Amalia S. Dutra

To begin development of a canine gene map and to define syntenic regions between the canine and human genomes, the technique of fluorescence in situ hybridization (FISH) was adapted to localize specific DNA sequences in canine metaphases. Genomic clones of canine origin as well as human yeast artificial chromosome clones were used to map canine loci. The immunoglobulin heavy-chain region was localized to canine chromosome 4qtel; the canine major histocompatibility locus, DLA, was found to be on chromosome 12qtel. Of particular interest, the canine X chromosome, which morphologically is highly similar to the human X, also showed syntenic localization of the clotting factor VIII (F8C) and factor IX (F9) genes to Xq28 and Xq26.3-->q27.1, respectively.

Fas-associated death domain protein (FADD)/MORT1 is a 23-kDa cytoplasmic protein containing a C-terminal death domain that interacts with the intracellular death domain of the Fas transmembrane receptor. Cross-linking of Fas mediates apoptosis in a variety of cells, primarily peripheral T lymphocytes, for which this pathway plays a major role in mature lymphocyte homeostasis. We report the characterization of the human FADD gene, which spans approximately 3.6 kb and contains two exons (286 and 341 bp) separated by a 2.0-kb intron. FADD was mapped to chromosome 11q13.3 by the independent techniques of PCR screening of somatic cell hybrid mapping panels and fluorescence in situ hybridization. In addition FADD was shown by fluorescence in situ hybridization to be amplified along with other 11q13.3 genes previously studied in the breast cancer cell line MDA-MB-134-VI, raising the possibility that overexpression of mutant FADD could contribute to poor prognosis and increased invasiveness of tumors. Its known role in apoptosis has made FADD a candidate susceptibility gene for autoimmune lymphoproliferative syndrome. Now that it has been colocalized in 11q13.3 with IDDM4, a diabetes susceptibility locus, alterations in FADD should also be considered as potential contributors to insulin-dependent familial diabetes. Elucidation of the map position and gene structure of FADD will make possible linkage and mutation analysis to study the role of this gene in human diseases.

The mouse Mitf gene encodes a transcription factor that is regulated by serine phosphorylation and is critical for the development of melanin-containing pigment cells. To test the role of phosphorylation at a particular serine, S73 in exon 2 of Mitf, we used a standard targeting strategy in mouse embryonic stem cells to change the corresponding codon into one encoding an alanine. By chance, we generated an allele in which 85,222 bp of wild-type Mitf sequence are duplicated and inserted into an otherwise correctly targeted Mitf gene. Depending on the presence or absence of a neomycin resistance cassette, this genomic rearrangement leads to animals with a white coat with or without pigmented spots or a gray coat with obligatory white and black spots. Several independent, genetically stable germline revertants that lacked the duplicated wild-type sequence but retained the targeted codon were then derived. These animals were normally pigmented, indicating that the serine-to-alanine mutation is not deleterious to melanocyte development. The fact that mosaic coat reversions occur in all mice lacking the neo-cassette and that approximately 1% of these transmit a reverted allele to their offspring places this mutation among those with the highest spontaneous reversion rates in mammals.