The importance of cancer metabolism has been appreciated for many years, but the intricacies of how metabolic pathways interconnect with oncogenic signaling are not fully understood. With a clear understanding of how metabolism contributes to tumorigenesis, we will be better able to integrate the targeting of these fundamental biochemical pathways into patient care. The mevalonate (MVA) pathway, paced by its rate-limiting enzyme, hydroxymethylglutaryl coenzyme A reductase (HMGCR), is required for the generation of several fundamental end-products including cholesterol and isoprenoids. Despite years of extensive research from the perspective of cardiovascular disease, the contribution of a dysregulated MVA pathway to human cancer remains largely unexplored. We address this issue directly by showing that dysregulation of the MVA pathway, achieved by ectopic expression of either full-length HMGCR or its novel splice variant, promotes transformation. Ectopic HMGCR accentuates growth of transformed and nontransformed cells under anchorage-independent conditions or as xenografts in immunocompromised mice and, importantly, cooperates with RAS to drive the transformation of primary mouse embryonic fibroblasts cells. We further explore whether the MVA pathway may play a role in the etiology of human cancers and show that high mRNA levels of HMGCR and additional MVA pathway genes correlate with poor prognosis in a meta-analysis of six microarray datasets of primary breast cancer. Taken together, our results suggest that HMGCR is a candidate metabolic oncogene and provide a molecular rationale for further exploring the statin family of HMGCR inhibitors as anticancer agents.HMGCR | hydroxymethylglutaryl coenzyme A reductase | cancer | metabolic oncogene | tumor metabolism

Signaling and repair of DNA double-strand breaks (DSBs) are critical for preventing immunodeficiency and cancer. These DNA breaks result from exogenous and endogenous DNA insults but are also programmed to occur during physiological processes such as meiosis and immunoglobulin heavy chain (IgH) class switch recombination (CSR). Recent studies reported that the E3 ligase RNF8 plays important roles in propagating DNA DSB signals and thereby facilitating the recruitment of various DNA damage response proteins, such as 53BP1 and BRCA1, to sites of damage. Using mouse models for Rnf8 mutation, we report that Rnf8 deficiency leads to impaired spermatogenesis and increased sensitivity to ionizing radiation both in vitro and in vivo. We also demonstrate the existence of alternative Rnf8-independent mechanisms that respond to irradiation and accounts for the partial recruitment of 53bp1 to sites of DNA damage in activated Rnf8−/− B cells. Remarkably, IgH CSR is impaired in a gene dose-dependent manner in Rnf8 mutant mice, revealing that these mice are immunodeficient. In addition, Rnf8−/− mice exhibit increased genomic instability and elevated risks for tumorigenesis indicating that Rnf8 is a novel tumor suppressor. These data unravel the in vivo pleiotropic effects of Rnf8.

Defective signaling or repair of DNA double-strand breaks has been associated with developmental defects and human diseases. The E3 ligase RING finger 168 (RNF168), mutated in the human radiosensitivity, immunodeficiency, dysmorphic features, and learning difficulties syndrome, was shown to ubiquitylate H2A-type histones, and this ubiquitylation was proposed to facilitate the recruitment of p53-binding protein 1 (53BP1) to the sites of DNA double-strand breaks. In contrast to more upstream proteins signaling DNA double-strand breaks (e.g., RNF8), deficiency of RNF168 fully prevents both the initial recruitment to and retention of 53BP1 at sites of DNA damage; however, the mechanism for this difference has remained unclear. Here, we identify mechanisms that regulate 53BP1 recruitment to the sites of DNA double-strand breaks and provide evidence that RNF168 plays a central role in the regulation of 53BP1 functions. RNF168 mediates K63-linked ubiquitylation of 53BP1 which is required for the initial recruitment of 53BP1 to sites of DNA double-strand breaks and for its function in DNA damage repair, checkpoint activation, and genomic integrity. Our findings highlight the multistep roles of RNF168 in signaling DNA damage.ubiquitin | G2/M checkpoint | HR repair pathways | NHEJ repair pathway T he DNA-damage response (DDR) is critical for genomic integrity (1) and is regulated by posttranslational modifications (PTMs) such as ubiquitylation of histones by the E3 ligases RING finger 8 (RNF8) and RING finger 168 (RNF168). Other PTMs important for DDR include dimethylation of histone H4 (H4K20me2), which allows p53-binding protein 1 (53BP1), a key mediator of DDR, to interact with chromatin.Mutations of RNF168 have been associated with the human radiosensitivity, immunodeficiency, dysmorphic features, and learning difficulties (RIDDLE) syndrome, (2-4). RNF168 has an N-terminal RING finger domain, three ubiquitin (Ub)-binding domains (UBDs); two motif interacting with Ub (MIU) domains; and one Ub interacting motif (UIM)-and MIU-related (UMI) UBD (3,5,6). Current data support RNF168 function in DNA double-strand break (DSB) signaling downstream of H2A.X, mediator of DNA damage checkpoint 1 (MDC1), and RNF8 and indicate its requirement for 53BP1 recruitment to DSB sites (3, 5). Through its UBDs, RNF168 recognizes RNF8 ubiquitylated non-nucleosomal protein(s) at DSB-flanking sites, leading to its recruitment at these sites of DNA damage (7). With the Ubconjugating enzyme UBC13, RNF168 initiates ubiquitylation of lysine (K) 13 or 15 of histones H2A and H2A.X, leading to the recruitment of DDR proteins, including 53BP1, to DSBs. Although H2A.X, MDC1, and RNF8 are important for the retention of 53BP1 at these DSBs, its initial and transient recruitment to DNA breaks still occurs in their absence (8-11). In contrast, deficiency of Rnf168 in mouse embryonic fibroblasts (MEFs) completely abolishes 53bp1 recruitment to DSB sites (12). Similar to 53bp1 −/− mice, but in contrast to H2a.x −/− mice (13), young Rnf168 −/− males are ferti...