Explore the vast range of titles available.

Cytidine base editors are powerful genetic tools that catalyse cytidine to thymidine conversion at specific genomic loci, and further improvement of the editing range and efficiency is critical for their broader applications. Through insertion of a non-sequence-specific single-stranded DNA-binding domain from Rad51 protein between Cas9 nickase and the deaminases, serial hyper cytidine base editors were generated with substantially increased activity and an expanded editing window towards the protospacer adjacent motif in both cell lines and mouse embryos. Additionally, hyeA3A-BE4max selectively catalysed cytidine conversion in TC motifs with a broader editing range and much higher activity (up to 257-fold) compared with eA3A-BE4max. Moreover, hyeA3A-BE4max specifically generated a C-to-T conversion without inducing bystander mutations in the haemoglobin gamma gene promoter to mimic a naturally occurring genetic variant for amelioration of β-haemoglobinopathy, suggesting the therapeutic potential of the improved base editors.Li and colleagues report base editor variants with improved targeting efficiencies and broader editing windows by fusing the original base editors with the single-stranded DNA-binding domain of Rad51.

Telomeres—repeated, noncoding nucleotide motifs and associated proteins that are found at the ends of eukaryotic chromosomes—mediate genome stability and determine cellular lifespan 1 . Telomeric-repeat-containing RNA (TERRA) is a class of long noncoding RNAs (lncRNAs) that are transcribed from chromosome ends 2 , 3 ; these RNAs in turn regulate telomeric chromatin structure and telomere maintenance through the telomere-extending enzyme telomerase 4 – 6 and homology-directed DNA repair 7 , 8 . The mechanisms by which TERRA is recruited to chromosome ends remain poorly defined. Here we develop a reporter system with which to dissect the underlying mechanisms, and show that the UUAGGG repeats of TERRA are both necessary and sufficient to target TERRA to chromosome ends. TERRA preferentially associates with short telomeres through the formation of telomeric DNA–RNA hybrid (R-loop) structures that can form in trans . Telomere association and R-loop formation trigger telomere fragility and are promoted by the recombinase RAD51 and its interacting partner BRCA2, but counteracted by the RNA-surveillance factors RNaseH1 and TRF1. RAD51 physically interacts with TERRA and catalyses R-loop formation with TERRA in vitro, suggesting a direct involvement of this DNA recombinase in the recruitment of TERRA by strand invasion. Together, our findings reveal a RAD51-dependent pathway that governs TERRA-mediated R-loop formation after transcription, providing a mechanism for the recruitment of lncRNAs to new loci in trans . Telomeric-repeat-containing RNA is recruited to telomeres by a mechanism that involves the DNA recombinase RAD51 and the formation of DNA–RNA hybrids, or R-loops—a process similar to that involved in homology-directed DNA repair.

The BRCA2 tumour suppressor protein preserves genomic integrity via interactions with the DNA-strand exchange RAD51 protein in homology-directed repair. The RAD51-binding TR2 motif at the BRCA2 C-terminus is essential for protection and restart of stalled replication forks. Biochemical evidence shows that TR2 recognises filamentous RAD51, but existing models of TR2 binding to RAD51 lack a structural basis. Here we used cryo-electron microscopy and structure-guided mutagenesis to elucidate the mechanism of TR2 binding to nucleoprotein filaments of human RAD51. We find that TR2 binds across the protomer interface in the filament, acting as a brace for adjacent RAD51 molecules. TR2 targets an acidic-patch motif on human RAD51 that serves as a recruitment hub in fission yeast Rad51 for recombination mediators Rad52 and Rad55-Rad57. Our findings provide a structural rationale for RAD51 filament stabilisation by BRCA2 and reveal a common recruitment mechanism of recombination mediators to the RAD51 filament. Here the authors report the cryoEM structure of the RAD51 nucleoprotein filament bound to the C-terminal TR2 domain of BRCA2. The structure explains how BRCA2 stabilises the filament and uncovers a conserved mechanism of filament binding by recombination mediators.

YTHDC1, a key protein in the m 6 A-related regulatory network within cells, is involved in multiple cellular processes such as chromatin-related regulation, RNA splicing, and nuclear export. Understanding its role in colorectal cancer (CRC) development and DNA damage repair is critical for the advancement of treatment strategies. Our study found that YTHDC1 was highly expressed in high-malignancy CRC tissues compared with low-malignancy ones. Upon silencing YTHDC1, we observed a pronounced suppression of the proliferation of CRC cell lines, accompanied by a substantial increase in cell apoptosis. Furthermore, we identified RAD51 as a crucial downstream target of YTHDC1. Knocking down YTHDC1 led to a notable decrease in RAD51 protein levels, and silencing RAD51 also inhibited cancer cell proliferation. Interestingly, RNA-sequencing data indicated that the YTHDC1 deletion did not affect RAD51 transcription. However, Western blot revealed that this deletion increased the ubiquitination of RAD51, likely due to the upregulated E3 ligase UBE3A. Ubiquitination experiments subsequently confirmed that RAD51 is indeed one of the substrates of UBE3A. In summary, our study provides novel insights into how YTHDC1 modulates the expression of RAD51 through post-translational modifications. These findings offer valuable information that may potentially contribute to the development of more effective therapeutic strategies for CRC.

Synthetic lethality is an approach to study selective cell killing based on genotype. Previous work in our laboratory has shown that loss of RAD52 is synthetically lethal with BRCA2 deficiency, while exhibiting no impact on cell growth and viability in BRCA2-proficient cells. We now show that this same synthetically lethal relationship is evident in cells with deficiencies in BRCA1 or PALB2, which implicates BRCA1, PALB2 and BRCA2 in an epistatic relationship with one another. When RAD52 was depleted in BRCA1- or PALB2-deficient cells, a severe reduction in plating efficiency was observed, with many abortive attempts at cell division apparent in the double-depleted background. In contrast, when RAD52 was depleted in a BRCA1- or PALB2-wildtype background, a negligible decrease in colony survival was observed. The frequency of ionizing radiation-induced RAD51 foci formation and double-strand break-induced homologous recombination (HR) was decreased by 3- and 10-fold, respectively, when RAD52 was knocked down in BRCA1- or PALB2-depleted cells, with minimal effect in BRCA1- or PALB2-proficient cells. RAD52 function was independent of BRCA1 status, as evidenced by the lack of any defect in RAD52 foci formation in BRCA1-depleted cells. Collectively, these findings suggest that RAD52 is an alternative repair pathway of RAD51-mediated HR, and a target for therapy in cells deficient in the BRCA1–PALB2–BRCA2 repair pathway.

Eukaryotic Rad51 protein is essential for homologous-recombination repair of DNA double-strand breaks. Rad51 recombinases first assemble onto single-stranded DNA to forma nucleoprotein filament, required for function in homology pairing and strand exchange. This filament assembly is the first regulation step in homologous recombination. Rad51 nucleation is kinetically slow, and several accessory factors have been identified to regulate this step. Swi5–Sfr1 (S5S1) stimulates Rad51-mediated homologous recombination by stabilizing Rad51 nucleoprotein filaments, but the mechanism of stabilization is unclear. We used single-molecule tethered particle motion experiments to show that mouse S5S1 (mS5S1) efficiently stimulates mouse RAD51 (mRAD51) nucleus formation and inhibits mRAD51 dissociation from filaments. We also used single-molecule fluorescence resonance energy transfer experiments to show that mS5S1 promotes stable nucleus formation by specifically preventing mRAD51 dissociation. This leads to a reduction of nucleation size from three mRAD51 to two mRAD51 molecules in the presence of mS5S1. Compared with mRAD51, fission yeast Rad51 (SpRad51) exhibits fast nucleation but quickly dissociates from the filament. SpS5S1 specifically reduces SpRad51 disassembly to maintain a stable filament. These results clearly demonstrate the conserved function of S5S1 by primarily stabilizing Rad51 on DNA, allowing both the formation of the stable nucleus and the maintenance of filament length.

Background Triple-negative breast cancer (TNBC) affects young women and is the most aggressive subtype of breast cancer (BC). TNBCs disproportionally affect women of African-American (AA) descent compared to other ethnicities. We have identified DNA repair gene RAD51 as a poor prognosis marker in TNBC and its posttranscriptional regulation through microRNAs (miRNAs). This study aims to delineate the mechanisms leading to RAD51 upregulation and develop novel therapeutic combinations to effectively treat TNBCs and reduce disparity in clinical outcomes. Methods Analysis of TCGA data for BC cohorts using the UALCAN portal and PrognoScan identified the overexpression of RAD51 in TNBCs. miRNA sequencing identified significant downregulation of RAD51-targeting miRNAs miR-214-5P and miR-142-3P. RT-PCR assays were used to validate the levels of miRNAs and RAD51, and immunohistochemical and immunoblotting techniques were used similarly for RAD51 protein levels in TNBC tissues and cell lines. Luciferase assays were performed under the control of RAD51 3’-UTR to confirm that miR-214-5P regulates RAD51 expression. To examine the effect of miR-214-5P-mediated downregulation of RAD51 on homologous recombination (HR) in TNBC cells, Dr-GFP reporter assays were performed. To assess the levels of olaparib-induced DNA damage responses in miR-214-5P, transfected cells, immunoblots, and immunofluorescence assays were used. Furthermore, COMET assays were used to measure DNA lesions and colony assays were performed to assess the sensitivity of BRCA-proficient TNBC cells to olaparib. Results In-silico analysis identified upregulation of RAD51 as a poor prognostic marker in TNBCs. miRNA-seq data showed significant downregulation of miR-214-5P and miR-142-3P in TNBC cell lines derived from AA women compared to Caucasian-American (CA) women. miR-214-5P mimics downregulated RAD51 expression and induces HR deficiency as measured by Dr-GFP assays in these cell lines. Based on these results, we designed a combination treatment of miR-214-5P and olaparib in HR-proficient AA TNBC cell lines using clonogenic survival assays. The combination of miR-214-5P and olaparib showed synergistic lethality compared to individual treatments in these cell lines. Conclusions Our studies identified a novel epigenetic regulation of RAD51 in TNBCs by miR-214-5P suggesting a novel combination therapies involving miR-214-5P and olaparib to treat HR-proficient TNBCs and to reduce racial disparity in therapeutic outcomes.

The RAD51 recombinase is a critical effector of Homologous Recombination (HR), which is an essential DNA repair mechanism for double-strand breaks. The RAD51 protein is recruited onto the DNA break by BRCA2 and forms homopolymeric filaments that invade the homologous chromatid and use it as a template for repair. RAD51 filaments are detectable by immunofluorescence as distinct foci in the cell nucleus, and their presence is a read out of HR proficiency. RAD51 is an essential gene, protecting cells from genetic instability. Its expression is low and tightly regulated in normal cells and, contrastingly, elevated in a large fraction of cancers, where its level of expression and activity have been linked with sensitivity to genotoxic treatment. In particular, BRCA-deficient tumors show reduced or obliterated RAD51 foci formation and increased sensitivity to platinum salt or PARP inhibitors. However, resistance to treatment sets in rapidly and is frequently based on a complete or partial restoration of RAD51 foci formation. Consequently, RAD51 could be a highly valuable therapeutic target. Here, we review the multiple levels of regulation that impact the transcription of the RAD51 gene, as well as the post-translational modifications that determine its expression level, recruitment on DNA damage sites and the efficient formation of homofilaments. Some of these regulation levels may be targeted and their impact on cancer cell survival discussed.

RAD51 recombinase activity plays a critical role for cancer cell proliferation and survival, and often contributes to drug‐resistance. Abnormally elevated RAD51 function and hyperactive homologous recombination (HR) rates have been found in a panel of cancers, including breast cancer and chronic myeloid leukaemia (CML). Directly targeting RAD51 and attenuating the deregulated RAD51 activity has therefore been proposed as an alternative and supplementary strategy for cancer treatment. Here we show that a newly identified small molecule, IBR2, disrupts RAD51 multimerization, accelerates proteasome‐mediated RAD51 protein degradation, reduces ionizing radiation‐induced RAD51 foci formation, impairs HR, inhibits cancer cell growth and induces apoptosis. In a murine imatinib‐resistant CML model bearing the T315I Bcr‐abl mutation, IBR2, but not imatinib, significantly prolonged animal survival. Moreover, IBR2 effectively inhibits the proliferation of CD34 + progenitor cells from CML patients resistant to known BCR‐ABL inhibitors. Therefore, small molecule inhibitors of RAD51 may suggest a novel class of broad‐spectrum therapeutics for difficult‐to‐treat cancers. Graphical Abstract A newly identified RAD51 inhibitor leading to degradation of RAD51 via the proteasome pathway inhibits cancer cell survival and greatly increases life spans in a mouse chronic myeloid leukaemia model.

Tumor suppressor protein BRCA2 interacts with RAD51 and functions in homologous recombination, but understanding its precise functions has been hampered by difficulties in purifying such a large protein. Now purified full-length human BRCA2 is shown to bind selectively to ssDNA, to promote RAD51 binding to ssDNA while reducing its association with dsDNA, and to stimulate RAD51-mediated DNA strand exchange. Individuals with BRCA2 mutations are predisposed to breast cancers owing to genome instability. To determine the functions of BRCA2, the human protein was purified. It was found to bind selectively to single-stranded DNA (ssDNA), and to ssDNA in tailed duplexes and replication fork structures. Monomeric and dimeric forms of BRCA2 were observed by EM. BRCA2 directed the binding of RAD51 recombinase to ssDNA, reduced the binding of RAD51 to duplex DNA and stimulated RAD51-mediated DNA strand exchange. These observations provide a molecular basis for the role of BRCA2 in the maintenance of genome stability.