SMC protein RecN drives translocation and remodelling of RecA filament for homology search

While the molecular repertoire of the homologous recombination pathway is well studied, the search mechanism that enables recombination between distant homologous regions is poorly understood. Here, we follow the dynamics of the recombinase RecA, an essential component of homology search, after induction of a single double-strand break on the Caulobacter chromosome. We find that the RecA-nucleoprotein filament translocates in a directional manner in the cell, undergoing several pole-to-pole traversals, until homology search is complete. Simultaneously, the filament undergoes dynamic remodelling; both translocation and dynamic remodelling are contingent on the action of the SMC protein RecN via its ATPase cycle. We provide a stochastic description of RecN regulated changes in filament length during translocation via modulation of RecA assembly-disassembly. Together, the observed RecN driven RecA dynamics points to a novel optimal search strategy.

The function of RAD52 N-terminal domain is essential for viability of BRCA-deficient cells

RAD52 is a member of the homologous recombination pathway that is important for survival of BRCA-deficient cells. Inhibition of RAD52 leads to lethality in BRCA-deficient cells. However, the exact mechanism of how RAD52 contributes to viability of BRCA-deficient cells remains unknown. Two major activities of RAD52 were previously identified: DNA or RNA pairing, which includes DNA/RNA annealing and strand exchange, and mediator, which is to assist RAD51 loading on RPA-covered ssDNA. Here, we report that the N-terminal domain (NTD) of RAD52 devoid of the potential mediator function is essential for maintaining viability of BRCA-deficient cells owing to its ability to promote DNA/RNA pairing. We show that RAD52 NTD forms nuclear foci upon DNA damage in BRCA-deficient human cells and promotes DNA double-strand break repair through two pathways: homology-directed repair (HDR) and single-strand annealing (SSA). Furthermore, we show that mutations in the RAD52 NTD that disrupt these activities fail to maintain viability of BRCA-deficient cells.

Clinical Outcome of Leiomyosarcomas With Somatic Alteration in Homologous Recombination Pathway Genes

PURPOSE To detect alterations in DNA damage repair (DDR) genes, measure homologous recombination deficiency (HRD), and correlate these findings with clinical outcome in patients with leiomyosarcoma (LMS). PATIENTS AND METHODS Patients with LMS treated at Memorial Sloan Kettering (MSK) Cancer Center who consented to prospective targeted next-generation sequencing with MSK-IMPACT were screened for oncogenic somatic variants in one of 33 DDR genes; where feasible, an experimental HRD score was calculated from IMPACT data. Progression-free survival (PFS) and overall survival (OS) were estimated after stratifying patients by DDR gene alteration status and HRD score. RESULTS Of 211 patients with LMS, 20% had an oncogenic DDR gene alteration. Univariable analysis of PFS in 117 patients who received standard frontline chemotherapy in the metastatic setting found that an altered homologous recombination pathway gene was significantly associated with shorter PFS (hazard ratio [HR], 1.79; 95% CI, 1.04 to 3.07; P = .035). Non- BRCA homologous recombination gene alteration was associated with shorter PFS (HR, 2.61; 95% CI, 1.35 to 5.04; P = .004) compared with BRCA-altered and wild-type homologous recombination genes. Univariable analysis of OS from diagnosis in the entire cohort of 211 patients found that age, tumor size, number of metastatic sites, localized disease, and non- BRCA homologous recombination gene alteration were significantly associated with OS. On multivariable analysis, non- BRCA homologous recombination pathway gene alteration remained significant (HR, 4.91; 95% CI, 2.47 to 9.76; P < .001). High HRD score was not associated with a different PFS or OS. CONCLUSION Patients with LMS with homologous recombination pathway gene alterations have poor clinical outcomes, particularly those with non- BRCA gene alterations. HRD score calculated from a targeted exome panel did not discern disparate clinical outcomes.

2-hexyl-4-pentynoic acid (HPTA), a novel radiosensitizer to breast cancer cells through increasing the instability of DNA repair proteins

Background Breast cancer is one of the most common malignant tumors in the world which is the main cause of cancer death for women. Radiotherapy is the main treatment. Although some drugs have been found to enhance the effect of radiotherapy, there are also obvious deficiencies. Therefore, recent applied clinical research has been focusing on locating a suitable radiosensitizer to breast cancer radiotherapy. Methods MTT, clonogenic survival assays, comet assays, immunofluorescence and western blot analyses were used to detect the effect of VPA / HPTA on DNA damage induced by radiotherapy for breast cancer through a variety of cell models( MCF7, EUFA423, HCC1937, DMBA-induced rat breast cancer-derived primary culture cell and DMBA-induced transformed human normal breast cell line). At the same time, flow cytometry, immunofluorescence and western blot analyses were used to investigate the effect of VPA / HPTA on DNA damage repair induced by radiation. In vivo experiment, the effect of HPTA as radiosensitizer was investigated by DMBA-induced breast cancer in rats. Finally, the possible mechanism of HPTA acting on target protein was proved by cycloheximide chase experiment. Results In this study, a derivative of valproic acid (VPA), 2-hexyl-4-pentynoic acid (HPTA), was demonstrated for the first time that low concentration of HPTA (15 µM) has radiosensitizing properties to breast cancer cells by multiple working models of breast cancer cell lines (in vivo), equivalent to a high concentration of VPA (500 µM). Mechanistic investigations revealed that HPTA induced radiosensitivity through inhibiting the BRCA1-Rad51-mediated homologous recombination pathway. These results were further manifested in breast cancer animal model (in vitro). Most importantly, our study found that HPTA influenced the stability of BRCA1 and Rad51 proteins via shorting their half-life. Conclusions Our findings support the proposition HPTA as an alternate, safe and effective radiosensitizer to tumor cells. Targeting BRCA1-Rad51-mediated homologous recombination pathway through HPTA may be a rational strategy to improve the radiotherapeutic efficacy of breast cancer.