Mutations in tetranucleotide repeats following DNA damage depend on repeat sequence and carcinogenic agent

Sporadic microsatellite mutations are frequently observed in lung, bladder, and head and neck tumors with intact DNA mismatch repair. AAAG tetranucleotide repeats appear to be especially prone to the accumulation of these mutations. We hypothesized that occurrences of microsatellite mutations in these cancers may be linked to DNA damage caused by exposure to carcinogens in tobacco smoke. To test this hypothesis, we developed a model system based on reactivation of green fluorescent protein (GFP) in which a plasmid vector carries a microsatellite repeat that places the GFP sequence out of frame for protein translation. In this reporter system, DNA slippage mutations can restore the GFP reading frame and become detectable by flow cytometry as GFP-positive cells. Pools of stably transfected RKO cells were treated at four dose levels each of gamma-irradiation, benzo(a)pyrene diol epoxide, N-methyl-N-nitro-N-nitrosoguanidine (MNNG), t-butyl hydrogen peroxide, and UV irradiation and assayed for GFP-positive cells 48 h later. We studied the microsatellite repeats AAAG, ATAG, CAGT, and CA, as well as a control sequence lacking any repetitive elements. A log-linear regression approach was used to discriminate between the effects of repeat unit and dose for each agent. A statistically significant increase in GFP-positive cells was found with increasing dose with all agents, although repeat unit-specific response patterns were only observed with MNNG, t-butyl hydrogen peroxide, and UV irradiation. With MNNG, significant differences in response were observed between dinucleotide and tetranucleotide repeat units. The effects of UV irradiation were consistent with the predicted number of pyrimidine dimers/repeat unit, with higher GFP activation in repeats that had large numbers of adjacent pyrimidines. We found no evidence to indicate that the AAAG repeat responded to any of the DNA-damaging agents with higher levels of GFP activation than other repeat units. These results provide evidence that DNA damage can induce slippage mutations and increase mutation rates in repeated sequences and that there are sequence-specific responses to different types of DNA damage. Our results are compatible with the hypothesis that sporadic microsatellite mutations in human cancer may reflect DNA damage caused by carcinogen exposure.