Two auxiliary factors promote Dmc1-driven DNA strand exchange via stepwise mechanisms

Positive and negative regulators of RAD51/DMC1 in homologous recombination and DNA replication

RAD51 recombinase plays a central role in homologous recombination (HR) by forming a nucleoprotein filament on single-stranded DNA (ssDNA) to catalyze homology search and strand exchange between the ssDNA and a homologous double-stranded DNA (dsDNA). The catalytic activity of RAD51 assembled on ssDNA is critical for the DNA-homology-mediated repair of DNA double-strand breaks in somatic and meiotic cells and restarting stalled replication forks during DNA replication. The RAD51-ssDNA complex also plays a structural role in protecting the regressed/reversed replication fork. Two types of regulators control RAD51 filament formation, stability, and dynamics, namely positive regulators, including mediators, and negative regulators, so-called remodelers. The appropriate balance of action by the two regulators assures genome stability. This review describes the roles of positive and negative RAD51 regulators in HR and DNA replication and its meiosis-specific homolog DMC1 in meiotic recombination. We also provide future study directions for a comprehensive understanding of RAD51/DMC1-mediated regulation in maintaining and inheriting genome integrity.

Meiosis: Dances Between Homologs

The raison d'être of meiosis is shuffling of genetic information via Mendelian segregation and, within individual chromosomes, by DNA crossing-over. These outcomes are enabled by a complex cellular program in which interactions between homologous chromosomes play a central role. We first provide a background regarding the basic principles of this program. We then summarize the current understanding of the DNA events of recombination and of three processes that involve whole chromosomes: homolog pairing, crossover interference, and chiasma maturation. All of these processes are implemented by direct physical interaction of recombination complexes with underlying chromosome structures. Finally, we present convergent lines of evidence that the meiotic program may have evolved by coupling of this interaction to late-stage mitotic chromosome morphogenesis.

Identification of a missense variant of MND1 in meiotic arrest and non-obstructive azoospermia

Meiotic arrest is a common pathologic phenotype of non-obstructive azoospermia (NOA), yet its genetic causes require further investigation. Meiotic nuclear divisions 1 (MND1) has been proved to be indispensable for meiotic recombination in many species. To date, only one variant of MND1 has been reported associated with primary ovarian insufficiency (POI), yet there has been no report of variants in MND1 associated with NOA. Herein, we identified a rare homozygous missense variant (NM_032117:c.G507C:p.W169C) of MND1 in two NOA-affected patients from one Chinese family. Histological analysis and immunohistochemistry demonstrated meiotic arrest at zygotene-like stage in prophase I and lack of spermatozoa in the proband's seminiferous tubules. In silico modeling demonstrated that this variant might cause possible conformational change in the leucine zippers 3 with capping helices (LZ3wCH) domain of MND1-HOP2 complex. Altogether, our study demonstrated that the MND1 variant (c.G507C) is likely responsible for human meiotic arrest and NOA. And our study provides new insights into the genetic etiology of NOA and mechanisms of homologous recombination repair in male meiosis.

Hop2-Mnd1 and Swi5-Sfr1 stimulate Dmc1 filament assembly using distinct mechanisms

In meiosis, Dmc1 recombinase and the general recombinase Rad51 are responsible for pairing homologous chromosomes and exchanging strands. Fission yeast (Schizosaccharomyces pombe) Swi5-Sfr1 and Hop2-Mnd1 stimulate Dmc1-driven recombination, but the stimulation mechanism is unclear. Using single-molecule fluorescence resonance energy transfer (smFRET) and tethered particle motion (TPM) experiments, we showed that Hop2-Mnd1 and Swi5-Sfr1 individually enhance Dmc1 filament assembly on single-stranded DNA (ssDNA) and adding both proteins together allows further stimulation. FRET analysis showed that Hop2-Mnd1 enhances the binding rate of Dmc1 while Swi5-Sfr1 specifically reduces the dissociation rate during the nucleation, about 2-fold. In the presence of Hop2-Mnd1, the nucleation time of Dmc1 filaments shortens, and doubling the ss/double-stranded DNA (ss/dsDNA) junctions of DNA substrates reduces the nucleation times in half. Order of addition experiments confirmed that Hop2-Mnd1 binds on DNA to recruit and stimulate Dmc1 nucleation at the ss/dsDNA junction. Our studies directly support the molecular basis of how Hop2-Mnd1 and Swi5-Sfr1 act on different steps during the Dmc1 filament assembly. DNA binding of these accessory proteins and nucleation preferences of recombinases thus dictate how their regulation can take place.

Synaptonemal Complex in Human Biology and Disease

The synaptonemal complex (SC) is a meiosis-specific multiprotein complex that forms between homologous chromosomes during prophase of meiosis I. Upon assembly, the SC mediates the synapses of the homologous chromosomes, leading to the formation of bivalents, and physically supports the formation of programmed double-strand breaks (DSBs) and their subsequent repair and maturation into crossovers (COs), which are essential for genome haploidization. Defects in the assembly of the SC or in the function of the associated meiotic recombination machinery can lead to meiotic arrest and human infertility. The majority of proteins and complexes involved in these processes are exclusively expressed during meiosis or harbor meiosis-specific subunits, although some have dual functions in somatic DNA repair and meiosis. Consistent with their functions, aberrant expression and malfunctioning of these genes have been associated with cancer development. In this review, we focus on the significance of the SC and their meiotic-associated proteins in human fertility, as well as how human genetic variants encoding for these proteins affect the meiotic process and contribute to infertility and cancer development.

The Hop2-Mnd1 Complex and Its Regulation of Homologous Recombination

Homologous recombination (HR) is essential for meiosis in most sexually reproducing organisms, where it is induced upon entry into meiotic prophase. Meiotic HR is conducted by the collaborative effort of proteins responsible for DNA double-strand break repair and those produced specifically during meiosis. The Hop2-Mnd1 complex was originally identified as a meiosis-specific factor that is indispensable for successful meiosis in budding yeast. Later, it was found that Hop2-Mnd1 is conserved from yeasts to humans, playing essential roles in meiosis. Accumulating evidence suggests that Hop2-Mnd1 promotes RecA-like recombinases towards homology search/strand exchange. This review summarizes studies on the mechanism of the Hop2-Mnd1 complex in promoting HR and beyond.

Genomic targets of HOP2 are enriched for features found at recombination hotspots

HOP2 is a conserved protein that plays a positive role in homologous chromosome pairing and a separable role in preventing illegitimate connections between nonhomologous chromosome regions during meiosis. We employed ChIP-seq to discover that Arabidopsis HOP2 binds along the length of all chromosomes, except for centromeric and nucleolar organizer regions, and no binding sites were detected in the organelle genomes. A large number of reads were assigned to the HOP2 locus itself, yet TAIL-PCR and SNP analysis of the aligned sequences indicate that many of these reads originate from the transforming T-DNA, supporting the role of HOP2 in preventing nonhomologous exchanges. The 292 ChIP-seq peaks are largely found in promoter regions and downstream from genes, paralleling the distribution of recombination hotspots, and motif analysis revealed that there are several conserved sequences that are also enriched at crossover sites. We conducted coimmunoprecipitation of HOP2 followed by LC-MS/MS and found enrichment for several proteins, including some histone variants and modifications that are also known to be associated with recombination hotspots. We propose that HOP2 may be directed to chromatin motifs near double strand breaks, where homology checks are proposed to occur.

Genetic landscape of a large cohort of Primary Ovarian Insufficiency: New genes and pathways and implications for personalized medicine

Background

Primary Ovarian Insufficiency (POI), a public health problem, affects 1-3.7% of women under 40 yielding infertility and a shorter lifespan. Most causes are unknown. Recently, genetic causes were identified, mostly in single families. We studied an unprecedented large cohort of POI to unravel its molecular pathophysiology.

Methods

375 patients with 70 families were studied using targeted (88 genes) or whole exome sequencing with pathogenic/likely-pathogenic variant selection. Mitomycin-induced chromosome breakages were studied in patients’ lymphocytes if necessary.

Findings

A high-yield of 29.3% supports a clinical genetic diagnosis of POI. In addition, we found strong evidence of pathogenicity for nine genes not previously related to a Mendelian phenotype or POI: ELAVL2, NLRP11, CENPE, SPATA33, CCDC150, CCDC185, including DNA repair genes: C17orf53(HROB), HELQ, SWI5 yielding high chromosomal fragility. We confirmed the causal role of BRCA2, FANCM, BNC1, ERCC6, MSH4, BMPR1A, BMPR1B, BMPR2, ESR2, CAV1, SPIDR, RCBTB1 and ATG7 previously reported in isolated patients/families. In 8.5% of cases, POI is the only symptom of a multi-organ genetic disease. New pathways were identified: NF-kB, post-translational regulation, and mitophagy (mitochondrial autophagy), providing future therapeutic targets. Three new genes have been shown to affect the age of natural menopause supporting a genetic link.

Interpretation

We have developed high-performance genetic diagnostic of POI, dissecting the molecular pathogenesis of POI and enabling personalized medicine to i) prevent/cure comorbidities for tumour/cancer susceptibility genes that could affect life-expectancy (37.4% of cases), or for genetically-revealed syndromic POI (8.5% of cases), ii) predict residual ovarian reserve (60.5% of cases). Genetic diagnosis could help to identify patients who may benefit from the promising in vitro activation-IVA technique in the near future, greatly improving its success in treating infertility.

Funding

Université Paris Saclay, Agence Nationale de Biomédecine.

MiR-4284 inhibits sensitivity to paclitaxel in human ovarian carcinoma SKOV3ip1 and HeyA8 cells by targeting DMC1

An increasing number of studies have confirmed that microRNAs (miRNAs) are involved in various biological processes, including tumor growth and drug resistance. MiR-4284 has been proved to be abnormally regulated in several cancers, but the function of miR-4284 in ovarian carcinoma (OC) is unclear. Paclitaxel resistance is a key obstacle in OC treatment. Here, the role of miR-4284 in cell sensitivity to paclitaxel in OC was investigated. Two OC cell lines (SKOV3ip1 and HeyA8) were utilized for the establishment of paclitaxel-resistant cell lines. Reverse transcription-quantitative PCR (RT-qPCR) was applied to analyze the levels of miR-4284 and potential mRNAs in OC cell lines. Western blotting was performed to evaluate the levels of DNA meiotic recombinase 1 (DMC1) protein and cell cycle-associated proteins. Identification of the relationship between miR-4284 and DMC1 was achieved by luciferase reporter assay. CCK-8 and flow cytometry assays were utilized for evaluating the impact of miR-4284 on the malignant characteristics of paclitaxel-resistant OC cells. MiR-4284 was upregulated in paclitaxel-resistant OC cell lines and correlated with an adverse prognosis in OC patients. Depletion of miR-4284 suppressed cell proliferation and cell cycle progression of paclitaxel-resistant OC. MiR-4284 targeted DMC1 which was downregulated in paclitaxel-resistant cells and reversed the inhibitory influence of miR-4284 silencing on the malignant characters of paclitaxel-resistant OC cells. MiR-4284 targets DMC1 to suppress sensitivity to paclitaxel in human OC cells.

Biochemical properties of fission yeast homologous recombination enzymes

Homologous recombination (HR) is a universal phenomenon conserved from viruses to humans. The mechanisms of HR are essentially the same in humans and simple unicellular eukaryotes like yeast. Two highly diverged yeast species, Saccharomyces cerevisiae and Schizosaccharomyces pombe, have proven exceptionally useful in understanding the fundamental mechanisms of eukaryotic HR by serving as a source for unique biological insights and also complementing each other. Here, we will review the features of S. pombe HR mechanisms in comparison to S. cerevisiae and other model organisms. Particular emphasis will be put on the biochemical characterization of HR mechanisms uncovered using S. pombe proteins.

A novel motif of Rad51 serves as an interaction hub for recombination auxiliary factors

Homologous recombination (HR) is essential for maintaining genome stability. Although Rad51 is the key protein that drives HR, multiple auxiliary factors interact with Rad51 to potentiate its activity. Here, we present an interdisciplinary characterization of the interactions between Rad51 and these factors. Through structural analysis, we identified an evolutionarily conserved acidic patch of Rad51. The neutralization of this patch completely abolished recombinational DNA repair due to defects in the recruitment of Rad51 to DNA damage sites. This acidic patch was found to be important for the interaction with Rad55-Rad57 and essential for the interaction with Rad52. Furthermore, biochemical reconstitutions demonstrated that neutralization of this acidic patch also impaired the interaction with Rad54, indicating that a single motif is important for the interaction with multiple auxiliary factors. We propose that this patch is a fundamental motif that facilitates interactions with auxiliary factors and is therefore essential for recombinational DNA repair.