Mutation rates in the complex microsatellite MYCL1 and related simple repeats in cultured human cells

Microsatellite instability is a phenotype observed in tumors cells that have defects in DNA mismatch repair (MMR). Most markers used for detecting microsatellite instability are mono- and dinucleotide repeats, but one tetranucleotide repeat (MYCL1) has been reported to be useful for this purpose. The MYCL1 repeat is actually a complex repeat, made up of approximately 14 GAAA tetranucleotides plus various other GA-rich repeats. In order to determine the nature of the instability of the this sequence, we have used a frameshift-reversion assay in MMR-proficient and -deficient human cells to compare the mutation rates and the types of mutation of MYCL1 to those of the related simple repeats (GAAA)17, (GA)17, and (CA)17. We found that the complex repeat was the most stable of the repeats examined in cells deficient in MMR; the tetranucleotide was less stable, while the dinucleotides were the least stable. In MMR-proficient cells, the relative rates were reversed; the MYCL1 repeat was the least stable, the tetranucleotide was more stable, and the dinucleotides were the most stable. These results suggest that MYCL1 and the pure tetranucleotide have relatively low rates of errors during replication, but that the errors in these repeats are corrected less efficiently than those in the smaller repeats. Because of their high rate of instability in MMR-proficient cells, MYCL1 and other tetranucleotide repeats appear to lack specificity for detection of tumors with defective MMR.