Genetic control of meiosis surveillance mechanisms in mammals

SIRT7 links H3K36ac epigenetic regulation with genome maintenance in the aging mouse testis

Reproductive aging is an increasing health concern affecting family planning and overall well-being. While extensively studied in females, the mechanisms driving male reproductive aging remain largely unexamined. Here we found that mammalian Sirtuin 7 (SIRT7) sustains spermatogenesis in an age-dependent manner through the control of histone 3 lysine 36 acetylation (H3K36ac). SIRT7 deficiency in mice resulted in increased levels of H3K36ac in spermatogonia and spermatocytes. In a germ cell line, SIRT7 deficiency disrupted nucleosome stability and increased vulnerability to genotoxic stress. Importantly, undifferentiated spermatogonia, which are required for continuous sperm production, decreased prematurely in Sirt7 -/- mice and showed genome damage accumulation. These changes were concurrent with age-dependent defects in homologous chromosome synapsis and partial meiotic arrest. Taken together, our results indicate that SIRT7 connects H3K36ac epigenetic regulation to long-term genome stability in male germ cells, ensuring steady-state spermatogenesis during the lengthy male reproductive lifespan.

Intercellular bridges are essential for transposon repression and meiosis in the male germline

Germ cell connectivity via intercellular bridges is a widely conserved feature across metazoans. However, its functional significance is poorly understood. Intercellular bridges are essential for fertility in male mice as genetic ablation of a critical bridge component, TEX14, causes spermatogenic failure, but the underlying reasons are unknown. Here we utilized a Tex14 hypomorph with reduced intercellular bridges along with Tex14-null mice that completely lack bridges to examine the roles of germ cell connectivity during spermatogenesis. We report that in males deficient for TEX14 and intercellular bridges, germ cells fail to complete meiotic DNA replication, synapsis and meiotic double-strand break repair. They also derepress retrotransposons and accumulate retrotransposon-encoded proteins during meiosis. Single-cell RNA-sequencing confirms sharing of transcripts between wild-type spermatids and demonstrates its partial attenuation in Tex14 hypomorphs, indicating that intercellular bridges enable cytoplasmic exchange between connected germ cells in testes. Our findings suggest that regulation of meiosis is non-cell-intrinsic and inform a model in which intercellular bridges influence critical meiotic events and protect germline genome integrity during spermatogenesis.

Transcriptional Integration of Meiotic Prophase I Progression and Early Oocyte Differentiation

Female reproductive senescence results from the regulated depletion of a finite pool of oocytes called the ovarian reserve. This pool of oocytes is initially established during fetal development, but the oocytes that comprise it must remain quiescent for decades until they are activated during maturation in adulthood. In order for developmentally competent oocytes to populate the ovarian reserve they must successfully initiate both meiosis and oogenesis. As the factors that regulate the timing and fidelity of these early events remain elusive, we assessed the precise function and timing of the transcriptional regulator TAF4b during meiotic prophase I progression in mouse fetal oocytes. Compared to matched controls, E14.5 Taf4b-deficient oocytes enter meiosis I in a timely manner however, their subsequent progression through the pachytene-to-diplotene transition of meiotic prophase I is compromised. Moreover, this disruption of meiotic progression is associated with the reduced ability of Taf4b-deficient oocytes to repair double-strand DNA breaks. Transcriptional profiling of Taf4b-deficient oocytes reveals that between E16.5 and E18.5 these oocytes fail to coordinate the reduction of meiotic gene expression and the induction of oocyte differentiation genes. These studies reveal that TAF4b promotes the formation of the ovarian reserve in part by orchestrating the timely transition to meiosis I arrest and oocyte differentiation, which are often perceived as separate events.

Chemically induced proximity reveals a Piezo-dependent meiotic checkpoint at the oocyte nuclear envelope

Sexual reproduction relies on robust quality control during meiosis. Assembly of the synaptonemal complex between homologous chromosomes (synapsis) regulates meiotic recombination and is crucial for accurate chromosome segregation in most eukaryotes. Synapsis defects can trigger cell cycle delays and, in some cases, apoptosis. We developed and deployed a chemically induced proximity system to identify key elements of this quality control pathway in Caenorhabditis elegans. Persistence of the polo-like kinase PLK-2 at pairing centers-specialized chromosome regions that interact with the nuclear envelope-induced apoptosis of oocytes in response to phosphorylation and destabilization of the nuclear lamina. Unexpectedly, the Piezo1/PEZO-1 channel localized to the nuclear envelope and was required to transduce this signal to promote apoptosis in maturing oocytes.

Comparative Analysis of Fluorescence In Situ Hybridization and Next-Generation Sequencing in Sperm Evaluation: Implications for Preimplantation Genetic Testing and Male Infertility

Pre-implantation genetic testing (PGT) is a crucial process for selecting embryos created through assisted reproductive technology (ART). Couples with chromosomal rearrangements, infertility, recurrent miscarriages, advanced maternal age, known single-gene disorders, a family history of genetic conditions, previously affected pregnancies, poor embryo quality, or congenital anomalies may be candidates for PGT. Preimplantation genetic testing for aneuploidies (PGT-A) enables the selection and transfer of euploid embryos, significantly enhancing implantation rates in assisted reproduction. Fluorescence in situ hybridization (FISH) is the preferred method for analyzing biopsied cells to identify these abnormalities. While FISH is a well-established method for identifying sperm aneuploidy, NGS offers a more comprehensive assessment of genetic material, potentially enhancing our understanding of male infertility. Chromosomal abnormalities, arising during meiosis, can lead to aneuploid sperm, which may hinder embryo implantation and increase miscarriage rates. This review provides a comparative analysis of fluorescence in situ hybridization (FISH) and next-generation sequencing (NGS) in sperm evaluations, focusing on their implications for preimplantation genetic testing. This analysis explores the strengths and limitations of FISH and NGS, aiming to elucidate their roles in improving ART outcomes and reducing the risk of genetic disorders in offspring. Ultimately, the findings will inform best practices in sperm evaluations and preimplantation genetic testing strategies.

Replication protein-A, RPA, plays a pivotal role in the maintenance of recombination checkpoint in yeast meiosis

DNA double-strand breaks (DSBs) activate DNA damage responses (DDRs) in both mitotic and meiotic cells. A single-stranded DNA (ssDNA) binding protein, Replication protein-A (RPA) binds to the ssDNA formed at DSBs to activate ATR/Mec1 kinase for the response. Meiotic DSBs induce homologous recombination monitored by a meiotic DDR called the recombination checkpoint that blocks the pachytene exit in meiotic prophase I. In this study, we further characterized the essential role of RPA in the maintenance of the recombination checkpoint during Saccharomyces cerevisiae meiosis. The depletion of an RPA subunit, Rfa1, in a recombination-defective dmc1 mutant, fully alleviates the pachytene arrest with the persistent unrepaired DSBs. RPA depletion decreases the activity of a meiosis-specific CHK2 homolog, Mek1 kinase, which in turn activates the Ndt80 transcriptional regulator for pachytene exit. These support the idea that RPA is a sensor of ssDNAs for the activation of meiotic DDR. Rfa1 depletion also accelerates the prophase I delay in the zip1 mutant defective in both chromosome synapsis and the recombination, consistent with the notion that the accumulation of ssDNAs rather than defective synapsis triggers prophase I delay in the zip1 mutant.

Gene expression programs in mammalian spermatogenesis

Mammalian spermatogenesis, probably the most complex of all cellular developmental processes, is an ideal model both for studying the specific mechanism of gametogenesis and for understanding the basic rules governing all developmental processes, as it entails both cell type-specific and housekeeping molecular processes. Spermatogenesis can be viewed as a mission with many tasks to accomplish, and its success is genetically programmed and ensured by the collaboration of a large number of genes. Here, I present an overview of mammalian spermatogenesis and the mechanisms underlying each step in the process, covering the cellular and molecular activities that occur at each developmental stage and emphasizing their gene regulation in light of recent studies.

Novel MEI1 mutations cause chromosomal and DNA methylation abnormalities leading to embryonic arrest and implantation failure

This study presents a case of a female infertile patient suffering from embryonic arrest and recurrent implantation failure. The primary objective was to assess the copy number variations (CNVs) and DNA methylation of her embryos. Genetic diagnosis was conducted by whole-exome sequencing and validated through Sanger sequencing. CNV evaluation of two cleavage stage embryos was performed using whole-genome sequencing, while DNA methylation and CNV assessment of two blastocysts were carried out using whole-genome bisulfite sequencing. We identified two novel pathogenic frameshift variants in the MEI1 gene (NM_152513.3, c.3002delC, c.2264_2268 + 11delGTGAGGTATGGACCAC) in the proband. These two variants were inherited from her heterozygous parents, consistent with autosomal recessive genetic transmission. Notably, two Day 3 embryos and two Day 6 blastocysts were all aneuploid, with numerous monosomy and trisomy events. Moreover, global methylation levels greatly deviated from the optimized window of 0.25-0.27, measuring 0.344 and 0.168 for the respective blastocysts. This study expands the mutational spectrum of MEI1 and is the first to document both aneuploidy and abnormal methylation levels in embryos from a MEI1-affected female patient presenting with embryonic arrest. Given that females affected by MEI1 mutations might experience either embryonic arrest or monospermic androgenetic hydatidiform moles due to the extrusion of all maternal chromosomes, the genetic makeup of the arrested embryos of MEI1 patients provides important clues for understanding the different disease mechanisms of the two phenotypes.

Chromatin diminution as a tool to study some biological problems

This work reveals the opportunities to obtain additional information about some biological problems through studying species that possess chromatin diminution. A brief review of the hypothesized biological significance of chromatin diminution is discussed. This article analyzes the biological role of chromatin diminution as it relates to the C-value enigma. It is proposed to consider chromatin diminution as a universal mechanism of genome reduction, reducing the frequency of recombination events in the genome, which leads to specialization and adaptation of the species to more narrow environmental conditions. A hypothesis suggesting the role of non-coding DNA in homologous recombination in eukaryotes is proposed. Cyclopskolensis Lilljeborg, 1901 (Copepoda, Crustacea) is proposed as a model species for studying the mechanisms of transformation of the chromosomes and interphase nuclei structure of somatic line cells due to chromatin diminution. Chromatin diminution in copepods is considered as a stage of irreversible differentiation of embryonic cells during ontogenesis. The process of speciation in cyclopoids with chromatin diminution is considered.

The crosstalk between telomeres and DNA repair mechanisms: an overview to mammalian somatic cells, germ cells, and preimplantation embryos

Telomeres are located at the ends of linear chromosomes and play a critical role in maintaining genomic stability by preventing premature activation of DNA repair mechanisms. Because of exposure to various genotoxic agents, telomeres can undergo shortening and genetic changes. In mammalian cells, the basic DNA repair mechanisms, including base excision repair, nucleotide excision repair, double-strand break repair, and mismatch repair, function in repairing potential damages in telomeres. If these damages are not repaired correctly in time, the unfavorable results such as apoptosis, cell cycle arrest, and cancerous transition may occur. During lifespan, mammalian somatic cells, male and female germ cells, and preimplantation embryos experience a number of telomeric damages. Herein, we comprehensively reviewed the crosstalk between telomeres and the DNA repair mechanisms in the somatic cells, germ cells, and embryos. Infertility development resulting from possible defects in this crosstalk is also discussed in the light of existing studies.

Dna2 removes toxic ssDNA-RPA filaments generated from meiotic recombination-associated DNA synthesis

During the repair of DNA double-strand breaks (DSBs), de novo synthesized DNA strands can displace the parental strand to generate single-strand DNAs (ssDNAs). Many programmed DSBs and thus many ssDNAs occur during meiosis. However, it is unclear how these ssDNAs are removed for the complete repair of meiotic DSBs. Here, we show that meiosis-specific depletion of Dna2 (dna2-md) results in an abundant accumulation of RPA and an expansion of RPA from DSBs to broader regions in Saccharomyces cerevisiae. As a result, DSB repair is defective and spores are inviable, although the levels of crossovers/non-crossovers seem to be unaffected. Furthermore, Dna2 induction at pachytene is highly effective in removing accumulated RPA and restoring spore viability. Moreover, the depletion of Pif1, an activator of polymerase δ required for meiotic recombination-associated DNA synthesis, and Pif1 inhibitor Mlh2 decreases and increases RPA accumulation in dna2-md, respectively. In addition, blocking DNA synthesis during meiotic recombination dramatically decreases RPA accumulation in dna2-md. Together, our findings show that meiotic DSB repair requires Dna2 to remove ssDNA-RPA filaments generated from meiotic recombination-associated DNA synthesis. Additionally, we showed that Dna2 also regulates DSB-independent RPA distribution.