Gene targeted DNA double-strand break induction by (125)I-labeled triplex-forming oligonucleotides is highly mutagenic following repair in human cells.

A parallel binding motif 16mer triplex-forming oligonucleotide (TFO) complementary to a polypurine-polypyrimidine target region near the 3'-end of the SupF gene of plasmid pSP189 was labeled with [5-(125)I]dCMP at position 15. Following triplex formation and decay accumulation, radiation-induced site-specific double-strand breaks (DSBs) were produced in the pSP189 SupF gene. Bulk damaged DNA and the isolated site-specific DSB-containing DNA were separately transfected into human WI38VA13 cells and allowed to repair prior to recovery and analysis of mutants. Bulk damaged DNA had a relatively low mutation frequency of 2.7 x 10(-3). In contrast, the isolated linear DNA containing site-specific DSBs had an unusually high mutation frequency of 7.9 x 10(-1). This was nearly 300-fold greater than that observed for the bulk damaged DNA mixture, and >1.5 x 10(4)-fold greater than background. The mutation spectra displayed a high proportion of deletion mutants targeted to the(125)I binding position within the SupF gene for both bulk damaged DNA and isolated linear DNA. Both spectra were characterized by complex mutations with mixtures of changes. However, mutations recovered from the linear site-specific DSB-containing DNA presented a much higher proportion of complex deletion mutations.