Cells from ERCC1-deficient mice show increased genome instability and a reduced frequency of S-phase-dependent illegitimate chromosome exchange but a normal frequency of homologous recombination.

The ERCC1 protein is essential for nucleotide excision repair in mammalian cells and is also believed to be involved in mitotic recombination. ERCC1-deficient mice, with their extreme runting and polyploid hepatocyte nuclei, have a phenotype that is more reminiscent of a cell cycle arrest/premature ageing disorder than the classic DNA repair deficiency disease, xeroderma pigmentosum. To understand the role of ERCC1 and the link between ERCC1-deficiency and cell cycle arrest, we have studied primary and immortalised embryonic fibroblast cultures from ERCC1-deficient mice and a Chinese hamster ovary ERCC1 mutant cell line. Mutant cells from both species showed the expected nucleotide excision repair deficiency, but the mouse mutant was only moderately sensitive to mitomycin C, indicating that ERCC1 is not essential for the recombination-mediated repair of interstrand cross links in the mouse. Mutant cells from both species had a high mutation frequency and the level of genomic instability was elevated in ERCC1-deficient mouse cells, both in vivo and in vitro. There was no evidence for an homologous recombination deficit in ERCC1 mutant cells from either species. However, the frequency of S-phase-dependent illegitimate chromatid exchange, induced by ultra violet light, was dramatically reduced in both mutants. In rodent cells the G1 arrest induced by ultra violet light is less extensive than in human cells, with the result that replication proceeds on an incompletely repaired template. Illegitimate recombination, resulting in a high frequency of chromatid exchange, is a response adopted by rodent cells to prevent the accumulation of DNA double strand breaks adjacent to unrepaired lesion sites on replicating DNA and allow replication to proceed. Our results indicate an additional role for ERCC1 in this process and we propose the following model to explain the growth arrest and early senescence seen in ERCC1-deficient mice. In the absence of ERCC1, spontaneously occurring DNA lesions accumulate and the failure of the illegitimate recombination process leads to the accumulation of double strand breaks following replication. This triggers the p53 response and the G2 cell cycle arrest, mediated by increased expression of the cyclin-dependent kinase inhibitor p21(cip1/waf1). The increased levels of unrepaired lesions and double strand breaks lead to an increased mutation frequency and genome instability.