Infrequent frameshift mutations in the simple repeat sequences of hMLH3 in hereditary nonpolyposis colorectal cancers

Background: A recently identified mismatch repair gene, hMLH3, contains two simple repeat sequence regions, (A)9 and (A)8, in its coding region. To clarify the role of hMLH3 in hereditary nonpolyposis colorectal cancer (HNPCC), we searched for hMLH3 somatic and germline mutations, particularly in the repeat regions, in 41 HNPCC patient cells. Methods: We analyzed the hMLH3 (A)9 and (A)8 repeats in 27 colorectal cancers with microsatellite instability (MSI) as well as in normal cells from 41 HNPCC patients by means of polymerase chain reaction-single-strand conformation polymorphism. hMSH3 (A)8 and hMSH6 (C)8 repeats were also examined in these cancers. Results: Frameshift mutations in the hMLH3 (A)9 repeat were observed in 4/27 (14.8%) cancers with MSI, all of which showed the severe MSI phenotype. No mutations in the (A)8 repeat were found in any case. The mutation frequency of the hMLH3 (A)9 repeat was similar to that of the hMSH6 (C)8 repeat (5/26, 19.2%), but was significantly lower than that of the hMSH3 (A)8 repeat (16/27, 59.3%) (P < 0.001). All four cancers with hMLH3 mutations exhibited germline hMSH2 and/or somatic hMSH3 mutations. No germline mutation in the hMLH3 (A)9 or (A)8 repeat was detected in normal cells from the 41 HNPCC patients. Conclusion: hMLH3 mutations were infrequently observed in HNPCC cancers with MSI and they may be secondary to other mismatch repair gene mutations. Hence hMLH3 may only play a small role in HNPCC tumorigenesis.