Relative rates of insertion and deletion mutations in a microsatellite sequence in cultured cells

A cell culture system has been used to determine the relative rates of insertions and deletions of integral numbers of dinucleotide repeats in a microsatellite sequence. A plasmid was constructed that contained 17 repeats of poly(dC-dA).poly(dG-dT) near the 5' end of a bacterial neomycin-resistance (neo) gene, such that the neo gene was translated in the (+1) reading frame. The plasmid was introduced into mismatch-repair-proficient and mismatch-repair-deficient mammalian cell lines. Rates of mutation to resistance to the neomycin analogue G418 were measured, and the nature of the mutations was determined. The mutations were all gains or losses of integral numbers of repeats, and mutations involving a single repeat greatly predominated over those involving multiple repeats. The data obtained from these studies were compared with results previously obtained with cells transfected with a similar plasmid in which the sequence of the oligonucleotide insert placed the neo gene in the (-1) reading frame. This experimental design made it possible to make direct comparisons between insertions and deletions of a single repeat unit. A significant excess of insertions over deletions was found in both repair-proficient and repair-deficient cell lines, although the few mutations involving more than two repeats were deletions.