Msh-2 suppresses in vivo mutation in a gene dose and lesion dependent manner

Mice deficient for the mismatch repair (MMR) gene Msh2 show accelerated tum ourigenesis and a reduced apoptotic response to DNA damage of methylation t ype, Here we examine the effect of mutation for Msh2 on in vivo mutation fr equencies in the intestine as determined by loss of function at the Dolicho s biflorus (Dlb-1) locus. Spontaneous mutation frequencies were scored in c ohorts of ageing mice either wild type or mutant for Msh2. In mice less tha n 1 year old, mutation frequencies were only elevated in Msh2 null mice. Ho wever, beyond this age heterozygous Msh2 mice showed significantly higher m utation frequencies than controls. These findings implicate a gene dose dep endent requirement for Msh2 in mutation suppression and prompted an analysi s of young Msh2 mutants following exposure to DNA damage. Following exposur e to N-methyl-N ' -nitro-N-nitrosoguanidine (MNNG), Msh2 deficient mice sho w a reduced apoptotic response and an increase in mutation frequency. Heter ozygotes did not differ from controls. Following exposure to cisplatin, no significant elevation was seen in mutation frequencies, even within homozyg otes. This is particularly surprising given the association between cisplat in resistance and MMR deficiency, These findings therefore demonstrate a co mplex reliance upon functional Msh2 in mutation surveillance. We have ident ified three separate scenarios. First, where retention of both Msh2 alleles ol er an extended period of time appears critical to the suppression of sp ontaneous mutation; second, 3 weeks following exposure to MNNG, where only complete loss of Msh2 results in elevated mutation; and finally following c isplatin exposure, where induced levels of mutation are independent of Msh2 status.