Mutations in TTC29, Encoding an Evolutionarily Conserved Axonemal Protein, Result in Asthenozoospermia and Male Infertility

Conserved genes regulating human sex differentiation, gametogenesis and fertilization

The study of the functional genome in mice and humans has been instrumental for describing the conserved molecular mechanisms regulating human reproductive biology, and for defining the etiologies of monogenic fertility disorders. Infertility is a reproductive disorder that includes various conditions affecting a couple's ability to achieve a healthy pregnancy. Recent advances in next-generation sequencing and CRISPR/Cas-mediated genome editing technologies have facilitated the identification and characterization of genes and mechanisms that, if affected, lead to infertility. We report established genes that regulate conserved functions in fundamental reproductive processes (e.g., sex determination, gametogenesis, and fertilization). We only cover genes the deletion of which yields comparable fertility phenotypes in both rodents and humans. In the case of newly-discovered genes, we report the studies demonstrating shared cellular and fertility phenotypes resulting from loss-of-function mutations in both species. Finally, we introduce new model systems for the study of human reproductive biology and highlight the importance of studying human consanguineous populations to discover novel monogenic causes of infertility. The rapid and continuous screening and identification of putative genetic defects coupled with an efficient functional characterization in animal models can reveal novel mechanisms of gene function in human reproductive tissues.

Gene-knockout by iSTOP enables rapid reproductive disease modeling and phenotyping in germ cells of the founder generation

Cytosine base editing achieves C•G-to-T•A substitutions and can convert four codons (CAA/CAG/CGA/TGG) into STOP-codons (induction of STOP-codons, iSTOP) to knock out genes with reduced mosaicism. iSTOP enables direct phenotyping in founders' somatic cells, but it remains unknown whether this works in founders' germ cells so as to rapidly reveal novel genes for fertility. Here, we initially establish that iSTOP in mouse zygotes enables functional characterization of known genes in founders' germ cells: Cfap43-iSTOP male founders manifest expected sperm features resembling human "multiple morphological abnormalities of the flagella" syndrome (i.e., MMAF-like features), while oocytes of Zp3-iSTOP female founders have no zona pellucida. We further illustrate iSTOP's utility for dissecting the functions of unknown genes with Ccdc183, observing MMAF-like features and male infertility in Ccdc183-iSTOP founders, phenotypes concordant with those of Ccdc183-KO offspring. We ultimately establish that CCDC183 is essential for sperm morphogenesis through regulating the assembly of outer dynein arms and participating in the intra-flagellar transport. Our study demonstrates iSTOP as an efficient tool for direct reproductive disease modeling and phenotyping in germ cells of the founder generation, and rapidly reveals the essentiality of Ccdc183 in fertility, thus providing a time-saving approach for validating genetic defects (like nonsense mutations) for human infertility.

Identification of novel homozygous asthenoteratospermia-causing ARMC2 mutations associated with multiple morphological abnormalities of the sperm flagella

PURPOSE

To identify the genetic causes of multiple morphological abnormalities in sperm flagella (MMAF) and male infertility in patients from two unrelated Han Chinese families.

METHODS

Whole-exome sequencing was conducted using blood samples from the two individuals with MMAF and male infertility. Hematoxylin and eosin staining and scanning electron microscopy were performed to evaluate sperm morphology. Ultrastructural and immunostaining analyses of the spermatozoa were performed. The HEK293T cells were used to confirm the pathogenicity of the variants.

RESULTS

We identified two novel homozygous missense ARMC2 variants: c.314C > T: p.P105L and c.2227A > G: p.N743D. Both variants are absent or rare in the human population genome data and are predicted to be deleterious. In vitro experiments indicated that both ARMC2 variants caused a slightly increased protein expression. ARMC2-mutant spermatozoa showed multiple morphological abnormalities (bent, short, coiled, absent, and irregular) in the flagella. In addition, the spermatozoa of the patients revealed a frequent absence of the central pair complex and disrupted axonemal ultrastructure.

CONCLUSION

We identified two novel ARMC2 variants that caused male infertility and MMAF in Han Chinese patients. These findings expand the mutational spectrum of ARMC2 and provide insights into the complex causes and pathogenesis of MMAF.

Coiled-coil domain-containing 38 is required for acrosome biogenesis and fibrous sheath assembly in mice

During spermiogenesis, haploid spermatids undergo dramatic morphological changes to form slender sperm flagella and cap-like acrosomes, which are required for successful fertilization. Severe deformities in flagella cause a male infertility syndrome, multiple morphological abnormalities of the flagella (MMAF), while acrosomal hypoplasia in some cases leads to sub-optimal embryonic developmental potential. However, evidence regarding the occurrence of acrosomal hypoplasia in MMAF is limited. Here, we report the generation of base-edited mice knocked out for coiled-coil domain-containing 38 (Ccdc38) via inducing a nonsense mutation and find that the males are infertile. The Ccdc38-KO sperm display acrosomal hypoplasia and typical MMAF phenotypes. We find that the acrosomal membrane is loosely anchored to the nucleus and fibrous sheaths are disorganized in Ccdc38-KO sperm. Further analyses reveal that Ccdc38 knockout causes a decreased level of TEKT3, a protein associated with acrosome biogenesis, in testes and an aberrant distribution of TEKT3 in sperm. We finally show that intracytoplasmic sperm injection overcomes Ccdc38-related infertility. Our study thus reveals a previously unknown role for CCDC38 in acrosome biogenesis and provides additional evidence for the occurrence of acrosomal hypoplasia in MMAF.

Lack of CCDC146, a ubiquitous centriole and microtubule-associated protein, leads to non-syndromic male infertility in human and mouse

From a cohort of 167 infertile patients suffering from multiple morphological abnormalities of the flagellum (MMAF), pathogenic bi-allelic mutations were identified in the CCDC146 gene. In somatic cells, CCDC146 is located at the centrosome and at multiple microtubule-related organelles during mitotic division, suggesting that it is a microtubule-associated protein (MAP). To decipher the molecular pathogenesis of infertility associated with CCDC146 mutations, a Ccdc146 knock-out (KO) mouse line was created. KO male mice were infertile, and sperm exhibited a phenotype identical to CCDC146 mutated patients. CCDC146 expression starts during late spermiogenesis. In the spermatozoon, the protein is conserved but is not localized to centrioles, unlike in somatic cells, rather it is present in the axoneme at the level of microtubule doublets. Expansion microscopy associated with the use of the detergent sarkosyl to solubilize microtubule doublets suggests that the protein may be a microtubule inner protein (MIP). At the subcellular level, the absence of CCDC146 impacted all microtubule-based organelles such as the manchette, the head-tail coupling apparatus (HTCA), and the axoneme. Through this study, a new genetic cause of infertility and a new factor in the formation and/or structure of the sperm axoneme were characterized.

CCDC65, encoding a component of the axonemal Nexin-Dynein regulatory complex, is required for sperm flagellum structure in humans

Sperm flagella share an evolutionary conserved microtubule-based structure with motile cilia expressed at the surface of several cell types, such as the airways epithelial cells. As a result, male infertility can be observed as an isolated condition or a syndromic trait, illustrated by Primary Cilia Dyskinesia (PCD). We report two unrelated patients showing multiple morphological abnormalities of the sperm flagella (MMAF) and carrying distinct homozygous truncating variants in the PCD-associated gene CCDC65. We characterized one of the identified variants (c.1208del; p.Asn403Ilefs*9), which induces the near absence of CCDC65 protein in patient sperm. In Chlamydomonas, CCDC65 ortholog (DRC2, FAP250) is a component of the Nexin-Dynein Regulatory complex (N-DRC), which interconnects microtubule doublets and coordinates dynein arms activity. In sperm cells from the patient, we also show the loss of GAS8, another component of the N-DRC, supporting a structural/functional link between the two proteins. Our work indicates that, similarly to ciliary axoneme, CCDC65 is required for sperm flagellum structure. Importantly, our work provides first evidence that mutations in the PCD-associated gene CCDC65 also cause asthenozoospermia.

From TgO/GABA-AT, GABA, and T-263 Mutant to Conception of Toxoplasma

Toxoplasma gondii causes morbidity, mortality, and disseminates widely via cat sexual stages. Here, we find T. gondii ornithine aminotransferase (OAT) is conserved across phyla. We solve TgO/GABA-AT structures with bound inactivators at 1.55 Å and identify an inactivator selective for TgO/GABA-AT over human OAT and GABA-AT. However, abrogating TgO/GABA-AT genetically does not diminish replication, virulence, cyst-formation, or eliminate cat's oocyst shedding. Increased sporozoite/merozoite TgO/GABA-AT expression led to our study of a mutagenized clone with oocyst formation blocked, arresting after forming male and female gametes, with "Rosetta stone"-like mutations in genes expressed in merozoites. Mutations are similar to those in organisms from plants to mammals, causing defects in conception and zygote formation, affecting merozoite capacitation, pH/ionicity/sodium-GABA concentrations, drawing attention to cyclic AMP/PKA, and genes enhancing energy or substrate formation in TgO/GABA-AT-related-pathways. These candidates potentially influence merozoite's capacity to make gametes that fuse to become zygotes, thereby contaminating environments and causing disease.

Functional and structural significance of the inner-arm-dynein subspecies d in ciliary motility

Motile cilia play various important physiological roles in eukaryotic organisms including cell motility and fertility. Inside motile cilia, large motor-protein complexes called "ciliary dyneins" coordinate their activities and drive ciliary motility. The ciliary dyneins include the outer-arm dyneins, the double-headed inner-arm dynein (IDA f/I1), and several single-headed inner-arm dyneins (IDAs a, b, c, d, e, and g). Among these single-headed IDAs, one of the ciliary dyneins, IDA d, is of particular interest because of its unique properties and subunit composition. In addition, defects in this subspecies have recently been associated with several types of ciliopathies in humans, such as primary ciliary dyskinesia and multiple morphologic abnormalities of the flagellum. In this mini-review, we discuss the composition, structure, and motor properties of IDA d, which have been studied in the model organism Chlamydomonas reinhardtii, and further discuss the relationship between IDA d and human ciliopathies. In addition, we provide future perspectives and discuss remaining questions regarding this intriguing dynein subspecies.

CCDC146 is required for sperm flagellum biogenesis and male fertility in mice

Multiple morphological abnormalities of the flagella (MMAF) is a severe disease of male infertility, while the pathogenetic mechanisms of MMAF are still incompletely understood. Previously, we found that the deficiency of Ccdc38 might be associated with MMAF. To understand the underlying mechanism of this disease, we identified the potential partner of this protein and found that the coiled-coil domain containing 146 (CCDC146) can interact with CCDC38. It is predominantly expressed in the testes, and the knockout of this gene resulted in complete infertility in male mice but not in females. The knockout of Ccdc146 impaired spermiogenesis, mainly due to flagellum and manchette organization defects, finally led to MMAF-like phenotype. Furthermore, we demonstrated that CCDC146 could interact with both CCDC38 and CCDC42. It also interacts with intraflagellar transport (IFT) complexes IFT88 and IFT20. The knockout of this gene led to the decrease of ODF2, IFT88, and IFT20 protein levels, but did not affect CCDC38, CCDC42, or ODF1 expression. Additionally, we predicted and validated the detailed interactions between CCDC146 and CCDC38 or CCDC42, and built the interaction models at the atomic level. Our results suggest that the testis predominantly expressed gene Ccdc146 is essential for sperm flagellum biogenesis and male fertility, and its mutations might be associated with MMAF in some patients.

Novel axonemal protein ZMYND12 interacts with TTC29 and DNAH1, and is required for male fertility and flagellum function

Male infertility is common and complex, presenting a wide range of heterogeneous phenotypes. Although about 50% of cases are estimated to have a genetic component, the underlying cause often remains undetermined. Here, from whole-exome sequencing on samples from 168 infertile men with asthenoteratozoospermia due to severe sperm flagellum, we identified homozygous ZMYND12 variants in four unrelated patients. In sperm cells from these individuals, immunofluorescence revealed altered localization of DNAH1, DNALI1, WDR66, and TTC29. Axonemal localization of ZMYND12 ortholog TbTAX-1 was confirmed using the Trypanosoma brucei model. RNAi knock-down of TbTAX-1 dramatically affected flagellar motility, with a phenotype similar to the sperm from men bearing homozygous ZMYND12 variants. Co-immunoprecipitation and ultrastructure expansion microscopy in T. brucei revealed TbTAX-1 to form a complex with TTC29. Comparative proteomics with samples from Trypanosoma and Ttc29 KO mice identified a third member of this complex: DNAH1. The data presented revealed that ZMYND12 is part of the same axonemal complex as TTC29 and DNAH1, which is critical for flagellum function and assembly in humans, and Trypanosoma. ZMYND12 is thus a new asthenoteratozoospermia-associated gene, bi-allelic variants of which cause severe flagellum malformations and primary male infertility.

The human sperm proteome-Toward a panel for male fertility testing

BACKGROUND

Although male factor accounts for 40%-50% of unintended childlessness, we are far from fully understanding the detailed causes. Usually, affected men cannot even be provided with a molecular diagnosis.

OBJECTIVES

We aimed at a higher resolution of the human sperm proteome for better understanding of the molecular causes of male infertility. We were particularly interested in why reduced sperm count decreases fertility despite many normal-looking spermatozoa and which proteins might be involved.

MATERIAL AND METHODS

Applying mass spectrometry analysis, we qualitatively and quantitatively examined the proteomic profiles of spermatozoa from 76 men differing in fertility. Infertile men had abnormal semen parameters and were involuntarily childless. Fertile subjects exhibited normozoospermia and had fathered children without medical assistance.

RESULTS

We discovered proteins from about 7000 coding genes in the human sperm proteome. These were mainly known for involvements in cellular motility, response to stimuli, adhesion, and reproduction. Numbers of sperm proteins showing at least threefold deviating abundances increased from oligozoospermia (N = 153) and oligoasthenozoospermia (N = 154) to oligoasthenoteratozoospermia (N = 368). Deregulated sperm proteins primarily engaged in flagellar assembly and sperm motility, fertilization, and male gametogenesis. Most of these participated in a larger network of male infertility genes and proteins.

DISCUSSION

We expose 31 sperm proteins displaying deviant abundances under infertility, which already were known before to have fertility relevance, including ACTL9, CCIN, CFAP47, CFAP65, CFAP251 (WDR66), DNAH1, and SPEM1. We propose 18 additional sperm proteins with at least eightfold differential abundance for further testing of their diagnostic potential, such as C2orf16, CYLC1, SPATA31E1, SPATA31D1, SPATA48, EFHB (CFAP21), and FAM161A.

CONCLUSION

Our results shed light on the molecular background of the dysfunctionality of the fewer spermatozoa produced in oligozoospermia and syndromes including it. The male infertility network presented may prove useful in further elucidating the molecular mechanism of male infertility.

TTC6-Mediated Stabilization of the Flagellum Annulus Ensures the Rapid and Directed Motion of Sperm

Sperm motility and structural integrity are essential for successful fertilization in vivo, and any hindrance of the correct assembly of the axoneme and peri-axonemal structures in the sperm flagellum can lead to fertility problems. While there has been considerable advancement in studying diseases related to the flagellum, the underlying mechanisms that control sperm movement are not yet fully understood. In this study, we reveal that the tetratricopeptide repeat protein 6 (Ttc6) gene, expressed mainly in the testes, plays a crucial role in maintaining male fertility in mice. We further demonstrate that the knockout of Ttc6 in mice results in decreased sperm motility and induces an abnormal circular swimming pattern, consequently leading to male subfertility. Morphological analysis showed an atypical hairpin-like appearance of the spermatozoa, and ultrastructural studies showed unsheathed flagella at the juncture between the midpiece and principal piece. Collectively, these findings suggest that TTC6 plays an essential role in maintaining the stability of the annulus region of the sperm flagellum, thus ensuring the swift and directed motion of sperm.

CFAP70 is a solid and valuable target for the genetic diagnosis of oligo-astheno-teratozoospermia in infertile men

BACKGROUND

Male infertility is a worldwide population health concern, but its aetiology remains largely understood. Although CFAP70 variants have already been reported in two oligo-astheno-teratozoospermia (OAT) individuals by sequencing, animal evidence to support CFAP70 as a credible OAT-pathogenic gene is lacking.

METHOD

Cfap70-KO mice were generated to explore the physiological role of CFAP70. CFAP70 variants were detected in infertile men with OAT by whole exome sequencing and Sanger sequencing confirmation. Cfap70-truncated mice were further generated to explore the pathogenicity of the nonsense variant of CFAP70 identified in the proband.

FINDINGS

Here, we demonstrate that Cfap70-KO mice are sterile mainly due to OAT and further identify a Chinese infertile man carrying a homozygous nonsense variant (c.2962C > T/p.R988X) of CFAP70. Cfap70-truncated mice lacking 5-8 tetratricopeptide repeats (TPRs) mimic the patient's symptoms. CFAP70 is required for the biogenesis of spermatid flagella partially by regulating the expression of OAT-associated proteins (e.g., QRICH2), assisting the cytoplasmic preassembly of the calmodulin- and radial spoke-associated complex (CSC), and controlling the manchette localization of axoneme-related proteins. Moreover, we suggest that CFAP70-associated male infertility could be overcome by intracytoplasmic sperm injection (ICSI) treatment.

INTERPRETATION

Overall, we demonstrate that CFAP70 is necessary to assemble spermatid flagella and that CFAP70 gene could be used as a diagnostic target for male infertility with OAT in the clinic.

FUNDING

This study was supported by the National Key Research and Development Project (2019YFA0802101 to S.C), Open Fund of Key Laboratory of Cell Proliferation and Regulation Biology, Ministry of Education (to S.C), Central Government to Guide Local Scientific and Technological Development (ZY21195023 to B.W), and Basic Research Projects of Central Scientific Research Institutes (to B.W).

Copy Number Variation and Structural Genomic Findings in 116 Cases of Sudden Unexplained Death between 1 and 28 Months of Age

In sudden unexplained death in pediatrics (SUDP) the cause of death is unknown despite an autopsy and investigation. The role of copy number variations (CNVs) in SUDP has not been well-studied. Chromosomal microarray (CMA) data are generated for 116 SUDP cases with age at death between 1 and 28 months. CNVs are classified using the American College of Medical Genetics and Genomics guidelines and CNVs in our cohort are compared to an autism spectrum disorder (ASD) cohort, and to a control cohort. Pathogenic CNVs are identified in 5 of 116 cases (4.3%). Variants of uncertain significance (VUS) favoring pathogenic CNVs are identified in 9 cases (7.8%). Several CNVs are associated with neurodevelopmental phenotypes including seizures, ASD, developmental delay, and schizophrenia. The structural variant 47,XXY is identified in two cases (2/69 boys, 2.9%) not previously diagnosed with Klinefelter syndrome. Pathogenicity scores for deletions are significantly elevated in the SUDP cohort versus controls (p = 0.007) and are not significantly different from the ASD cohort. The finding of pathogenic or VUS favoring pathogenic CNVs, or structural variants, in 12.1% of cases, combined with the observation of higher pathogenicity scores for deletions in SUDP versus controls, suggests that CMA should be included in the genetic evaluation of SUDP.

Biallelic CFAP61 variants cause male infertility in humans and mice with severe oligoasthenoteratozoospermia

BACKGROUND

The genetic causes for most male infertility due to severe oligoasthenoteratozoospermia (OAT) remain unclear.

OBJECTIVE

To identify the genetic cause of male infertility characterised by OAT.

METHODS

Variant screening was performed by whole-exome sequencing from 325 infertile patients with OAT and 392 fertile individuals. In silico and in vitro analyses were performed to evaluate the impacts of candidate disease-causing variants. A knockout mouse model was generated to confirm the candidate disease-causing gene, and intracytoplasmic sperm injection (ICSI) was used to evaluate the efficiency of clinical treatment.

RESULTS

We identified biallelic CFAP61 variants (NM_015585.4: c.1654C>T (p.R552C) and c.2911G>A (p.D971N), c.144-2A>G and c.1666G>A (p.G556R)) in two (0.62%) of the 325 OAT-affected men. In silico bioinformatics analysis predicted that all four variants were deleterious, and in vitro functional analysis confirmed the deleterious effects of the mutants. Notably, H&E staining and electron microscopy analyses of the spermatozoa revealed multiple morphological abnormalities of sperm flagella, the absence of central pair microtubules and mitochondrial sheath malformation in sperm flagella from man with CFAP61 variants. Further immunofluorescence assays revealed markedly reduced CFAP61 staining in the sperm flagella. In addition, Cfap61-deficient mice showed the OAT phenotype, suggesting that loss of function of CFAP61 was the cause of OAT. Two individuals accepted ICSI therapy using their own ejaculated sperm, and one of them succeeded in fathering a healthy baby.

CONCLUSIONS

Our findings indicate that CFAP61 is essential for spermatogenesis and that biallelic CFAP61 variants lead to male infertility in humans and mice with OAT.

New Mutations in DNHD1 Cause Multiple Morphological Abnormalities of the Sperm Flagella

Male infertility is a common and complex disease and presents as a wide range of heterogeneous phenotypes. Multiple morphological abnormalities of the sperm flagellum (MMAF) phenotype is a peculiar condition of extreme morphological sperm defects characterized by a mosaic of sperm flagellum defects to a total asthenozoospermia. At this time, about 40 genes were associated with the MMAF phenotype. However, mutation prevalence for most genes remains individually low and about half of individuals remain without diagnosis, encouraging us to pursue the effort to identify new mutations and genes. In the present study, an a cohort of 167 MMAF patients was analyzed using whole-exome sequencing, and we identified three unrelated patients with new pathogenic mutations in DNHD1, a new gene recently associated with MMAF. Immunofluorescence experiments showed that DNHD1 was totally absent from sperm cells from DNHD1 patients, supporting the deleterious effect of the identified mutations. Transmission electron microscopy reveals severe flagellum abnormalities of sperm cells from one mutated patient, which appeared completely disorganized with the absence of the central pair and midpiece defects with a shortened and misshapen mitochondrial sheath. Immunostaining of IFT20 was not altered in mutated patients, suggesting that IFT may be not affected by DNHD1 mutations. Our data confirmed the importance of DNHD1 for the function and structural integrity of the sperm flagellum. Overall, this study definitively consolidated its involvement in MMAF phenotype on a second independent cohort and enriched the mutational spectrum of the DNHD1 gene.

Clinical detection, diagnosis and treatment of morphological abnormalities of sperm flagella: A review of literature

Sperm carries male genetic information, and flagella help move the sperm to reach oocytes. When the ultrastructure of the flagella is abnormal, the sperm is unable to reach the oocyte and achieve insemination. Multiple morphological abnormalities of sperm flagella (MMAF) is a relatively rare idiopathic condition that is mainly characterized by multiple defects in sperm flagella. In the last decade, with the development of high-throughput DNA sequencing approaches, many genes have been revealed to be related to MMAF. However, the differences in sperm phenotypes and reproductive outcomes in many cases are attributed to different pathogenic genes or different pathogenic mutations in the same gene. Here, we will review information about the various phenotypes resulting from different pathogenic genes, including sperm ultrastructure and encoding proteins with their location and functions as well as assisted reproductive technology (ART) outcomes. We will share our clinical detection and diagnosis experience to provide additional clinical views and broaden the understanding of this disease.

Novel biallelic mutations in TTC29 cause asthenoteratospermia and male infertility

BACKGROUND

Multiple morphological abnormalities of the sperm flagella (MMAF), which is characterized as asthenoteratospermia involving absent, short, bent, coiled, and/or irregular-caliber flagella, is a rare recessive inherited disorder associated with male infertility. To date, genetic causes of MMAF cases are not fully explored.

METHODS

Whole-exome sequencing was conducted to identify pathogenic variants in a patient with MMAF. The functional effect of the identified mutations was investigated by immunofluorescence staining and western blotting. Intracytoplasmic sperm injection was used to assist fertilization for the patient with MMAF.

RESULTS

We identified novel biallelic mutations, a splicing variant NC_000004.12:g.146937593C>T (c.254+1G>A), and a nonsense mutation NM_001300761.4:c.1185C>G (NP_001287690.1:p.Tyr395*), in TTC29 from an infertile patient. In addition to the typical MMAF phenotype, the patient also presented aberrant morphology of sperm heads. Further functional experiments confirmed the absence of TTC29 expression in the spermatozoa. We also explored the specific expression pattern of TTC29 in human and mouse spermatogenesis. The outcome of intracytoplasmic sperm injection in the patient was unsuccessful, while additional female risk factors should not be excluded.

CONCLUSIONS

Our study revealed the novel biallelic mutations in TTC29 in a MMAF patient, which findings expand the mutational spectrum of TTC29 and further contribute to the diagnosis, genetic counseling, and prognosis of male infertility.

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.

Oligogenic heterozygous inheritance of sperm abnormalities in mouse

Male infertility is an important health concern that is expected to have a major genetic etiology. Although high-throughput sequencing has linked gene defects to more than 50% of rare and severe sperm anomalies, less than 20% of common and moderate forms are explained. We hypothesized that this low success rate could at least be partly due to oligogenic defects – the accumulation of several rare heterozygous variants in distinct, but functionally connected, genes. Here, we compared fertility and sperm parameters in male mice harboring one to four heterozygous truncating mutations of genes linked to multiple morphological anomalies of the flagellum (MMAF) syndrome. Results indicated progressively deteriorating sperm morphology and motility with increasing numbers of heterozygous mutations. This first evidence of oligogenic inheritance in failed spermatogenesis strongly suggests that oligogenic heterozygosity could explain a significant proportion of asthenoteratozoospermia cases. The findings presented pave the way to further studies in mice and man.

Homozygous SPAG6 variants can induce nonsyndromic asthenoteratozoospermia with severe MMAF

BACKGROUND

Multiple morphological abnormalities of the sperm flagella (MMAF) is a subtype of severe asthenoteratozoospermia with poorly understood genetic etiology. SPAG6 is a core axonemal component that plays a critical role in the formation of cilia and sperm flagella. Previous studies have reported that mutations in SPAG6 cause primary ciliary dyskinesia (PCD), but the association between SPAG6 gene variants and the MMAF phenotype has not yet been described.

METHODS

We performed whole-exome sequencing (WES) in two unrelated Han Chinese men with MMAF. Sanger sequencing was used to validate the candidate variants. Routine semen analysis was carried out according to the WHO guidelines (5^th Edition). Sperm morphology was assessed using modified Papanicolaou staining. Scanning and transmission electron microscopy (S/TEM) was performed to observe the ultrastructural defects of the sperm flagella. Western blot analysis and immunofluorescence (IF) of spermatozoa were performed to examine the expression of SPAG6 protein. Assisted fertilization with intracytoplasmic sperm injection (ICSI) was applied.

RESULTS

Two homozygous SPAG6 variants were identified by WES and Sanger validation in two patients with MMAF phenotype (F1 II-1: c.308C > A, p. A103D; F2 II-1: c. 585delA, p. K196Sfs*6). Semen analysis showed progressive rates of less than 1%, and most of the spermatozoa presented MMAF by Papanicolaou staining. TEM revealed that the overall axonemal ultrastructure was disrupted and primarily presented an abnormal "9 + 0" configuration. No other PCD-related symptoms were found on physical examination and medical consultations, as well as lung CT screening. The level of SPAG6 protein was significantly decreased in the spermatozoa, and IF analysis revealed that SPAG6 staining was extremely weak and discontinuous in the sperm flagella of the two patients. Notably, F1 II-1 and his wife conceived successfully after undergoing ICSI.

CONCLUSIONS

Our research provides new evidence for a potential correlation between SPAG6 variants and the MMAF phenotype.

PCD Genes-From Patients to Model Organisms and Back to Humans

Primary ciliary dyskinesia (PCD) is a hereditary genetic disorder caused by the lack of motile cilia or the assembxly of dysfunctional ones. This rare human disease affects 1 out of 10,000-20,000 individuals and is caused by mutations in at least 50 genes. The past twenty years brought significant progress in the identification of PCD-causative genes and in our understanding of the connections between causative mutations and ciliary defects observed in affected individuals. These scientific advances have been achieved, among others, due to the extensive motile cilia-related research conducted using several model organisms, ranging from protists to mammals. These are unicellular organisms such as the green alga Chlamydomonas, the parasitic protist Trypanosoma, and free-living ciliates, Tetrahymena and Paramecium, the invertebrate Schmidtea, and vertebrates such as zebrafish, Xenopus, and mouse. Establishing such evolutionarily distant experimental models with different levels of cell or body complexity was possible because both basic motile cilia ultrastructure and protein composition are highly conserved throughout evolution. Here, we characterize model organisms commonly used to study PCD-related genes, highlight their pros and cons, and summarize experimental data collected using these models.

Mitochondrial regulation during male germ cell development

Mitochondria tailor their morphology to execute their specialized functions in different cell types and/or different environments. During spermatogenesis, mitochondria undergo continuous morphological and distributional changes with germ cell development. Deficiencies in these processes lead to mitochondrial dysfunction and abnormal spermatogenesis, thereby causing male infertility. In recent years, mitochondria have attracted considerable attention because of their unique role in the regulation of piRNA biogenesis in male germ cells. In this review, we describe the varied characters of mitochondria and focus on key mitochondrial factors that play pivotal roles in the regulation of spermatogenesis, from primordial germ cells to spermatozoa, especially concerning metabolic shift, stemness and reprogramming, mitochondrial transformation and rearrangement, and mitochondrial defects in human sperm. Further, we discuss the molecular mechanisms underlying these processes.

Genetic diagnosis, sperm phenotype and ICSI outcome in case of severe asthenozoospermia with multiple morphological abnormalities of the flagellum

STUDY QUESTION

Are ICSI outcomes impaired in cases of severe asthenozoospermia with multiple morphological abnormalities of the flagellum (MMAF phenotype)?

SUMMARY ANSWER

Despite occasional technical difficulties, ICSI outcomes for couples with MMAF do not differ from those of other couples requiring ICSI, irrespective of the genetic defect.

WHAT IS KNOWN ALREADY

Severe asthenozoospermia, especially when associated with the MMAF phenotype, results in male infertility. Recent findings have confirmed that a genetic aetiology is frequently responsible for this phenotype. In such situations, pregnancies can be achieved using ICSI. However, few studies to date have provided detailed analyses regarding the flagellar ultrastructural defects underlying this phenotype, its genetic aetiologies, and the results of ICSI in such cases of male infertility.

STUDY DESIGN, SIZE, DURATION

We performed a retrospective study of 25 infertile men exhibiting severe asthenozoospermia associated with the MMAF phenotype identified through standard semen analysis. They were recruited at an academic centre for assisted reproduction in Paris (France) between 2009 and 2017. Transmission electron microscopy (TEM) and whole exome sequencing (WES) were performed in order to determine the sperm ultrastructural phenotype and the causal mutations, respectively. Finally 20 couples with MMAF were treated by assisted reproductive technologies based on ICSI.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Patients with MMAF were recruited based on reduced sperm progressive motility and increased frequencies of absent, short, coiled or irregular flagella compared with those in sperm from fertile control men. A quantitative analysis of the several ultrastructural defects was performed for the MMAF patients and for fertile men. The ICSI results obtained for 20 couples with MMAF were compared to those of 378 men with oligoasthenoteratozoospermia but no MMAF as an ICSI control group.

MAIN RESULTS AND THE ROLE OF CHANCE

TEM analysis and categorisation of the flagellar anomalies found in these patients provided important information regarding the structural defects underlying asthenozoospermia and sperm tail abnormalities. In particular, the absence of the central pair of axonemal microtubules was the predominant anomaly observed more frequently than in control sperm (P < 0.01). Exome sequencing, performed for 24 of the 25 patients, identified homozygous or compound heterozygous pathogenic mutations in CFAP43, CFAP44, CFAP69, DNAH1, DNAH8, AK7, TTC29 and MAATS1 in 13 patients (54.2%) (11 affecting MMAF genes and 2 affecting primary ciliary dyskinesia (PCD)-associated genes). A total of 40 ICSI cycles were undertaken for 20 MMAF couples, including 13 cycles (for 5 couples) where a hypo-osmotic swelling (HOS) test was required due to absolute asthenozoospermia. The fertilisation rate was not statistically different between the MMAF (65.7%) and the non-MMAF (66.0%) couples and it did not differ according to the genotype or the flagellar phenotype of the subjects or use of the HOS test. The clinical pregnancy rate per embryo transfer did not differ significantly between the MMAF (23.3%) and the non-MMAF (37.1%) groups. To date, 7 of the 20 MMAF couples have achieved a live birth from the ICSI attempts, with 11 babies born without any birth defects.

LIMITATIONS, REASONS FOR CAUTION

The ICSI procedure outcomes were assessed retrospectively on a small number of affected subjects and should be confirmed on a larger cohort. Moreover, TEM analysis could not be performed for all patients due to low sperm concentrations, and WES results are not yet available for all of the included men.

WIDER IMPLICATIONS OF THE FINDINGS

An early and extensive phenotypic and genetic investigation should be considered for all men requiring ICSI for severe asthenozoospermia. Although our study did not reveal any adverse ICSI outcomes associated with MMAF, we cannot rule out that some rare genetic causes could result in low fertilisation or pregnancy rates.

STUDY FUNDING/COMPETING INTEREST(S)

No external funding was used for this study and there are no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

A novel stop-gain mutation in ARMC2 is associated with multiple morphological abnormalities of the sperm flagella

RESEARCH QUESTION

Male infertility is a global issue worldwide and multiple morphological abnormalities of the sperm flagella (MMAF) is one of the most severe forms of the qualitative sperm defects with a heterogeneous genetic cause that has not been completely understood. Can whole-exome sequencing (WES) reveal novel genetic causes contributing to MMAF in a consanguineous Pakistani family, comprising three infertile brothers?

DESIGN

WES and bioinformatic analysis were conducted to screen potential pathogenic variants. The identified variant was validated by Sanger sequencing in all available family members Transmission electron microscopy analyses was carried out to examine the flagella ultrastructure of spermatozoa from patient.

RESULTS

WES and Sanger sequencing identified a novel homozygous stop-gain mutation (ENST00000392644.4, c.182C>G, p.S61X) in ARMC2, which is expected to lead to loss of protein functions. Transmission electron microscopy analyses revealed that the flagellar ultrastructure of the patient's spermatozoa was disorganized along with a complete absence of central pair complex (CPC), suggesting that ARMC2 is involved in the assembly, stability of the axonemal complex, or both, particularly the CPC.

CONCLUSION

We report that a familial stop-gain mutation in ARMC2 is associated with male infertility in humans caused by MMAF accompanied with loss of CPCs and axonemal disorganization. We provide genetic evidence that ARMC2 is essential for human spermatogenesis and its mutation may be pathogenic for MMAF. These findings will improve the knowledge about the genetic basis of MMAF and provide information for genetic counselling of this disease.

Homozygous mutations in CCDC34 cause male infertility with oligoasthenoteratozoospermia in humans and mice

BACKGROUND

Oligoasthenoteratozoospermia is a typical feature of sperm malformations leading to male infertility. Only a few genes have been clearly identified as pathogenic genes of oligoasthenoteratozoospermia.

METHODS AND RESULTS

Here, we identified a homozygous frameshift variant (c.731dup, p.Asn244Lysfs*3) in CCDC34, which is preferentially expressed in the human testis, using whole-exome sequencing in a cohort of 100 Chinese men with multiple morphological abnormalities of the sperm flagella (MMAF). In an additional cohort of 167 MMAF-affected men from North Africa, Iran and France, we identified a second subject harbouring a homozygous CCDC34 frameshift variant (c.799_817del, p.Glu267Lysfs*72). Both affected men presented a typical MMAF phenotype with an abnormally low sperm concentration (ie, oligoasthenoteratozoospermia). Transmission electron microscopy analysis of the sperm flagella affected by CCDC34 deficiency further revealed dramatic disorganisation of the axoneme. Immunofluorescence assays of the spermatozoa showed that CCDC34 deficiency resulted in almost absent staining of CCDC34 and intraflagellar transport-B complex-associated proteins (such as IFT20 and IFT52). Furthermore, we generated a mouse Ccdc34 frameshift mutant using CRISPR-Cas9 technology. Ccdc34-mutated (Ccdc34^mut/mut ) male mice were sterile and presented oligoasthenoteratozoospermia with typical MMAF anomalies. Intracytoplasmic sperm injection has good pregnancy outcomes in both humans and mice.

CONCLUSIONS

Our findings support that CCDC34 is crucial to the formation of sperm flagella and that biallelic deleterious mutations in CCDC34/Ccdc34 cause male infertility with oligoasthenoteratozoospermia in humans and mice.

Exome sequencing reveals variants in known and novel candidate genes for severe sperm motility disorders

STUDY QUESTION

What are the causative genetic variants in patients with male infertility due to severe sperm motility disorders?

SUMMARY ANSWER

We identified high confidence disease-causing variants in multiple genes previously associated with severe sperm motility disorders in 10 out of 21 patients (48%) and variants in novel candidate genes in seven additional patients (33%).

WHAT IS KNOWN ALREADY

Severe sperm motility disorders are a form of male infertility characterised by immotile sperm often in combination with a spectrum of structural abnormalities of the sperm flagellum that do not affect viability. Currently, depending on the clinical sub-categorisation, up to 50% of causality in patients with severe sperm motility disorders can be explained by pathogenic variants in at least 22 genes.

STUDY DESIGN, SIZE, DURATION

We performed exome sequencing in 21 patients with severe sperm motility disorders from two different clinics.

PARTICIPANTS/MATERIALS, SETTING, METHOD

Two groups of infertile men, one from Argentina (n = 9) and one from Australia (n = 12), with clinically defined severe sperm motility disorders (motility <5%) and normal morphology values of 0–4%, were included. All patients in the Argentine cohort were diagnosed with DFS-MMAF, based on light and transmission electron microscopy. Sperm ultrastructural information was not available for the Australian cohort. Exome sequencing was performed in all 21 patients and variants with an allele frequency of <1% in the gnomAD population were prioritised and interpreted.

MAIN RESULTS AND ROLE OF CHANCE

In 10 of 21 patients (48%), we identified pathogenic variants in known sperm assembly genes: CFAP43 (3 patients); CFAP44 (2 patients), CFAP58 (1 patient), QRICH2 (2 patients), DNAH1 (1 patient) and DNAH6 (1 patient). The diagnostic rate did not differ markedly between the Argentinian and the Australian cohort (55% and 42%, respectively). Furthermore, we identified patients with variants in the novel human candidate sperm motility genes: DNAH12, DRC1, MDC1, PACRG, SSPL2C and TPTE2. One patient presented with variants in four candidate genes and it remains unclear which variants were responsible for the severe sperm motility defect in this patient.

LARGE SCALE DATA

N/A

LIMITATIONS, REASONS FOR CAUTION

In this study, we described patients with either a homozygous or two heterozygous candidate pathogenic variants in genes linked to sperm motility disorders. Due to unavailability of parental DNA, we have not assessed the frequency of de novo or maternally inherited dominant variants and could not determine the parental origin of the mutations to establish in all cases that the mutations are present on both alleles.

WIDER IMPLICATIONS OF THE FINDINGS

Our results confirm the likely causal role of variants in six known genes for sperm motility and we demonstrate that exome sequencing is an effective method to diagnose patients with severe sperm motility disorders (10/21 diagnosed; 48%). Furthermore, our analysis revealed six novel candidate genes for severe sperm motility disorders. Genome-wide sequencing of additional patient cohorts and re-analysis of exome data of currently unsolved cases may reveal additional variants in these novel candidate genes.

STUDY FUNDING/COMPETING INTEREST(S)

This project was supported in part by funding from the Australian National Health and Medical Research Council (APP1120356) to M.K.O.B., J.A.V. and R.I.M.L., The Netherlands Organisation for Scientific Research (918-15-667) to J.A.V., the Royal Society and Wolfson Foundation (WM160091) to J.A.V., as well as an Investigator Award in Science from the Wellcome Trust (209451) to J.A.V. and Grants from the National Research Council of Argentina (PIP 0900 and 4584) and ANPCyT (PICT 9591) to H.E.C. and a UUKi Rutherford Fund Fellowship awarded to B.J.H.

Mutational landscape of DNAH1 in Chinese patients with multiple morphological abnormalities of the sperm flagella: cohort study and literature review

PURPOSE

Multiple morphological abnormalities of the sperm flagella (MMAF) are important causes of male infertility. Mutations in DNAH1 are the main causative factors proven so far. We aim to determine the mutational landscape of DNAH1 in Chinese patients with MMAF.

METHODS

Forty-one Chinese patients with MMAF were enrolled and underwent a 10-gene next-generation sequencing panel screening.

RESULTS

Only the DNAH1 gene was found to have mutations in 12 of these unrelated individuals (29%). Combining published data from two other cohorts of Chinese men with MMAF, we suggest that p.P3909fs*33, p.R868X, p.Q1518X, p.E3284K, and p.R4096L are hotspot mutations. A polymorphism-rs12163565 (G>A)- showed linkage to p.P3909fs*33, suggesting that this involved a founder effect. Four of the 12 patients with DNAH1 mutations were able to use intracytoplasmic sperm injection with their partners and all were successful in obtaining embryos.

CONCLUSIONS

Hotspot mutations were identified for Chinese patients with MMAF. MMAF sub-phenotypes might be associated with different combinations of DNAH1 mutations.

Composition and function of ciliary inner-dynein-arm subunits studied in Chlamydomonas reinhardtii

Motile cilia (also interchangeably called "flagella") are conserved organelles extending from the surface of many animal cells and play essential functions in eukaryotes, including cell motility and environmental sensing. Large motor complexes, the ciliary dyneins, are present on ciliary outer-doublet microtubules and drive movement of cilia. Ciliary dyneins are classified into two general types: the outer dynein arms (ODAs) and the inner dynein arms (IDAs). While ODAs are important for generation of force and regulation of ciliary beat frequency, IDAs are essential for control of the size and shape of the bend, features collectively referred to as waveform. Also, recent studies have revealed unexpected links between IDA components and human diseases. In spite of their importance, studies on IDAs have been difficult since they are very complex and composed for several types of IDA motors, each unique in composition and location in the axoneme. Thanks in part to genetic, biochemical, and structural analysis of Chlamydomonas reinhardtii, we are beginning to understand the organization and function of the ciliary IDAs. In this review, we summarize the composition of Chlamydomonas IDAs particularly focusing on each subunit, and discuss the assembly, conservation, and functional role(s) of these IDA subunits. Furthermore, we raise several additional questions/challenges regarding IDAs, and discuss future perspectives of IDA studies.

A missense mutation in IFT74, encoding for an essential component for intraflagellar transport of Tubulin, causes asthenozoospermia and male infertility without clinical signs of Bardet-Biedl syndrome

Cilia and flagella are formed around an evolutionary conserved microtubule-based axoneme and are required for fluid and mucus clearance, tissue homeostasis, cell differentiation and movement. The formation and maintenance of cilia and flagella require bidirectional transit of proteins along the axonemal microtubules, a process called intraflagellar transport (IFT). In humans, IFT defects contribute to a large group of systemic diseases, called ciliopathies, which often display overlapping phenotypes. By performing exome sequencing of a cohort of 167 non-syndromic infertile men displaying multiple morphological abnormalities of the sperm flagellum (MMAF) we identified two unrelated patients carrying a homozygous missense variant adjacent to a splice donor consensus site of IFT74 (c.256G > A;p.Gly86Ser). IFT74 encodes for a core component of the IFT machinery that is essential for the anterograde transport of tubulin. We demonstrate that this missense variant affects IFT74 mRNA splicing and induces the production of at least two distinct mutant proteins with abnormal subcellular localization along the sperm flagellum. Importantly, while IFT74 deficiency was previously implicated in two cases of Bardet-Biedl syndrome, a pleiotropic ciliopathy with variable expressivity, our data indicate that this missense mutation only results in primary male infertility due to MMAF, with no other clinical features. Taken together, our data indicate that the nature of the mutation adds a level of complexity to the clinical manifestations of ciliary dysfunction, thus contributing to the expanding phenotypical spectrum of ciliopathies.

The sodium/proton exchanger SLC9C1 (sNHE) is essential for human sperm motility and fertility

Asthenozoospermia, defined by the absence or reduction of sperm motility, constitutes the most frequent cause of human male infertility. This pathological condition is caused by morphological and/or functional defects of the sperm flagellum, which preclude proper sperm progression. While in the last decade many causal genes were identified for asthenozoospermia associated with severe sperm flagellar defects, the causes of purely functional asthenozoospermia are still poorly defined. We describe here the case of an infertile man, displaying asthenozoospermia without major morphological flagellar anomalies and carrying a homozygous splicing mutation in SLC9C1 (sNHE), which we identified by whole-exome sequencing. SLC9C1 encodes a sperm-specific sodium/proton exchanger, which in mouse regulates pH homeostasis and interacts with the soluble adenylyl cyclase (sAC), a key regulator of the signalling pathways involved in sperm motility and capacitation. We demonstrate by means of RT-PCR, immunodetection and immunofluorescence assays on patient's semen samples that the homozygous splicing mutation (c.2748 + 2 T > C) leads to in-frame exon skipping resulting in a deletion in the cyclic nucleotide-binding domain of the protein. Our work shows that in human, similar to mouse, SLC9C1 is required for sperm motility. Overall, we establish a homozygous truncating mutation in SLC9C1 as a novel cause of human asthenozoospermia and infertility.

Novel bi-allelic variants in DNAH2 cause severe asthenoteratozoospermia with multiple morphological abnormalities of the flagella

RESEARCH QUESTION

Multiple morphological abnormalities of the flagella (MMAF) is characterized by excessive immotile spermatozoa with severe flagellar abnormalities in the ejaculate. Previous studies have reported a heterogeneous genetic profile associated with MMAF. What other genetic variants might explain the cause of MMAF?

DESIGN

Whole-exome sequencing was conducted in a cohort of 90 Chinese patients with MMAF. The pathogenicity of identified mutations was assessed through electron microscopy and immunofluorescent examinations.

RESULTS

Three unrelated men with bi-allelic DNAH2 variants were identified. Sanger sequencing verified that the six novel variants originated from every parent. All these variants were located at the conserved domains of DNAH2 and predicted to be deleterious by bioinformatic tools. Haematoxylin and eosin staining and scanning electron microscopy revealed that spermatozoa harbouring DNAH2 variants displayed severely aberrant morphology mainly with absent and short flagella (≥78%). Moreover, transmission electron microscopy revealed the obvious absence of a central pair of microtubules and inner dynein arms in the spermatozoa with mutated DNAH2. Immunofluorescence data further validated these findings, showing reduced DNAH2 protein expression in the spermatozoa with DNAH2 variants, compared with normal spermatozoa. Intracytoplasmic sperm injection using spermatozoa from the three men with mutated DNAH2 resulted in blastocyst formation in all cases. Embryo transfer was carried out in two couples, both resulting in clinical pregnancy.

CONCLUSIONS

These experimental and clinical data suggest that bi-allelic DNAH2 variants might induce MMAF-associated asthenoteratozoospermia, which can be overcome through intracytoplasmic sperm injection. These findings contribute to the knowledge of the genetic landscape of asthenoteratozoospermia and clinical counselling of male infertility.

Screening, identification and interaction analysis of key MicroRNAs and genes in Asthenozoospermia

Background: Asthenozoospermia, one of the most common causes of male infertility, is a complicate multifactorial pathological condition that genetic factors are involved in. However, the epigenetic signature and mechanism of asthenozoospermia still remain limited. Our study aimed to confirm the key microRNAs (miRNAs) and genes in asthenozoospermia and demonstrate the underlying epigenetic regulatory mechanisms. Methods: We screened out and pooled previous studies to extracted potential differentially expressed miRNAs (DEMs). GSE22331 and a published profile dataset were integrated to identify differentially expressed genes (DEGs). Pathway and gene ontology analysis were performed using DAVID. A protein-protein network (PPI) was constructed using STRING. The target genes of DEMs were predicted using TargetScan and the miRNA-mRNA network was built. Results: We reported 3 DEMs and 423 DEGs by pooling included dataset and published studies. Pathway analysis showed that these DEGs might participate in signaling pathways regulating pluripotency of stem cells, Wnt signaling pathway and Notch signaling pathway. 25 hub genes were identified, and the most significant gene was BDNF. We screened out the overlapped DEGs between the predicted target genes of 3 DEMs and the 423 DEGs. Finally, a potential miRNA-mRNA regulatory network was constructed. Conclusion: This study firstly pooled several published studies and a GEO dataset to determine the significance of potential miRNAs and genes, such as miR-374b, miR-193a, miR-34b, BDNF, NTRK2, HNRNPD and EFTUD2 in regulating asthenozoospermia and underscore their interactions in the pathophysiological mechanism. Our results provided theoretical basis and new clues for potential therapeutic treatment in asthenozoospermia. Validations in vivo and in vitro are required in future studies.

Genetic underpinnings of asthenozoospermia

Asthenozoospermia (AZS), defined by reduced motility or absent sperm motility, is one of the main causes of male infertility. This condition may be divided into isolated AZS in the absence of other symptoms and syndromic AZS, which is characterized by several concurrent clinical symptoms. Sperm motility depends on fully functional flagellum, energy availability, and the crosstalk of several signaling pathways; therefore, mutations in genes involved in flagellar assembly and motile regulation can cause AZS. Thus, it is crucial to understand the genetic causes and mechanisms contributing to AZS. In this review, we summarize the current knowledge about the particular genes and mechanisms involved in intact flagellum, energy availability, and signaling transduction that could cause human AZS and discuss the respective gene defects known to be responsible for these abnormalities. Additionally, we discuss intracytoplasmic sperm injection outcomes and offspring health where available in these cases.

Molecular genetics of infertility: loss-of-function mutations in humans and corresponding knockout/mutated mice

BACKGROUND

Infertility is a major issue in human reproductive health, affecting an estimated 15% of couples worldwide. Infertility can result from disorders of sex development (DSD) or from reproductive endocrine disorders (REDs) with onset in infancy, early childhood or adolescence. Male infertility, accounting for roughly half of all infertility cases, generally manifests as decreased sperm count (azoospermia or oligozoospermia), attenuated sperm motility (asthenozoospermia) or a higher proportion of morphologically abnormal sperm (teratozoospermia). Female infertility can be divided into several classical types, including, but not limited to, oocyte maturation arrest, premature ovarian insufficiency (POI), fertilization failure and early embryonic arrest. An estimated one half of infertility cases have a genetic component; however, most genetic causes of human infertility are currently uncharacterized. The advent of high-throughput sequencing technologies has greatly facilitated the identification of infertility-associated gene mutations in patients over the past 20 years.

OBJECTIVE AND RATIONALE

This review aims to conduct a narrative review of the genetic causes of human infertility. Loss-of-function mutation discoveries related to human infertility are summarized and further illustrated in tables. Corresponding knockout/mutated animal models of causative genes for infertility are also introduced.

SEARCH METHODS

A search of the PubMed database was performed to identify relevant studies published in English. The term 'mutation' was combined with a range of search terms related to the core focus of the review: infertility, DSD, REDs, azoospermia or oligozoospermia, asthenozoospermia, multiple morphological abnormalities of the sperm flagella (MMAF), primary ciliary dyskinesia (PCD), acephalic spermatozoa syndrome (ASS), globozoospermia, teratozoospermia, acrosome, oocyte maturation arrest, POI, zona pellucida, fertilization defects and early embryonic arrest.

OUTCOMES

Our search generated ∼2000 records. Overall, 350 articles were included in the final review. For genetic investigation of human infertility, the traditional candidate gene approach is proceeding slowly, whereas high-throughput sequencing technologies in larger cohorts of individuals is identifying an increasing number of causative genes linked to human infertility. This review provides a wide panel of gene mutations in several typical forms of human infertility, including DSD, REDs, male infertility (oligozoospermia, MMAF, PCD, ASS and globozoospermia) and female infertility (oocyte maturation arrest, POI, fertilization failure and early embryonic arrest). The causative genes, their identified mutations, mutation rate, studied population and their corresponding knockout/mutated mice of non-obstructive azoospermia, MMAF, ASS, globozoospermia, oocyte maturation arrest, POI, fertilization failure and early embryonic arrest are further illustrated by tables. In this review, we suggest that (i) our current knowledge of infertility is largely obtained from knockout mouse models; (ii) larger cohorts of clinical cases with distinct clinical characteristics need to be recruited in future studies; (iii) the whole picture of genetic causes of human infertility relies on both the identification of more mutations for distinct types of infertility and the integration of known mutation information; (iv) knockout/mutated animal models are needed to show whether the phenotypes of genetically altered animals are consistent with findings in human infertile patients carrying a deleterious mutation of the homologous gene; and (v) the molecular mechanisms underlying human infertility caused by pathogenic mutations are largely unclear in most current studies.

WILDER IMPLICATIONS

It is important to use our current understanding to identify avenues and priorities for future research in the field of genetic causes of infertility as well as to apply mutation knowledge to risk prediction, genetic diagnosis and potential treatment for human infertility.

Bi-allelic Loss-of-function Variants in CFAP58 Cause Flagellar Axoneme and Mitochondrial Sheath Defects and Asthenoteratozoospermia in Humans and Mice

Multiple morphological abnormalities of the sperm flagella (MMAF) is a severe form of asthenoteratozoospermia. Although recent studies have revealed several MMAF-associated genes and demonstrated MMAF to be a genetically heterogeneous disease, at least one-third of the cases are still not well understood for their etiology. Here, we identified bi-allelic loss-of-function variants in CFAP58 by using whole-exome sequencing in five (5.6%) unrelated individuals from a cohort of 90 MMAF-affected Chinese men. Each of the men harboring bi-allelic CFAP58 variants presented typical MMAF phenotypes. Transmission electron microscopy demonstrated striking flagellar defects with axonemal and mitochondrial sheath malformations. CFAP58 is predominantly expressed in the testis and encodes a cilia- and flagella-associated protein. Immunofluorescence assays showed that CFAP58 localized at the entire flagella of control sperm and predominantly concentrated in the mid-piece. Immunoblotting and immunofluorescence assays showed that the abundances of axoneme ultrastructure markers SPAG6 and SPEF2 and a mitochondrial sheath protein, HSP60, were significantly reduced in the spermatozoa from men harboring bi-allelic CFAP58 variants. We generated Cfap58-knockout mice via CRISPR/Cas9 technology. The male mice were infertile and presented with severe flagellar defects, consistent with the sperm phenotypes in MMAF-affected men. Overall, our findings in humans and mice strongly suggest that CFAP58 plays a vital role in sperm flagellogenesis and demonstrate that bi-allelic loss-of-function variants in CFAP58 can cause axoneme and peri-axoneme malformations leading to male infertility. This study provides crucial insights for understanding and counseling of MMAF-associated asthenoteratozoospermia.

Bi-allelic DNAH8 Variants Lead to Multiple Morphological Abnormalities of the Sperm Flagella and Primary Male Infertility

Sperm malformation is a direct factor for male infertility. Multiple morphological abnormalities of the flagella (MMAF), a severe form of asthenoteratozoospermia, are characterized by immotile spermatozoa with malformed and/or absent flagella in the ejaculate. Previous studies indicated genetic heterogeneity in MMAF. To further define genetic factors underlying MMAF, we performed whole-exome sequencing in a cohort of 90 Chinese MMAF-affected men. Two cases (2.2%) were identified as carrying bi-allelic missense DNAH8 variants, variants which were either absent or rare in the control human population and were predicted to be deleterious by multiple bioinformatic tools. Re-analysis of exome data from a second cohort of 167 MMAF-affected men from France, Iran, and North Africa permitted the identification of an additional male carrying a DNAH8 homozygous frameshift variant. DNAH8 encodes a dynein axonemal heavy-chain component that is expressed preferentially in the testis. Hematoxylin-eosin staining and electron microscopy analyses of the spermatozoa from men harboring bi-allelic DNAH8 variants showed a highly aberrant morphology and ultrastructure of the sperm flagella. Immunofluorescence assays performed on the spermatozoa from men harboring bi-allelic DNAH8 variants revealed the absent or markedly reduced staining of DNAH8 and its associated protein DNAH17. Dnah8-knockout male mice also presented typical MMAF phenotypes and sterility. Interestingly, intracytoplasmic sperm injections using the spermatozoa from Dnah8-knockout male mice resulted in good pregnancy outcomes. Collectively, our experimental observations from humans and mice demonstrate that DNAH8 is essential for sperm flagellar formation and that bi-allelic deleterious DNAH8 variants lead to male infertility with MMAF.

The genetic architecture of morphological abnormalities of the sperm tail

Spermatozoa contain highly specialized structural features reflecting unique functions required for fertilization. Among them, the flagellum is a sperm-specific organelle required to generate the motility, which is essential to reach the egg. The flagellum integrity is, therefore, critical for normal sperm function and flagellum defects consistently lead to male infertility due to reduced or absent sperm motility defined as asthenozoospermia. Multiple morphological abnormalities of the flagella (MMAF), also called short tails, is among the most severe forms of sperm flagellum defects responsible for male infertility and is characterized by the presence in the ejaculate of spermatozoa being short, coiled, absent and of irregular caliber. Recent studies have demonstrated that MMAF is genetically heterogeneous which is consistent with the large number of proteins (over one thousand) localized in the human sperm flagella. In the past 5 years, genomic investigation of the MMAF phenotype allowed the identification of 18 genes whose mutations induce MMAF and infertility. Here we will review information about those genes including their expression pattern, the features of the encoded proteins together with their localization within the different flagellar protein complexes (axonemal or peri-axonemal) and their potential functions. We will categorize the identified MMAF genes following the protein complexes, functions or biological processes they may be associated with, based on the current knowledge in the field.