MOLECULAR ANALYSIS OF MUTATIONS IN MUTATOR COLORECTAL-CARCINOMA CELL-LINES

The nature of mutations occurring in two colorectal carcinoma cell lines deficient in mismatch repair and displaying mutator phenotypes was determined. One of the lines (HCT116) exhibited a higher level of microsatellite instability than the second (DLD-1), although the rate of mutation at the selectable locus encoding the purine salvage enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT) was equally elevated (about 350-450-fold relative to mismatch repair proficient cell lines). Transitions were the major class of mutations in the two mutator lines. In DLD-1 these mutations recurred at several sites that appeared to be hotspots. Frameshifts at a run of six guanine residues in the coding sequence for HPRT constituted 35% of mutations in HCT116. These frameshifts were highly unstable and reverted to wild type at high frequency. Larger deletions were also detected in HCT116. Although these deletions constituted a small proportion of mutations compared with the other types, our data suggest that the rate of deletion is elevated relative to mismatch repair proficient (hMLH1(+)) cell lines. These observations suggest that the gene(s) altered in DLD-1 may preferentially affect the repair of base mismatches while the alteration(s) in HCT116 may affect the repair of both mismatches and frameshifts.