DIFFERENCES IN THE SPECTRUM OF SPONTANEOUS MUTATIONS IN THE HPRT GENEBETWEEN TUMOR-CELLS OF THE MICROSATELLITE MUTATOR PHENOTYPE

We have determined the frequency and spectrum of spontaneous mutations at the hprt locus in LoVo, HCT116, LS180 and DLD-1 colon carcinoma ce ll lines exhibiting microsatellite genetic instability. Each cell line has a different mutator gene. LoVo and HCT116 cells have mutated hMSH 2 and hMLH1 genes, respectively, which account for the majority of her editary non-polyposis colorectal cancer (HNPCC). LS180 cells are wild type for these genes and also for hPMS1 and hPMS2 mismatch repair gene s. DLD-1 cells harbor a mutated GTBP mismatch binding factor and a mut ated DNA Polymerase delta. The mutation rate at the hprt locus was sev eral hundred fold higher in these cell lines relative to control cell lines without microsatellite instability. The mutations were frameshif ts (deletions and insertions of a single nucleotide in short repeats) and single base substitutions (transversions and transitions). Some mu tations were shared by these four cell lines. However, every cell line also exhibited a distinctive spectrum of mutations suggesting that ea ch mutator gene induces a particular mutator phenotype. These results also suggest that the frequency and spectrum of somatic mutations in t umor cells of the microsatellite mutator phenotype may have diagnostic applications to discriminate among the diverse underlying mutator gen es.