Homologous recombination-deficient mutation cluster in tumor suppressor RAD51C identified by comprehensive analysis of cancer variants

Mutations in homologous recombination (HR) genes, including BRCA1, BRCA2, and the RAD51 paralog RAD51C, predispose to tumorigenesis and sensitize cancers to DNA-damaging agents and poly(ADP ribose) polymerase inhibitors. However, ∼800 missense variants of unknown significance have been identified for RAD51C alone, impairing cancer risk assessment and therapeutic strategies. Here, we interrogated >50 RAD51C missense variants, finding that mutations in residues conserved with RAD51 strongly predicted HR deficiency and disrupted interactions with other RAD51 paralogs. A cluster of mutations was identified in and around the Walker A box that led to impairments in HR, interactions with three other RAD51 paralogs, binding to single-stranded DNA, and ATP hydrolysis. We generated structural models of the two RAD51 paralog complexes containing RAD51C, RAD51B-RAD51C-RAD51D-XRCC2 and RAD51C-XRCC3. Together with our functional and biochemical analyses, the structural models predict ATP binding at the interface of RAD51C interactions with other RAD51 paralogs, similar to interactions between monomers in RAD51 filaments, and explain the failure of RAD51C variants in binding multiple paralogs. Ovarian cancer patients with variants in this cluster showed exceptionally long survival, which may be relevant to the reversion potential of the variants. This comprehensive analysis provides a framework for RAD51C variant classification. Importantly, it also provides insight into the functioning of the RAD51 paralog complexes.

Abstract AP05: OVARIAN CANCER-ASSOCIATED RAD51D MUTATIONS WHICH IMPAIR ITS INTERACTION WITH XRCC2 RESULT IN DNA REPAIR DEFICIENCY

The proficiency of ovarian cancer cells to repair DNA double-strand breaks (DSBs) by homologous recombination (HR) is a key determinant in predicting response to targeted therapies such as PARP inhibitors (PARPi). The RAD51 paralogs act downstream of BRCA1/2 to facilitate HR. Numerous epidemiological studies have linked mutations in the RAD51 paralogs with hereditary ovarian cancer predisposition. Despite their substantial links to cancer predisposition and development, RAD51 paralog function during HR has remained elusive, in part due to limitations in studying recombination events downstream of RAD51 filament formation. Here we investigate the impact of cancer-associated mutations in the RAD51 paralog, RAD51D, using yeast 2/3-hybrid assays to screen for altered protein-protein interactions. Following the identification of mutations that disrupt the interaction between RAD51D and XRCC2 in yeast, we validated the interaction by co-immunoprecipitation in human cells. Importantly, we determined the impact of these mutations on HR-proficiency using a direct-repeat recombination assay. By characterizing the impact of cancer-associated mutations in the RAD51 paralogs on HR-proficiency, we aim to develop more effective predictive models for therapeutic sensitivity and resistance in patients who harbor similar mutations in these essential genes.

Citation Format: Robert A. Baldock, Catherine A. Pressimone, Jared M. Baird, Anton Y. Khodakov, Yoav Karpenshif, Edwige B. Garcin, Stéphanie Gon, Mauro Modesti, Kara A. Bernstein. OVARIAN CANCER-ASSOCIATED RAD51D MUTATIONS WHICH IMPAIR ITS INTERACTION WITH XRCC2 RESULT IN DNA REPAIR DEFICIENCY [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr AP05.

Abstract A08: A novel system determines the functional significance of ovarian tumor mutations in the homologous recombination gene RAD51C

Homologous recombination (HR) deficiency is associated with hereditary ovarian carcinomas. HR deficiency can occur through somatic and germline mutations in BRCA1, BRCA2, and other associated genes such as RAD51C. Understanding the impact of HR gene mutations on DNA double-strand break (DSB) repair is critical for developing targeted therapeutic strategies for these patients. Although BRCA1 and BRCA2 deficiency has been well characterized, limited functional analysis has occurred in patients harboring mutations in RAD51C. RAD51C mutations are found in familial breast and ovarian cancers and in Fanconi anemia-like syndrome, FANCO. To determine which RAD51C-tumor associated mutations disrupt HR, we utilized a novel RAD51C conditional knockout breast epithelial cell line (MCF10A) generated by Dr. Maria Jasin. In collaboration with the Jasin laboratory, we complemented these cells with RAD51C and RAD51C cancer-associated mutants. Using yeast-two-hybrid and co-immunoprecipitation strategies, we identified RAD51C mutations that disrupt RAD51C protein-protein interactions. For example, we identified RAD51C mutations that impair RAD51C interaction with either its binding partner RAD51B, RAD51D, or XRCC3. We next measured RAD51C mutant cells for viability and HR proficiency at an I-SceI-induced DSB using a DR-GFP reporter assay. We identified RAD51C mutations that impact HR repair, suggesting that patients harboring these mutations would be good candidates for PARPi. Our goal is to identify specific RAD51C mutations that disrupt HR to predict which RAD51C-deficient tumors will respond to therapy.

Citation Format: Meghan R. Sullivan, Rohit Prakash, Michael J. Mihalevic, Jared M. Baird, Maria Jasin, Kara A. Bernstein. A novel system determines the functional significance of ovarian tumor mutations in the homologous recombination gene RAD51C. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr A08.

Tryptophan biosynthesis is important for resistance to replicative stress in Saccharomyces cerevisiae

Acute tryptophan depletion is used to induce low levels of serotonin in the brain. This method has been widely used in psychiatric studies to evaluate the effect of low levels of serotonin, and is generally considered a safe and reversible procedure. Here we use the budding yeast Saccharomyces cerevisiae to study the effects of tryptophan depletion on growth rate upon exposure to DNA-damaging agents. Surprisingly, we found that budding yeast undergoing tryptophan depletion were more sensitive to DNA-damaging agents such as methyl methanesulphonate (MMS) and hydroxyurea (HU). We found that this defect was independent of several DNA repair pathways, such as homologous recombination, base excision repair and translesion synthesis, and that this damage sensitivity was not due to impaired S-phase signalling. Upon further analysis, we found that the DNA-damage sensitivity of tryptophan depletion was likely due to impaired protein synthesis. These studies describe an important source of variance in budding yeast when using tryptophan as an auxotrophic marker, particularly on studies focusing on DNA repair, and suggest that further testing of the effect of tryptophan depletion on DNA repair in mammalian cells is warranted. Copyright © 2016 John Wiley & Sons, Ltd.

Nonwoven fiber barriers for control of cabbage maggot and onion maggot (Diptera: Anthomyiidae)

We investigated the use of nonwoven fiber barriers for control of cabbage maggot, Delia radicum (L.), and onion maggot, D. antiqua (Meigen). The barriers consist of arrangements of minute fibers loosely intertwined in "web" form. Results from a greenhouse experiment showed that manually applied graphite fibers placed at the base of broccoli plants reduced the number of D. radicum eggs by 64-98%, and that efficacy increased with greater fiber density. Using a melt extrusion process, we devised a method for on-site creation of nonwoven fibers of ethylene vinyl acetate (EVA). In field trials with broccoli and onion plants, EVA fibers significantly reduced the number of cabbage and onion maggots infesting plants. Fiber barriers provided comparable control to standard insecticide applications. The addition of blue, yellow, red, or black pigments, as well as optical brighteners that absorb UV light did not enhance fiber efficacy. Incorporation of capsaicin olfactory repellent to EVA also did not enhance fiber efficacy. Nonwoven fiber barriers may offer an alternative to insecticides for control of cabbage maggot and onion maggot and possibly other insect pests. Additional research is needed to improve the application process and to identify economically feasible and biodegradable compounds for fibers.

Replication of trance logic using a modified experimental design: highly hypnotizable subjects in both real and simulator groups

A potentially useful method for diagnosing the presence of hypnosis has been the comparison of hypnotic Ss and simulators. The present study attempted to replicate the hypnotic-simulator differentiation in regard to trance logic, as originally described by Orne (1959), using a modified design in which Ss in the 2 conditions were matched for hypnotizability scores. Orne's doubled person hallucination (DPH) suggestion was presented, and the results showed significant differences between the 2 treatment conditions in both DPHs and spontaneous transparency reports. One interesting case of DPH occurred in the simulator group with one S in whom a hypnotic-like state was self-induced. In contrast to the placid acceptance of DPH among Ss in the hypnotic condition, the experience was a confusing and mildly disturbing one for this S, who was not expecting any unusual perceptual experiences. Trance logic results from the "metasuggestion," experienced through participation in a formal induction procedure, that hypnosis entails new rules of experience and behavior.

Ocular Changes Experimentally Produced in Thyroparathyroidectomized Dogs, with Special Reference to Intra-Ocular Tension and Blood-Pressure: A Preliminary Report

Images