DNA double-strand breaks (DSBs) are harmful lesions leading to genomic instability or diversity. Non-homologous end-joining (NHEJ) is a prominent DSB repair pathway, which has long been considered to be error-prone. However, recent data have pointed to the intrinsic precision of NHEJ. Three reasons can account for the apparent fallibility of NHEJ: 1) the existence of a highly error-prone alternative end-joining process; 2) the adaptability of canonical C-NHEJ (Ku- and Xrcc4/ligase IV–dependent) to imperfect complementary ends; and 3) the requirement to first process chemically incompatible DNA ends that cannot be ligated directly. Thus, C-NHEJ is conservative but adaptable, and the accuracy of the repair is dictated by the structure of the DNA ends rather than by the C-NHEJ machinery. We present data from different organisms that describe the conservative/versatile properties of C-NHEJ. The advantages of the adaptability/versatility of C-NHEJ are discussed for the development of the immune repertoire and the resistance to ionizing radiation, especially at low doses, and for targeted genome manipulation.

A two-hybrid screen was used to identify Saccharomyces cerevisiae genes encoding proteins that interact with MSH2. One gene was found to encode a homologue of Schizosaccharomyces pombe EXO1, a double-stranded DNA-specific 5′–3′ exonuclease. S. cerevisiae EXO1 interacted with both S. cerevisiae and human MSH2 in two-hybrid and coimmunoprecipitation experiments. exo1 mutants showed a mutator phenotype, and epistasis analysis was consistent with EXO1 functioning in the MSH2 -dependent mismatch repair pathway. exo1 mutations were lethal in combination with rad27 mutations, and overexpression of EXO1 suppressed both the temperature sensitive and mutator phenotypes of rad27 mutants.

Germ‐line mutations in the BRCA1 gene strongly predispose women to breast cancer (lifetime risk up to 80%). Furthermore, the BRCA1 protein is absent or present at very low levels in about one third of sporadic breast cancers. However, the mechanisms underlying BRCA1 somatic inactivation appear multiple and are still not fully understood. We report here the involvement of miR‐146a and miR‐146b‐5p that bind to the same site in the 3′UTR of BRCA1 and down‐regulate its expression as demonstrated using reporter assays. This was further confirmed with the endogenous BRCA1 gene by transfecting microRNA (miRNA) precursors or inhibitors in mammary cell lines. This down‐regulation was accompanied by an increased proliferation and a reduced homologous recombination rate, two processes controlled by BRCA1. Furthermore, we showed that the highest levels of miR‐146a and/or miR‐146b‐5p are found in basal‐like mammary tumour epithelial cell lines and in triple negative breast tumours, which are the closest to tumours arising in carriers of BRCA1 mutations. This work provides further evidence for the involvement of miRNAs in sporadic breast cancer through down‐regulation of BRCA1.

We report crosstalk between three senescence‐inducing conditions, DNA damage response (DDR) defects, oxidative stress (OS) and nuclear shape alterations. The recessive autosomal genetic disorder Ataxia telangiectasia (A‐T) is associated with DDR defects, endogenous OS and premature ageing. Here, we find frequent nuclear shape alterations in A‐T cells, as well as accumulation of the key nuclear architecture component lamin B1. Lamin B1 overexpression is sufficient to induce nuclear shape alterations and senescence in wild‐type cells, and normalizing lamin B1 levels in A‐T cells reciprocally reduces both nuclear shape alterations and senescence. We further show that OS increases lamin B1 levels through p38 Mitogen Activated Protein kinase activation. Lamin B1 accumulation and nuclear shape alterations also occur during stress‐induced senescence and oncogene‐induced senescence (OIS), two canonical senescence situations. These data reveal lamin B1 as a general molecular mediator that controls OS‐induced senescence, independent of established Ataxia Telangiectasia Mutated (ATM) roles in OIS. There is a Have you seen? (March 2012) associated with this Article.