SPECTRA OF SPONTANEOUS MUTATIONS AT THE HPRT LOCUS IN COLORECTAL-CARCINOMA CELL-LINES DEFECTIVE IN MISMATCH REPAIR

Spectra of spontaneous mutations at the hypoxanthineguanine phosphorib osyltransferase (hprt) locus in colon carcinoma cell lines HCT116 and HCT-15 deficient in mismatch repair and displaying mutator phenotypes were determined, HCT116 and HCT-15 cells, respectively, harbour a muta tion in the mismatch repair gene hMLH1 and GTBP, The mutation frequenc y at the hprt locus in both cell lines was elevated by about two order s, but the microsatellite instability in HCT116 cells was one order hi gher than in HCT-15 cells, Except for one mutant of HCT-15, all the mu tations (114/115) were point mutations; base substitutions of various types and frameshifts (deletions/insertions of less than a few bases, predominantly of +/-1 bp). Base substitutions (57%) and frameshifts (4 3%) occurred at a comparable rate in HCT116, whereas base substitution s (92%) were the major mutational events in HCT-15. Most frameshifts i n HCT116 occurred at sites of monotonous or short tandem repeating seq uences, and two of these sites, where there was a run of six Gs and fo ur As, were hot spots, Three hot spot sites of base substitutions were found in HCT-15; two of them at splice acceptor sites, the other at t he CpG site shared with HCT116. The distinct mutation spectra of the H CT116 and HCT-15 cell lines may reflect functional differences in the hMLH1 and GTBP gene products in mismatch repair, The gene product GTBP may be involved in the preferential repair of base mismatches, and ML H1 in the repair of both base mismatches and deletions/insertions of l ess than a few bases, These results suggest that mismatch repair defic iency affects the microsatellite stability as widely reported in color ectal tumour cells, but that it may not severely affect chromosome int egrity as the karyotypes of these tumour cells are, unlike other tumou r cells, relatively stable.