PRMT5-Dependent Methylation of the TIP60 Coactivator RUVBL1 Is a Key Regulator of Homologous Recombination

The Cancer/Testes (CT) Antigen HORMAD1 promotes Homologous Recombinational DNA Repair and Radioresistance in Lung adenocarcinoma cells

The Cancer/Testes (CT) Antigen HORMAD1 is germ cell-restricted and plays developmental roles in generation and processing of meiotic DNA Double Strand Breaks (DSB). Many tumors aberrantly overexpress HORMAD1 yet the potential impact of this CT antigen on cancer biology is unclear. We tested a potential role of HORMAD1 in genome maintenance in lung adenocarcinoma cells. We show that HORMAD1 re-distributes to nuclear foci and co-localizes with the DSB marker γH2AX in response to ionizing radiation (IR) and chemotherapeutic agents. The HORMA domain and C-term disordered oligomerization motif are necessary for localization of HORMAD1 to IR-induced foci (IRIF). HORMAD1-depleted cells are sensitive to IR and camptothecin. In reporter assays, Homologous Recombination (HR)-mediated repair of targeted ISce1-induced DSBs is attenuated in HORMAD1-depleted cells. In Non-Homologous End Joining (NHEJ) reporter assays, HORMAD1-depletion does not affect repair of ISce1-induced DSB. Early DSB signaling events (including ATM phosphorylation and formation of γH2AX, 53BP1 and NBS1 foci) are intact in HORMAD1-depleted cells. However, generation of RPA-ssDNA foci and redistribution of RAD51 to DSB are compromised in HORMAD1-depleted cells, suggesting that HORMAD1 promotes DSB resection. HORMAD1-mediated HR is a neomorphic activity that is independent of its meiotic partners (including HORMAD2 and CCDC36. Bioinformatic analysis of TCGA data show that similar to known HR pathway genes HORMAD1 is overexpressed in lung adenocarcinomas. Overexpression of HR genes is associated with specific mutational profiles (including copy number variation). Taken together, we identify HORMAD1-dependent DSB repair as a new mechanism of radioresistance and a probable determinant of mutability in lung adenocarcinoma.

X-ray structure of full-length human RuvB-Like 2 - mechanistic insights into coupling between ATP binding and mechanical action

RuvB-Like transcription factors function in cell cycle regulation, development and human disease, such as cancer and heart hyperplasia. The mechanisms that regulate adenosine triphosphate (ATP)-dependent activity, oligomerization and post-translational modifications in this family of enzymes are yet unknown. We present the first crystallographic structure of full-length human RuvBL2 which provides novel insights into its mechanistic action and biology. The ring-shaped hexameric RuvBL2 structure presented here resolves for the first time the mobile domain II of the human protein, which is responsible for protein-protein interactions and ATPase activity regulation. Structural analysis suggests how ATP binding may lead to domain II motion through interactions with conserved N-terminal loop histidine residues. Furthermore, a comparison between hsRuvBL1 and 2 shows differences in surface charge distribution that may account for previously described differences in regulation. Analytical ultracentrifugation and cryo electron microscopy analyses performed on hsRuvBL2 highlight an oligomer plasticity that possibly reflects different physiological conformations of the protein in the cell, as well as that single-stranded DNA (ssDNA) can promote the oligomerization of monomeric hsRuvBL2. Based on these findings, we propose a mechanism for ATP binding and domain II conformational change coupling.

Multiple roles of the splicing complex SF3B in DNA end resection and homologous recombination

The appropriate repair of DNA double strand breaks is critical for genome maintenance. Thus, several cellular pathways collaborate to orchestrate a coordinated response. These include the repair of the breaks, which could be achieved by different mechanisms. A key protein involved in the regulation of the repair of broken chromosomes is CtIP. Here, we have found new partners of CtIP involved in the regulation of DNA break repair through affecting DNA end resection. We focus on the splicing complex SF3B and show that its depletion impairs DNA end resection and hampers homologous recombination. Functionally, SF3B controls CtIP function at, as least, two levels: by affecting CtIP mRNA levels and controlling CtIP recruitment to DNA breaks, in a way that requires ATM-mediated phosphorylation of SF3B2 at serine 289. Indeed, overexpression of CtIP rescues the resection defect caused by SF3B downregulation. Strikingly, other SF3B depletion phenotypes, such as impaired homologous recombination or cellular sensitivity to DNA damaging agents, are independent of CtIP levels, suggesting a more general role of SF3B in controlling the response to chromosome breaks.

Linking PRMT5 to breast cancer stem cells: New therapeutic opportunities?

The arginine methyltransferase PRMT5 has been increasingly associated with cancer development. Here we describe our recent findings that PRMT5 is a critical regulator of breast cancer stem cell survival via the epigenetic regulation of FOXP1. Consequently, PRMT5 inhibitors could potentially eradicate cancer stem cells thereby preventing tumour relapse.

PRMT5 Is a Critical Regulator of Breast Cancer Stem Cell Function via Histone Methylation and FOXP1 Expression

Breast cancer progression, treatment resistance, and relapse are thought to originate from a small population of tumor cells, breast cancer stem cells (BCSCs). Identification of factors critical for BCSC function is therefore vital for the development of therapies. Here, we identify the arginine methyltransferase PRMT5 as a key in vitro and in vivo regulator of BCSC proliferation and self-renewal and establish FOXP1, a winged helix/forkhead transcription factor, as a critical effector of PRMT5-induced BCSC function. Mechanistically, PRMT5 recruitment to the FOXP1 promoter facilitates H3R2me2s, SET1 recruitment, H3K4me3, and gene expression. Our findings are clinically significant, as PRMT5 depletion within established tumor xenografts or treatment of patient-derived BCSCs with a pre-clinical PRMT5 inhibitor substantially reduces BCSC numbers. Together, our findings highlight the importance of PRMT5 in BCSC maintenance and suggest that small-molecule inhibitors of PRMT5 or downstream targets could be an effective strategy eliminating this cancer-causing population.

Put Your Mark Where Your Damage Is: Acetyl-CoA Production by ACLY Promotes DNA Repair

In this issue of Molecular Cell, Sivanand et al. (2017) describe the importance for nuclear ACLY-mediated production of acetyl-CoA, which promotes histone acetylation, BRCA1 recruitment, and subsequent HR-mediated DNA repair in response to DNA damage, thus drawing a direct link between DNA repair and cellular metabolism.

Histone methylation and the DNA damage response

Preserving genome function and stability are paramount for ensuring cellular homeostasis, an imbalance in which can promote diseases including cancer. In the presence of DNA lesions, cells activate pathways referred to as the DNA damage response (DDR). As nuclear DNA is bound by histone proteins and organized into chromatin in eukaryotes, DDR pathways have evolved to sense, signal and repair DNA damage within the chromatin environment. Histone proteins, which constitute the building blocks of chromatin, are highly modified by post-translational modifications (PTMs) that regulate chromatin structure and function. An essential histone PTM involved in the DDR is histone methylation, which is regulated by histone methyltransferase (HMT) and histone demethylase (HDM) enzymes that add and remove methyl groups on lysine and arginine residues within proteins respectively. Methylated histones can alter how proteins interact with chromatin, including their ability to be bound by reader proteins that recognize these PTMs. Here, we review histone methylation in the context of the DDR, focusing on DNA double-strand breaks (DSBs), a particularly toxic lesion that can trigger genome instability and cell death. We provide a comprehensive overview of histone methylation changes that occur in response to DNA damage and how the enzymes and reader proteins of these marks orchestrate the DDR. Finally, as many epigenetic pathways including histone methylation are altered in cancer, we discuss the potential involvement of these pathways in the etiology and treatment of this disease.

The Role of Pontin and Reptin in Cellular Physiology and Cancer Etiology

Pontin (RUVBL1, TIP49, TIP49a, Rvb1) and Reptin (RUVBL2, TIP48, TIP49b, Rvb2) are highly conserved ATPases of the AAA+ (ATPases Associated with various cellular Activities) superfamily and are involved in various cellular processes that are important for oncogenesis. First identified as being upregulated in hepatocellular carcinoma and colorectal cancer, their overexpression has since been shown in multiple cancer types such as breast, lung, gastric, esophageal, pancreatic, kidney, bladder as well as lymphatic, and leukemic cancers. However, their exact functions are still quite unknown as they interact with many molecular complexes with vastly different downstream effectors. Within the nucleus, Pontin and Reptin participate in the TIP60 and INO80 complexes important for chromatin remodeling. Although not transcription factors themselves, Pontin and Reptin modulate the transcriptional activities of bona fide proto-oncogenes such as MYC and β-catenin. They associate with proteins involved in DNA damage repair such as PIKK complexes as well as with the core complex of Fanconi anemia pathway. They have also been shown to be important for cell cycle progression, being involved in assembly of telomerase, mitotic spindle, RNA polymerase II, and snoRNPs. When the two ATPases localize to the cytoplasm, they were reported to promote cancer cell invasion and metastasis. Due to their various roles in carcinogenesis, it is not surprising that Pontin and Reptin are proving to be important biomarkers for diagnosis and prognosis of various cancers. They are also current targets for the development of new therapeutic anticancer drugs.

Nuclear Acetyl-CoA Production by ACLY Promotes Homologous Recombination

While maintaining the integrity of the genome and sustaining bioenergetics are both fundamental functions of the cell, potential crosstalk between metabolic and DNA repair pathways is poorly understood. Since histone acetylation plays important roles in DNA repair and is sensitive to the availability of acetyl coenzyme A (acetyl-CoA), we investigated a role for metabolic regulation of histone acetylation during the DNA damage response. In this study, we report that nuclear ATP-citrate lyase (ACLY) is phosphorylated at S455 downstream of ataxia telangiectasia mutated (ATM) and AKT following DNA damage. ACLY facilitates histone acetylation at double-strand break (DSB) sites, impairing 53BP1 localization and enabling BRCA1 recruitment and DNA repair by homologous recombination. ACLY phosphorylation and nuclear localization are necessary for its role in promoting BRCA1 recruitment. Upon PARP inhibition, ACLY silencing promotes genomic instability and cell death. Thus, the spatial and temporal control of acetyl-CoA production by ACLY participates in the mechanism of DNA repair pathway choice.