| Document Type: | Doctor of Philosophy |
| Name: | Qing Tang |
| Email address: | renntiger@altavista.net |
| Title: | Genetic control of eye development, a case study focused on the murine
mutation Belly spot and tail (Bst) |
| Degree: | Doctor of Philosophy |
| Program: | Anatomy and Neurobiology |
| Research Advisor: | Daniel Goldowitz, Ph.D. |
| Advisor's email: | dgold@nb.utmem.edu |
| Committee Members: | John Cox, Ph.D. |
| Eldon E. Geisert, Jr., Ph.D. | |
| D. J. Surmeier, Ph.D. | |
| Robert Williams, Ph.D. | |
| Keywords: | retina development, mouse mutation, gene regulation, gene interaction, compound mutants, developmental timing |
| Availability: | Release the entire work for World Access. |
The Belly spot and tail (Bst) semi-dominant mutation, mapped to mouse
Chromosome 16, leads to developmental defects of the eye, skeleton, and
coat
pigmentation. In the eye, the mutant phenotype is characterized by
the presence of retinal colobomas, a paucity of retinal ganglion cells,
and axon misrouting. The severity of defects in the Bst/+ retina is variable
among individuals and is often asymmetric. In order to determine the role
of the Bst locus during retinal morphogenesis, we searched for the earliest
observable defects in the developing eye. We examined the retinas of Bst/+
and +/+ littermates from embryonic day 9.5 (E9.5) through E13.5, and measured
retinal size, cell density, cell death, mitotic index and cell birth index.
We have found that development of the Bst/+ retina is notably dilatory
by as early as E10.5. The affected retinas are smaller than their wildtype
counterparts, and optic fissure fusion is delayed. In the mutant, there
is a marked lag in the exit of retinal cells from the mitotic cycle, even
though there are no observable differences in the rate of cellular proliferation
or cell death between the two groups. We hypothesize that Bst regulates
retinal cell differentiation, and that variability of structural defects
in the mutant, such as those affecting optic fissure fusion, is a reflection
of the extent of developmental delay brought about by the Bst mutation.
In an effort to determine the role of Bst within the network of genes
controlling eye development, we examined the effects of Bst in relation
to two genes believed to
regulate eye development during the same developmental period (E9-13):
the boundary gene Pax2, which plays a role during optic stalk fissure fusion,
and the proneural gene Hes1, which has been shown to regulate retinal ganglion
cell differentiation. We cross-mated hemizygous Bst/+ mutant mice with
hemizygous Krd/+ (kidney and retinal defects, in which Pax2 is deleted)
mice and Hes1 +/- knockout mice to produce Bst/+ Krd/+ and Bst/+ Hes1 +/-
compound heterozygous mutants. We find that there is marked ectopic expression
of Pax2 protein in the Bst/+ retina, and a potentiation of the retinal
defects in Bst/+ Krd/+ compound mutant offspring compared to the parental
mutant phenotypes; which indicates that the Bst mutation could have resulted
in a loss of positional cues for subsequent morphogenetic events, such
as those dependent upon Pax2 expression. In contrast, the level of Hes1
mRNA in the Bst/+ mutant appears normal. In the Bst/+ Hes1
+/- compound mutant offspring, there are fewer instances of ocular
defects compared to those of Bst/+ Hes1 +/+ littermates. Given that Hes1
is believed to be an inhibitor of cellular differentiation, the partially
improved Bst/+ Hes1 +/- retinal phenotype is consistent with the hypothesis
that the effects of the two mutant alleles are antagonistic.
Taken together, our results suggest that the Bst locus is involved in
the regulation
of cellular differentiation during early eye development.
Revised 23 May 2002