Mammary carcinogenesis in transgenic mice expressing a dominant-negative mutant of DNA polymerase beta in their mammary glands

DNA polymerase beta (polbeta) is a major contributor to mammalian DNA damage repair through its gap-filling DNA synthesis and 5'-deoxyribose phosphate lyase activities. In this way, polbeta plays pivotal roles in the repair of oxidative DNA damage, replication, embryonic survival, neuronal development, meiosis, apoptosis and telomere function. A 36 kDa truncated polbetaDelta protein is expressed in human colorectal, breast, lung and renal carcinomas, but not in normal matched tissues. Interestingly, a binary protein-protein complex of polbetaDelta and X-ray cross-complementing group 1 acts as dominant-negative mutant. In this study, the potential tumorigenic activity of polbetaDelta was examined in nude and transgenic mouse models. Mouse embryonic fibroblasts (MEFs) expressing polbetaDelta in the absence of endogenous polbeta exhibited increased susceptibility to N-methyl-N-nitrosourea (MNU)-induced morphological transformation as compared with cells expressing wild-type (WT) polbeta. This was accompanied by reduced gap-filling DNA synthesis activity. Anchorage-independent transformed cells derived from polbetaDelta-expressing MEFs induced 100% tumor occurrence in nude mice. To support these data, we established transgenic mice expressing polbetaDelta specifically in the mammary glands from a whey acidic protein promoter-driven transgene. This is the first report of transgenic mice with tissue-specific expression of polbetaDelta. MNU-induced tumor formation was analyzed in transgenic mice expressing polbetaDelta together with endogenous WT polbeta in their mammary glands and in normal control mice expressing only WT polbeta. The latent period of tumor appearance was markedly shorter and tumor incidence was significantly higher in transgenic animals than in control animals treated under the same conditions. These results indicate that cells expressing the mutant polbetaDelta display an enhanced sensitivity to MNU that probably underlies an increased susceptibility to tumorigenesis.