Role of mismatch repair proteins in genomic stability, cancer and meiosis

Abstract

Mismatch repair (MMR) proteins are important for the maintenance of genomic stability in all organisms. A major role of MMR proteins is to correct DNA mismatches that arise during replication. In addition to DNA repair, MMR proteins play also important roles in DNA damage signaling, recombination and meiosis. Mutations in the MMR genes are frequently associated with Hereditary Non-Polyposis Colorectal Cancer (HNPCC).;Exonuclease 1 (Exo1) is a 5'-3' exonuclease that plays a role in the late step of MMR. Inactivation of Exo1 in mice results in defects in DNA mismatch repair, increased cancer susceptibility and defects in meiosis that result in both male and female sterility. To investigate the role of Exo1 in recombination, I performed a detailed analysis of DSB repair in Exo1 -/- cells. This analysis showed that Exo1 plays multiple roles in DSB repair and functions in Homologous Recombination (HR), Non-Homologous End Joining (NHEJ) and Single Strand Annealing (SSA). In addition, I demonstrated the involvement of Exo1 in the suppression of homeologous recombination in mammalian cells. These results suggest that inactivation of the Exo1 DSB repair function plays a role in increased genomic instability and contributes to cancer susceptibility in Exo1-/- mice.;At least fifty percent of MMR mutations implicated in HNPCC involve the MLH1 gene. The analysis of Mlh1-deficient mice showed that loss of Mlh1 function results in MMR deficiency, increased cancer predisposition and infertility. To further analyze the role of Mlh1 in these processes we generated a mouse line carrying the human recurrent G67R missense mutation located within the conserved ATPase domain. The analysis of Mlh1G67R mutant animals showed that, similar to Mlh1-deficient mice, Mlh1 G67R/G67R homozygous mice are sterile and prone to tumor development. In this study we show that ATPase processing of Mlh1 is essential for normal MMR function and proper localization of Mlh1 on meiotic chromosomes. However, in contrast to Mlh1-/- cells, Mlh1G67R/G67R mutant cells remain sensitive to DNA damaging agents and display normal apoptotic response. Our results demonstrate that a single point mutation in Mlh1 can have differential effects on MMR functions and meiosis and also modulate the tumor spectrum.