Mismatch repair, G(2)/M cell cycle arrest and lethality after DNA damage

The role of the mismatch repair pathway in DNA replication is well defined but its involvement in processing DNA damage induced by chemical or physica l agents is less clear. DNA repair and cell cycle control are tightly linke d and it has been suggested that mismatch repair is necessary to activate t he G(2)/M checkpoint in the presence of certain types of DNA damage. We inv estigated the proposed role for mismatch repair (MMR) in activation of the G(2)/M checkpoint following exposure to DNA-damaging agents. We compared th e response of MMR-proficient HeLa and Raji cells with isogenic variants def ective in either the hMutL alpha or hMutS alpha complex. Different agents w ere used: the cross-linker N-(2-chloroethyl)-N'-cyclohexyl-N-nitrosourea (C CNU), gamma-radiation and the monofunctional methylating agent N-methyl-N-n itrosourea (MNU), MMR-defective cells are relatively sensitive to CCNU, whi le no differences in survival between repair-proficient and -deficient cell s were observed after exposure to gamma-radiation. Analysis of cell cycle d istribution indicates that G(2) arrest is induced at least as efficiently i n MMR-defective cells after exposure to either CCNU or ionizing radiation. As expected, MNU does not induce G(2) accumulation in MMR-defective cells, which are known to be highly tolerant to killing by methylating agents, ind icating that MNU-induced cell cycle alterations are strictly dependent on t he cytotoxic processing of methylation damage by MMR, Conversely, activatio n of the G(2)/M checkpoint after DNA damage induced by CCNU and gamma-radia tion does not depend on functional MMR. In addition, the absence of a simpl e correlation between the extent of G(2) arrest and cell killing by these a gents suggests that G(2) arrest reflects the processing by MMR of both leth al and non-lethal DNA damage.