Microsatellite instability and mutations in DNA mismatch repair genes in sporadic colorectal cancers

Purpose: This study was designed to investigate the frequency of mutations in DNA mismatch repair genes in sporadic colorectal cancers. Methods: Genomic DNAs procured from paraffin blocks of the pathologic specimens from 230 consecutive patients with colorectal cancer were examined for their microsatellite instability status using a mononucleotide microsatellite marker, BAT-26, and also evaluated expressions of hMLH1, hMSH2, and hMSH6 proteins by immunohistochemical staining. Any of these 230 patients did not have family histories of hereditary nonpolyposis colorectal cancer, familial adenomatous polyposis, colorectal cancer, or hereditary nonpolyposis colorectal-elated cancers, such as endometrial, small bowel, and ureteral and renal pelvic cancers. When microsatellite instability was positive, mutations in the simple repeated sequences of TGF-betaRII, BAX, IGF IIR, hMSH3, and hMSH6 genes were examined. In microsatellite instability-positive or staining-negative cases, polymerase chain reaction-single-strand conformation polymorphism and DNA sequencing detected mutations of hMLH1, hMSH2, and hMSH6 genes. If mutations were found in tumor tissue samples, we tested for a germline mutation with a microdissected corresponding normal tissue. Results: Among 230 cases of sporadic colorectal cancer, 21 (9.1 percent) manifested microsatellite instability. In the immuncohistochemical staining, 20 (8.6 percent) showed loss of expressions. An 20 staining-negative cases were microsatellite instability-positive. Only 1 of 21 (4.8 percent) microsatellite instability-positive cases showed intact staining for three proteins. The frame-shift mutations of the simple repetitive sequences were found in 17 cases (81.0 percent) in TGF-betaRII, 11 (52.4 percent) in BAX, 5 (23.8 percent) in IGF IIR, 7 (33.3 percent) in hMSH3, and 8 (38.1 percent) in hMSH6 genes. Germline mutation was observed in only one case, which accounts for 4.8 percent among positive microsatellite instability and 0.4 percent of total patients, and was found in hMSH2. Five somatic mutations (2 in hMLH1, 2 in hMSH2, and 1 in hMSH6) also were found. Conclusion: The results indicated that a germline mutation of DNA mismatch repair gene was a rare event in sporadic colorectal cancers.