CELLS FROM ERCC1-DEFICIENT MICE SHOW INCREASED GENOME INSTABILITY ANDA 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 mamma lian cells and is also believed to be involved in mitotic recombinatio n. ERCC1-deficient mice, with their extreme runting and polyploid hepa tocyte nuclei, have a phenotype that is more reminiscent of a cell cyc le arrest/premature ageing disorder than the classic DNA repair defici ency disease, xeroderma pigmentosum. To understand the role of ERCC1 a nd the link between ERCC1-deficiency and cell cycle arrest, we have st udied primary and immortalised embryonic fibroblast cultures from ERCC 1-deficient mice and a Chinese hamster ovary ERCC1 mutant cell line. M utant 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 recombi nation-mediated repair of interstrand cross links in the mouse. Mutant cells from both species had a high mutation frequency and the level o f genomic instability was elevated in ERCC1-deficient mouse cells, bot h in vivo and in vitro. There was no evidence for an homologous recomb ination deficit in ERCC1 mutant cells from either species. However, th e frequency of S-phase-dependent illegitimate chromatid exchange, indu ced by ultra violet light, was dramatically reduced in both mutants. I n rodent cells the G(1) arrest induced by ultra violet light is less e xtensive than in human cells, with the result that replication proceed s on an incompletely repaired template. Illegitimate recombination, re sulting in a high frequency of chromatid exchange, is a response adopt ed by rodent cells to prevent the accumulation of DNA double strand br eaks adjacent to unrepaired lesion sites on replicating DNA and allow replication to proceed. Our results indicate an additional role for ER CC1 in this process and we propose the following model to explain the growth arrest and early senescence seen in ERCC1-deficient mice. In th e absence of ERCC1, spontaneously occurring DNA lesions accumulate and the failure of the illegitimate recombination process leads to the ac cumulation of double strand breaks following replication, This trigger s the p53 response and the G(2) cell cycle arrest, mediated by increas ed expression of the cyclin-dependent kinase inhibitor p21(cip1/waf1). The increased levels of unrepaired lesions and double strand breaks l ead to an increased mutation frequency and genome instability.