High-resolution structure of the presynaptic RAD51 filament on single-stranded DNA by electron cryo-microscopy

Homologous DNA recombination (HR) by the RAD51 recombinase enables error-free DNA break repair. To execute HR, RAD51 first forms a presynaptic filament on single-stranded (ss) DNA, which catalyses pairing with homologous double-stranded (ds) DNA. Here, we report a structure for the presynaptic human RAD51 filament at 3.5-5.0Å resolution using electron cryo-microscopy. RAD51 encases ssDNA in a helical filament of 103Å pitch, comprising 6.4 protomers per turn, with a rise of 16.1Å and a twist of 56.2°. Inter-protomer distance correlates with rotation of an α-helical region in the core catalytic domain that is juxtaposed to ssDNA, suggesting how the RAD51-DNA interaction modulates protomer spacing and filament pitch. We map Fanconi anaemia-like disease-associated RAD51 mutations, clarifying potential phenotypes. We predict binding sites on the presynaptic filament for two modules present in each BRC repeat of the BRCA2 tumour suppressor, a critical HR mediator. Structural modelling suggests that changes in filament pitch mask or expose one binding site with filament-inhibitory potential, rationalizing the paradoxical ability of the BRC repeats to either stabilize or inhibit filament formation at different steps during HR. Collectively, our findings provide fresh insight into the structural mechanism of HR and its dysregulation in human diseaseThis is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/nar/gkw78

The correlation of breakpoint cluster region rearrangement and p210 phl/abl expression with morphological analysis of Ph-negative chronic myeloid leukemia and other myeloproliferative diseases

The chromosome 22 derivative, the Philadelphia (Ph) chromosome, results from a reciprocal translocation t(9;22)(q34;q11) and is associated with chronic myeloid leukemia (CML). The translocation can be identified at the DNA level in Ph-positive CML by using a probe to the breakpoint cluster region (bcr). In addition, as a result of this translocation an abl-related 210-kd protein with protein tyrosine kinase (PTK) activity is produced. We analyzed 28 cases of Ph-negative CML for rearrangement of the chromosome 22 sequences and found that eight of the 28 show rearrangement of the bcr. When 12 of the Ph-negative cases were independently reviewed, five were indistinguishable from Ph-positive CML on the basis of morphology, peripheral blood film and clinical details. These five also showed bcr rearrangement. The other seven were reclassified as six atypical CML (aCML) and one chronic myelomonocytic leukemia (CMML). None of these seven showed bcr rearrangement. In addition 11 cases of bcr- CML were assayed for abl-related PTK, and no detectable activity was present, whereas p210 phl/abl PTk was observed both in Ph-positive (three cases examined) and Ph-negative, bcr+ (four cases examined) CML. Therefore, bcr+ CML, whether or not the Ph chromosome is cytogenetically apparent, involves a similar molecular alteration and produces the 210-kd protein with enhanced PTK activity. Furthermore, these cases can be distinguished from Ph-negative bcr- CML by careful evaluation of clinical and hematologic data.link_to_subscribed_fulltex

PIDD orchestrates translesion DNA synthesis in response to UV irradiation

PIDD has been implicated in survival and apoptotic pathways in response to DNA damage, and a role for PIDD was recently identified in non-homologous end-joining (NHEJ) repair induced by γ-irradiation. Here, we present an interaction of PIDD with PCNA, first identified in a proteomics screen. PCNA has essential functions in DNA replication and repair following UV irradiation. Translesion synthesis (TLS) is a process that prevents UV irradiation-induced replication blockage and is characterized by PCNA monoubiquitination and interaction with the TLS polymerase eta (polη). Both of these processes are inhibited by p21. We report that PIDD modulates p21-PCNA dissociation, and promotes PCNA monoubiquitination and interaction with polη in response to UV irradiation. Furthermore, PIDD deficiency leads to a defect in TLS that is associated, both in vitro and in vivo, with cellular sensitization to UV-induced apoptosis. Thus, PIDD performs key functions upon UV irradiation, including TLS, NHEJ, NF-κB activation and cell death

Sequential recruitment of the repair factors during NER: the role of XPG in initiating the resynthesis step

To address the biochemical mechanisms underlying the coordination between the various proteins required for nucleotide excision repair (NER), we employed the immobilized template system. Using either wild-type or mutated recombinant proteins, we identified the factors involved in the NER process and showed the sequential comings and goings of these factors to the immobilized damaged DNA. Firstly, we found that PCNA and RF-C arrival requires XPF 5′ incision. Moreover, the positioning of RF-C is facilitated by RPA and induces XPF release. Concomitantly, XPG leads to PCNA recruitment and stabilization. Our data strongly suggest that this interaction with XPG protects PCNA and Polδ from the effect of inhibitors such as p21. XPG and RPA are released as soon as Polδ is recruited by the RF-C/PCNA complex. Finally, a ligation system composed of FEN1 and Ligase I can be recruited to fully restore the DNA. In addition, using XP or trichothiodystrophy patient-derived cell extracts, we were able to diagnose the biochemical defect that may prove to be important for therapeutic purposes

Unscheduled DNA synthesis: the clinical and functional assay for global genomic DNA nucleotide excision repair

The unscheduled DNA synthesis (UDS) assay measures the ability of a cell to perform global genomic nucleotide excision repair (NER). This chapter provides instructions for the application of this technique by creating 6-4 photoproducts and pyrimidine dimers using UV-C irradiation. This procedure is designed specifically for quantification of the 6-4 photoproducts. Repair is quantified by the amount of radioactive thymidine incorporated during repair synthesis after this insult, and radioactivity is evaluated by grain counting after autoradiography. The results are used to clinically diagnose human DNA repair deficiency disorders and provide a basis for investigation of repair deficiency in human tissues or tumors. No other functional assay is available that directly measures the capacity to perform NER on the entire genome without the use of specific antibodies. Since live cells are required for this assay, explant culture techniques must be previously established. Host cell reactivation (HCR), as discussed in Chapter 37, is not an equivalent technique, as it measures only transcription-coupled repair (TCR) at active genes, a small subset of total NER