A genome-wide IR-induced RAD51 foci RNAi screen identifies CDC73 involved in chromatin remodeling for DNA repair.

To identify new regulators of homologous recombination repair, we carried out a genome-wide short-interfering RNA screen combined with ionizing irradiation using RAD51 foci formation as readout. All candidates were confirmed by independent short-interfering RNAs and validated in secondary assays like recombination repair activity and RPA foci formation. Network analysis of the top modifiers identified gene clusters involved in recombination repair as well as components of the ribosome, the proteasome and the spliceosome, which are known to be required for effective DNA repair. We identified and characterized the RNA polymerase II-associated protein CDC73/Parafibromin as a new player in recombination repair and show that it is critical for genomic stability. CDC73 interacts with components of the SCF/Cullin and INO80/NuA4 chromatin-remodeling complexes to promote Histone ubiquitination. Our findings indicate that CDC73 is involved in local chromatin decondensation at sites of DNA damage to promote DNA repair. This function of CDC73 is related to but independent of its role in transcriptional elongation.

Breast cancer stem-like cells show dominant homologous recombination due to a larger S-G2 fraction.

The concept of cancer stem cells is generally accepted in different malignancies. We have previously shown that the MDA-MB231 breast cancer cells were more radiation resistant when sorted for the two stem cell markers CD24 and ESA. In this study, we examined a possible mechanism that might underlie this phenotype by looking at cell cycle profile and the effect this has on DNA repair pathways. The cell cycle profile showed that there were more CD24(-) ESA(+) sorted MDA-MB231 cells in the S- and G(2)-phases compared with the unsorted cells, 60 and 38% respectively. Cyclin D and E protein levels supported the cell cycle profile and highlighted the possible involvement of homologous recombination (HR) repair in the radioresistant phenotype. To further support this, CD24(-) ESA(+) sorted MDA-MB231 cells demonstrated statistically significant more RAD51 and less γ-H2AX foci 2 h post 4Gy ionising radiation, compared with the unsorted population. Inhibition of the HR pathway effectively sterilised the CD24(- ) ESA(+) sorted MDA-MB231 cells but had no effect on the unsorted cells or MDA468 control breast cancer cell line. Although the changes we saw were specific to MDA-MB231, these results merit further investigation and can be crucial in identifying a mechanism responsible for cancer stem cells treatment resistance in primary tumors.

Selective inhibitors of the JMJD2 histone demethylases: combined nondenaturing mass spectrometric screening and crystallographic approaches.

Ferrous ion and 2-oxoglutarate (2OG) oxygenases catalyze the demethylation of N(epsilon)-methylated lysine residues in histones. Here we report studies on the inhibition of the JMJD2 subfamily of histone demethylases, employing binding analyses by nondenaturing mass spectrometry (MS), dynamic combinatorial chemistry coupled to MS, turnover assays, and crystallography. The results of initial binding and inhibition assays directed the production and analysis of a set of N-oxalyl-d-tyrosine derivatives to explore the extent of a subpocket at the JMJD2 active site. Some of the inhibitors were shown to be selective for JMJD2 over the hypoxia-inducible factor prolyl hydroxylase PHD2. A crystal structure of JMJD2A in complex with one of the potent inhibitors was obtained; modeling other inhibitors based on this structure predicts interactions that enable improved inhibition for some compounds.