The second mutation of SYCE1 gene associated with autosomal recessive nonobstructive azoospermia

Mouse modeling of familial human SYCE1 c.197-2A>G splice site mutation leads to meiotic recombination failure and non-obstructive azoospermia

Infertility affects a considerable number of couples at reproductive age, with an incidence of 10-15%. Approximately 25% of cases are classified as idiopathic infertility. Often, errors during the meiotic stage appear to be related to idiopathic infertility. A crucial component during the first meiotic prophase is the synaptonemal complex (SC), which plays a fundamental role in homologous chromosome pairing and meiotic recombination. In many studies with infertile patients, mutations affecting SC-coding genes have been identified. The generation of humanized models has high physiological relevance, helping to clarify the molecular bases of pathology, which in turn is essential for the development of therapeutic procedures. Here, we report the generation and characterization of genetically modified mice carrying a mutation equivalent to SYCE1 c.197-2A>G, previously found in male infertile patients, aiming to determine the actual effects of this mutation on reproductive capacity and to study the underlying molecular mechanisms. Homozygous mutants were infertile. SYCE1 protein was not detected and Syce1 transcript presented minimal levels, suggesting transcript degradation underlying the infertility mechanism. Additionally, homozygous mutants showed impaired homologous chromosome synapsis, meiotic arrest before the pachytene stage, and increased apoptosis of meiotic cells. This study validates the variant as pathogenic and causative of infertility, since the observed dramatic phenotype was attributable to this single homozygous point mutation, when compared to WT and heterozygous littermates. Moreover, although this homozygous point mutation has been only found in infertile men thus far, we anticipate that if it were present in women, it would cause infertility as well, as homozygous female mice also exhibited an infertility phenotype.

Novel Mutations Reduce Expression of Meiotic Regulators SYCE1 and BOLL in Testis of Azoospermic Men from West Bengal, India

We investigated the polymorphisms/mutations in synaptonemal complex central element protein 1 (SYCE1) and CDC25A mRNA-binding protein (BOLL) to test whether they increase the risk of azoospermia among Bengali-speaking men from West Bengal, India. Sanger's dideoxy sequencing was used to genotype 140 azoospermic individuals who tested negative for Y chromosome microdeletion and 120 healthy controls. In both cases and controls, qRT-PCR was used to determine the expression summary of SYCE1 and BOLL. The perceived harmful consequences of identified mutations were inferred using in silico analysis. Suitable statistical approaches were used to conduct the association study. We found SYCE1 177insT (ON245141), 10650T > G (ON257012), 10093insT (ON257013), 10653insG (ON292504), rs10857748A > G, rs10857749G > A, and rs10857750T > A and BOLL 7708T > A (ON245141insT), rs72918816T > C, and rs700655C > T variants with the prevalence of azoospermia. Data from qRT-PCR and in silico studies projected that the variations would either disrupt the transcript's natural splice junctions or cause probable damage to the structure of the genes' proteins. SYCE1 gene variants [177insT (ON245141), 10650T > G (ON257012), 10093insT (ON257013), 10653insG (ON292504), rs10857748A > G, rs10857749G > A, rs10857750T > A] and BOLL gene variants [7708T > A (ON245141insT), rs72918816T > C, rs700655C > T] reduce the expression of respective gene in testicular tissue among azoospermic male as revealed from qRT-PCR result. These genetic variations could be utilized as screening tools for male infertility to determine the best course of treatment in routine ART practise.

Cloning, expression analysis and localization of DAZL gene implicated in germ cell development of male Hezuo pig

Deleted in azoospermia-like (DAZL) is essential for mammalian testicular function and spermatogenesis. To explore the molecular characterization, expression patterns, and cellular localization of the DAZL in Hezuo pig testes, testicular tissue was isolated from Hezuo pig at five development stages including 30 days old (30 d), 90 days old (90 d), 120 days old (120 d), 180 days old (180 d), and 240 days old (240 d). DAZL cDNA was first cloned using the RT-PCR method, and its molecular characterization was analyzed using relevant bioinformatics software. Subsequently, the expression patterns and cellular localization of DAZL were evaluated using quantitative real-time PCR (qRT-PCR), Western blot, and immunohistochemistry. The cloning and sequence analysis showed that the Hezuo pig DAZL cDNA fragment contained 888 bp open reading frame (ORF) capable of encoding 295 amino acid residues and exhibited high identities with some other mammals. The qRT-PCR and Western blot results indicated that DAZL was specifically expressed in Hezuo pig testes, and DAZL levels of both mRNA and protein were expressed at all five reproductive stages of Hezuo pig testes, with extremely significant higher expression levels in 90 d, 120 d, 180 d, and 240 d than those in 30 d (p < 0.01). Additionally, immunohistochemistry results revealed that DAZL protein was mainly localized in gonocytes at 30 d testes, primary spermatocytes, and spermatozoon at other developmental stages, and Leydig cells throughout five development stages. Together, these results suggested that DAZL may play an important role by regulating the proliferation or differentiation of gonocytes, development of primary spermatocytes and spermatozoon, and functional maintenance of Leydig cells in testicular development and spermatogenesis of Hezuo pig. Nevertheless, the specific regulatory mechanisms underlying these phenomena still requires further investigated and verified.

Meiotic Chromosome Structure, the Synaptonemal Complex, and Infertility

In meiosis, homologous chromosome synapsis is mediated by a supramolecular protein structure, the synaptonemal complex (SC), that assembles between homologous chromosome axes. The mammalian SC comprises at least eight largely coiled-coil proteins that interact and self-assemble to generate a long, zipper-like structure that holds homologous chromosomes in close proximity and promotes the formation of genetic crossovers and accurate meiotic chromosome segregation. In recent years, numerous mutations in human SC genes have been associated with different types of male and female infertility. Here, we integrate structural information on the human SC with mouse and human genetics to describe the molecular mechanisms by which SC mutations can result in human infertility. We outline certain themes in which different SC proteins are susceptible to different types of disease mutation and how genetic variants with seemingly minor effects on SC proteins may act as dominant-negative mutations in which the heterozygous state is pathogenic.

Genetic variants underlying spermatogenic arrests in men with non-obstructive azoospermia

Spermatogenic arrest is a severe form of non-obstructive azoospermia (NOA), which occurs in 10-15% of infertile men. Interruption in spermatogenic progression at premeiotic, meiotic, or postmeiotic stage can lead to arrest in men with NOA. Recent studies have intensively focused on defining genetic variants underlying these spermatogenic arrests by making genome/exome sequencing. A number of variants were discovered in the genes involving in mitosis, meiosis, germline differentiation and other basic cellular events. Herein, defined variants in NOA cases with spermatogenic arrests and created knockout mouse models for the related genes are comprehensively reviewed. Also, importance of gene panel-based screening for NOA cases was discussed. Screening common variants in these infertile men with spermatogenic arrests may contribute to elucidating the molecular background and designing novel treatment strategies.

Analysis of copy number variation in men with non-obstructive azoospermia

BACKGROUND

Recent findings demonstrate that single nucleotide variants can cause non-obstructive azoospermia (NOA). In contrast, copy number variants (CNVs) were only analysed in few studies in infertile men. Some have reported a higher prevalence of CNVs in infertile versus fertile men.

OBJECTIVES

This study aimed to elucidate if CNVs are associated with NOA.

MATERIALS AND METHODS

We performed array-based comparative genomic hybridisation (aCGH) in 37 men with meiotic arrest, 194 men with Sertoli cell-only phenotype, and 21 control men. We filtered our data for deletions affecting genes and prioritised the affected genes according to the literature search. Prevalence of CNVs was compared between all groups. Exome data of 2,030 men were screened to detect further genetic variants in prioritised genes. Modelling was performed for the protein encoded by the novel candidate gene TEKT5 and we stained for TEKT5 in human testicular tissue.

RESULTS

We determined the cause of infertility in two individuals with homozygous deletions of SYCE1 and in one individual with a heterozygous deletion of SYCE1 combined with a likely pathogenic missense variant on the second allele. We detected heterozygous deletions affecting MLH3, EIF2B2, SLX4, CLPP and TEKT5, in one subject each. CNVs were not detected more frequently in infertile men compared with controls.

DISCUSSION

While SYCE1 and MLH3 encode known meiosis-specific proteins, much less is known about the proteins encoded by the other identified candidate genes, warranting further analyses. We were able to identify the cause of infertility in one out of the 231 infertile men by aCGH and in two men by using exome sequencing data.

CONCLUSION

As aCGH and exome sequencing are both expensive methods, combining both in a clinical routine is not an effective strategy. Instead, using CNV calling from exome data has recently become more precise, potentially making aCGH dispensable.

Novel copy number variations within SYCE1 caused meiotic arrest and non-obstructive azoospermia

Background

Non-obstructive azoospermia (NOA) is the most severe disease in male infertility, but the genetic causes for majority of NOA remain unknown.

Methods

Two Chinese NOA-affected patients were recruited to identify the genetic causal factor of infertility. Whole-exome sequencing (WES) was conducted in the two patients with NOA. Sanger sequencing and CNV array were used to ascertain the WES results. Hematoxylin and eosin (H&E) staining and immunofluorescence (IF) were carried out to evaluate the stage of spermatogenesis arrested in the affected cases.

Results

Novel heterozygous deletion (LOH) within SYCE1 (seq[GRCh37] del(10)(10q26.3)chr10:g.135111754_135427143del) and heterozygous loss of function (LoF) variant in SYCE1 (NM_001143763: c.689_690 del:p.F230fs) were identified in one NOA-affected patient. While homozygous deletion within SYCE1 (seq[GRCh37] del(10)(10q26.3)chr10:g.135340247_135379115del) was detected in the other patient with meiotic arrest. H&E and IF staining demonstrated that the spermatogenesis was arrested at pachytene stage in the two patients with NOA, suggesting these two novel CNVs within SYCE1 could lead to meiotic defect and NOA.

Conclusions

We identified that two novel CNVs within SYCE1 are associated with meiotic arrest and male infertility. Thus, our study expands the knowledge of variants in SYCE1 and provides a new insight to understand the genetic etiologies of NOA.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01288-8.

Multiomics analysis of male infertility

Infertility affects 8-12% of couples globally, and the male factor is a primary cause in approximately 50% of couples. Male infertility is a multifactorial reproductive disorder, which can be caused by paracrine and autocrine factors, hormones, genes, and epigenetic changes. Recent studies in rodents and most notably in humans using multiomics approach have yielded important insights into understanding the biology of spermatogenesis. Nonetheless, the etiology and pathogenesis of male infertility are still largely unknown. In this review, we summarized and critically evaluated findings based on the use of advanced technologies to compare normal and obstructive azoospermia (OA) versus non-obstructive azoospermia (NOA) men, including whole-genome bisulfite sequencing (WGBS), single cell RNA-seq (scRNA-seq), whole exome sequencing (WES), and ATAC-seq. It is obvious that the multiomics approach is the method of choice for basic research and clinical studies including clinical diagnosis of male infertility.

Identification of sex determination locus in sea cucumber Apostichopus japonicus using genome-wide association study

Background

Sex determination mechanisms are complicated and diverse across taxonomic categories. Sea cucumber Apostichopus japonicus is a benthic echinoderm, which is the closest group of invertebrates to chordate, and important economic and ecologically aquaculture species in China. A. japonicus is dioecious, and no phenotypic differences between males and females can be detected before sexual maturation. Identification of sex determination locus will broaden knowledge about sex-determination mechanism in echinoderms, which allows for the identification of sex-linked markers and increases the efficiency of sea cucumber breeding industry.

Results

Here, we integrated assembly of a novel chromosome-level genome and resequencing of female and male populations to investigate the sex determination mechanisms of A. japonicus. We built a chromosome-level genome assembly AJH1.0 using Hi-C technology. The assembly AJH1.0 consists of 23 chromosomes ranging from 22.4 to 60.4 Mb. To identify the sex-determination locus of A. japonicus, we conducted genome-wide association study (GWAS) and analyses of distribution characteristics of sex-specific SNPs and fixation index F_ST. The GWAS analysis showed that multiple sex-associated loci were located on several chromosomes, including chromosome 4 (24.8%), followed by chromosome 9 (10.7%), chromosome 17 (10.4%), and chromosome 18 (14.1%). Furthermore, analyzing the homozygous and heterozygous genotypes of plenty of sex-specific SNPs in females and males confirmed that A. japonicus might have a XX/XY sex determination system. As a physical region of 10 Mb on chromosome 4 included the highest number of sex-specific SNPs and higher F_ST values, this region was considered as the candidate sex determination region (SDR) in A. japonicus.

Conclusions

In the present study, we integrated genome-wide association study and analyses of sex-specific variations to investigate sex determination mechanisms. This will bring novel insights into gene regulation during primitive gonadogenesis and differentiation and identification of master sex determination gene in sea cucumber. In the sea cucumber industry, investigation of molecular mechanisms of sex determination will be helpful for artificial fertilization and precise breeding.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08632-3.

In silico analysis of a novel pathogenic variant c.7G > A in C14orf39 gene identified by WES in a Pakistani family with azoospermia

Infertility is a multifactorial disorder that affects approximately 12% of couples of childbearing ages worldwide. Few studies have been conducted to understand the genetic causes of infertility in depth. The synaptonemal complex (SC), which is essential for the progression of meiosis, is a conserved tripartite structure that binds homologous chromosomes together and is thus required for fertility. This study investigated genetic causes of infertility in a Pakistani consanguineous family containing two patients suffering from non-obstructive azoospermia (NOA). We performed whole-exome sequencing, followed by Sanger sequencing, and identified a novel pathogenic variant (c.7G > A [p.D3N]) in the SC coding gene C14orf39, which was recessively co-segregated with NOA. In silico analysis revealed that charges on wild-type residues were lost, which may result in loss of interactions with other molecules and residues, and a reduction in protein stability occurred, which was caused by the p.D3N mutation. The novel variant generated the mutant protein C14ORF39^D3N, and homozygous mutations in C14orf39 resulted in NOA. The transcriptome profile of C14ORF39 shows that it is specifically expressed in early brain development, which suggests that research in this area is required to study other functions of C14ORF39 in addition to its role in the germline. This research highlights the conserved role of C14orf39/SIX6OS1 in assembly of the SC and its indispensable role in facilitating genetic diagnosis in patients with infertility, which may enable the development of future treatments.

Testis-specific fascin component FSCN3 is dispensable for mouse spermatogenesis and fertility

BACKGROUND

Fascins belong to a family of actin-bundling proteins that are involved in a wide range of biological functions. FSCN3, a newly identified testis-specific actin-bundling protein, is specifically expressed in elongated spermatids. However, its in vivo function in mouse spermiogenesis remains unknown.

METHODS AND RESULTS

We generated Fscn3 knockout mice through CRISPR/Cas9 gene-editing technology. Fscn3^-/- mice displayed normal testis morphology and testis to bodyweight ratio, and sperm concentrations did not differ significantly between Fscn3^+/+ and Fscn3^-/- mice. Fertility assays consistently revealed that Fscn3^-/- mice are completely fertile and their reproductive status does not differ from that of wild-type. Moreover, hematoxylin and eosin staining of the testis sections of Fscn3^-/- mice detected various germ cells, ranging from spermatogonia to mature spermatozoa. Furthermore, the swimming velocity of the sperm of Fscn3^-/- mice was comparable to that of their wild-type littermates. Both Fscn3^+/+ and Fscn3^-/-mice had normal sperm morphology, indicating that the disruption of Fscn3 does not affect sperm morphology. The analysis of meiotic prophase I progression demonstrated normal prophase-I phases (leptonema to diplonema) in both Fscn3^+/+ and Fscn3^-/- mice, suggesting that Fscn3 is not essential for meiosis I.

CONCLUSION

Our study provides the first evidence that FSCN3 is a testis-specific actin-bundling protein that is not required for mouse spermatogenesis. Our results will help reproductive biologists focus their efforts on genes that are crucial for fertility and avoid research duplication.

A rare frameshift mutation in SYCP1 is associated with human male infertility

Proper assembly of the synaptonemal complex is essential for successful meiosis, and impairments in the process lead to infertility. Meiotic transverse filament proteins encoded by the SYCP1 (synaptonemal complex protein 1) gene are one of the main components of the synaptonemal complex and play an important role in correct synapsis and recombination. Family-based whole exome sequencing revealed a rare homozygous SYCP1 frameshift mutation (c.2892delA: p.K967Nfs*1) in two men with severe oligozoospermia, followed by validation and segregation through Sanger sequencing. This single nucleotide deletion not only changes lysine 967 (K) into asparagine (N) but also causes a premature stop codon, which leads to deletion of 968-976 residues from the end of the C-tail region of the SYCP1 protein. Although, sycp1 knockout male mice are reported to be sterile with a complete lack of spermatids and spermatozoa, to date no SYCP1 variant has been associated with human oligozoospermia. HADDOCK analysis indicated that this mutation decreases the ability of the truncated SYCP1 protein to bind DNA. Immunodetection of ϒH2AX signal, in SYCP1 mutant semen cells and a 40% DNA fragmentation index might indicate that a small number of DNA double-strand breaks, which require SYCP1 and/or synapsis to be repaired, are not efficiently repaired, resulting in defects in differentiation of germline cells and appearance of the oligozoospermia phenotype. To our knowledge, this is the first report of homozygous SYCP1 mutation that decreases sperm count. Further studies are required to determine the function of the SYCP1 mutation, which is potentially associated with human oligozoospermia.

Identification of deleterious variants in patients with male infertility due to idiopathic non-obstructive azoospermia

BACKGROUND

Non-obstructive azoospermia (NOA) is the most severe type of male infertility, affecting 1% of men worldwide. Most of its etiologies remain idiopathic. Although genetic studies have identified dozens of NOA genes, monogenic mutations can also account for a small proportion of idiopathic NOA cases. Hence, this genetic study was conducted to explore the causes of monogenic variants of NOA in a cohort of Chinese patients.

METHODS

Following the screening using chromosomal karyotyping, Y chromosome microdeletion analyses, and sex hormone assessments, subsequent whole-exome sequencing analysis was performed in 55 unrelated idiopathic NOA patients with male infertility to explore potential deleterious variants associated with spermatogenesis. We also performed Sanger sequencing to demonstrate the variants. Testicular biopsy or microsurgical testicular sperm extraction was also performed to confirm the diagnosis of NOA and identify spermatozoa. Hematoxylin and eosin staining was performed to assess the histopathology of spermatogenesis.

RESULTS

Abnormal testicular pathological phenotypes included Sertoli cell-only syndrome, maturation arrest, and hypospermatogenesis. Using bioinformatics analysis, we detected novel variants in two recessive genes, FANCA (NM_000135, c.3263C > T, c.1729C > G) and SYCE1 (NM_001143763, c.689_690del); one X-linked gene, TEX11 (NM_031276, c.466A > G, c.559_560del); and two dominant genes, DMRT1 (NM_021951, c.425C > T, c.340G > A) and PLK4 (NM_001190799, c.2785A > G), in eight patients, which corresponded to 14.55% (8/55) of the patients.

CONCLUSION

This study presented some novel variants of known pathogenic genes for NOA. Further, it expanded the variant spectrum of NOA patients, which might advance clinical genetic counseling in the future.

Variations of C14ORF39 and SYCE1 Identified in Idiopathic Premature Ovarian Insufficiency and Nonobstructive Azoospermia

CONTEXT

Premature ovarian insufficiency (POI) and nonobstructive azoospermia (NOA) are the most severe diseases causing irreversible infertility in females and males, respectively. The contribution of synaptonemal complex (SC) gene variations in the pathogenesis of sporadic patients with POI and NOA has not been systematically illustrated.

OBJECTIVE

To investigate the role of SC genes in the pathogenesis of sporadic POI and NOA.

DESIGN

Genetic and functional study.

SETTING

University-based reproductive medicine center.

PATIENT(S)

A total of 1030 patients with sporadic POI and 400 patients with sporadic NOA.

INTERVENTION(S)

The variations of SC genes were filtered in the in-house database of whole exome sequencing performed in 1030 patients with sporadic POI and 400 patients with sporadic NOA. The pathogenic or likely pathogenic variations following recessive inheritance mode were selected according to American College of Medical Genetics and Genomics (ACMG) guidelines and confirmed by Sanger sequencing. The pathogenic effects of the variations were verified by functional studies.

MAIN OUTCOME MEASURE(S)

ACMG classification and functional characteristics.

RESULT(S)

Two homozygous variations of C14ORF39 and 2 recessive variations of SYCE1 were first identified in sporadic patients with POI and NOA, respectively. Functional studies showed the C14ORF39 variations significantly accelerated the protein degradation and the variations in SYCE1 disrupted its interaction with SYCP1 or C14ORF39, both of which affected SC assembly and meiosis.

CONCLUSION(S)

Our study identified novel pathogenic variations of C14ORF39 and SYCE1 in sporadic patients with POI or NOA, highlighting the essential role of SC genes in the maintenance of ovarian and testicular function.

Novel exon mutation in SYCE1 gene is associated with non-obstructive azoospermia

Non‐obstructive azoospermia (NOA) is a common cause of male infertility, and genetic problems, such as chromosomal abnormalities and gene mutations, are important causes of NOA. Our centre received a case of NOA, in which no mature sperm was found during microdissection testicular sperm extraction. A postoperative pathological examination revealed that testicular spermatogenesis was blocked. Target region capture combined with high‐throughput sequencing was used to screen for male infertility‐related gene mutations. Sanger sequencing further confirmed that the SYCE1 gene, a central component of the synaptonemal complex (SC) during meiosis, had a homozygous deletion mutation in the tenth exon (c.689_690del; p.F230fs). Through molecular biological studies, we discovered altered expression and nuclear localization of the endogenous mutant SYCE1. To verify the effects in vitro, wild‐ and mutated‐type SYCE1 vectors were constructed and transfected into a human cell line. The results showed that the expression and molecular weight were decreased for SYCE1 containing c.689_690del. In addition, mutated SYCE1 was abnormally located in the cytoplasm rather than in the nucleus. In summary, our research suggests that the novel homozygous mutation (c.689_690del; p.F230fs) altered the SYCE1 expression pattern and may have disturbed SC assembly, leading to male infertility and to a barrier to gamete formation. We reported for the first time that a frameshift mutation occurred in the exon region of SYCE1 in an NOA patient. This study is beneficial for accurate NOA diagnosis and the development of corresponding gene therapy strategies.

The formation and repair of DNA double-strand breaks in mammalian meiosis

Programmed DNA double-strand breaks (DSBs) are necessary for meiosis in mammals. A sufficient number of DSBs ensure the normal pairing/synapsis of homologous chromosomes. Abnormal DSB repair undermines meiosis, leading to sterility in mammals. The DSBs that initiate recombination are repaired as crossovers and noncrossovers, and crossovers are required for correct chromosome separation. Thus, the placement, timing, and frequency of crossover formation must be tightly controlled. Importantly, mutations in many genes related to the formation and repair of DSB result in infertility in humans. These mutations cause nonobstructive azoospermia in men, premature ovarian insufficiency and ovarian dysgenesis in women. Here, we have illustrated the formation and repair of DSB in mammals, summarized major factors influencing the formation of DSB and the theories of crossover regulation.

A homozygous loss-of-function mutation in FBXO43 causes human non-obstructive azoospermia

Non-obstructive azoospermia (NOA) represents one of the most serious forms of male infertility caused by spermatogenic failure. Despite multiple genes found to be associated with human NOA, the genetic basis of this idiopathic disease remains largely unknown. FBXO43 is a direct inhibitor of the anaphase-promoting complex/cyclosome (APC/C) E3 ligase and crucially important in mouse spermatogenesis. In this study, for the first time, we identified a homozygous nonsense mutation in FBXO43 c.1747C > T:p.Gln583X in two NOA brothers from a Chinese consanguineous family via whole-exome sequencing. FBXO43 was absent from testicular tissue of the proband, and FBXO43-immunostaining signals were invisible in the affected seminiferous tubules. Furthermore, in humans, FBXO43 defects cause meiotic arrest within early diplotene of prophase I. The results here demonstrate the pathogenicity of this loss-of-function mutation and confirmed that spermatocytes were unable to complete meiotic divisions without FBXO43 in humans. In mouse testicular protein extracts, three subunits of the APC/C, including ANAPC2, ANAPC8 and ANAPC10, were validated to interact directly with FBXO43, whereas no interactions were detected for FBXO43 and SKP1. This study furthers our understanding of the genetic basis of human NOA and provides insights into FBXO43 and male infertility.

The organization, regulation, and biological functions of the synaptonemal complex

The synaptonemal complex (SC) is a meiosis-specific proteinaceous macromolecular structure that assembles between paired homologous chromosomes during meiosis in various eukaryotes. The SC has a highly conserved ultrastructure and plays critical roles in controlling multiple steps in meiotic recombination and crossover formation, ensuring accurate meiotic chromosome segregation. Recent studies in different organisms, facilitated by advances in super-resolution microscopy, have provided insights into the macromolecular structure of the SC, including the internal organization of the meiotic chromosome axis and SC central region, the regulatory pathways that control SC assembly and dynamics, and the biological functions exerted by the SC and its substructures. This review summarizes recent discoveries about how the SC is organized and regulated that help to explain the biological functions associated with this meiosis-specific structure.

FAM9B serves as a novel meiosis-related protein localized in meiotic chromosome cores and is associated with human gametogenesis

Meiosis is a complex process involving the expression and interaction of numerous genes in a series of highly orchestrated molecular events. Fam9b localized in Xp22.3 has been found to be expressed in testes. However, FAM9B expression, localization, and its role in meiosis have not been previously reported. In this study, FAM9B expression was evaluated in the human testes and ovaries by RT-PCR, qPCR, and western blotting. FAM9B was found in the nuclei of primary spermatocytes in testes and specifically localized in the synaptonemal complex (SC) region of spermatocytes. FAM9B was also evident in the follicle cell nuclei and diffusely dispersed in the granular cell cytoplasm. FAM9B was partly co-localized with SYCP3, which is essential for both formation and maintenance of lateral SC elements. In addition, FAM9B had a similar distribution pattern and co-localization as γH2AX, which is a novel biomarker for DNA double-strand breaks during meiosis. All results indicate that FAM9B is a novel meiosis-associated protein that is co-localized with SYCP3 and γH2AX and may play an important role in SC formation and DNA recombination during meiosis. These findings offer a new perspective for understanding the molecular mechanisms involved in meiosis of human gametogenesis.

Familial primary ovarian insufficiency associated with an SYCE1 point mutation: defective meiosis elucidated in humanized mice

More than 50% of cases of primary ovarian insufficiency (POI) and nonobstructive azoospermia in humans are classified as idiopathic infertility. Meiotic defects may relate to at least some of these cases. Mutations in genes coding for synaptonemal complex (SC) components have been identified in humans, and hypothesized to be causative for the observed infertile phenotype. Mutation SYCE1 c.721C>T (former c.613C>T)-a familial mutation reported in two sisters with primary amenorrhea-was the first such mutation found in an SC central element component-coding gene. Most fundamental mammalian oogenesis events occur during the embryonic phase, and eventual defects are identified many years later, thus leaving few possibilities to study the condition's etiology and pathogenesis. Aiming to validate an approach to circumvent this difficulty, we have used the CRISPR/Cas9 technology to generate a mouse model with an SYCE1 c.721C>T equivalent genome alteration. We hereby present the characterization of the homozygous mutant mice phenotype, compared to their wild type and heterozygous littermates. Our results strongly support a causative role of this mutation for the POI phenotype in human patients, and the mechanisms involved would relate to defects in homologous chromosome synapsis. No SYCE1 protein was detected in homozygous mutants and Syce1 transcript level was highly diminished, suggesting transcript degradation as the basis of the infertility mechanism. This is the first report on the generation of a humanized mouse model line for the study of an infertility-related human mutation in an SC component-coding gene, thus representing a proof of principle.

Conception by fertility treatment and offspring deoxyribonucleic acid methylation

OBJECTIVE

To investigate whether deoxyribonucleic acid (DNA) methylation at birth and in childhood differ by conception using assisted reproductive technologies (ART) or ovulation induction compared with those in children conceived without fertility treatment.

DESIGN

Upstate KIDS is a matched exposure cohort which oversampled on newborns conceived by treatment.

SETTING

New York State (excluding New York City).

PATIENT(S)

This analysis included 855 newborns and 152 children at approximately 9 years of age.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

DNA methylation levels were measured using the Illumina EPIC platform. Single CpG and regional analyses at imprinting genes were conducted.

RESULT(S)

Compared to no fertility treatment, ART was associated with lower mean DNA methylation levels at birth in 11 CpGs (located in/near SYCE1, SPRN, KIAA2013, MYO1D, GET1/WRB-SH4BGR, IGF1R, SORD, NECAB3/ACTL10, and GET1) and higher mean methylation level in 1 CpG (KLK4; all false discovery rate P<.05). The strongest association (cg17676129) was located at SYCE1, which codes for a synaptonemal complex that plays a role in meiosis and therefore infertility. This CpG remained associated with newborn hypomethylation when the analysis was limited to those conceived with ICSI, but this may be because of underlying male infertility. In addition, nine regions in maternally imprinted genes (IGF1R, PPIEL, SVOPL GNAS, L3MBTL, BLCAP, HYMAI/PLAGL1, SNU13, and MEST) were observed to have decreased mean DNA methylation levels among newborns conceived by ART. In childhood, hypomethylation of the maternally imprinted gene, GNAS, persisted. No CpGs or regions were associated with ovulation induction.

CONCLUSION(S)

ART but not ovulation induction was associated with hypomethylation at birth, but only one difference at an imprinting region appeared to persist in childhood.

Shared genetics between nonobstructive azoospermia and primary ovarian insufficiency

OBJECTIVE

Primary ovarian insufficiency (POI) and Non-obstructive azoospermia (NOA) both represent disease states of early, and often complete, failure of gametogenesis. Because oogenesis and spermatogenesis share the same conserved steps in meiosis I, it is possible that inherited defects in meiosis I could lead to shared causes of both POI and NOA. Currently, known genes that contribute to both POI and NOA are limited. In this review article, we provide a systematic review of genetic mutations in which both POI and NOA phenotypes exist.

EVIDENCE REVIEW

A PubMed literature review was conducted from January 1, 2000 through October 2020. We included all studies that demonstrated human cases of POI or NOA due to a specific genetic mutation either within the same family or in separate families.

RESULTS

We identified 33 papers that encompassed 10 genes of interest with mutations implicated in both NOA and POI. The genes were all involved in processes of meiosis I.

CONCLUSION

Mutations in genes involved in processes of meiosis I may cause both NOA and POI. Identifying these unique phenotypes among shared genotypes leads to biologic plausibility that the key error occurs early in gametogenesis with an etiology shared among both male and female offspring. From a clinical standpoint, this shared relationship may help us better understand and identify individuals at high risk for gonadal failure within families and suggests that clinicians obtain history for opposite sex family members when approaching a new diagnosis of POI or NOA.

Autosomal recessive loci contribute significantly to quantitative variation of male fertility in a dairy cattle population

BACKGROUND

Cattle are ideally suited to investigate the genetics of male fertility. Semen from individual bulls is used for thousands of artificial inseminations for which the fertilization success is monitored. Results from the breeding soundness examination and repeated observations of semen quality complement the fertility evaluation for each bull.

RESULTS

In a cohort of 3881 Brown Swiss bulls that had genotypes at 683,609 SNPs, we reveal four novel recessive QTL for male fertility on BTA1, 18, 25, and 26 using haplotype-based association testing. A QTL for bull fertility on BTA1 is also associated with sperm head shape anomalies. All other QTL are not associated with any of the semen quality traits investigated. We perform complementary fine-mapping approaches using publicly available transcriptomes as well as whole-genome sequencing data of 125 Brown Swiss bulls to reveal candidate causal variants. We show that missense or nonsense variants in SPATA16, VWA3A, ENSBTAG00000006717 and ENSBTAG00000019919 are in linkage disequilibrium with the QTL. Using whole-genome sequence data, we detect strong association (P = 4.83 × 10^- 12) of a missense variant (p.Ile193Met) in SPATA16 with male fertility. However, non-coding variants exhibit stronger association at all QTL suggesting that variants in regulatory regions contribute to variation in bull fertility.

CONCLUSION

Our findings in a dairy cattle population provide evidence that recessive variants may contribute substantially to quantitative variation in male fertility in mammals. Detecting causal variants that underpin variation in male fertility remains difficult because the most strongly associated variants reside in poorly annotated non-coding regions.

Genetics of Azoospermia

Azoospermia affects 1% of men, and it can be due to: (i) hypothalamic-pituitary dysfunction, (ii) primary quantitative spermatogenic disturbances, (iii) urogenital duct obstruction. Known genetic factors contribute to all these categories, and genetic testing is part of the routine diagnostic workup of azoospermic men. The diagnostic yield of genetic tests in azoospermia is different in the different etiological categories, with the highest in Congenital Bilateral Absence of Vas Deferens (90%) and the lowest in Non-Obstructive Azoospermia (NOA) due to primary testicular failure (~30%). Whole-Exome Sequencing allowed the discovery of an increasing number of monogenic defects of NOA with a current list of 38 candidate genes. These genes are of potential clinical relevance for future gene panel-based screening. We classified these genes according to the associated-testicular histology underlying the NOA phenotype. The validation and the discovery of novel NOA genes will radically improve patient management. Interestingly, approximately 37% of candidate genes are shared in human male and female gonadal failure, implying that genetic counselling should be extended also to female family members of NOA patients.

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.

Preconception genome medicine: current state and future perspectives to improve infertility diagnosis and reproductive and health outcomes based on individual genomic data

BACKGROUND

Our genetic code is now readable, writable and hackable. The recent escalation of genome-wide sequencing (GS) applications in population diagnostics will not only enable the assessment of risks of transmitting well-defined monogenic disorders at preconceptional stages (i.e. carrier screening), but also facilitate identification of multifactorial genetic predispositions to sub-lethal pathologies, including those affecting reproductive fitness. Through GS, the acquisition and curation of reproductive-related findings will warrant the expansion of genetic assessment to new areas of genomic prediction of reproductive phenotypes, pharmacogenomics and molecular embryology, further boosting our knowledge and therapeutic tools for treating infertility and improving women's health.

OBJECTIVE AND RATIONALE

In this article, we review current knowledge and potential development of preconception genome analysis aimed at detecting reproductive and individual health risks (recessive genetic disease and medically actionable secondary findings) as well as anticipating specific reproductive outcomes, particularly in the context of IVF. The extension of reproductive genetic risk assessment to the general population and IVF couples will lead to the identification of couples who carry recessive mutations, as well as sub-lethal conditions prior to conception. This approach will provide increased reproductive autonomy to couples, particularly in those cases where preimplantation genetic testing is an available option to avoid the transmission of undesirable conditions. In addition, GS on prospective infertility patients will enable genome-wide association studies specific for infertility phenotypes such as predisposition to premature ovarian failure, increased risk of aneuploidies, complete oocyte immaturity or blastocyst development failure, thus empowering the development of true reproductive precision medicine.

SEARCH METHODS

Searches of the literature on PubMed Central included combinations of the following MeSH terms: human, genetics, genomics, variants, male, female, fertility, next generation sequencing, genome exome sequencing, expanded carrier screening, secondary findings, pharmacogenomics, controlled ovarian stimulation, preconception, genetics, genome-wide association studies, GWAS.

OUTCOMES

Through PubMed Central queries, we identified a total of 1409 articles. The full list of articles was assessed for date of publication, limiting the search to studies published within the last 15 years (2004 onwards due to escalating research output of next-generation sequencing studies from that date). The remaining articles' titles were assessed for pertinence to the topic, leaving a total of 644 articles. The use of preconception GS has the potential to identify inheritable genetic conditions concealed in the genome of around 4% of couples looking to conceive. Genomic information during reproductive age will also be useful to anticipate late-onset medically actionable conditions with strong genetic background in around 2-4% of all individuals. Genetic variants correlated with differential response to pharmaceutical treatment in IVF, and clear genotype-phenotype associations are found for aberrant sperm types, oocyte maturation, fertilization or pre- and post-implantation embryonic development. All currently known capabilities of GS at the preconception stage are reviewed along with persisting and forthcoming barriers for the implementation of precise reproductive medicine.

WIDER IMPLICATIONS

The expansion of sequencing analysis to additional monogenic and polygenic traits may enable the development of cost-effective preconception tests capable of identifying underlying genetic causes of infertility, which have been defined as 'unexplained' until now, thus leading to the development of a true personalized genomic medicine framework in reproductive health.

Meiotic chromosome synapsis depends on multivalent SYCE1-SIX6OS1 interactions that are disrupted in cases of human infertility

Meiotic reductional division depends on the synaptonemal complex (SC), a supramolecular protein assembly that mediates homologous chromosomes synapsis and promotes crossover formation. The mammalian SC has eight structural components, including SYCE1, the only central element protein with known causative mutations in human infertility. We combine mouse genetics, cellular, and biochemical studies to reveal that SYCE1 undergoes multivalent interactions with SC component SIX6OS1. The N terminus of SIX6OS1 binds and disrupts SYCE1's core dimeric structure to form a 1:1 complex, while their downstream sequences provide a distinct second interface. These interfaces are separately disrupted by SYCE1 mutations associated with nonobstructive azoospermia and premature ovarian failure (POF), respectively. Mice harboring SYCE1's POF mutation and a targeted deletion within SIX6OS1's N terminus are infertile with failure of chromosome synapsis. We conclude that both SYCE1-SIX6OS1 binding interfaces are essential for SC assembly, thus explaining how SYCE1's reported clinical mutations give rise to human infertility.