Oligogenic heterozygous inheritance of sperm abnormalities in mouse

Genetic variants in QRICH2 gene among Jordanians with sperm motility disorders

Sperm motility, a key determinant of male fertility, is often impaired by genetic variations affecting flagellar formation. The glutamine-rich protein 2 (QRICH2) gene encodes a protein essential for sperm flagella biogenesis and structural integrity. This study investigates genetic variations within exon 3 of the QRICH2 gene, identifying novel heterozygous variants associated with sperm tail-specific abnormalities and motility impairments. Among 34 individuals diagnosed with asthenozoospermia (ASZ) and 26 individuals with normal sperm parameters (NZ) from Jordan, eight unique heterozygous variants (c.123 G>T, c.133 G>C, c.138A>G, c.170A>C, c.189C>G, c.190T>C, c.195A>T, and c.204A>T) were exclusive to the ASZ group, while four variants (c.136 G>A, c.145A>C, c.179T>G, and c.180T>G) were found only in NZ. These variants were absent from major genetic databases, suggesting their potential novelty, while two variants (c.206C>T and c.189C>T) were linked to known SNP cluster IDs rs73996306 and rs1567790525, respectively. Four non-synonymous SNPs (c.136 G>A, c.145A>C, c.170A>C, and c.204A>T) were predicted to be functionally and structurally damaging, underscoring their significance. Additionally, five variants overlapped with previously reported mutation sites, indicating potential mutation hotspots. Statistical analysis revealed a significant association between QRICH2 mutations and tail defects (p < 0.021). These findings highlight the critical role of heterozygous QRICH2 mutations in mild-to-moderate ASZ, even in NZ individuals. Despite some carriers meeting WHO criteria for NZ, notable morphological abnormalities suggest the need for refined diagnostic benchmarks. Screening for QRICH2 mutations is essential for accurate molecular diagnosis and should be integrated into genetic counseling, particularly in regions like Jordan. Further research into the cumulative effects of heterozygous mutations and their environmental interactions is needed to expand our understanding of idiopathic male infertility and to enhance diagnostic and therapeutic strategies for male infertility.

Exploring the therapeutic effect of melatonin targeting common biomarkers in testicular germ cell tumor, prostate adenocarcinoma, and male infertility: an integrated biology approach

Globally, male infertility (MI) is a major concern. Several other comorbidities related to MI are testicular germ cell tumor (TGCT) and prostate adenocarcinoma (PRAD). This study focuses on finding the common biomarkers among these diseases and their interaction with Melatonin (MLT). The differential expressed genes were retrieved using the GEPIA2 database for TGCT and PRAD, whereas the DISGENET database for MI-related genes. InteractiVenn was performed in response to identify the common genes. The STAG3, RNF212, DDX3Y, DPY19L2, TPCN1, KLK3, GNRH1, DMD, CCDC146, and DNAH1 are found to be involved in all these diseases. The gene ontologies and pathway enrichment analysis were done for these significant genes in response to identifying and accessing the involvement of these genes in other processes. MLT is a neuroendocrine hormone with high therapeutic properties. MLT showed the best binding energy with DDX3Y among all the proteins. Molecular dynamic simulation (MDS) of MLT with DDX3Y was performed and found to be -52.382 ± 13.110 kJ/mol binding energy. The RMSD, RMSF, SASA, RG, H-bond, FEL, PCA, and MM-PBSA analysis confirm the stability and compactness of the DDX3Y-MLT complex. The MDS results indicate that MLT is a promising therapeutic option for enhancing DDX3Y expression, which will support spermatogenesis. Additionally, the hub genes were identified based on MCC parameters from the merged interactive network of common genes in response to finding significant genes that can be a potential biomarker for the diagnosis of diseases.

A comprehensive study of the sperm head defects in MMAF condition and their impact on embryo development in mice

Among rare cases of teratozoospermia, MMAF (multiple morphological abnormalities of the flagellum) syndrome is a complex genetic disorder involving at least 70 different genes. As the name suggests, patients with MMAF syndrome produce spermatozoa with multiple flagellar defects, rendering them immobile and non-fertilizing, leading to complete infertility in affected men. The only viable treatment option is ICSI. What is less understood is the presence of the various types of head defects in the spermatozoa, which are consistently present. Due to the involvement of numerous genes and the limited number of patients with MMAF syndrome, research on head defects and their impact on embryonic development remains insufficiently explored. To address these questions, a comparative study was conducted under controlled experimental conditions using four knockout (KO) mouse lines targeting Cfap43, Cfap44, Armc2, and Ccdc146 genes, all associated with MMAF syndrome in humans and mice. Each KO line underwent a detailed examination of nuclear defects, including morphology, DNA compaction, chromosomal architecture, and ploidy. The study revealed significant heterogeneity among the four lineages, with the extent of defects varying depending on the lineage, ranked as Ccdc146-/- > Cfap43-/- > Armc2-/- ≈ Cfap44-/-. The developmental potential of sperm from males in each lineage was assessed by injecting them into wild-type oocytes, and embryo development was monitored up to the blastocyst stage. Sperm from all KO lines exhibited a marked decrease in supporting embryo development compared to the wild-type, with developmental failure rates ranked as follows: Ccdc146 > Cfap43 > Armc2 > Cfap44-deficient sperm. The degree of developmental failure thus correlated with the severity of nuclear defects, and zygotes produced with sperm from Ccdc146-/- and Cfap43-/- mice showed the highest rates of developmental impairment. These findings from preclinical models highlight the heterogeneous nature of MMAF syndrome, both in terms of sperm nuclear defects and developmental potentials. Genetic characterization in humans is therefore crucial for improving therapeutic counselling in affected individuals.

Insight into the complexity of male infertility: a multi-omics review

Male infertility is a reproductive disorder, accounting for 40-50% of infertility. Currently, in about 70% of infertile men, the cause remains unknown. With the introduction of novel omics and advancement in high-throughput technology, potential biomarkers are emerging. The main purpose of our work was to overview different aspects of omics approaches in association with idiopathic male infertility and highlight potential genes, transcripts, non-coding RNA, proteins, and metabolites worth further exploring. Using the Gene Ontology (GO) analysis, we aimed to compare enriched GO terms from each omics approach and determine their overlapping. A PubMed database screening for the literature published between February 2014 and June 2022 was performed using the keywords: male infertility in association with different omics approaches: genomics, epigenomics, transcriptomics, ncRNAomics, proteomics, and metabolomics. A GO enrichment analysis was performed using the Enrichr tool. We retrieved 281 global studies: 171 genomics (DNA level), 21 epigenomics (19 of methylation and two histone residue modifications), 15 transcriptomics, 31 non-coding RNA, 29 proteomics, two protein posttranslational modification, and 19 metabolomics studies. Gene ontology comparison showed that different omics approaches lead to the identification of different molecular factors and that the corresponding GO terms, obtained from different omics approaches, do not overlap to a larger extent. With the integration of novel omics levels into the research of idiopathic causes of male infertility, using multi-omic systems biology approaches, we will be closer to finding the potential biomarkers and consequently becoming aware of the entire spectrum of male infertility, their cause, prognosis, and potential treatment.

Constitutive Androstane Receptor Regulates Germ Cell Homeostasis, Sperm Quality, and Male Fertility via Akt-Foxo1 Pathway

Male sexual function can be disrupted by exposure to exogenous compounds that cause testicular physiological alterations. The constitutive androstane receptor (Car) is a receptor for both endobiotics and xenobiotics involved in detoxification. However, its role in male fertility, particularly in regard to the reprotoxic effects of environmental pollutants, remains unclear. This study aims to investigate the role of the Car signaling pathway in male fertility. In vivo, in vitro, and pharmacological approaches are utilized in wild-type and Car-deficient mouse models. The results indicate that Car inhibition impaired male fertility due to altered sperm quality, specifically histone retention, which is correlated with an increased percentage of dying offspring in utero. The data highlighted interactions among Car, Akt, Foxo1, and histone acetylation. This study demonstrates that Car is crucial in germ cell homeostasis and male fertility. Further research on the Car signaling pathway is necessary to reveal unidentified causes of altered fertility and understand the harmful impact of environmental molecules on male fertility and offspring health.

Genetic etiological spectrum of sperm morphological abnormalities

PURPOSE

Male infertility manifests in the form of a reduction in sperm count, sperm motility, or the loss of fertilizing ability. While the loss of sperm production can have mixed reasons, sperm structural defects, cumulatively known as teratozoospermia, have predominantly genetic bases. The aim of the present review is to undertake a comprehensive analysis of the genetic mutations leading to sperm morphological deformities/teratozoospermia.

METHODS

We undertook literature review for genes involved in sperm morphological abnormalities. The genes were classified according to the type of sperm defects they cause and on the basis of the level of evidence determined by the number of human studies and the availability of a mouse knockout.

RESULTS

Mutations in the SUN5, CEP112, BRDT, DNAH6, PMFBP1, TSGA10, and SPATA20 genes result in acephalic sperm; mutations in the DPY19L2, SPATA16, PICK1, CCNB3, CHPT1, PIWIL4, and TDRD9 genes cause globozoospermia; mutations in the AURKC gene cause macrozoospermia; mutations in the WDR12 gene cause tapered sperm head; mutations in the RNF220 and ADCY10 genes result in small sperm head; mutations in the AMZ2 gene lead to vacuolated head formation; mutations in the CC2D1B and KIAA1210 genes lead to pyriform head formation; mutations in the SEPT14, ZPBP1, FBXO43, ZCWPW1, KATNAL2, PNLDC1, and CCIN genes cause amorphous head; mutations in the SEPT12, RBMX, and ACTL7A genes cause deformed acrosome formation; mutations in the DNAH1, DNAH2, DNAH6, DNAH17, FSIP2, CFAP43, AK7, CHAP251, CFAP65, ARMC2 and several other genes result in multiple morphological abnormalities of sperm flagella (MMAF).

CONCLUSIONS

Altogether, mutations in 31 genes have been reported to cause head defects and mutations in 62 genes are known to cause sperm tail defects.

Phenotypic continuum and poor intracytoplasmic sperm injection intracytoplasmic sperm injection prognosis in patients harboring HENMT1 variants

BACKGROUND

Small RNAs interacting with PIWI (piRNAs) play a crucial role in regulating transposable elements and translation during spermatogenesis and are essential in male germ cell development. Disruptions in the piRNA pathway typically lead to severe spermatogenic defects and thus male infertility. The HENMT1 gene is a key player in piRNAs primary biogenesis and dysfunction of HENMT1 protein in meiotic and haploid germ cells resulted in the loss of piRNA methylation, piRNA instability, and TE de-repression. Henmt1-knockout mice exhibit a severe oligo-astheno-teratozoospermia (OAT) phenotype, whereas patients with HENMT1 variants display more severe azoospermia phenotypes, ranging from meiotic arrest to hypospermatogenesis. Through whole-exome sequencing (WES) of infertile patient cohorts, we identified two new patients with variants in the HENMT1 gene presenting spermatozoa in their ejcaulate, providing us the opportunity to study spermatozoa from these patients.

OBJECTIVES

Investigate the spermatozoa of two patients harboring an HENMT1 variant to determine whether or not these scarce spermatozoa could be used with assisted reproductive technologies.

MATERIALS AND METHODS

HENMT1 variants identified by WES were validated through Sanger sequencing. Comprehensive semen analysis was conducted, and sperm cells were subjected to transmission electron microscopy for structural examination, in situ hybridization for aneuploidy assessment, and aniline blue staining for DNA compaction status. Subsequently, we assessed their suitability for in vitro fertilization using intracytoplasmic sperm injection (IVF-ICSI).

RESULTS

Our investigations revealed a severe OAT phenotype similar to knockout mice, revealing altered sperm concentration, mobility, morphology, aneuploidy and nuclear compaction defects. Multiple IVF-ICSI attempts were also performed, but no live births were achieved.

DISCUSSION

We confirm the crucial role of HENMT1 in spermatogenesis and highlight a phenotypic continuum associated with HENMT1 variants. Unfortunately, the clinical outcome of these genetic predispositions remains unfavorable, regardless of the patient's phenotype.

CONCLUSION

The presence of spermatozoa is insufficient to anticipate ICSI pregnancy success in HENMT1 patients.

Hypogonadotropic hypogonadism in male tilapia lacking a functional rln3b gene

Relaxin 3 is a neuropeptide that plays a crucial role in reproductive functions of mammals. Previous studies have confirmed that rln3a plays an important role in the male reproduction of tilapia. To further understand the significance of its paralogous gene rln3b in male fertility, we generated a homozygous mutant line of rln3b in Nile tilapia. Our findings indicated that rln3b mutation delayed spermatogenesis and led to abnormal testes structure. Knocking out rln3b gene resulted in a decrease in sperm count, sperm motility and male fish fertility. TUNEL detection revealed a small amount of apoptosis in the testes of rln3b-/- male fish at 390 days after hatching (dah). RT-qPCR analysis demonstrated that mutation of rln3b gene caused a significant downregulation of steroid synthesis-related genes such as cyp17a1, cyp11b2, germ cell marker gene, Vasa, and gonadal somatic cell marker genes of amh and amhr2. Furthermore, we found a significant down-regulation of hypothalamic-pituitary-gonadal (HPG) axis-related genes, while a significantly up-regulation of the dopamine synthetase gene in the rln3b-/- male fish. Taken together, our data strongly suggested that Rln3b played a crucial role in the fertility of XY tilapia by regulating HPG axis genes.

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.

Effects of Tcte1 knockout on energy chain transportation and spermatogenesis: implications for male infertility

STUDY QUESTION

Is the Tcte1 mutation causative for male infertility?

SUMMARY ANSWER

Our collected data underline the complex and devastating effect of the single-gene mutation on the testicular molecular network, leading to male reproductive failure.

WHAT IS KNOWN ALREADY

Recent data have revealed mutations in genes related to axonemal dynein arms as causative for morphology and motility abnormalities in spermatozoa of infertile males, including dysplasia of fibrous sheath (DFS) and multiple morphological abnormalities in the sperm flagella (MMAF). The nexin-dynein regulatory complex (N-DRC) coordinates the dynein arm activity and is built from the DRC1-DRC7 proteins. DRC5 (TCTE1), one of the N-DRC elements, has already been reported as a candidate for abnormal sperm flagella beating; however, only in a restricted manner with no clear explanation of respective observations.

STUDY DESIGN SIZE DURATION

Using the CRISPR/Cas9 genome editing technique, a mouse Tcte1 gene knockout line was created on the basis of the C57Bl/6J strain. The mouse reproductive potential, semen characteristics, testicular gene expression levels, sperm ATP, and testis apoptosis level measurements were then assessed, followed by visualization of N-DRC proteins in sperm, and protein modeling in silico. Also, a pilot genomic sequencing study of samples from human infertile males (n = 248) was applied for screening of TCTE1 variants.

PARTICIPANTS/MATERIALS SETTING METHODS

To check the reproductive potential of KO mice, adult animals were crossed for delivery of three litters per caged pair, but for no longer than for 6 months, in various combinations of zygosity. All experiments were performed for wild-type (WT, control group), heterozygous Tcte1+/- and homozygous Tcte1-/- male mice. Gross anatomy was performed on testis and epididymis samples, followed by semen analysis. Sequencing of RNA (RNAseq; Illumina) was done for mice testis tissues. STRING interactions were checked for protein-protein interactions, based on changed expression levels of corresponding genes identified in the mouse testis RNAseq experiments. Immunofluorescence in situ staining was performed to detect the N-DRC complex proteins: Tcte1 (Drc5), Drc7, Fbxl13 (Drc6), and Eps8l1 (Drc3) in mouse spermatozoa. To determine the amount of ATP in spermatozoa, the luminescence level was measured. In addition, immunofluorescence in situ staining was performed to check the level of apoptosis via caspase 3 visualization on mouse testis samples. DNA from whole blood samples of infertile males (n = 137 with non-obstructive azoospermia or cryptozoospermia, n = 111 samples with a spectrum of oligoasthenoteratozoospermia, including n = 47 with asthenozoospermia) was extracted to perform genomic sequencing (WGS, WES, or Sanger). Protein prediction modeling of human-identified variants and the exon 3 structure deleted in the mouse knockout was also performed.

MAIN RESULTS AND THE ROLE OF CHANCE

No progeny at all was found for the homozygous males which were revealed to have oligoasthenoteratozoospermia, while heterozygous animals were fertile but manifested oligozoospermia, suggesting haploinsufficiency. RNA-sequencing of the testicular tissue showed the influence of Tcte1 mutations on the expression pattern of 21 genes responsible for mitochondrial ATP processing or linked with apoptosis or spermatogenesis. In Tcte1-/- males, the protein was revealed in only residual amounts in the sperm head nucleus and was not transported to the sperm flagella, as were other N-DRC components. Decreased ATP levels (2.4-fold lower) were found in the spermatozoa of homozygous mice, together with disturbed tail:midpiece ratios, leading to abnormal sperm tail beating. Casp3-positive signals (indicating apoptosis) were observed in spermatogonia only, at a similar level in all three mouse genotypes. Mutation screening of human infertile males revealed one novel and five ultra-rare heterogeneous variants (predicted as disease-causing) in 6.05% of the patients studied. Protein prediction modeling of identified variants revealed changes in the protein surface charge potential, leading to disruption in helix flexibility or its dynamics, thus suggesting disrupted interactions of TCTE1 with its binding partners located within the axoneme.

LARGE SCALE DATA

All data generated or analyzed during this study are included in this published article and its supplementary information files. RNAseq data are available in the GEO database (https://www.ncbi.nlm.nih.gov/geo/) under the accession number GSE207805. The results described in the publication are based on whole-genome or exome sequencing data which includes sensitive information in the form of patient-specific germline variants. Information regarding such variants must not be shared publicly following European Union legislation, therefore access to raw data that support the findings of this study are available from the corresponding author upon reasonable request.

LIMITATIONS REASONS FOR CAUTION

In the study, the in vitro fertilization performance of sperm from homozygous male mice was not checked.

WIDER IMPLICATIONS OF THE FINDINGS

This study contains novel and comprehensive data concerning the role of TCTE1 in male infertility. The TCTE1 gene is the next one that should be added to the 'male infertility list' because of its crucial role in spermatogenesis and proper sperm functioning.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by National Science Centre in Poland, grants no.: 2015/17/B/NZ2/01157 and 2020/37/B/NZ5/00549 (to M.K.), 2017/26/D/NZ5/00789 (to A.M.), and HD096723, GM127569-03, NIH SAP #4100085736 PA DoH (to A.N.Y.). The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

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.

Risk of Sperm Disorders and Impaired Fertility in Frozen-Thawed Bull Semen: A Genome-Wide Association Study

Cryopreservation is a widely used method of semen conservation in animal breeding programs. This process, however, can have a detrimental effect on sperm quality, especially in terms of its morphology. The resultant sperm disorders raise the risk of reduced sperm fertilizing ability, which poses a serious threat to the long-term efficacy of livestock reproduction and breeding. Understanding the genetic factors underlying these effects is critical for maintaining sperm quality during cryopreservation, and for animal fertility in general. In this regard, we performed a genome-wide association study to identify genomic regions associated with various cryopreservation sperm abnormalities in Holstein cattle, using single nucleotide polymorphism (SNP) markers via a high-density genotyping assay. Our analysis revealed a significant association of specific SNPs and candidate genes with absence of acrosomes, damaged cell necks and tails, as well as wrinkled acrosomes and decreased motility of cryopreserved sperm. As a result, we identified candidate genes such as POU6F2, LPCAT4, DPYD, SLC39A12 and CACNB2, as well as microRNAs (bta-mir-137 and bta-mir-2420) that may play a critical role in sperm morphology and disorders. These findings provide crucial information on the molecular mechanisms underlying acrosome integrity, motility, head abnormalities and damaged cell necks and tails of sperm after cryopreservation. Further studies with larger sample sizes, genome-wide coverage and functional validation are needed to explore causal variants in more detail, thereby elucidating the mechanisms mediating these effects. Overall, our results contribute to the understanding of genetic architecture in cryopreserved semen quality and disorders in bulls, laying the foundation for improved animal reproduction and breeding.

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.

Identification of IQCH as a calmodulin-associated protein required for sperm motility in humans

Sperm fertilization ability mainly relies on proper sperm progression through the female genital tract and capacitation, which involves phosphorylation signaling pathways triggered by calcium and bicarbonate. We performed exome sequencing of an infertile asthenozoospermic patient and identified truncating variants in MAP7D3, encoding a microtubule-associated protein, and IQCH, encoding a protein of unknown function with enzymatic and signaling features. We demonstrate the deleterious impact of both variants on sperm transcripts and proteins from the patient. We show that, in vitro, patient spermatozoa could not induce the phosphorylation cascades associated with capacitation. We also provide evidence for IQCH association with calmodulin, a well-established calcium-binding protein that regulates the calmodulin kinase. Notably, we describe IQCH spatial distribution around the sperm axoneme, supporting its function within flagella. Overall, our work highlights the cumulative pathological impact of gene mutations and identifies IQCH as a key protein required for sperm motility and capacitation.

The Spectrum of the Heterozygous Effect in Biallelic Mendelian Diseases-The Symptomatic Heterozygote Issue

Heterozygous carriers of pathogenic/likely pathogenic variants in autosomal recessive disorders seem to be asymptomatic. However, in recent years, an increasing number of case reports have suggested that mild and unspecific symptoms can occur in some heterozygotes, as symptomatic heterozygotes have been identified across different disease types, including neurological, neuromuscular, hematological, and pulmonary diseases. The symptoms are usually milder in heterozygotes than in biallelic variants and occur "later in life". The status of symptomatic heterozygotes as separate entities is often disputed, and alternative diagnoses are considered. Indeed, often only a thin line exists between dual, dominant, and recessive modes of inheritance and symptomatic heterozygosity. Interestingly, recent population studies have found global disease effects in heterozygous carriers of some genetic variants. What makes the few heterozygotes symptomatic, while the majority show no symptoms? The molecular basis of this phenomenon is still unknown. Possible explanations include undiscovered deep-splicing variants, genetic and environmental modifiers, digenic/oligogenic inheritance, skewed methylation patterns, and mutational burden. Symptomatic heterozygotes are rarely reported in the literature, mainly because most did not undergo the complete diagnostic procedure, so alternative diagnoses could not be conclusively excluded. However, despite the increasing accessibility to high-throughput technologies, there still seems to be a small group of patients with mild symptoms and just one variant of autosomes in biallelic diseases. Here, we present some examples, the current state of knowledge, and possible explanations for this phenomenon, and thus argue against the existing dominant/recessive classification.

Reproductive genomics of the mouse: implications for human fertility and infertility

Genetic analyses of mammalian gametogenesis and fertility have the potential to inform about two important and interrelated clinical areas: infertility and contraception. Here, we address the genetics and genomics underlying gamete formation, productivity and function in the context of reproductive success in mammalian systems, primarily mouse and human. Although much is known about the specific genes and proteins required for meiotic processes and sperm function, we know relatively little about other gametic determinants of overall fertility, such as regulation of gamete numbers, duration of gamete production, and gamete selection and function in fertilization. As fertility is not a binary trait, attention is now appropriately focused on the oligogenic, quantitative aspects of reproduction. Multiparent mouse populations, created by complex crossing strategies, exhibit genetic diversity similar to human populations and will be valuable resources for genetic discovery, helping to overcome current limitations to our knowledge of mammalian reproductive genetics. Finally, we discuss how what we know about the genomics of reproduction can ultimately be brought to the clinic, informing our concepts of human fertility and infertility, and improving assisted reproductive technologies.

First-line Evaluation of Sperm Parameters in Mice ( Mus musculus )

Infertility has become a major public health problem, with a male factor involved in about half the cases. Mice are the most widely used animal model in reproductive biology research laboratories, but changes in sperm parameters in mice can be subtle and, in the absence of official guidelines, it is important that analyses are carried out in a strict and reproductive manner. This protocol successively details the different steps required to obtain spermatozoa under good conditions, the measurement of sperm motility using a Computer Assisted Sperm Analysis System (CASA) device, the calculation of sperm concentration in the epididymides using a sperm counting cell, and the examination of sperm morphology. The combination of these assays provides an overview of the basic sperm parameters in mice. This is both a diagnostic and a decision-making tool for researchers to orient their scientific strategy according to the observed abnormalities.