Quantitative evaluation of incomplete preweaning lethality in mice by using the CRISPR/Cas9 system.

Various molecular biology techniques implementing genome editing have made it possible to generate mouse mutants for nearly all known genes; as a result, the International Mouse Phenotyping Consortium (IMPC) database listing the phenotypes of genetically modified mice has been established. Among mouse phenotypes, lethality is crucial to evaluate the importance of genes in mouse survival. Although many genes are reported to show "preweaning lethality, incomplete penetrance" in the IMPC database, the survival rates of homozygous knockout mice are highly variable. Here, we propose the lethal allele index (LAI), the ratio of the observed number of mice with homozygous knockout (KO) to the theoretically predicted number of homozygous KO mice, as a simple quantitative indicator of preweaning lethality. Among the mice mutants registered as incompletely lethal in IMPC, the LAI calculated from the genotypes of F mice tended to be lower in disease-related genes, and correlated with the frequency of loss-of-function (LOF) alleles in humans. In genome-edited mice using CRISPR/Cas9, the number of mice with homozygous frameshift alleles seemed to be associated with lethality. We edited the Ehd1 gene in cell lines as well as mice using CRISPR/Cas9, and found that the genotype distribution was significantly different. The LAI calculated from these data was similar to the value calculated from the IMPC data. These findings support the potential usefulness of the LAI as an index of preweaning lethality in genome-edited mice.