DNA Repair in fish from polluted estuaries

A cancer-resistant fish, the mummichog Fundulus heteroclitus, was assayed for its ability to repair damage to its genome caused by an alkylating agent. Although DNA may be alkylated at several sites, the persistence of O6-alkylguanine has been correlated with carcinogenesis. O6-alkylguanine, if not repaired before the next round of replication, can lead to base transition mutations. The DNA repair protein, O6-alkylguanine transferase (O6-AGT), removes alkyl groups from the O6 position of guanine. Populations from polluted and relatively pristine estuaries were compared. O6-AGT activity was assayed by incubating extracts of liver from feral fish with previously methylated calf thymus DNA. After incubation, the DNA was depurinated by acid hydrolysis and the O6-methylguanine (O6-MeG) was separated and quantified by HPLC. Activity was measured by the reduction of O6-MeG in the incubated sample as compared to controls. Both populations demonstrated O6-AGT activity in their livers. Both had activity levels which were not as high as those seen in mammals. The reference population had O6-AGT levels inversely correlated with size, which is expected since younger fish have more DNA synthesis in their livers. The polluted population did not have a significant correlation with size, and generally exhibited less activity than the reference population. The lower O6-AGT activity in the polluted population suggests an environmental impact, and does not answer the question as to why this species is cancer resistant.