Mismatch Repair Assay

DNA mismatch repair plays an important role in mutation avoidance by recognizing and correcting mismatched bases and loops prior to their fixation as mutations. When mismatch repair is defective, cells exhibit elevated rates of spontaneous mutations. For example, microsatellite sequences exhibit frequent gains or losses of simple repeat units in tumor cells from patients with hereditary nonpolyposis colon cancer (HNPCC; 1 –3 ). Defects in mismatch repair have also been identified as an important route to tolerance of certain cytotoxic methylating agents (reviewed in 4 ). Cell lines that are deficient for methytransferase activity normally exhibit extreme sensitivity to N -methyl-N -nitrosourea (MNU) or 1-methyl-3-nitro-1-nitrosoguanidine (MNNG). However resistant subpopulations can arise spontaneously, and these subpopulations have been shown to be mismatch repair defective (5 ,6 ). To determine unambiguously that both HNPCC (7 ) and alkylation tolerance were owing to mismatch repair-defects, it was crucial to prove that mismatch repair activity was reduced or eliminated in mutant cells.