129-Derived Mouse Strains Express an Unstable but Catalytically Active DNA Polymerase Iota Variant

Abstract

? 2015, American Society for Microbiology.Mice derived from the 129 strain have a nonsense codon mutation in exon 2 of the polymerase iota (Pol?) gene and are therefore considered Pol? deficient. When we amplified Pol? mRNA from 129/SvJ or 129/Ola testes, only a small fraction of the full-length cDNA contained the nonsense mutation; the major fraction corresponded to a variant Pol? isoform lacking exon 2. Pol? mRNA lacking exon 2 contains an open reading frame, and the corresponding protein was detected using a polyclonal antibody raised against the C terminus of the murine Pol? protein. The identity of the corresponding protein was further confirmed by mass spectrometry. Although the variant protein was expressed at only 5 to 10% of the level of wild-type Pol?, it retained de novo DNA synthesis activity, the capacity to form replication foci following UV irradiation, and the ability to rescue UV light sensitivity in Pol? -/- embryonic fibroblasts derived from a new, fully deficient Pol? knockout (KO) mouse line. Furthermore, in vivo treatment of 129-derived male mice with Velcade, a drug that inhibits proteasome function, stabilized and restored a substantial amount of the variant Pol? in these animals, indicating that its turnover is controlled by the proteasome. An analysis of two xeroderma pigmentosum-variant (XPV) cases corresponding to missense mutants of Pol?, a related translesion synthesis (TLS) polymerase in the same family, similarly showed a destabilization of the catalytically active mutant protein by the proteasome. Collectively, these data challenge the prevailing hypothesis that 129-derived strains of mice are completely deficient in Pol? activity. The data also document, both for 129-derived mouse strains and for some XPV patients, new cases of genetic defects corresponding to the destabilization of an otherwise functional protein, the phenotype of which is reversible by proteasome inhibition.