Random mutagenesis of proximal mouse chromosome 5 uncovers predominantly embryonic lethal mutations

A region-specific ENU mutagenesis screen was conducted to elucidate the functional content of proximal mouse Chr S. We used the visibly marked, recessive, lethal inversion RUMP White (Rw) as a balancer in a three-generation breeding scheme to identify recessive mutations within the -50 megabases spanned by Rw. A total of 1003 pedigrees were produced, representing the largest inversion screen performed in mice. Test-class animals, homozygous for the ENU-mutagenized proximal Chr 5 and visibly distinguishable from nonhomozygous littemates, were screened for fertility, hearing, vestibular function, DNA repair, behavior, and dysmorphology. Lethals were identifiable by failure to derive test-class animals within a pedigree. Embryonic lethal mutations (total of 34) were overwhelmingly the largest: class of mutants recovered. We characterized them with respect to the time of embryonic death, revealing that most act at midgestation (8.5-10.5) or sooner. To position the mutations within the Rw region and to guide allelism tests, we performed complementation analyses with a set of new and existing chromosomal deletions, as well as standard recombinational mapping on a Subset of the mutations. By pooling the data from this and other region-specific mutagenesis projects, we calculate that the mouse genome contains -3479-4825 embryonic lethal genes, or about 13.7%-19% of all genes.