Reprogramming of Meiotic Chromatin Architecture during Spermatogenesis.

Chromatin organization undergoes drastic reconfiguration during gametogenesis. However, the molecular reprogramming of three-dimensional chromatin structure in this process remains poorly understood for mammals, including primates. Here, we examined three-dimensional chromatin architecture during spermatogenesis in rhesus monkey using low-input Hi-C. Interestingly, we found that topologically associating domains (TADs) undergo dissolution and reestablishment in spermatogenesis. Strikingly, pachytene spermatocytes, where synapsis occurs, are strongly depleted for TADs despite their active transcription state but uniquely show highly refined local compartments that alternate between transcribing and non-transcribing regions (refined-A/B). Importantly, such chromatin organization is conserved in mouse, where it remains largely intact upon transcription inhibition. Instead, it is attenuated in mutant spermatocytes, where the synaptonemal complex failed to be established. Intriguingly, this is accompanied by the restoration of TADs, suggesting that the synaptonemal complex may restrict TADs and promote local compartments. Thus, these data revealed extensive reprogramming of higher-order meiotic chromatin architecture during mammalian gametogenesis.

M1AP interacts with the mammalian ZZS complex and promotes male meiotic recombination.

Following meiotic recombination, each pair of homologous chromosomes acquires at least one crossover, which ensures accurate chromosome segregation and allows reciprocal exchange of genetic information. Recombination failure often leads to meiotic arrest, impairing fertility, but the molecular basis of recombination remains elusive. Here, we report a homozygous M1AP splicing mutation (c.1074 + 2T > C) in patients with severe oligozoospermia owing to meiotic metaphase I arrest. The mutation abolishes M1AP foci on the chromosome axes, resulting in decreased recombination intermediates and crossovers in male mouse models. M1AP interacts with the mammalian ZZS (an acronym for yeast proteins Zip2-Zip4-Spo16) complex components, SHOC1, TEX11, and SPO16. M1AP localizes to chromosomal axes in a SPO16-dependent manner and colocalizes with TEX11. Ablation of M1AP does not alter SHOC1 localization but reduces the recruitment of TEX11 to recombination intermediates. M1AP shows cytoplasmic localization in fetal oocytes and is dispensable for fertility and crossover formation in female mice. Our study provides the first evidence that M1AP acts as a copartner of the ZZS complex to promote crossover formation and meiotic progression in males.

Homozygous mutations in C14orf39/SIX6OS1 cause non-obstructive azoospermia and premature ovarian insufficiency in humans.

Human infertility is a multifactorial disease that affects 8%-12% of reproductive-aged couples worldwide. However, the genetic causes of human infertility are still poorly understood. Synaptonemal complex (SC) is a conserved tripartite structure that holds homologous chromosomes together and plays an indispensable role in the meiotic progression. Here, we identified three homozygous mutations in the SC coding gene C14orf39/SIX6OS1 in infertile individuals from different ethnic populations by whole-exome sequencing (WES). These mutations include a frameshift mutation (c.204_205del [p.His68Glnfs∗2]) from a consanguineous Pakistani family with two males suffering from non-obstructive azoospermia (NOA) and one female diagnosed with premature ovarian insufficiency (POI) as well as a nonsense mutation (c.958G>T [p.Glu320∗]) and a splicing mutation (c.1180-3C>G) in two unrelated Chinese men (individual P3907 and individual P6032, respectively) with meiotic arrest. Mutations in C14orf39 resulted in truncated proteins that retained SYCE1 binding but exhibited impaired polycomplex formation between C14ORF39 and SYCE1. Further cytological analyses of meiosis in germ cells revealed that the affected familial males with the C14orf39 frameshift mutation displayed complete asynapsis between homologous chromosomes, while the affected Chinese men carrying the nonsense or splicing mutation showed incomplete synapsis. The phenotypes of NOA and POI in affected individuals were well recapitulated by Six6os1 mutant mice carrying an analogous mutation. Collectively, our findings in humans and mice highlight the conserved role of C14ORF39/SIX6OS1 in SC assembly and indicate that the homozygous mutations in C14orf39/SIX6OS1 described here are responsible for infertility of these affected individuals, thus expanding our understanding of the genetic basis of human infertility.

Nuclear translocation of MTL5 from cytoplasm requires its direct interaction with LIN9 and is essential for male meiosis and fertility.

Meiosis is essential for the generation of gametes and sexual reproduction, yet the factors and underlying mechanisms regulating meiotic progression remain largely unknown. Here, we showed that MTL5 translocates into nuclei of spermatocytes during zygotene-pachytene transition and ensures meiosis advances beyond pachytene stage. MTL5 shows strong interactions with MuvB core complex components, a well-known transcriptional complex regulating mitotic progression, and the zygotene-pachytene transition of MTL5 is mediated by its direct interaction with the component LIN9, through MTL5 C-terminal 443-475 residues. Male Mtl5c-mu/c-mu mice expressing the truncated MTL5 (p.Ser445Arg fs*3) that lacks the interaction with LIN9 and is detained in cytoplasm showed male infertility and spermatogenic arrest at pachytene stage, same as that of Mtl5 knockout mice, indicating that the interaction with LIN9 is essential for the nuclear translocation and function of MTL5 during meiosis. Our data demonstrated MTL5 translocates into nuclei during the zygotene-pachytene transition to initiate its function along with the MuvB core complex in pachytene spermatocytes, highlighting a new mechanism regulating the progression of male meiosis.

Identification of pathogenic mutations from nonobstructive azoospermia patients†.

It is estimated that approximately 25% of nonobstructive azoospermia (NOA) cases are caused by single genetic anomalies, including chromosomal aberrations and gene mutations. The identification of these mutations in NOA patients has always been a research hot spot in the area of human infertility. However, compared with more than 600 genes reported to be essential for fertility in mice, mutations in approximately 75 genes have been confirmed to be pathogenic in patients with male infertility, in which only 14 were identified from NOA patients. The small proportion suggested that there is much room to improve the methodology of mutation screening and functional verification. Fortunately, recent advances in whole exome sequencing and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas9 have greatly promoted research on the etiology of human infertility and made improvements possible. In this review, we have summarized the pathogenic mutations found in NOA patients and the efforts we have made to improve the efficiency of mutation screening from NOA patients and functional verification with the application of new technologies.

Biallelic HFM1 variants cause non-obstructive azoospermia with meiotic arrest in humans by impairing crossover formation to varying degrees.

STUDY QUESTION: Do variants in helicase for meiosis 1 (HFM1) account for male infertility in humans? SUMMARY ANSWER: Biallelic variants in HFM1 cause human male infertility owing to non-obstructive azoospermia (NOA) with impaired crossover formation and meiotic metaphase I (MMI) arrest. WHAT IS KNOWN ALREADY: HFM1 encodes an evolutionarily conserved DNA helicase that is essential for crossover formation and completion of meiosis. The null mutants of Hfm1 or its ortholog in multiple organisms displayed spermatogenic arrest at the MMI owing to deficiencies in synapsis and severe defects in crossover formation. Although HFM1 variants were found in infertile men with azoospermia or oligozoospermia, the causal relationship has not yet been established with functional evidence. STUDY DESIGN, SIZE, DURATION: A Pakistani family, having two infertile brothers born to consanguineous parents, and three unrelated Chinese men diagnosed with NOA were recruited for pathogenic variants screening. PARTICIPANTS/MATERIALS, SETTING, METHODS: All the patients were diagnosed with idiopathic NOA and, for the Chinese patients, meiotic defects were confirmed by histological analyses and/or immunofluorescence staining on testicular sections. Exome sequencing and subsequent bioinformatic analyses were performed to screen for candidate pathogenic variants. The pathogenicity of identified variants was assessed and studied in vivo in mice carrying the equivalent mutations. MAIN RESULTS AND THE ROLE OF CHANCE: Six variants (homozygous or compound heterozygous) in HFM1 were identified in the three Chinese patients with NOA and two brothers with NOA from the Pakistani family. Testicular histological analysis revealed that spermatogenesis is arrested at MMI in patients carrying the variants. Mice modeling the HFM1 variants identified in patients recapitulated the meiotic defects of patients, confirming the pathogenicity of the identified variants. These Hfm1 variants led to various reductions of HFM1 foci on chromosome axes and resulted in varying degrees of synapsis and crossover formation defects in the mutant male mice. In addition, Hfm1 mutant female mice displayed infertility or subfertility with oogenesis variously affected. LIMITATIONS, REASONS FOR CAUTION: A limitation of the current study is the small sample size. Owing to the unavailability of fresh testicular samples, the defects of synapsis and crossover formation could not be detected in spermatocytes of patients. Owing to the unavailability of antibodies, we could not quantify the impact of these variants on HFM1 protein levels. WIDER IMPLICATIONS OF THE FINDINGS: Our findings provide direct clinical and in vivo functional evidence that HFM1 variants cause male infertility in humans and also suggest that HFM1 may regulate meiotic crossover formation in a dose-dependent manner. Noticeably, our findings from mouse models showed that HFM1 variants could impair spermatogenesis and oogenesis with a varying degree of severity and might also be compatible with the production of a few spermatozoa in men and subfertility in women, extending the phenotypic spectrum of patients with HFM1 variants. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by the National Natural Science Foundation of China (31890780, 32070850, 32061143006, 32000587 and 31900398) and the Fundamental Research Funds for the Central Universities (YD2070002007 and YD2070002012). The authors declare no potential conflicts of interest. TRIAL REGISTRATION NUMBER: N/A.

Histone acetyltransferase KAT8 is essential for mouse oocyte development by regulating reactive oxygen species levels.

Proper oocyte development is crucial for female fertility and requires timely and accurate control of gene expression. K (lysine) acetyltransferase 8 (KAT8), an important component of the X chromosome dosage compensation system in Drosophila, regulates gene activity by acetylating histone H4 preferentially at lysine 16. To explore the function of KAT8 during mouse oocyte development, we crossed Kat8flox/flox mice with Gdf9-Cre mice to specifically delete Kat8 in oocytes. Oocyte Kat8 deletion resulted in female infertility, with follicle development failure in the secondary and preantral follicle stages. RNA-seq analysis revealed that Kat8 deficiency in oocytes results in significant downregulation of antioxidant genes, with a consequent increase in reactive oxygen species. Intraperitoneal injection of the antioxidant N-acetylcysteine rescued defective follicle and oocyte development resulting from Kat8 deficiency. Chromatin immunoprecipitation assays indicated that KAT8 regulates antioxidant gene expression by direct binding to promoter regions. Taken together, our findings demonstrate that KAT8 is essential for female fertility by regulating antioxidant gene expression and identify KAT8 as the first histone acetyltransferase with an essential function in oogenesis.

Npat-dependent programmed Sertoli cell proliferation is indispensable for testis cord development and germ cell mitotic arrest.

As the major somatic cell type, Sertoli cells undergo active proliferation and play essential roles to establish testis cord at fetal stage. They also function to maintain germ cell development throughout the life of testicular development. However, the significance of Sertoli cell number for testis cord development and gonocyte fate is still unclear. Nuclear protein ataxia-telangiectasia (NPAT, also known as p220), a substrate of cyclin E/cyclin-dependent kinase 2, is well known as a regulator of cell proliferation through regulating histone expression. To study the role of NPAT during Sertoli cell development, we generated a mouse strain carrying conditional floxed Npat alleles, when crossing with anti-Müllerian hormone-cre, leading to the specific deletion of Npat in Sertoli cells. Npat disruption in Sertoli cells inhibited the programmed proliferation of fetal Sertoli cells resulting in disruption of developing testis cords, and subsequent postnatal mutant testes were severely hypoplastic. Germ cells, which are presumed to be in quiescent status during perinatal stage, exited G0 phase arrest and re-enter mitotic cell cycle prematurely. Of particular note, some germ cells possessed the meiotic signal in Npat-deficient testes. Our data thus indicates that the function of Npat-dependent Sertoli cells is essential at multiple steps in testis development, and this study also identifies Sertoli cells as a major regulator of germ cell development, which are required to maintain a local growth niche to repress premature mitosis and meiosis of gonocytes.-Jiang, X., Yin, S., Fan, S., Bao, J., Jiao, Y., Ali, A., Iqbal, F., Xu, J., Zhang, Y., Shi, Q. Npat-dependent programmed Sertoli cell proliferation is indispensable for testis cord development and germ cell mitotic arrest.

A homozygous FANCM frameshift pathogenic variant causes male infertility.

PURPOSE: Fanconi anemia (FA) genes play important roles in spermatogenesis. In mice, disruption of Fancm impairs male fertility and testicular integrity, but whether FANCM pathogenic variants (PV) similarly affect fertility in men is unknown. Here we characterize a Pakistani family having three infertile brothers, two manifesting oligoasthenospermia and one exhibiting azoospermia, born to first-cousin parents. A homozygous PV in FANCM (c.1946_1958del, p.P648Lfs*16) was found cosegregating with male infertility. Our objective is to validate that FANCM p.P648Lfs*16 is the PV causing infertility in this family. METHODS: Exome and Sanger sequencing were used for PV screening. DNA interstrand crosslink (ICL) sensitivity was assessed in lymphocytes from patients. A mouse model carrying a PV nearly equivalent to that in the patients (FancmΔC/ΔC) was generated, followed by functional analysis in spermatogenesis. RESULTS: The loss-of-function FANCM PV increased ICL sensitivity in lymphocytes of patients and FancmΔC/ΔC spermatogonia. Adult FancmΔC/ΔC mice showed spermatogenic failure, with germ cell loss in 50.2% of testicular tubules and round-spermatid maturation arrest in 43.5% of tubules. In addition, neither bone marrow failure nor cancer/tumor was detected in all the patients or adult FancmΔC/ΔC mice. CONCLUSION: These findings revealed male infertility to be a novel phenotype of human patients with a biallelic FANCM PV.

Correction: A homozygous FANCM frameshift pathogenic variant causes male infertility.

Hao Win, Hui Ma and Sajjad Hussain were incorrectly affiliated to 'Department of Radiation Oncology, The Houston Methodist Research Institute, Houston, TX 77030 USA'. These authors should only have been affiliated to 'Hefei National Laboratory for Physical Sciences at Microscale, The First Affiliated Hospital of USTC, USTC-SJH Joint Center for Human Reproduction and Genetics, The CAS Key Laboratory of Innate Immunity and Chronic Diseases, School of Life Sciences, CAS Center for Excellence in Molecular Cell Science, Collaborative Innovation Center of Genetics and Development, University of Science and Technology of China, Hefei 230027, China'. They were also not noted as contributing equally to the paper. Both these errors have now been corrected in the PDF and HTML versions of the paper.

A novel recombination protein C12ORF40/REDIC1 is required for meiotic crossover formation.

During meiosis, at least one crossover must occur per homologous chromosome pair to ensure normal progression of meiotic division and accurate chromosome segregation. However, the mechanism of crossover formation is not fully understood. Here, we report a novel recombination protein, C12ORF40/REDIC1, essential for meiotic crossover formation in mammals. A homozygous frameshift mutation in C12orf40 (c.232_233insTT, p.Met78Ilefs*2) was identified in two infertile men with meiotic arrest. Spread mouse spermatocyte fluorescence immunostaining showed that REDIC1 forms discrete foci between the paired regions of homologous chromosomes depending on strand invasion and colocalizes with MSH4 and later with MLH1 at the crossover sites. Redic1 knock-in (KI) mice homozygous for mutation c.232_233insTT are infertile in both sexes due to insufficient crossovers and consequent meiotic arrest, which is also observed in our patients. The foci of MSH4 and TEX11, markers of recombination intermediates, are significantly reduced numerically in the spermatocytes of Redic1 KI mice. More importantly, our biochemical results show that the N-terminus of REDIC1 binds branched DNAs present in recombination intermediates, while the identified mutation impairs this interaction. Thus, our findings reveal a crucial role for C12ORF40/REDIC1 in meiotic crossover formation by stabilizing the recombination intermediates, providing prospective molecular targets for the clinical diagnosis and therapy of infertility.

Loss-of-function variants in KCTD19 cause non-obstructive azoospermia in humans.

Azoospermia is a significant cause of male infertility, with non-obstructive azoospermia (NOA) being the most severe type of spermatogenic failure. NOA is mostly caused by congenital factors, but our understanding of its genetic causes is very limited. Here, we identified a frameshift variant (c.201_202insAC, p.Tyr68Thrfs∗17) and two nonsense variants (c.1897C>T, p.Gln633∗; c.2005C>T, p.Gln669∗) in KCTD19 (potassium channel tetramerization domain containing 19) from two unrelated infertile Chinese men and a consanguineous Pakistani family with three infertile brothers. Testicular histological analyses revealed meiotic metaphase I (MMI) arrest in the affected individuals. Mice modeling KCTD19 variants recapitulated the same MMI arrest phenotype due to severe disrupted individualization of MMI chromosomes. Further analysis showed a complete loss of KCTD19 protein in both Kctd19 mutant mouse testes and affected individual testes. Collectively, our findings demonstrate the pathogenicity of the identified KCTD19 variants and highlight an essential role of KCTD19 in MMI chromosome individualization.

RAD51AP2 is required for efficient meiotic recombination between X and Y chromosomes.

Faithful segregation of X and Y chromosomes requires meiotic recombination to form a crossover between them in the pseudoautosomal region (PAR). Unlike autosomes that have approximately 10-fold more double-strand breaks (DSBs) than crossovers, one crossover must be formed from the one or two DSBs in PARs, implying the existence of a sex chromosome­specific recombination mechanism. Here, we found that RAD51AP2, a meiosis-specific partner of RAD51, is specifically required for the crossover formation on the XY chromosomes, but not autosomes. The decreased crossover formation between X and Y chromosomes in Rad51ap2 mutant mice results from compromised DSB repair in PARs due to destabilization of recombination intermediates rather than defects in DSB generation or synapsis. Our findings provide direct experimental evidence that XY recombination may use a PAR-specific DSB repair mechanism mediated by factors that are not essential for recombination on autosomes.

ZFP541 maintains the repression of pre-pachytene transcriptional programs and promotes male meiosis progression.

The DSB machinery, which induces the programmed DNA double-strand breaks (DSBs) in the leptotene and zygotene stages during meiosis, is suppressed before the onset of the pachytene stage. However, the biological significance and underlying mechanisms remain largely unclear. Here, we report that ZFP541 is indispensable for the suppression of DSB formation after mid-pachytene. The deletion of Zfp541 in mice causes the aberrant recruitment of DSB machinery to chromosome axes and generation of massive DSBs in late pachytene and diplotene spermatocytes, leading to meiotic arrest at the diplotene stage. Integrated analysis of single-cell RNA sequencing (scRNA-seq) and chromatin immunoprecipitation (ChIP) sequencing data indicate that ZFP541 predominantly binds to promoters of pre-pachytene genes, including meiotic DSB formation-related genes (e.g., Prdm9 and Mei1) and their upstream activators (e.g., Meiosin and Rxra), and maintains their repression in pachytene spermatocytes. Our results reveal that ZFP541 functions as a transcriptional regulator in pachytene spermatocytes, orchestrating the transcriptome to ensure meiosis progression.

A TOP6BL mutation abolishes meiotic DNA double-strand break formation and causes human infertility.

Meiosis is pivotal for sexual reproduction and fertility. Meiotic programmed DNA double-strand breaks (DSBs) initiate homologous recombination, ensuring faithful chromosome segregation and generation of gametes. However, few studies have focused on meiotic DSB formation in human reproduction. Here, we report four infertile siblings born to a consanguineous marriage, with three brothers suffering from non-obstructive azoospermia and one sister suffering from unexplained infertility with normal menstrual cycles and normal ovary sizes with follicular activity. An autosomal recessive mutation in TOP6BL was found co-segregating with infertility in this family. Investigation of one male patient revealed failure in programmed meiotic DSB formation and meiotic arrest prior to pachytene stage of prophase I. Mouse models carrying similar mutations to that in patients recapitulated the spermatogenic abnormalities of the patient. Pathogenicity of the mutation in the female patient was supported by observations in mice that meiotic programmed DSBs failed to form in mutant oocytes and oocyte maturation failure due to absence of meiotic recombination. Our study thus illustrates the phenotypical characteristics and the genotype-phenotype correlations of meiotic DSB formation failure in humans.

RPL10L Is Required for Male Meiotic Division by Compensating for RPL10 during Meiotic Sex Chromosome Inactivation in Mice.

The mammalian sex chromosomes have undergone profound changes during their evolution from an ancestral pair of autosomes [1-4]. Specifically, the X chromosome has acquired a paradoxical sex-biased function by redistributing gene contents [5, 6] and has generated a disproportionately high number of retrogenes that are located on autosomes and exhibit male-biased expression patterns [6]. Several selection-based models have been proposed to explain this phenomenon, including a model of sexual antagonism driving X inactivation (SAXI) [6-8] and a compensatory mechanism based on meiotic sex chromosome inactivation (MSCI) [6, 8-11]. However, experimental evidence correlating the function of X-chromosome-derived autosomal retrogenes with evolutionary forces remains limited [12-17]. Here, we show that the deficiency of Rpl10l, a murine autosomal retrogene of Rpl10 with testis-specific expression, disturbs ribosome biogenesis in late-prophase spermatocytes and prohibits the transition from prophase into metaphase of the first meiotic division, resulting in male infertility. Rpl10l expression compensates for the lack of Rpl10, which exhibits a broad expression pattern but is subject to MSCI during spermatogenesis. Importantly, ectopic expression of RPL10L prevents the death of cultured RPL10-deficient somatic cells, and Rpl10l-promoter-driven transgenic expression of Rpl10 in spermatocytes restores spermatogenesis and fertility in Rpl10l-deficient mice. Our results demonstrate that Rpl10l plays an essential role during the meiotic stage of spermatogenesis by compensating for MSCI-mediated transcriptional silencing of Rpl10. These data provide direct evidence for the compensatory hypothesis and add novel insight into the evolution of X-chromosome-derived autosomal retrogenes and their role in male fertility.