Induction of frameshift mutations in cultured mammalian cells within a transfected sequence containing a poly(dC-dA).poly(dT-dG) microsatellite.

A cultured mouse cell line with an integrated copy of a plasmid that contains a short dinucleotide repeat sequence (microsatellite) has been used to determine the frequencies and types of mutation induced by two frameshift mutagens. The presence of the microsatellite, which consists of 17 repeats of a poly(dC-dA).poly(dT-dG) sequence, disrupts the reading frame of a gene coding for neomycin resistance. Revertants were selected in G418, and mutations were analyzed by PCR. ICR-170 was found to increase the reversion frequency by ten- to 15-fold at its LD50, although most of the frameshifts that it induced were single-base insertions outside the microsatellite sequence. NA-AAF brought about a more modest increase in mutation frequency, but nearly all of the revertants in the NA-AAF-treated cultures had insertions or deletions of multiples of two base pairs within the DNA segment that included the microsatellite. This system can be modified to include different short tandem repeat sequences as targets for testing of compounds that are suspected of having frameshift-inducing activities.