Signalling cell cycle arrest and cell death through the MMR System

Abstract

Loss of DNA mismatch repair (MMR) in mammalian cells, as well as having a causative role in cancer, has been linked to resistance to certain DNA damaging agents including clinically important cytotoxic chemotherapeutics. MMR-deficient cells exhibit defects in G2/M cell cycle arrest and cell killing when treated with these agents. MMR-dependent cell cycle arrest occurs, at least for low doses of alkylating agents, only after the second S-phase following DNA alkylation, suggesting that two rounds of DNA replication are required to generate a checkpoint signal. These results point to an indirect role for MMR proteins in damage signalling where aberrant processing of mismatches leads to the generation of DNA structures (single-strand gaps and/or double-strand breaks) that provoke checkpoint activation and cell killing. Significantly, recent studies have revealed that the role of MMR proteins in mismatch repair can be uncoupled from the MMR-dependent damage responses. Thus, there is a threshold of expression of MSH2 or MLH1 required for proper checkpoint and cell-death signalling, even though sub-threshold levels are sufficient for fully functional MMR repair activity. Segregation is also revealed through the identification of mutations in MLH1 or MSH2 that provide alleles functional in MMR but not in DNA damage responses and mutations in MSH6 that compromise MMR but not in apoptotic responses to DNA damaging agents. These studies suggest a direct role for MMR proteins in recognizing and signalling DNA damage responses that is independent of the MMR catalytic repair process. How MMR-dependent G2 arrest may link to cell death remains elusive and we speculate that it is perhaps the resolution of the MMR-dependent G2 cell cycle arrest following DNA damage that is important in terms of cell survival.

MLH1 expression sensitises ovarian cancer cells to cell death mediated by XIAP inhibition

“…The presence of wt MLH1 (OVCAR3) or a re-expression in an MLH1-defective A2780 variant did not significantly alter the proliferation rate in cisplatin response, but significantly increased sensitivity to 6-TG, a chemotherapeutic purine nucleoside analogue, the primary mechanism of action of which is dependent on the presence of a functional DNA MMR system (Hawn et al, 1995;Buermeyer et al, 1999;Yan et al, 2003) consistent with earlier reports Branch et al, 2000). However, both 6-TG and cisplatin induced an MLH1-dependent apoptosis and arrest in the G2/M phase of cell cycle (our unpublished observations), consistent with earlier reports in OVCAR3 Kolfschoten et al, 2002) (O'Brien andBrown, 2006). The MLH1 proteolysis in cells undergoing apoptosis correlated with the increased expression of active forms of caspase-3 and -9.…”

“…A number of studies in vitro (Aebi et al, 1996;Brown et al, 1997;Fink et al, 1998b;O'Brien and Brown, 2006) have reported a correlation between MMR deficiency and platinum resistance. Indeed, studies link MMR deficiency with reduced CR to platinumbased chemotherapy (Fink et al, 1998b;Strathdee et al, 2001;Gifford et al, 2004).…”

“…The tumour set from this study comprised only 44% serous EOC analysed by immunohistochemistry compared with our report wherein 100% serous EOCs were analysed by microarray, possibly attributing the discrepancy to multiple histological types. A common thread, however, is the potential of an alternate MMR-independent role of MLH1 in acquired cisplatin resistance, a mechanism still unclear (O'Brien and Brown, 2006).…”

“…Consistent with this, cisplatin-resistant ovarian cell lines have been observed to acquire an MSI phenotype and are defective in strand-specific MMR (Watanabe et al, 2001). Several reports link MLH1 status and clinical response (CR) in cancer patients (O'Brien and Brown, 2006). In contrast, some clinical reports (Scartozzi et al, 2003;Helleman et al, 2006) suggest no direct association between cisplatin response and MMR status in primary ovarian cancers.…”

MSH2 is required for cell proliferation, cell cycle control and cell invasiveness in colorectal cancer cells

“…MSH2 binds to CHK1 and CHK2 both in vivo and in vitro [17]. ATM (ataxia-telangiectasia-mutated kinase), CHK1 and CHK2 have all been implicated in triggering the G2/M checkpoint response, and MMR genes are presumed to play a role in the G2/M cell cycle checkpoint [18]. Campregher et al [19] used an in vitro co-culture model that mimicked intestinal inflammation in ulcerative colitis, in which the activated neutrophils caused an accumulation of target cells in G2/M, which was consistent with the installation of a DNA-damage checkpoint.…”