Nonpermissiveness for mouse embryonic stem (ES) cell derivation circumvented by a single backcross to 129/Sv strain: establishment of ES cell lines bearing the Om(d) conditional lethal mutation

The inbred mouse strain DDK carries a conditional early embryonic lethal mu tation that is manifested when DDK females are crossed to males of other in bred strains but not in the corresponding reciprocal crosses. It has been s hown that embryonic lethality could be assigned to a single genetic locus c alled Ovum mutant (Om), on Chromosome (Chr) II near Syca I. In the course o f our study of the molecular mechanisms underlying the embryonic lethality, we were interested in deriving an embryonic stem cell bearing the Om mutat ion in the homozygous state (Om(d)/Om(d)). However, it turned out that DDK is nonpermissive for ES cell establishment, with a standard protocol. Here we show that permissiveness could be obtained using Om(d)/Om(d) blastocysts with a 75% 129/Sv and 25% DDK genetic background. Several germline-compete nt Om(d)/Om(d) ES cell lines have been derived from blastocysts of this gen otype. Such a scenario could be extended to the generation of ES cell lines bearing any mutation present in an otherwise nonpermissive mouse strain.