Interactions of the Nijmegen breakage syndrome protein with ATM and BRCA1

Fanconi anemia group J helicase and MRE11 nuclease interact to facilitate the DNA damage response

FANCJ mutations are linked to Fanconi anemia (FA) and increase breast cancer risk. FANCJ encodes a DNA helicase implicated in homologous recombination (HR) repair of double-strand breaks (DSBs) and interstrand cross-links (ICLs), but its mechanism of action is not well understood. Here we show with live-cell imaging that FANCJ recruitment to laser-induced DSBs but not psoralen-induced ICLs is dependent on nuclease-active MRE11. FANCJ interacts directly with MRE11 and inhibits its exonuclease activity in a specific manner, suggesting that FANCJ regulates the MRE11 nuclease to facilitate DSB processing and appropriate end resection. Cells deficient in FANCJ and MRE11 show increased ionizing radiation (IR) resistance, reduced numbers of γH2AX and RAD51 foci, and elevated numbers of DNA-dependent protein kinase catalytic subunit foci, suggesting that HR is compromised and the nonhomologous end-joining (NHEJ) pathway is elicited to help cells cope with IR-induced strand breaks. Interplay between FANCJ and MRE11 ensures a normal response to IR-induced DSBs, whereas FANCJ involvement in ICL repair is regulated by MLH1 and the FA pathway. Our findings are discussed in light of the current model for HR repair.

ISG15 deregulates autophagy in genotoxin-treated ataxia telangiectasia cells

Ataxia-telangiectasia (A-T) is a cerebellar neurodegenerative disorder; however, the basis for the neurodegeneration in A-T is not well established. Lesions in the ubiquitin and autophagy pathways are speculated to contribute to the neurodegeneration in other neurological diseases and may have a role in A-T neurodegeneration. Our recent studies revealed that the constitutively elevated ISG15 pathway impairs targeted proteasome-mediated protein degradation in A-T cells. Here, we demonstrate that the basal autophagy pathway is activated in the ubiquitin pathway-compromised A-T cells. We also show that genotoxic stress triggers aberrant degradation of the proteasome and autophagy substrates (autophagic flux) in A-T cells. Inhibition of autophagy at an early stage using 3-methyladenine blocked UV-induced autophagic flux in A-T cells. On the other hand, bafilomycin A1, which inhibits autophagy at a late stage, failed to block UV-induced autophagic flux, suggesting that overinduction of autophagy may underlie aberrant autophagic flux in A-T cells. The ISG15-specific shRNA that restored proteasome function restores autophagic function in A-T cells. These findings suggest that autophagy compensates for the ISG15-dependent ablation of proteasome-mediated protein degradation in A-T cells. Genotoxic stress overactivates this compensatory mechanism, triggering aberrant autophagic flux in A-T cells. Supporting the model, we show that autophagy is activated in the brain tissues of human A-T patients. This highlights a plausible causal contribution of a novel "ISG15 proteinopathy" in A-T neuronal cell death.

The role of BRCA1 in DNA damage response

BRCA1 is a well-established tumor suppressor gene, which is frequently mutated in familial breast and ovarian cancers. The gene product of BRCA1 functions in a number of cellular pathways that maintain genomic stability, including DNA damage-induced cell cycle checkpoint activation, DNA damage repair, protein ubiquitination, chromatin remodeling, as well as transcriptional regulation and apoptosis. In this review, we discuss recent advances regarding our understanding of the role of BRCA1 in tumor suppression and DNA damage response, including DNA damage-induced cell cycle checkpoint activation and DNA damage repair.

Cell cycle-dependent complex formation of BRCA1.CtIP.MRN is important for DNA double-strand break repair

BRCA1 plays an important role in the homologous recombination (HR)-mediated DNA double-strand break (DSB) repair, but the mechanism is not clear. Here we describe that BRCA1 forms a complex with CtIP and MRN (Mre11/Rad50/Nbs1) in a cell cycle-dependent manner. Significantly, the complex formation, especially the ionizing radiation-enhanced association of BRCA1 with MRN, requires cyclin-dependent kinase activity. CtIP directly interacts with Nbs1. The in vivo association of BRCA1 with MRN is largely dependent on the association of CtIP with the BRCT domains at the C terminus of BRCA1, whereas the N terminus of BRCA1 also contributes to its association with MRN. CtIP, as well as the interaction of BRCA1 with CtIP and MRN, is critical for IR-induced single-stranded DNA formation and cellular resistance to radiation. Consistently, CtIP itself is required for efficient HR-mediated DSB repair, like BRCA1 and MRN. These studies suggest that the complex formation of BRCA1.CtIP.MRN is important for facilitating DSB resection to generate single-stranded DNA that is needed for HR-mediated DSB repair. Because cyclin-dependent kinase is important for establishing IR-enhanced interaction of MRN with BRCA1, we propose that the cell cycle-dependent complex formation of BRCA1, CtIP, and MRN contributes to the activation of HR-mediated DSB repair in the S and G(2) phases of the cell cycle.

The role of BRCA1 in non-homologous end-joining

From the genotypic viewpoint, single nucleotide polymorphisms in the genes of the non-homologous end-joining (NHEJ) pathway, which is important in the repair of DNA double-strand breaks, have been shown to be associated with increased breast cancer risk. However, more phenotypic evidence is needed to strengthen the link between defective NHEJ genes and breast cancer development. Recently, BRCA1-deficient mouse embryonic fibroblasts were found to have significantly reduced NHEJ activity, suggesting an accessory role of BRCA1 in NHEJ. Since BRCA1 is a well-documented breast cancer susceptibility gene, this association between NHEJ and BRCA1 not only suggests a role of BRCA1 in NHEJ, but also provides support for the tumorigenic contribution of the NHEJ pathway to breast cancer development. Interestingly, the phenotypic data show that BRCA1 may promote only specific subtypes of NHEJ, e.g. in vivo precise and terminal end-joining capacities, and have either a suppressive or no effect on others. However, these findings have remained inconclusive, and the lack of consistency between these results may be at least partly explained by the use of different assays, which may measure different subtypes of NHEJ, and of different cell lines investigated. Although some insights have been obtained, the whole picture of NHEJ repair in mammalian cells is far from complete, and the questions of how many subpathways are involved or how we can investigate each subpathway have not yet been adequately addressed.