Rapid gene-specific repair of cisplatin lesions at the human DUG/DHFR locus comprising the divergent upstream gene and dihydrofolate reductase gene during early G1 phase of the cell cycle assayed by using the exonucleolytic activity of T4 DNA polymerase

A novel assay to detect strand-specific DNA repair after cellular exposure to cisplatin at IC50 levels, is used to measure rapid repair in the divergent upstream gene (DUG), a human MutS homolog, and in the bidirectional promoter for dihydrofolate reductase gene (DHFR) and the contiguous upstream DUG. Single-stranded DNA capable of hybridizing to gene-specific probes is generated enzymatically by the 3'-5' exonuclease activity of T4 DNA polymerase. The presence of cisplatin lesions inhibit the exonucleolytic activity of T4 DNA polymerase and block the formation of single-stranded DNA. This decreases the amount of complementary sequence produced when assayed by gene-specific probe hybridization. With the progression of repair, increasing quantities of single-stranded DNA become available for probe hybridization. This assay was applied to human A2780 ovarian carcinoma cells treated with cisplatin at the beginning of G1 phase. A dose-response experiment showed that the assay was applicable down to cisplatin concentrations of 2.5 microM. To assay for strand-specific gene repair, the synchronized cells were treated with cisplatin and then allowed time to repair in drug-free medium. Extensive removal of cisplatin lesions after 2 hr of cellular repair during early G1 phase in the DUG and the DUG/DHFR promoter was measured, with no evidence of repair in the unexpressed delta-globin gene. The extent of preferential DNA repair was much more distinct than has been observed previously at high-drug dosage in asynchronous cells.