Tight Regulation of Srs2 Helicase Activity Is Crucial for Proper Functioning of DNA Repair Mechanisms.

Proper DNA damage repair is one of the most vital and fundamental functions of every cell. Several different repair mechanisms exist to deal with various types of DNA damage, in various stages of the cell cycle and under different conditions. Homologous recombination is one of the most important repair mechanisms in all organisms. Srs2, a regulator of homologous recombination, is a DNA helicase involved in DNA repair, cell cycle progression and genome integrity. Srs2 can remove Rad51 from ssDNA, and is thought to inhibit unscheduled recombination. However, Srs2 has to be precisely regulated, as failure to do so is toxic and can lead to cell death. We noticed that a very slight elevation of the levels of Srs2 (by addition of a single extra copy of the SRS2 gene) leads to hyper-sensitivity of yeast cells to methyl methanesulfonate (MMS, a DNA damaging agent). This effect is seen in haploid, but not in diploid, cells. We analyzed the mechanism that controls haploid/diploid sensitivity and arrived to the conclusion that the sensitivity requires the activity of RAD59 and RDH54, whose expression in diploid cells is repressed. We carried out a mutational analysis of Srs2 to determine the regions of the protein required for the sensitization to genotoxins. Interestingly, Srs2 needs the HR machinery and its helicase activity for its toxicity, but does not need to dismantle Rad51. Our work underscores the tight regulation that is required on the levels of Srs2 activity, and the fact that Srs2 helicase activity plays a more central role in DNA repair than the ability of Srs2 to dismantle Rad51 filaments.

A high-throughput chemical screen with FDA approved drugs reveals that the antihypertensive drug Spironolactone impairs cancer cell survival by inhibiting homology directed repair.

DNA double-strand breaks (DSBs) are the most severe type of DNA damage. DSBs are repaired by non-homologous end-joining or homology directed repair (HDR). Identifying novel small molecules that affect HDR is of great importance both for research use and therapy. Molecules that elevate HDR may improve gene targeting whereas inhibiting molecules can be used for chemotherapy, since some of the cancers are more sensitive to repair impairment. Here, we performed a high-throughput chemical screen for FDA approved drugs, which affect HDR in cancer cells. We found that HDR frequencies are increased by retinoic acid and Idoxuridine and reduced by the antihypertensive drug Spironolactone. We further revealed that Spironolactone impairs Rad51 foci formation, sensitizes cancer cells to DNA damaging agents, to Poly (ADP-ribose) polymerase (PARP) inhibitors and cross-linking agents and inhibits tumor growth in xenografts, in mice. This study suggests Spironolactone as a new candidate for chemotherapy.