Targeted disurption of the Huntington's disease gene (Hdh) results in early embryonic death in transgenic mouse embryos. Thus the protein, Huntingtin must play an important role in early embryonic development. One possible explanation is that the Huntington's Disease gene interacts with other genes and this leads to embryonic death. One class of interacting gene could be homeobox genes, since these genes are transcription factors and they are known to be involved in early embryonic patterning. The RNA transcripts for the homeobox gene Otx2 are normally progressively restricted to the cranial end of the early mouse embryo. The aim of this study is to characterize the altered expression patterns of Otx2 in mouse embryos lacking a normal copy of the Hdh gene.

Mouse embryos were collected at E7.5 days (n=22) and 8.5 days (n=20) gestation from timemated heterozygous parents carrying one ocpy of the disrupted Hdh gene. Embryos were fixed, embedded in wax, sectioned and some stained with H&E. The H&E sections were evaluated for signs of abnormal morphology. Adjacent sections were placed on slides and used in radioactive situ hybridization. We used an antisense probe for mouse Otx2 and a sense control probe. Embryos were examined and photographed with a coumpound microscope using brightfield and darkfield illumination.

In morphologically normal embryos, the Otx2 gene is expressed at E7.5 in the early headfolds but not in the tail region. Abnormal E7.5 embryos had more homogenous signal in the ectodermal layer and it was not possible to distinguish headfolds. At E8.5 Otx2 is expressed in the early forebrain and midbrain with a sharp boundary at the midhindbrain junction. The 8.5d abnormal embryos expressed Otx2 in a similar pattern to E7.5 embryos thus it appeared development had arrested at least 24 hours earlier.

The homogenous Otx2 expression in the abnormal embryos correlates with their delayed or arrested development. It is also of note that level of expression of Otx2 transcripts is unchanged in the morphologically abnormal embryos. These two observations do not support the hypothesis that the Otx2 gene is directly regulated by the Hdh gene.