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The role of DNA mismatch repair genes in genome stability and carcinogenesis Baross-Francis, Agnes

Abstract

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.

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