Biochemical analysis of the N-terminal domain of human RAD54B

RAD54B mutations enhance the sensitivity of ovarian cancer cells to poly(ADP-ribose) polymerase (PARP) inhibitors

Synthetic lethal targeting of homologous recombination (HR)–deficient ovarian cancers (OvCas) with poly(ADP-ribose) polymerase inhibitors (PARPis) has attracted considerable attention. Olaparib was the first PARPi approved by the Food and Drug Administration, offering significant clinical benefits in BRCA1/2-deficient OvCas. However, only approximately 20% of OvCa patients harbor BRCA1/2 mutations. Given the shared roles that BRCA1/2 have with other HR regulators, alterations in HR genes may also contribute to “BRCAness profiles” in OvCas. RAD54B has been considered a key player in HR repair, although its roles and therapeutic potential in cancers need further investigation. Here, we identified 22 frequently mutated HR genes by whole-exome sequencing of OvCa tissues from 82 patients. To our surprise, 7.3% of patients were found to harbor mutations of RAD54B, the third-highest mutated gene among patients. We determined that RAD54B-mutated tumor tissues harbored more DNA double-strand breaks than normal tissues. Additionally, we found that RAD54B knockdown inhibited HR repair, enhanced sensitivities of OvCa cells with increased DNA double-strand breaks to olaparib, and induced apoptosis. Enhanced inhibitory effects of olaparib on the growth of ES2 xenograft tumors were further demonstrated by RAD54B knockdown. Finally, we show that restoration with wildtype RAD54B rather than RAD54B^N593S and RAD54B^H219Y, identified in patients, abolished the effects of RAD54B knockdown, indicating these RAD54B mutants probably malfunctioned in HR repair. Our investigations may offer insight into the contributions of RAD54B mutations to synthetic lethality with olaparib treatment in OvCas, enrich the gene list for “HR deficiency scoring,” and expand the applications of PARPis.

Rad51 filament dynamics and its antagonistic modulators

Rad51 recombinase is the central player in homologous recombination, the faithful repair pathway for double-strand breaks and key event during meiosis. Rad51 forms nucleoprotein filaments on single-stranded DNA, exposed by a double-strand break. These filaments are responsible for homology search and strand invasion, which lead to homology-directed repair. Due to its central roles in DNA repair and genome stability, Rad51 is modulated by multiple factors and post-translational modifications. In this review, we summarize our current understanding of the dynamics of Rad51 filaments, the roles of other factors and their modes of action in modulating key stages of Rad51 filaments: formation, stability and disassembly.

Inhibition of RAD54B suppresses proliferation and promotes apoptosis in hepatoma cells

RAD54 homolog B is a central motor protein of homologous recombination (HR), which plays an important role in the development and evolution of human cancer. Recent studies have indicated that RAD54B is aberrantly expressed in a variety of cancers and exhibits numerous biological functions, including participating in the repair of DNA double strand breaks. However, the expression and function of RAD54B in liver cancer have not yet been reported. The aim of the present study was to investigate the expression of RAD54B and elucidate its role in liver cancer cell lines by inhibiting RAD54B using a lentivirus-mediated shRNA interference system. We also assessed the effect of RAD54B on cell proliferation, colony formation, cell cycle distribution and cell apoptosis in BEL-7404 and SMMC-7721 cell lines using shRAD54B or shCtrl transfection. Furthermore, we analyzed the relationship between the expression of RAD54B protein, as measured by immunohistochemical staining, and the prognosis of patients with hepatoma. We found that RAD54B was highly expressed in liver cancer cell lines compared with the normal hepatic cell line LO2. Similarly, positive expression of RAD54B, which is associated with poor prognosis, was also observed in 52/83 samples of liver cancer tissue. Additionally, RAD54B downregulation significantly inhibited cell proliferation and colony formation, while also inducing G1/S cell cycle arrest and apoptosis in BEL-7404 and SMMC-7721 cells. These results indicated that RAD54B has oncogenic properties, and may be a potential treatment target for liver cancer patients.

Biochemical attributes of mitotic and meiotic presynaptic complexes

Homologous recombination (HR) is a universally conserved mechanism used to maintain genomic integrity. In eukaryotes, HR is used to repair the spontaneous double strand breaks (DSBs) that arise during mitotic growth, and the programmed DSBs that form during meiosis. The mechanisms that govern mitotic and meiotic HR share many similarities, however, there are also several key differences, which reflect the unique attributes of each process. For instance, even though many of the proteins involved in mitotic and meiotic HR are the same, DNA target specificity is not: mitotic DSBs are repaired primarily using the sister chromatid as a template, whereas meiotic DBSs are repaired primarily through targeting of the homologous chromosome. These changes in template specificity are induced by expression of meiosis-specific HR proteins, down-regulation of mitotic HR proteins, and the formation of meiosis-specific chromosomal structures. Here, we compare and contrast the biochemical properties of key recombination intermediates formed during the pre-synapsis phase of mitotic and meiotic HR. Throughout, we try to highlight unanswered questions that will shape our understanding of how homologous recombination contributes to human cancer biology and sexual reproduction.

Genome-wide analyses of non-syndromic cleft lip with palate identify 14 novel loci and genetic heterogeneity

Non-syndromic cleft lip with palate (NSCLP) is the most serious sub-phenotype of non-syndromic orofacial clefts (NSOFC), which are the most common craniofacial birth defects in humans. Here we conduct a GWAS of NSCLP with multiple independent replications, totalling 7,404 NSOFC cases and 16,059 controls from several ethnicities, to identify new NSCLP risk loci, and explore the genetic heterogeneity between sub-phenotypes of NSOFC. We identify 41 SNPs within 26 loci that achieve genome-wide significance, 14 of which are novel (RAD54B, TMEM19, KRT18, WNT9B, GSC/DICER1, PTCH1, RPS26, OFCC1/TFAP2A, TAF1B, FGF10, MSX1, LINC00640, FGFR1 and SPRY1). These 26 loci collectively account for 10.94% of the heritability for NSCLP in Chinese population. We find evidence of genetic heterogeneity between the sub-phenotypes of NSOFC and among different populations. This study substantially increases the number of genetic susceptibility loci for NSCLP and provides important insights into the genetic aetiology of this common craniofacial malformation.

Non-syndromic cleft lip with palate is a common birth defect of unknown aetiology. Here, the authors discover 14 new genes associated with this condition, and show genetic heterogeneity in this and other non-syndromic orofacial clefting disorders.

High RAD54B expression: an independent predictor of postoperative distant recurrence in colorectal cancer patients

We recently reported a specific mechanism that RAD54B, an important factor in homologous recombination, promotes genomic instability via the degradation of p53 protein in vitro. However, clinical significance of RAD54Bin colorectal cancer (CRC) remains unclear. Thus we analyzed RAD54B geneexpression in CRC patients. Using the training set (n = 123), the optimal cut-off value for stratification was determined, and validated in another cohort (n = 89). Kaplan-Meier plots showed that distant recurrence free survival was significantly lesser in high RAD54B expression group compared with that of low expression group in both training (P = 0.0013) and validation (P = 0.024) set. Multivariate analysis using Cox proportional-hazards model showed that high RAD54B expression was an independent predictor in both training (hazard ratio, 4.31; 95% CI, 1.53-13.1; P = 0.0060) and validation (hazard ratio, 3.63; 95% CI, 1.23-10.7; P = 0.021) set. In addition, a negative significant correlation between RAD54B and CDKN1A, a target gene of p53, was partially confirmed, suggesting that RAD54B functions via the degradation of p53 protein even in clinical samples. This study first demonstrated RAD54B expression has potential to serve as a novel prognostic biomarker, particularly for distant recurrence in CRC patients.

Dbl2 Regulates Rad51 and DNA Joint Molecule Metabolism to Ensure Proper Meiotic Chromosome Segregation

To identify new proteins required for faithful meiotic chromosome segregation, we screened a Schizosaccharomyces pombe deletion mutant library and found that deletion of the dbl2 gene led to missegregation of chromosomes during meiosis. Analyses of both live and fixed cells showed that dbl2Δ mutant cells frequently failed to segregate homologous chromosomes to opposite poles during meiosis I. Removing Rec12 (Spo11 homolog) to eliminate meiotic DNA double-strand breaks (DSBs) suppressed the segregation defect in dbl2Δ cells, indicating that Dbl2 acts after the initiation of meiotic recombination. Analyses of DSBs and Holliday junctions revealed no significant defect in their formation or processing in dbl2Δ mutant cells, although some Rec12-dependent DNA joint molecules persisted late in meiosis. Failure to segregate chromosomes in the absence of Dbl2 correlated with persistent Rad51 foci, and deletion of rad51 or genes encoding Rad51 mediators also suppressed the segregation defect of dbl2Δ. Formation of foci of Fbh1, an F-box helicase that efficiently dismantles Rad51-DNA filaments, was impaired in dbl2Δ cells. Our results suggest that Dbl2 is a novel regulator of Fbh1 and thereby Rad51-dependent DSB repair required for proper meiotic chromosome segregation and viable sex cell formation. The wide conservation of these proteins suggests that our results apply to many species.

Meiosis produces haploid gametes from diploid precursor cells. This reduction of chromosome number is achieved by two successive divisions after only a single round of DNA replication. To identify novel regulators of meiosis, we screened a library of fission yeast deletion mutants and found that deletion of the dbl2 gene led to missegregation of chromosomes during meiosis. Analysis of live dbl2Δ cells by fluorescence microscopy showed that chromosomes frequently failed to segregate during the first meiotic division. Further cytological and biochemical analyses revealed that this segregation defect is due to persistent intermediates of DNA double-strand break repair, also called DNA joint molecules. Our results indicate that Dbl2 is required for formation of Fbh1 DNA helicase foci at the sites of DNA double-strand break repair in order to process DNA joint molecules and allow segregation of chromosomes during meiotic divisions. Our bioinformatics searches revealed that Dbl2 is highly conserved in fungi, animals and plants, suggesting that Dbl2 plays a similar role in other organisms–the formation of viable sex cells and healthy progeny.

The Overexpression of FEN1 and RAD54B May Act as Independent Prognostic Factors of Lung Adenocarcinoma

Synthetic lethality arises when a combination of mutations in two or more genes leads to cell death. However, the prognostic role of concordant overexpression of synthetic lethality genes in protein level rather than a combination of mutations is not clear. In this study, we explore the prognostic role of combined overexpression of paired genes in lung adenocarcinoma. We used immunohistochemical staining to investigate 24 paired genes in 93 lung adenocarcinoma patients and Kaplan-Meier analysis and Cox proportional hazards models to evaluate their prognostic roles. Among 24 paired genes, only FEN1 (Flap endonuclease 1) and RAD54B (RAD54 homolog B) were overexpressed in lung adenocarcinoma patients with poor prognosis. Patients with expression of both FEN1 and RAD54B were prone to have advanced nodal involvement and significantly poor prognosis (HR = 2.35, P = 0.0230). These results suggest that intensive follow up and targeted therapy might improve clinical outcome for patients who show expression of both FEN1 and RAD54B.

Human PSF concentrates DNA and stimulates duplex capture in DMC1-mediated homologous pairing

PSF is considered to have multiple functions in RNA processing, transcription and DNA repair by mitotic recombination. In the present study, we found that PSF is produced in spermatogonia, spermatocytes and spermatids, suggesting that PSF may also function in meiotic recombination. We tested the effect of PSF on homologous pairing by the meiosis-specific recombinase DMC1, and found that human PSF robustly stimulated it. PSF synergistically enhanced the formation of a synaptic complex containing DMC1, ssDNA and dsDNA during homologous pairing. The PSF-mediated DMC1 stimulation may be promoted by its DNA aggregation activity, which increases the local concentrations of ssDNA and dsDNA for homologous pairing by DMC1. These results suggested that PSF may function as an activator for the meiosis-specific recombinase DMC1 in higher eukaryotes.

Functions of the Snf2/Swi2 family Rad54 motor protein in homologous recombination

Homologous recombination is a central pathway to maintain genomic stability and is involved in the repair of DNA damage and replication fork support, as well as accurate chromosome segregation during meiosis. Rad54 is a dsDNA-dependent ATPase of the Snf2/Swi2 family of SF2 helicases, although Rad54 lacks classical helicase activity and cannot carry out the strand displacement reactions typical for DNA helicases. Rad54 is a potent and processive motor protein that translocates on dsDNA, potentially executing several functions in recombinational DNA repair. Rad54 acts in concert with Rad51, the central protein of recombination that performs the key reactions of homology search and DNA strand invasion. Here, we will review the role of the Rad54 protein in homologous recombination with an emphasis on mechanistic studies with the yeast and human enzymes. We will discuss how these results relate to in vivo functions of Rad54 during homologous recombination in somatic cells and during meiosis. This article is part of a Special Issue entitled: Snf2/Swi2 ATPase structure and function.