The Lower Bound to the Evolution of Mutation Rates

Despite substantial attention from theoreticians, the evolutionary mechanisms that drive intra- and interspecific variation in the mutation rate remain unclear. It has often been argued that mutation rates associated with the major replicative polymerases have been driven down to their physiological limits, defined as the point at which further enhancement in replication fidelity incurs a cost in terms of reproductive output, but no evidence in support of this argument has emerged for cellular organisms. Here, it is suggested that the lower barrier to mutation rate evolution may ultimately be defined not by molecular limitations but by the power of random genetic drift. As the mutation rate is reduced to a very low level, a point will eventually be reached at which the small advantage of any further reduction is overwhelmed by the power of drift. This hypothesis is consistent with a number of observations, including the inverse relationship between the per-site mutation rate and genome size in microbes, the negative scaling between the per-site mutation rate and effective population size in eukaryotes, and the elevated error rates associated with less frequently deployed polymerases and repair pathways

The role of DNA mismatch repair genes in genome stability and carcinogenesis

DNA mismatch repair (MMR) deficiency is associated with an increased mutational burden and predisposition to certain malignancies. Relatively little is known, however, about mutant frequencies within MMR-deficient primary tumors. Thymic lymphomas from Msh2-/- mice were thus analyzed using a lacI-based transgenic shuttle-phage mutation detection system. All tumors exhibited greatly elevated lacI gene mutant frequencies, ranging from 3.2- to 17.4-fold above the ~15-fold elevations present within normal Msh2-/- thymi. In addition, individual lacI genes harboring multiple changes were found in the tumors. To investigate whether hypermutation was a feature of all tumors arising in mismatch repair-deficient mice, lacI transgene mutant frequencies were obtained from tumors of mice deficient for Pms2 and/or Msh2. While lacI gene hypermutation was again clearly evident in Msh2+/- Pms2-/- and Msh2-/- Pms2-/- thymic lymphomas, three non-thymic Msh2 deficient tumors failed to show elevated frequencies of mutation in lacI when compared to a normal tissue within the respective mice. The elevated mutant frequencies in the lymphoid tumors, and the finding of multiple clustered mutations in lacI genes from these tumors, suggested that they were possibly generated by a lymphoma-specific hypermutational mechanism. Similar to Msh2 deficient mice, mice rendered deficient in Mlh1 or Pms2 via gene targeting are also prone to tumorigenesis, particularly lymphomas. According to the recent model of mammalian MMR, these two proteins function as a heterodimer. However, while Mlh1-/- mice develop small intestinal adenomas and adenocarcinomas, Pms2-/- animals remain free of such tumors. To establish whether this discrepancy might be associated with a quantitative and/or qualitative difference in genomic instability between Mlh1-/- and Pms2-/- mice, we determined small intestinal epithelial cell DNA mutant frequencies and spectra using the lacI reporter system. We found that C:G->T:A transitions were significantly elevated in Mlh1-/- versus Pms2-/- mice, leading to a 1.5-fold lacI mutant frequency increase in these animals. We hypothesize that this finding may explain, in part, why Mlh1-/- mice, but not Pms2-/- mice, develop tumors at this site. Furthermore, the difference in the lacI mutational spectrum of Mlh1-/- and Pms2-/- mice suggests that MLH1 may be involved in a PMS2-independent repair pathway particularly towards DNA lesions that result in C:G->T:A transition mutations.Medicine, Faculty ofMedical Genetics, Department ofGraduat

Mbd4 inactivation increases C→T transition mutations and promotes gastrointestinal tumor formation

Mbd4 (methyl-CpG binding domain 4) is a novel mammalian repair enzyme that has been implicated biochemically in the repair of mismatched G-T residues at methylated CpG sites. In addition, the human protein has been shown to interact with the DNA mismatch repair protein MLH1. To clarify the role of Mbd4 in DNA repair in vivo and to examine the impact of Mbd4 inactivation on gastrointestinal (GI) tumorigenesis, we introduced a null mutation into the murine Mbd4 gene by gene targeting. Heterozygous and homozygous Mbd4 mutant mice develop normally and do not show increased cancer susceptibility or reduced survival. Although Mbd4 inactivation did not increase microsatellite instability (MSI) in the mouse genome, it did result in a 2- to 3-fold increase in C→T transition mutations at CpG sequences in splenocytes and epithelial cells of the small intestinal mucosa. The combination of Mbd4 deficiency with a germ line mutation in the adenomatous polyposis coli (Apc) gene increased the tumor number in the GI tract and accelerated tumor progression. The change in the GI cancer phenotype was associated with an increase in somatic C→T mutations at CpG sites within the coding region of the wild-type Apc allele. These studies indicate that, although inactivation of Mbd4 does not by itself cause cancer predisposition in mice, it can alter the mutation spectrum in cancer cells and modify the cancer predisposition phenotype