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RESEARCH PRODUCT

Mechanisms and consequences of methylating agent-induced SCEs and chromosomal aberrations: a long road traveled and still a far way to go.

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Since the milestone work of Evans and Scott, demonstrating the replication dependence of alkylation-induced aberrations, and Obe and Natarajan, pointing to the critical role of DNA double-strand breaks (DSBs) as the ultimate trigger of aberrations, the field has grown extensively. A notable example is the identification of DNA methylation lesions provoking chromosome breakage (clastogenic) effects, which made it possible to model clastogenic pathways evoked by genotoxins. Experiments with repair-deficient mutants and transgenic cell lines revealed both O<sup>6</sup>-methylguanine (O<sup>6</sup>MeG) and N- methylpurines as critical lesions. For S<sub>N</sub>2 agents such as methyl- methanesulfonate (MMS), base N-methylation lesions are most critical, likely because of the formation of apurinic sites blocking replication. For S<sub>N</sub>1 agents, such as N-methyl-N′-nitro-N-nitrosoguanidine (MNNG), O<sup>6</sup>-methylguanine (O<sup>6</sup>MeG) plays the major role both in recombination and clastogenicity in the post-treatment cell cycle, provided the lesion is not pre-replicatively repaired by O<sup>6</sup>-methylguanine-DNA methyltransferase (MGMT). The conversion probability of O<sup>6</sup>MeG into SCEs and chromosomal aberrations is estimated to be about 30:1 and >10,000:1 respectively, indicating this mispairing pro-mutagenic lesion to be highly potent in inducing recombination giving rise to SCEs. O<sup>6</sup>MeG needs replication and mismatch repair to become converted into a critical secondary genotoxic lesion. Here it is proposed that this secondary lesion can be tolerated by a process termed <i>recombination bypass</i>. This process is supposed to be important in the tolerance of lesions that can not be processed by translesion synthesis accomplished by low-fidelity DNA polymerases. Recombination bypass results in SCEs and might represent an alternative pathway of tolerance of non-instructive lesions. In the case of O<sup>6</sup>MeG-derived secondary lesions, recombination bypass appears to protect against cell killing since SCEs are already induced with low, non-toxic doses of MNNG. Saturation of lesion tolerance by recombination bypass or translesion synthesis may cause block of DNA replication leading to DSBs at stalled replication forks, which result in chromatid-type aberrations. Along with this model, several putative consequences of methylation-induced aberrations will be discussed such as cell death by apoptosis as well its role in tumor promotion and progression.