Mutations in hMSH6 alone are not sufficient to cause the microsatellite instability in colorectal cancer cell lines

Microsatellite instability (MSI) at simple repeated sequences characterises a distinct mechanism of carcinogenesis in hereditary nonpolyposis colorect al cancer (HNPCC), as well as sporadic colorectal cancers displaying MSI. S uch MSI is associated with mutations of hMSH2 and hMLH1, and somatic frames hift mutations in TGF-beta RII, IGFIIR, BAX, hMSH3 and hMSH6 at simple repe ated sequences in coding regions. The aim of this study was to look for a c orrelation between mutations in mismatch repair genes and frameshift mutati ons in colorectal cancer cell lines with MSI. Using 22 colorectal cancer ce ll lines, we examined the MSI status at mononucleotide repeat microsatellit e markers and mutations in hMSH2 and hMLH1, TGF-beta RII, IGFIIR, BAX, hMSH 3 and hMSH6. Thirteen of 22 lines (59%) displayed MSI. In these 13 lines sh owing MSI, 10 lines (77%) had mutations in TGF-beta RII, nine lines (69%) i n BAX, seven lines (54%) in hMSH6, and six lines (46%) in hMSH3, while muta tions in the IGFIIR gene were identified in only two lines (15%). Of the 13 lines with MSI, six lines (46%) harboured mutations/deletions in hMSH2 (tw o nonsense mutations, three deletions and one no expression of transcripts) and three of these cell Lines (50%) had mutations both in the hMSH2 and hM SH3 genes. Two cell lines (15%) had mutations/deletions in hMLH1 (one misse nse mutation and one deletion) and these two cell Lines also had mutations in hMSH3. One line had a mutation in hMSH3 only, although this line showed MSI and had mutations in TGF-beta RII, IGFIIR and BAX. All lines with mutat ions in hMLH1, hMSH2, TGF-beta RII, IGFIIR, BAX and hMSH3 genes showed MSI. However, of the nine lines without MSI, two (22%) had homozygous mutations in hMSH6. In these two cell Lines, no mutations were identified in hMLH1, hMSH2, TGF-beta RII, IGFIIR, BAX and hMSH3. Our results indicate that mutat ions in hMLH1, hMSH2 and hMSH3 are associated with MSI, but mutations in hM SH6 are not. We conclude that mutations in hMSH6 alone are not sufficient t o cause MSI, although protein functional effects of these mutations should still be examined. (C) 1999 Elsevier Science Ltd. All rights reserved.