that control the development of body segments.
a hox gene is called a hox gene because it contains a homeobox which is a region of conserved nucleotide sequence usually 180bp long located within the coding sequence
Genes in the HOX family are distributed in four unlinked gene clusters.
Each gene cluster contains 9 - 11 genes.
Oh the Horror! Genes resulting in "leg head" fruit flies found in Mammalian Genome.
Homeotic selector genes were first discovered by researchers investigating mutations in Drosophila melanogaster which caused the flies to develop strange Phenotypes such as legs growing out of their heads instead of antennae
The genes in question code for transcription factors that act as on-off switches.
The transcription factors instruct the segments of the fly to develop into one tissue or another.
Mutations in the genes cause the homeotic selector gene to be expressed at an improper time and place resulting in the strange phenotypes.
By using these Drosophila homeotic selector genes as probes in cross-hybridization experiments,
it was discovered that the genomes of every species of multicellular organism from C.elegans to Homo Sapiens (including our friend the mouse) contain homologs for these genes!
There are numerous homologs in mice as seen here
the homologs are lined up vertically
Class Question number 1: What do homeotic selector genes do in drosophila development?
Answer: Determine the proper differentiation of tissues in specific body segments
class Question number 2: Do Mice or humans have obvious body segments like drosophila?
Answer: NO!!!!!!
Since homeotic selector genes are involved in the development of body segments and mice do not have defined body segments we must ask :
what is the function of homeotic selector (hox) genes in mice?
In the quest to find out the function of the hox genes in mice, researchers ran into one monster of a problem:
There were no known mutations at any of the hox loci!
The solution: convert a clone of a gene into a tool to analyse the gene's expression.
Step 1: label and denature a clone
Step 2: perform in situ hybridization on RNA present in fixed tissue samples taken from embryos at different stages of development
Step 3: analyse results for patterns of RNA expression in specific tissues at specific stages of development and interpret the results in reference to the known homeotic selector genes
upon examination of the expression of the hox genes geneticists discovered that each gene is transcribed along a specific portion of the embryonic axis
The hox genes were found to line up in clusters along chromosomes in an order that parallels the body part they control
Their expression along the axis corresponds to their position in the cluster
The further the gene is located towards the 3' end of the cluster the more extensive its expression becomes.
At this point we can create a hypothesis as to what the hox genes do:
remember in drosophila these genes regulated the differentiation of tissues in specific body segments
in mice these genes may control the development of different sections of the embryonic axis
from the above diagram we can see many genes may be expressed over many segments but these segments still develop differently from one another
what's the deal??!!!
Those bright scientists found an explanation for this:
they figure that the only gene that counts when many are present in the same segment is the gene that is most 5' in the cluster
the expression of the most 5' gene is epistatic (meaning hides the expression of) the other more 3' genes in the segment
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The bottom gene, HoxD13, is epistatic to all the genes above it so that it is the only one expressed in that segment. When it ends HoxD12 becomes the next most 5' gene and so on... |
Review time:
What do we know about the Hox genes in mice so far?
1: hox genes are hypothesized to control the development of different sections of the embryonic axis
2: some hox genes are expressed across many segments of the embryonic axis, yet those segments develop differently
class question number 3: what the heck is our hypothesis?
Answer: hox genes function in the development of the embryonic axis of mice and that the most 5' gene is responsible for the development of that segment
Class question number 4: what was the monster problem geneticists faced in the quest to discover the function of the Hox genes?
Answer: No known mutations at the Hox loci!!!
To study the functions of genes researchers often compare mutants which lack a gene to wild type organisms.
What ever the mutant phenotype is missing is probably due to its missing gene.
To understand the function of the hox genes ideally researchers must be able to study embryos who do not express a specific hox gene at all or who express it at an unusual time and place.
But once again there are no known mutations!!!!
Researchers used a hoxA1 regulatory region and attached it to a hoxD4 coding sequence
in a wild type fetus:
hoxa1
normally controls the development
of the occipital
bone at the base of the skull (region "e")
hoxd4 normally controls the development of c1 and c2 vertebrae
class question number 5: what do you think the transgene described above would do?
Answer: the function of the hoxd4 gene should be expressed where hoxa1 normally would be
class question number 6: what physical traits would you expect to see if this transgene is used?
Answer: well, let'‘s just see........
This is a picture of a transgenic fetus with the transgene mentioned above added to its genome
when
compared to a wild type fetus we can see that the
hoxd4 gene was in fact expressed in the occipital
region.
vertebrae appear where the occipital bone should be!!!
n
Now to compare the wild type and transgenic fetuses
we now have our first piece of evidence that the hox genes play a role in the development of the embryonic axis!!
recall: that the development of each segment depends on the most 5' gene in the cluster of genes expressed
class question number 7: what would happen to a segment's development if a gene was knocked out?
Answer: the segment would resemble the previous segment!!!
If
we performed a knock out experiment and knocked
out hoxd12 the sacral segment
would look like the caudal segment
why? Remember that the most 5' gene is epistatic to the 3' genes
therefore when hoxd12 is removed hoxd13 becomes the most 5' gene
now we have our second piece of evidence we can now say with confidence that our hypothesis was true.
The function of the hox genes and their homologs in other species is not in determining the differentiation of any specific tissue
but rather to provide positional information for other genes to use so they may produce the specific tissues in the right places!
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