Coupling crossover and synaptonemal complex in meiosis

DNA cytosine methyltransferases differentially regulate genome-wide hypermutation and interhomolog recombination in Trichoderma reesei meiosis

Trichoderma reesei is an economically important enzyme producer with several unique meiotic features. spo11, the initiator of meiotic double-strand breaks (DSBs) in most sexual eukaryotes, is dispensable for T. reesei meiosis. T. reesei lacks the meiosis-specific recombinase Dmc1. Rad51 and Sae2, the activator of the Mre11 endonuclease complex, promote DSB repair and chromosome synapsis in wild-type and spo11Δ meiosis. DNA methyltransferases (DNMTs) perform multiple tasks in meiosis. Three DNMT genes (rid1, dim2 and dimX) differentially regulate genome-wide cytosine methylation and C:G-to-T:A hypermutations in different chromosomal regions. We have identified two types of DSBs: type I DSBs require spo11 or rid1 for initiation, whereas type II DSBs do not rely on spo11 and rid1 for initiation. rid1 (but not dim2) is essential for Rad51-mediated DSB repair and normal meiosis. rid1 and rad51 exhibit a locus heterogeneity (LH) relationship, in which LH-associated proteins often regulate interconnectivity in protein interaction networks. This LH relationship can be suppressed by deleting dim2 in a haploid rid1Δ (but not rad51Δ) parental strain, indicating that dim2 and rid1 share a redundant function that acts earlier than rad51 during early meiosis. In conclusion, our studies provide the first evidence of the involvement of DNMTs during meiotic initiation and recombination.

Chromatin binding by HORMAD proteins regulates meiotic recombination initiation

The meiotic chromosome axis coordinates chromosome organization and interhomolog recombination in meiotic prophase and is essential for fertility. In S. cerevisiae, the HORMAD protein Hop1 mediates the enrichment of axis proteins at nucleosome-rich islands through a central chromatin-binding region (CBR). Here, we use cryoelectron microscopy to show that the Hop1 CBR directly recognizes bent nucleosomal DNA through a composite interface in its PHD and winged helix-turn-helix domains. Targeted disruption of the Hop1 CBR-nucleosome interface causes a localized reduction of axis protein binding and meiotic DNA double-strand breaks (DSBs) in axis islands and leads to defects in chromosome synapsis. Synthetic effects with mutants of the Hop1 regulator Pch2 suggest that nucleosome binding delays a conformational switch in Hop1 from a DSB-promoting, Pch2-inaccessible state to a DSB-inactive, Pch2-accessible state to regulate the extent of meiotic DSB formation. Phylogenetic analyses of meiotic HORMADs reveal an ancient origin of the CBR, suggesting that the mechanisms we uncover are broadly conserved.

DNA double-strand break genetic variants in patients with premature ovarian insufficiency

Premature ovarian insufficiency (POI) is a clinically heterogeneous disease that may seriously affect the physical and mental health of women of reproductive age. POI primarily manifests as ovarian function decline and endocrine disorders in women prior to age 40 and is an established cause of female infertility. It is crucial to elucidate the causative factors of POI, not only to expand the understanding of ovarian physiology, but also to provide genetic counselling and fertility guidance to affected patients. Factors leading to POI are multifaceted with genetic factors accounting for 7% to 30%. In recent years, an increasing number of DNA damage-repair-related genes have been linked with the occurrence of POI. Among them, DNA double-strand breaks (DSBs), one of the most damaging to DNA, and its main repair methods including homologous recombination (HR) and non-homologous end joining (NHEJ) are of particular interest. Numerous genes are known to be involved in the regulation of programmed DSB formation and damage repair. The abnormal expression of several genes have been shown to trigger defects in the overall repair pathway and induce POI and other diseases. This review summarises the DSB-related genes that may contribute to the development of POI and their potential regulatory mechanisms, which will help to further establish role of DSB in the pathogenesis of POI and provide theoretical guidance for the study of the pathogenesis and clinical treatment of this disease.

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.

Acrylamide-induced meiotic arrest of spermatocytes in adolescent mice by triggering excessive DNA strand breaks: Potential therapeutic effects of resveratrol

Background: Baked carbohydrate-rich foods are the main source of acrylamide (AA) in the general population and are widely consumed by teenagers. Considering the crucial development of the reproductive system during puberty, the health risks posed by AA in adolescent males have raised public concern.Methods: In this study, we exposed 3-week-old male pubertal mice to AA for 4 weeks to evaluate its effect on spermatogenesis using computer-assisted sperm analysis (CASA) and historical analysis. Flow cytometric analysis and meiocyte spreading assay were conducted to assess meiosis in mice. The expression of meiosis-related proteins and double-strand break (DSB) proteins were evaluated by immunoblot analyses. Additionally, isolated spermatocytes were used to explore the role of resveratrol in AA-induced damages of meiosis.Results: Our results showed that AA decreased the testicular and epididymal indexes, reduced sperm count and motility, and induced morphological disruption of the testes in pubertal mice. Subsequent meiotic analysis revealed that AA increased the proportion of 4C spermatocytes and decreased the proportion of 1C spermatids. The expression levels of meiosis-related proteins (SYCP3, Cyclin A1 and CDK2) were downregulated, and signaling proteins (γH2AX, p-CHK2 and p-ATM) expression levels were upregulated in AA-treated mice testes. Similar expression patterns were observed in primary spermatocytes treated with AA and these effects were reversed significantly by resveratrol.Conclusions: Our results indicate that AA induces meiotic arrest via persistent activation of DSBs, which may contribute to AA-compromised spermatogenesis. Resveratrol could serve as a potential therapeutic agent against AA-induced meiotic toxicity. These data highlight the importance of natural product supplementation for treating AA-related reproductive toxicity.

Variations of wheat (Triticum aestivum L.) chromosomes caused by the 5A chromosomes with complex cytological structure

To study the effects of structural alterations of chromosomes caused by tandem repeats on the meiotic recombination, the wheat (Triticum aestivum L.) 5A chromosomes with different structure from ten wheat cultivars were used to investigate their meiotic recombination using non-denaturing fluorescence in situ hybridization (ND-FISH) technology. Fifteen cross combinations were carried out and they were divided into seven F₁ categories. The structural difference between the intercalary regions of the long arms of the two 5A chromosomes (5AL) in the F₁ categories III, VI, and VII was greater than that in the categories I and II, subsequently, the recombination frequencies in the distal regions of the 5AL arm in the progenies from the three categories were significantly lower than that from the categories I and II. For the two 5A chromosomes in the F₁ categories VI and VII, the structural differences in the distal regions of both of the two arms were greater than that in the categories IV and V. So, the recombination frequencies in the intercalary region of the 5AL arm in the progeny from the categories IV and V were higher than that in the progeny from the categories VI and VII. The breakage of 5A chromosome together with the 5A translocations and the breakage of some other chromosomes were observed in the progeny from the F₁ categories V, VI, and VII. These chromosomal variations were not observed in the progenies from the other four F₁ categories. In conclusion, the smaller structural difference between the 5A chromosomes in distal regions of the two arms resulted in a higher recombination frequency in interstitial region and vice versa. The 5A chromosome with complex cytological structure can be used to induce genetic variations of wheat genome.

Beyond Editing, CRISPR/Cas9 For Protein Localization: An Educational Primer for Use with "A dCas9-Based System Identifies a Central Role for Ctf19 in Kinetochore-Derived Suppression of Meiotic Recombination"

CRISPR/Cas9 has dramatically changed how we conduct genetic research, providing a tool for precise sequence editing. However, new applications of CRISPR/Cas9 have emerged that do not involve nuclease activity. In the accompanying article "A dCas9-Based System Identifies a Central Role for Ctf19 in Kinetochore-Derived Suppression of Meiotic Recombination", Kuhl and colleagues utilize a catalytically dead Cas9 to localize proteins at specific genomic locations. The authors seek to understand the role of kinetochore proteins in the suppression of meiotic recombination, a phenomenon that has been observed in centromere regions. By harnessing the power of CRISPR/Cas9 to bind specific genomic sequences, Kuhl et al. localized individual kinetochore proteins to areas of high meiotic recombination and assessed their role in suppression. This primer article provides undergraduate students with background information on chromosomes, meiosis, recombination and CRISPR/Cas9 to support their reading of the Kuhl et al. study. This primer is intended to help students and instructors navigate the study's experimental design, interpret the results, and appreciate the broader scope of meiotic recombination and CRISPR/Cas9. Questions are included to facilitate discussion of the study.