WIDESPREAD DISTRIBUTION OF CELLS CONTAINING HUMAN DNA IN EMBRYOS DERIVED FROM MOUSE EGGS INJECTED WITH HUMAN-CHROMOSOME FRAGMENTS

The possibility that metaphase chromosomes can serve as a source of ge netic material for making transgenic mice was suggested by our previou s finding of the incorporation of human satellite DNA into mouse embry os that were injected with microdissected human centromeric fragments. In the present study, we further examined whether this chromosome tra nsfer method can be used to generate transgenic mice containing a port ion of human chromosome 4 spanning the Huntington's disease (HD) gene. For this purpose, we used an improved method of metaphase chromosome preparation that may minimize the potential for DNA damage. Using meta phase chromosomes prepared in this manner, chromosome fragments spanni ng the region of chromosome 4 containing the HD gene were microdissect ed, retrieved, and injected into fertilized mouse eggs. The injected e ggs exhibited good viability and developed with a high efficiency when implanted into foster mothers. To determine whether the human DNA fro m the injected chromosome fragment had been incorporated into the mous e genome, embryos were harvested at 12.5 days of gestation (dg) and an alyzed by in situ hybridization using a human Alu repetitive DNA probe . This analysis showed that most of the embryos contained cells with h uman Alu repeats. However, all of the embryos were mosaic, and the lev el of mosaicism was such that we were not able to determine the precis e chromosomal origin of the human DNA insert. We discuss the possible basis for the mosaicism and the potential value of such mosaic animals for studying Huntington's disease.