Phenotype-based identification of mouse chromosome instability mutants

There is increasing evidence that defects in DNA double-strand-break (DSB) repair can cause chromosome instability, which may result in cancer. To identify novel DSB repair genes in mice, we performed a phenotype-driven mutagenesis screen for chromosome instability mutants using a flow cytometric peripheral blood micronucleus assay. Micronucleus levels were used as a quantitative indicator of chromosome damage in vivo. Among offspring derived from males mutagenized with the germline mutagen N-ethyl-N-nitrosourea (ENU), we identified a recessive mutation conferring elevated levels of spontaneous and radiation- or mitomycin C-induced micronuclei. This mutation, named chaos1 (chromosome aberration occurring spontaneously 1), was genetically mapped to a 1.3-Mb interval on chromosome 16 containing Polq, encoding DNA polymerase θ. We identified a nonconservative mutation in the ENU-derived allele, making it a strong candidate for chaos1. POLQ is homologous to Drosophila MUS308, which is essential for normal DNA interstrand crosslink repair and is unique in that it contains both a helicase and a DNA polymerase domain. While cancer susceptibility of chaos1 mutant mice is still under investigation, these data provide a practical paradigm for using a forward genetic approach to discover new potential cancer susceptibility genes using the surrogate biomarker of chromosome instability as a screen.