Efficacy of the combination of a PARP inhibitor and UVC on cancer cells as imaged by focus formation by the DNA repair-related protein 53BP1 linked to green fluorescent protein

Background: The ability to image DNA repair in cancer cells after irradiation, as well as its inhibition by potential therapeutic agents, is important for the further development of effective cancer therapy. 53BP1 is a DNA repair protein that is overexpressed and forms foci when doublestranded DNA breaks occur in DNA. Materials and Methods: The re-localization of green fluorescent protein (GFP) fused to the chromatin-binding domain of 53BP1 to form foci was imaged after UVC irradiation of breast and pancreatic cancer cells expressing 53BP1-GFP using confocal microscopy. Results: During live-cell imaging, 53BP1-GFP focus formation was observed within 10 minutes after UVC irradiation. Most 53BP1 foci resolved by 100 minutes. To block UVC-induced double-strand break repair in cancer cells, poly(ADP-ribose) polymerase (PARP) was targeted with ABT-888 (veliparib). PARP inhibition markedly enhanced UVC-irradiation-induced persistence of 53BP1-foci, even beyond 100 minutes after UVC irradiation, and reduced proliferation of breast and pancreatic cancer cells. Conclusion: Confocal microscopy of 53BP1-GFP is a powerful method for imaging UVC-induced DNA damage and repair, as well as inhibition of repair.