The phenotypic and molecular consequences of a HOXA13 mutation in Hypodactyly mice.

Hypodactyly (Hoxa13Hd) is a spontaneous mouse mutation affecting the limbs and urogenital system. Homozygotes have one digit on each paw and only 5% survive to birth; rare survivors of both sexes are infertile. The Hoxa13Hd mutation is a 50 base-pair deletion within the coding sequence of Hoxa13, an AbdB-like Hox gene transcription factor that is critical for embryonic growth and patterning. Hoxa13 Hd is more severe than the engineered null mutation of Hoxa13. In this thesis, I have explored the phenotypic and molecular consequences of Hoxa13Hd on limb and reproductive tract development and fetal lethality. In limbs, the severe phenotype is caused both by a lack of wild-type HOXA13 protein (loss-of-function) and by the production of a novel mutant form of HOXA13 (gain-of-function). In transgenic mice expressing the mutant RNA throughout developing limb buds, the novel protein severely reduced the size of limb skeletal structures. The mutant protein appears to act by promoting mesenchymal cell death and altering the expression of other important limb patterning genes such as Hoxd13. The infertility of Hoxa13Hd/Hd mice results from profound hypoplasia of caudal reproductive structures. In female mice, the overall size of the vagina was severely reduced and an anterior transformation of cervical epithelial tissue to a uterine-like stroma was observed. Males have marked reduction in the size of the proximal segment of the penian bone that undergoes endochondral ossification, but not the distal segment that undergoes intramembranous ossification. To investigate the cause of fetal lethality I determined the time of death and assessed Hoxa13 expression in the placenta. Most Hoxa13Hd/Hd fetuses die between E14.5 and E17.5, and rare surviving mutant females outnumber males by 2.6 to 1 at weaning. HOXA13 is expressed in the placenta in a developmentally-regulated manner with highest levels observed at E11.5. Although the Hoxa13Hd mRNA is highly expressed in the developing placenta, no histological abnormalities were observed and the cause of fetal lethality remains unclear. Western blot analyses revealed a significant maternal contribution of HOXA13 to the placenta. These experiments suggest that translation of HOXA13 protein may be regulated differently in the placenta compared to the limb buds.Ph.D.Biological SciencesGeneticsMolecular biologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131979/2/9938515.pd