SUMMARY Lysine63-linked ubiquitin (K63-Ub) chains represent a particular ubiquitin topology that mediates proteasome-independent signaling events. The deubiquitinating enzyme (DUB) BRCC36 segregates into distinct nuclear and cytoplasmic complexes that are specific for K63-Ub hydrolysis. RAP80 targets the five-member nuclear BRCC36 complex to K63-Ub chains at DNA double-strand breaks. The alternative four-member BRCC36 containing complex (BRISC) lacks a known targeting moiety. Here we identify Serine Hydroxymethyltransferase (SHMT) as a heretofore-unappreciated component that fulfills this function. SHMT directs BRISC activity at K63-Ub chains conjugated to the type 1 interferon (IFN) receptor chain 1 (IFNAR1). BRISC-SHMT2 complexes localize to and deubiquitinate actively engaged IFNAR1, thus limiting its K63-Ub mediated internalization and lysosomal degradation. BRISC deficient cells and mice exhibit attenuated responses to IFN and are protected from IFN-associated immunopathology. These studies reveal a novel mechanism of DUB regulation, and suggest a therapeutic use of BRISC inhibitors for treating pathophysiologic processes driven by elevated IFN responses.

Cell division cycle 25 (CDC25) phosphatases are key actors in eukaryotic cell cycle control. They are responsible for the dephosphorylations that activate the cyclin-dependent kinases (CDK) at specific stages of the cell cycle. Human CDC25A, CDC25B and CDC25C are also central targets and regulators of the G2/M checkpoint mechanisms activated in response to DNA injury. The expression and activity of these enzymes is finely regulated by multiple mechanisms including post-translational modifications, interactions with regulatory partners, control of their intracellular localization, and cell cycle-regulated degradation. Altered expression of these phosphatases is associated with checkpoint bypass and genetic instability. Accordingly, increased expression of CDC25A and CDC25B is found in many high-grade tumors and is correlated with poor prognosis in human cancers. This review summarizes our current knowledge within this domain and discusses the data that support therapeutic strategies targeting CDC25 activity in the treatment of cancer.

Breast cancer is the most common cancer in women in developed countries and has a well-established genetic component. Germline mutations in a network of genes encoding BRCA1, BRCA2, and their interacting partners confer hereditary susceptibility to breast cancer. Abraxas directly interacts with the BRCA1 BRCT (BRCA1 carboxyl-terminal) repeats and contributes to BRCA1-dependent DNA damage responses, making Abraxas a candidate for yet unexplained disease susceptibility. Here, we have screened 125 Northern Finnish breast cancer families for coding region and splice-site Abraxas mutations and genotyped three tagging single-nucleotide polymorphisms within the gene from 991 unselected breast cancer cases and 868 female controls for common cancer-associated variants. A novel heterozygous alteration, c.1082G>A (Arg361Gln), that results in abrogated nuclear localization and DNA response activities was identified in three breast cancer families and in one additional familial case from an unselected breast cancer cohort, but not in healthy controls (P = 0.002). On the basis of its exclusive occurrence in familial cancers, disease cosegregation, evolutionary conservation, and disruption of critical BRCA1 functions, the recurrent Abraxas c.1082G>A mutation connects to cancer predisposition. These findings contribute to the concept of a BRCA-centered tumor suppressor network and provide the identity of Abraxas as a new breast cancer susceptibility gene.