DNA cleavage by In-111-labeled oligodeoxyribonucleotide

We studied the fine structure of DNA damage produced by the decay of In-111 incorporated into duplex and tripler DNA strands to evaluate the usefulnes s of this radionuclide for sequence-specific DNA cleavage. Methods: Oligode oxyribonucleotides (ODNs) were prepared with In-111 attached by diethylenet riaminepentaacetic acid (DTPA) at the 5' end or 3' end through a long chemi cal linker or to an internal nucleotide position through a short linker. Su bsequent formation of DNA duplexes and triplexes was confirmed by gel elect rophoresis. The In-111-induced breaks were assayed in denaturing polyacryla mide gel electrophoresis with a single-nucleotide resolution. Results: In-1 11-labeled oligonucleotides of high specific activity (740-1554 TBq/mmol) w ere synthesized. The presence of the bulky In-111-DTPA group did not impede duplex or tripler formation. Localized DNA breaks were observed in all dup lexes and triplexes formed. The majority of DNA breaks in duplex formations were located within +/-10 nucleotides from the site of attachment of the I n-111-bearing linker. The yield of DNA breaks per decay was 0.38 in a duple x with internally modified ODNs. This is nearly 2 times less than the yield of DNA breaks in the same duplex with I-125 attached through the same link er. The yield of DNA breaks in the pyrimidine and purine strands of DNA tri plexes with In-111 attached to the tripler-forming ODNs through the linkers of different length varied from 0.05 to 0.10. The distribution of DNA brea ks was wider in comparison with the duplex experiment. The lower yields of breaks per In-111 decay compared with I-125 may be not only the result of l ower deposited energy but also of the ionic repulsion of the negatively cha rged In-111-DTPA group from the DNA strands. Conclusion: We have shown that decay of In-111 produces highly localized DNA breaks. In-111 introduced in to triplex- and duplex-forming ODNs through hydrocarbon linkers produces se quence-specific DNA strand breaks with an efficiency nearly comparable with that of I-125. These findings are supportive of our proposed use of In-111 -ODNs for gene-specific radiotherapy.