Mutant ZP1 in familial infertility

Evaluation of an Updated Gene Panel as a Diagnostic Tool for Both Male and Female Infertility

In recent years, an increasing number of genes associated with male and female infertility have been identified. The genetics of infertility is no longer limited to the analysis of karyotypes or specific genes, and it is now possible to analyse several dozen infertility genes simultaneously. Here, we present the diagnostic activity over the past two years including 140 patients (63 women and 77 men). Targeted sequencing revealed causative variants in 17 patients, representing an overall diagnostic rate of 12.1%, with prevalence rates in females and males of 11% and 13%, respectively. The gene-disease relationship (GDR) was re-evaluated for genes due to the addition of new patients and/or variants in the actual study. Five genes changed categories: two female genes (MEIOB and TBPL2) moved from limited to moderate; two male genes (SOHLH1 and GALNTL5) moved from no evidence to strong and from limited to moderate; and SEPTIN12, which was unable to classify male infertility, was reclassified as limited. Many infertility genes have yet to be identified. With the increasing integration of genetics in reproductive medicine, the scope of intervention extends to include other family members, in addition to individual patients or couples. Genetic counselling consultations and appropriate staffing will need to be established in fertility centres. Trial registration number: Not applicable.

Cellular mechanisms of monozygotic twinning: clues from assisted reproduction

BACKGROUND

Monozygotic (MZ) twins are believed to arise from the fission of a single fertilized embryo at different stages. Monochorionic MZ twins, who share one chorion, originate from the splitting of the inner cell mass (ICM) within a single blastocyst. In the classic model for dichorionic MZ twins, the embryo splits before compaction, developing into two blastocysts. However, there are a growing number of ART cases where a single blastocyst transfer results in dichorionic MZ twins, indicating that embryo splitting may occur even after blastocyst formation.

OBJECTIVE AND RATIONALE

For monochorionic MZ twins, we conducted a comprehensive analysis of the cellular mechanisms involved in ICM splitting, drawing from both ART cases and animal experiments. In addition, we critically re-examine the classic early splitting model for dichorionic MZ twins. We explore cellular mechanisms leading to two separated blastocysts in ART, potentially causing dichorionic MZ twins.

SEARCH METHODS

Relevant studies including research articles, reviews, and conference papers were searched in the PubMed database. Cases of MZ twins from IVF clinics were found by using combinations of terms including 'monozygotic twins' with 'IVF case report', 'ART', 'single embryo transfer', or 'dichorionic'. The papers retrieved were categorized based on the implicated mechanisms or as those with unexplained mechanisms. Animal experiments relating to MZ twins were found using 'mouse embryo monozygotic twins', 'mouse 8-shaped hatching', 'zebrafish janus mutant', and 'nine-banded armadillo embryo', along with literature collected through day-to-day reading. The search was limited to articles in English, with no restrictions on publication date or species.

OUTCOMES

For monochorionic MZ twins, ART cases and mouse experiments demonstrate evidence that a looser ICM in blastocysts has an increased chance of ICM separation. Physical forces facilitated by blastocoel formation or 8-shaped hatching are exerted on the ICM, resulting in monochorionic MZ twins. For dichorionic MZ twins, the classic model resembles artificial cloning of mouse embryos in vitro, requiring strictly controlled splitting forces, re-joining prevention, and proper aggregation, which allows the formation of two separate human blastocysts under physiological circumstances. In contrast, ART procedures involving the transfer of a single blastocysts after atypical hatching or vitrified-warmed cycles might lead to blastocyst separation. Differences in morphology, molecular mechanisms, and timing across various animal model systems for MZ twinning can impede this research field. As discussed in future directions, recent developments of innovative in vitro models of human embryos may offer promising avenues for providing fundamental novel insights into the cellular mechanisms of MZ twinning during human embryogenesis.

WIDER IMPLICATIONS

Twin pregnancies pose high risks to both the fetuses and the mother. While single embryo transfer is commonly employed to prevent dizygotic twin pregnancies in ART, it cannot prevent the occurrence of MZ twins. Drawing from our understanding of the cellular mechanisms underlying monochorionic and dichorionic MZ twinning, along with insights into the genetic mechanisms, could enable improved prediction, prevention, and even intervention strategies during ART procedures.

REGISTRAITON NUMBER

N/A.

Genetic factors of oocyte maturation arrest: an important cause for recurrent IVF/ICSI failures

Oocyte maturation arrest (OMA) is a common phenotype observed in IVF/ICSI cycles, characterized by the production of immature oocytes which lead to infertility. Previous studies have demonstrated that genetic factors play an important role in OMA, but the genetic mechanisms underlying a group of patients remain to be elucidated. In the recent issue of Journal of Assisted Reproduction and Genetics, Hu et al. and Wan et al. identified novel PATL2 or ZFP36L2 variants in OMA patients, respectively. By conducting in vitro experiments, they demonstrated the destructive effect of the variants on protein function. These findings expand the mutational spectrum of PATL2 and ZFP36L2, and provide precise reference for genetic counseling of OMA patients.

Novel PATL2 variants cause female infertility with oocyte maturation defect

PURPOSE

Oocyte maturation defect (OOMD) is a rare cause of in vitro fertilization failure characterized by the production of immature oocytes. Compound heterozygous or homozygous PATL2 mutations have been associated with oocyte arrest at the germinal vesicle (GV), metaphase I (MI), and metaphase II (MII) stages, as well as morphological changes.

METHODS

In this study, we recruited three OOMD cases and conducted a comprehensive multiplatform laboratory investigation.

RESULTS

Whole exome sequence (WES) revealed four diagnostic variants in PATL2, nonsense mutation c.709C > T (p.R237*) and frameshift mutation c.1486_1487delinsT (p.A496Sfs*4) were novel mutations that have not been reported previously. Furthermore, the pathogenicity of these variants was predicted using in silico analysis, which indicated detrimental effects. Molecular dynamic analysis suggested that the A496S variant disrupted the hydrophobic segment, leading to structural changes that affected the overall protein folding and stability. Additionally, biochemical and molecular experiments were conducted on cells transfected with wild-type (WT) or mutant PATL2 (p.R237* and p.A496Sfs*4) plasmid vectors.

CONCLUSIONS

The results demonstrated that PATL2A496Sfs*4 and PATL2R237* had impacts on protein size and expression level. Interestingly, expression levels of specific genes involved in oocyte maturation and early embryonic development were found to be simultaneously deregulated. The findings in our study expand the variation spectrum of the PATL2 gene, provide solid evidence for counseling on future pregnancies in affected families, strongly support the application of in the diagnosis of OOMD, and contribute to the understanding of PATL2 function.

Single-cell RNA sequencing of human oocytes reveals a differential transcriptomic profile associated with agar-like zona pellucida

BACKGROUND

Agar-like zona pellucida (ZP) is the most common type of abnormal ZP, and is one of the causes of low fertility or infertility. However, the molecular mechanism of agar-like ZP is unclear. Single-cell RNA-sequencing (scRNA-seq) analysis was used to assess the cellular and molecular landscape of oocytes with agar-like ZP.

METHODS

Human metaphase I (MI) oocytes were collected from four patients with agar-like ZP and four healthy donors. Total RNA was isolated, cDNA was synthesized, and libraries were generated and subsequently sequenced on a HiSeq 2500 instrument. The scRNA-seq data were analyzed with R software.

RESULTS

We identified 1320 genes that were differentially expressed between agar-like ZP oocytes and healthy donor oocytes. Gene Ontology term enrichment results showed that the genes downregulated in agar-like ZP oocytes were significantly related to extracellular matrix organization, while the genes upregulated in agar-like ZP oocytes were significantly related to the regulation of response to DNA damage stimulus. The Kyoto Encyclopedia of Genes and Genomes enrichment results showed that genes were enriched in the ECM-receptor interaction pathway and focal adhesion pathway. Other signaling pathways important in oocyte development were also enriched, such as PI3K-Akt. Differential expression analysis identified UBC, TLR4, RELA, ANXA5, CAV1, KPNA2, CCNA2, ACTA2, FYN and ITGB3 as genetic markers of oocytes with agar-like ZP.

CONCLUSIONS

Our findings suggest that agar-like ZP oocytes exhibit significant downregulation of genes involved in the ECM-receptor interaction signaling pathway and focal adhesion pathway, which could lead to aberrant ZP formation, while the upregulated genes were significantly related to regulation of the response to DNA damage stimulus. Agar-like ZP formation may interfere with the normal exchange of signals between oocytes and perivitelline granulosa cells, thereby preventing cumulus cells from participating in oocyte DNA damage repair and causing MI arrest.

ZP1-Y262C mutation causes abnormal zona pellucida formation and female infertility in humans

Defective oocyte maturation is a common cause of female infertility. The loss of the zona pellucida (ZP) represents a specific condition of impaired oocyte maturation. The extracellular matrix known as the ZP envelops mammalian oocytes and preimplantation embryos, exerting significant influence on oogenesis, fertilization, and embryo implantation. However, the genetic factors leading to the loss of the ZP in oocytes are not well understood. This study focused on patients who underwent oocyte retrieval surgery after ovarian stimulation and were found to have abnormal oocyte maturation without the presence of the ZP. Ultrasonography was performed during the surgical procedure to evaluate follicle development. Peripheral blood samples from the patient were subjected to exome sequencing. Here, a novel, previously unreported heterozygous mutation in the ZP1 gene was identified. Within the ZP1 gene, we discovered a novel heterozygous mutation (ZP1 NM_207341.4:c.785A>G (p.Y262C)), specifically located in the trefoil domain. Bioinformatics comparisons further revealed conservation of the ZP1-Y262C mutation across different species. Model predictions of amino acid mutations on protein structure and cell immunofluorescence/western blot experiments collectively confirmed the detrimental effects of the ZP1-Y262C mutation on the function and expression of the ZP1 protein. The ZP1-Y262C mutation represents the novel mutation in the trefoil domain of the ZP1 protein, which is associated with defective oocyte maturation in humans. Our report enhances comprehension regarding the involvement of ZP-associated genes in female infertility and offers enriched understanding for the genetic diagnosis of this condition.

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.

Mitochondrial deoxyguanosine kinase is required for female fertility in mice

Mitochondrial homeostasis plays a pivotal role in oocyte maturation and embryonic development. Deoxyguanosine kinase (DGUOK) is a nucleoside kinase that salvages purine nucleosides in mitochondria and is critical for mitochondrial DNA replication and homeostasis in non-proliferating cells. Dguok loss-of-function mutations and deletions lead to hepatocerebral mitochondrial DNA deletion syndrome. However, its potential role in reproduction remains largely unknown. In this study, we find that Dguok knockout results in female infertility. Mechanistically, DGUOK deficiency hinders ovarian development and oocyte maturation. Moreover, DGUOK deficiency in oocytes causes a significant reduction in mitochondrial DNA copy number and abnormal mitochondrial dynamics and impairs germinal vesicle breakdown. Only few DGUOK-deficient oocytes can extrude their first polar body during in vitro maturation, and these oocytes exhibit irregular chromosome arrangements and different spindle lengths. In addition, DGUOK deficiency elevates reactive oxygen species levels and accelerates oocyte apoptosis. Our findings reveal novel physiological roles for the mitochondrial nucleoside salvage pathway in oocyte maturation and implicate DGUOK as a potential marker for the diagnosis of female infertility.

Positive selection in gamete interaction proteins in Carnivora

The absence of robust interspecific isolation barriers among pantherines, including the iconic South American jaguar (Panthera onca), led us to study molecular evolution of typically rapidly evolving reproductive proteins within this subfamily and related groups. In this study, we delved into the evolutionary forces acting on the zona pellucida (ZP) gamete interaction protein family and the sperm-oocyte fusion protein pair IZUMO1-JUNO across the Carnivora order, distinguishing between Caniformia and Feliformia suborders and anticipating few significant diversifying changes in the Pantherinae subfamily. A chromosome-resolved jaguar genome assembly facilitated coding sequences, enabling the reconstruction of protein evolutionary histories. Examining sequence variability across more than 30 Carnivora species revealed that Feliformia exhibited significantly lower diversity compared to its sister taxa, Caniformia. Molecular evolution analyses of ZP2 and ZP3, subunits directly involved in sperm-recognition, unveiled diversifying positive selection in Feliformia, Caniformia and Pantherinae, although no significant changes were linked to sperm binding. Structural cross-linking ZP subunits, ZP4 and ZP1 exhibited lower levels or complete absence of positive selection. Notably, the fusion protein IZUMO1 displayed prominent positive selection signatures and sites in basal lineages of both Caniformia and Feliformia, extending along the Caniformia subtree but absent in Pantherinae. Conversely, JUNO did not exhibit any positive selection signatures across tested lineages and clades. Eight Caniformia-specific positive selected sites in IZUMO1 were detected within two JUNO-interaction clusters. Our findings provide for the first time insights into the evolutionary trajectories of ZP proteins and the IZUMO1-JUNO gamete interaction pair within the Carnivora order.

Embryological characteristics and clinical outcomes of oocytes with different degrees of abnormal zona pellucida during assisted reproductive treatment

Abnormalities in the zona pellucida (ZP) adversely affect oocyte maturation, embryo development and pregnancy outcomes. However, the assessment of severity is challenging. To evaluate the effects of different degrees of ZP abnormalities on embryo development and clinical outcomes, in total, 590 retrieval cycles were scored and divided into four categories (control, mild, moderate and severe) based on three parameters: perivitelline space, percentage of immature oocytes and percentage of oocytes with abnormal morphology. As the severity of abnormal ZP increased, both the number of retrieved oocytes and mature oocytes decreased. The fertilization rate did not differ significantly among groups. The rates of embryo cleavage and day-3 high-quality embryos in the mild group and the moderate group did not vary significantly between the two groups but were significantly higher than those in the severe group. The blastulation rates of the abnormal ZP groups were similar; however, they were lower than those of the control group. Moreover, the cycle cancellation rate of the severe abnormal ZP group was as high as 66.20%, which was significantly higher than that of the other three groups. Although the rates of cumulative clinical pregnancy and live births were lower than those in the control group, they were comparable among the abnormal ZP groups. There were no differences in the neonatal outcomes of the different groups. Together, ZP abnormalities show various degrees of severity, and in all patients regardless of the degree of ZP abnormalities who achieve available embryos, there will be an opportunity to eventually give birth.

Genetic mechanisms of fertilization failure and early embryonic arrest: a comprehensive review

BACKGROUND

Infertility and pregnancy loss are longstanding problems. Successful fertilization and high-quality embryos are prerequisites for an ongoing pregnancy. Studies have proven that every stage in the human reproductive process is regulated by multiple genes and any problem, at any step, may lead to fertilization failure (FF) or early embryonic arrest (EEA). Doctors can diagnose the pathogenic factors involved in FF and EEA by using genetic methods. With the progress in the development of new genetic technologies, such as single-cell RNA analysis and whole-exome sequencing, a new approach has opened up for us to directly study human germ cells and reproductive development. These findings will help us to identify the unique mechanism(s) that leads to FF and EEA in order to find potential treatments.

OBJECTIVE AND RATIONALE

The goal of this review is to compile current genetic knowledge related to FF and EEA, clarifying the mechanisms involved and providing clues for clinical diagnosis and treatment.

SEARCH METHODS

PubMed was used to search for relevant research articles and reviews, primarily focusing on English-language publications from January 1978 to June 2023. The search terms included fertilization failure, early embryonic arrest, genetic, epigenetic, whole-exome sequencing, DNA methylation, chromosome, non-coding RNA, and other related keywords. Additional studies were identified by searching reference lists. This review primarily focuses on research conducted in humans. However, it also incorporates relevant data from animal models when applicable. The results were presented descriptively, and individual study quality was not assessed.

OUTCOMES

A total of 233 relevant articles were included in the final review, from 3925 records identified initially. The review provides an overview of genetic factors and mechanisms involved in the human reproductive process. The genetic mutations and other genetic mechanisms of FF and EEA were systematically reviewed, for example, globozoospermia, oocyte activation failure, maternal effect gene mutations, zygotic genome activation abnormalities, chromosome abnormalities, and epigenetic abnormalities. Additionally, the review summarizes progress in treatments for different gene defects, offering new insights for clinical diagnosis and treatment.

WIDER IMPLICATIONS

The information provided in this review will facilitate the development of more accurate molecular screening tools for diagnosing infertility using genetic markers and networks in human reproductive development. The findings will also help guide clinical practice by identifying appropriate interventions based on specific gene mutations. For example, when an individual has obvious gene mutations related to FF, ICSI is recommended instead of IVF. However, in the case of genetic defects such as phospholipase C zeta1 (PLCZ1), actin-like7A (ACTL7A), actin-like 9 (ACTL9), and IQ motif-containing N (IQCN), ICSI may also fail to fertilize. We can consider artificial oocyte activation technology with ICSI to improve fertilization rate and reduce monetary and time costs. In the future, fertility is expected to be improved or restored by interfering with or supplementing the relevant genes.

A heterozygous ZP2 mutation causes zona pellucida defects and female infertility in mouse and human

The zona pellucida (ZP) is an extracellular glycoprotein matrix surrounding mammalian oocytes. Recently, numerous mutations in genes encoding ZP proteins have been shown to be possibly related to oocyte abnormality and female infertility; few reports have confirmed the functions of these mutations in living animal models. Here, we identified a novel heterozygous missense mutation (NM_001376231.1:c.1616C>T, p.Thr539Met) in ZP2 from a primary infertile female. We showed that the mutation reduced ZP2 expression and impeded ZP2 secretion in cell lines. Furthermore, we constructed the mouse model with the mutation (Zp2T541M) using CRISPR-Cas9. Zp2WT/T541M female mice had normal fertility though generated oocytes with the thin ZP, whereas Zp2T541M female mice were completely infertile due to degeneration of oocytes without ZP. Additionally, ZP deletion impaired folliculogenesis and caused female infertility in Zp2T541M mice. Our study not only expands the spectrum of ZP2 mutation sites but also, more importantly, increases the understanding of pathogenic mechanisms of ZP2 mutations.

Case report: A novel homozygous variant in ZP3 is associated with human empty follicle syndrome

Empty follicle syndrome (EFS) is a rare condition in female infertility. It is characterized by the inability to retrieve oocytes from visibly large, normally developing follicles in the ovaries, despite ovarian stimulation. The genetic factors contributing to this syndrome remain unclear. This study focused on patients who underwent three consecutive ovarian stimulation procedures for oocyte retrieval but experienced unsuccessful outcomes, despite the presence of observable large follicles. Ultrasound examinations were conducted to assess follicular development during each procedure. In order to investigate potential genetic causes, we performed whole exome sequencing on peripheral blood samples from the patient. Interestingly, we identified that this patient carries a homozygous mutation in the ZP3 genes. Within the ZP3 gene, we identified a homozygous variant [NM_001110354.2, c.176T>A (p.L59H)] specifically located in the zona pellucida (ZP) domain. Further analysis, including bioinformatics methods and protein structure modeling, was carried out to investigate the conservation of the ZP3L59H variant across different species. This homozygous variant exhibited a high degree of conservation across various species. Importantly, the homozygous ZP3L59H variant was associated with the occurrence of empty follicle syndrome in affected female patients. The homozygous ZP3L59H variant represents a newly discovered genetic locus implicated in the development of human empty follicle syndrome. Our findings contribute to a deeper understanding of the role of zona pellucida-related genes in infertility and provide valuable insights for the genetic diagnosis of female infertility.

Female infertility from oocyte maturation arrest: assembling the genetic puzzle

Assisted reproduction procedures often encounter an issue called oocyte maturation arrest (OMA), which is manifested as failed IVF/ICSI attempts using oocytes from some infertile women. In this issue of EMBO Molecular Medicine, Wang et al identify infertile women bearing novel DNA sequence variants in a gene called PABPC1L, which is essential for translation of maternal mRNAs. By conducting a series of in vitro and in vivo experiments, they demonstrated certain variants as being causal for OMA, confirming a conserved requirement for PABPC1L in human oocyte maturation. This study offers a promising therapeutic target for treating OMA patients.

Identification of zona pellucida defects revealed a novel loss-of-function mutation in ZP2 in humans and rats

Introduction

Human zona pellucida (ZP) plays an important role in reproductive process. Several rare mutations in the encoding genes (ZP1, ZP2, and ZP3) have been demonstrated to cause women infertility. Mutations in ZP2 have been reported to cause ZP defects or empty follicle syndrome. We aimed to identify pathogenic variants in an infertile woman with a thin zona pellucida (ZP) phenotype and investigated the effect of ZP defects on oocyte gene transcription.

Methods

We performed whole-exome sequencing and Sanger sequencing of genes were performed for infertilite patients characterized by fertilization failure in routine in vitro fertilization (IVF). Immunofluorescence (IF) and intracytoplasmic sperm injection (ICSI) were used in the mutant oocytes. Single-cell RNA sequencing was used to investigate transcriptomes of the gene-edited (Zp2mut/mut) rat model. Biological function enrichment analysis, quantitative real-time PCR (qRT-PCR), and IF were performed.

Results

We identified a novel homozygous nonsense mutation of ZP2 (c.1924C > T, p.Arg642X) in a patient with non-consanguineous married parents. All oocytes showed a thin or no ZP under a light microscope and were fertilized after ICSI. The patient successfully conceived by receiving the only two embryos that developed to the blastocyst stage. The immunofluorescence staining showed an apparently abnormal form of the stopped oocytes. We further demonstrated a total of 374 differentially expressed genes (DEGs) in the transcriptome profiles of Zp2mut/mut rats oocytes and highlighted the signal communication between oocytes and granulosa cells. The pathway enrichment results of DEGs showed that they were enriched in multiple signaling pathways, especially the transforming growth factor-β (TGF-β) signaling pathway in oocyte development. qRT-PCR, IF, and phosphorylation analysis showed significantly downregulated expressions of Acvr2b, Smad2, p38MAPK, and Bcl2 and increased cleaved-caspase 3 protein expression.

Discussion

Our findings expanded the known mutational spectrum of ZP2 associated with thin ZP and natural fertilization failure. Disruption of the integrity of the ZP impaired the TGF-β signaling pathway between oocytes and surrounding granulosa cells, leading to increased apoptosis and decreased developmental potential of oocytes.

Bi-allelic pathogenic variants in PABPC1L cause oocyte maturation arrest and female infertility

Oocyte maturation arrest is one of the important causes of female infertility, but the genetic factors remain largely unknown. PABPC1L, a predominant poly(A)-binding protein in Xenopus, mouse, and human oocytes and early embryos prior to zygotic genome activation, plays a key role in translational activation of maternal mRNAs. Here, we identified compound heterozygous and homozygous variants in PABPC1L that are responsible for female infertility mainly characterized by oocyte maturation arrest in five individuals. In vitro studies demonstrated that these variants resulted in truncated proteins, reduced protein abundance, altered cytoplasmic localization, and reduced mRNA translational activation by affecting the binding of PABPC1L to mRNA. In vivo, three strains of Pabpc1l knock-in (KI) female mice were infertile. RNA-sequencing analysis showed abnormal activation of the Mos-MAPK pathway in the zygotes of KI mice. Finally, we activated this pathway in mouse zygotes by injecting human MOS mRNA, and this mimicked the phenotype of KI mice. Our findings reveal the important roles of PABPC1L in human oocyte maturation and add a genetic potential candidate gene to be screened for causes of infertility.

A novel compound heterozygous mutation in TUBB8 causing early embryonic developmental arrest

PURPOSE

Mutations in the β-tubulin isotype, TUBB8, can cause female infertility. Although several mutations of TUBB8 have been reported, the full spectrum for guiding genetics counseling still needs to be further explored. Here, we sought to identify novel variants in TUBB8 and their phenotypic effects on microtubule network structure in vitro.

METHODS

Whole-exome sequence analysis was performed in two families with infertility to detect pathogenic variants, with validation by Sanger sequencing. All gene variants and protein structures were predicted in silico. Cells were transfected with wild-type and mutants, and immunofluorescence analysis was performed to visualize microtubule network changes.

RESULTS

We detected a novel compound heterozygous mutation, c.915_916delCC (p.Arg306Serfs*21) and c.82C > T (p.His28Tyr), and a benign heterozygous variant c.1286C > T (p.Thr429Met) in TUBB8 in the two families. Female patients with p.Arg306Serfs*21 and p.His28Tyr were infertile with early embryonic developmental arrest. The female patient with p.Thr429Met gave birth to a healthy baby in the second in vitro fertilization frozen embryo transfer cycle. The p.Arg306Serfs*21 mutation was predicted to cause large structural alteration in the TUBB8 protein and was confirmed to produce a truncated and trace protein by western blot analysis. Immunofluorescence analysis of transfected HeLa cells showed that p.Arg306Serfs*21 significantly disrupted microtubule structure.

CONCLUSIONS

Our findings expand the known mutational spectrum of TUBB8 associated with early embryonic developmental arrest and female infertility.

Genetic screening and analysis of TUBB8 variants in females seeking ART

RESEARCH QUESTION

More than 100 variants have been identified in the TUBB8 gene, which account for approximately 30% of infertile women with oocyte maturation defects. But what is the correlation between the highly phenotypic diversity and genetic variability? Are there other variants in TUBB8 related to female infertility?

DESIGN

TUBB8 resequencing was performed in 80 female subjects who were experiencing infertility and were seeking treatment with assisted reproductive technologies (ART), or had ever experienced ART failure due to oocyte maturation defects. All variants were evaluated with pedigree analysis, population frequency, in-silico analysis and molecular modelling. The effects of the variants on oocytes/arrested embryos were assessed by morphological observations, immunostaining, embryo biopsies and chromosome euploidy analysis.

RESULTS

Nine missense variants and two frameshift variants from an additional 15 families were identified, including four novel variants and seven previously reported recurrent variants. These TUBB8 variants were related to highly variable phenotypes, including abnormalities in oocyte maturation or morphology, fertilization failure, embryonic development abnormalities and implantation failure. Also further clarified were the incomplete penetrance of heterozygous p.E108K, the likely benign significance of heterozygous p.A313V and the clinical effect of a novel variant of p.R380C.

CONCLUSIONS

This study significantly expands the variant spectrum of the TUBB8 gene and, together with the available findings on TUBB8 variants and female infertility, will potentially facilitate the genetic counselling of infertile women in future.

Zona pellucida glycoproteins: Relevance in fertility and development of contraceptive vaccines

Mammalian zona pellucida (ZP) is composed of three to four glycoproteins, which plays an important role during fertilization. Mutations in the genes encoding zona proteins are reported in women with empty follicle syndrome, degenerated oocytes and those with an abnormal or no ZP further emphasizing their relevance during fertility. Immunization with either native or recombinant ZP glycoproteins/proteins leads to curtailment of fertility in various animal species. Observed infertility is frequently associated with ovarian pathology characterized by follicular atresia and degenerative changes in ZP, which may be due to oophoritogenic T cell epitope(s) within ZP glycoproteins. To avoid ovarian dystrophy, B cell epitopes of ZP glycoproteins have been mapped by using bio-effective monoclonal antibodies. Immunization with the immunogens encompassing the mapped B cell epitopes by and large led to amelioration of follicular atresia. However, their use for human application will require more rigorous research to establish their safety and reversibility of the contraceptive effect. Nonetheless, to minimize human-animal conflicts, ZP-based contraceptive vaccines have been used successfully in the population management of free-ranging animal species such as feral horses, white-tailed deer and elephants. To control zoonotic diseases, attempts are also underway to control the population of other animal species including stray dogs, which acts as one of the major vectors for the rabies virus.

Novel variants in ZP1, ZP2 and ZP3 associated with empty follicle syndrome and abnormal zona pellucida

RESEARCH QUESTION

Which genetic variants might explain the causes of empty follicle syndrome (EFS) and abnormal zona pellucida (ZP) and affect the success of treatment with assisted reproductive technologies (ART)?

DESIGN

Whole-exome sequencing was performed in probands with EFS and abnormal ZP. Sanger sequencing was used for variant validation. Using HEK-293T cells, the effects of ZP1 and ZP2 variants on protein expression were explored by western blotting, and the effect of the ZP1 variant on protein location was investigated via immunofluorescence. The protein structure was also analysed to investigate the pathogenicity of variants.

RESULTS

A homozygous nonsense variant in ZP1 (c.874C>T, p.Gln292*) was detected in a patient with EFS. A novel homozygous frameshift variant in ZP2 (c.836_837delAG, p.Glu279Valfs*6) and a novel heterozygous missense variant in ZP3 (c.1159G>A, p.Val387Met) were identified in two patients with ZP morphological abnormalities, respectively. Western blotting and immunofluorescence analysis showed that the ZP1 variant results in a premature stop codon, leading to the truncated ZP1 protein. The ZP2 variant, which is situated in the N-terminus, triggers the degradation of a premature termination protein. Additionally, the patient with the ZP3 variant achieved clinical pregnancy following intracytoplasmic sperm injection treatment.

CONCLUSIONS

These findings expand the mutational spectrum of ZP1, ZP2 and ZP3, and provide new evidence for genetic diagnosis of female infertility. The targeted genetic diagnosis of ZP genes is recommended to choose appropriate fertilization methods and improve success rates of treatment with ART.

A ZP1 gene mutation in a patient with empty follicle syndrome: A case report and literature review

Genuine empty follicle syndrome (gEFS) is a rare cause of female infertility; it is defined as the presence of cumulus-oocyte complexes (COCs) in follicular fluid but the absence of oocytes after denudation in an in vitro fertilization (IVF) programme. Mutations in one of the four genes encoding zona pellucida (ZP) proteins have been implicated in gEFS. The objectives of the present study were to explore the molecular basis of idiopathic infertility in a 35-year-old woman with gEFS (observed after four ovarian retrievals), compare her phenotype and genotype with those of other patients described in the literature, and discuss therapeutic approaches that could be adopted by reproductive health centres in this situation. Sequencing of the ZP genes revealed a new homozygous missense variant in ZP1: c.1097G > A;p.(Arg366Gln). The variant is located in the ZP-N domain, which is essential for ZP protein polymerization. An immunohistochemical assessment of an ovarian biopsy confirmed the absence of ZP1 protein. The novel variant appears to prevent ZP assembly, which would explain the absence of normal oocytes after denudation in our patient (and despite the retrieval of COCs). ZP gene sequencing should be considered for patients with a phenotype suggestive of gEFS. An etiological genetic diagnosis enables appropriate genetic counselling and a switch to an IVF programme (with a suitable denudation technique) or an oocyte donation programme.

A novel homozygous C-terminal deletion in BTG4 causes zygotic cleavage failure and female infertility

PURPOSE

We aimed to identify pathogenic variants in a female patient with primary infertility and recurrent failure of in vitro fertilization with zygotic cleavage failure.

METHODS

The genomic DNA from the affected individual was subjected to whole-exome sequencing and the variant was confirmed by Sanger sequencing. The functional effect of the identified variant was further investigated in 293 T cells.

RESULTS

We identified a novel homozygous deletion in BTG4 (c.580_616del) in the affected individual. The deletion results in frameshift and replacement of the last 29 residues (aa195-223) with 66 random amino acids. The mutated amino acid residues are highly conserved among mammalian species. Co-immunoprecipitation in 293 T cells showed that the mutation abolished the interaction between BTG4 and PABPN1L.

CONCLUSION

This study conforms previous studies and expands the mutational spectrum of BTG4. Our findings prove the functional importance of the C-terminal of BTG4. BTG4 is a potential diagnostic and therapeutic target for patients suffering from zygotic cleavage failure.

Can successful pregnancy be achieved and predicted from patients with identified ZP mutations? A literature review

BACKGROUND

In mammals, normal fertilization depends on the structural and functional integrity of the zona pellucida (ZP), which is an extracellular matrix surrounding oocytes. Mutations in ZP may affect oogenesis, fertilization and early embryonic development, which may cause female infertility.

METHODS

A PubMed literature search using the keywords 'zona pellucida', 'mutation' and 'variant' limited to humans was performed, with the last research on June 30, 2022. The mutation types, clinical phenotypes and pregnancy outcomes were summarized and analyzed. The naive Bayes classifier was used to predict clinical pregnancy outcomes for patients with ZP mutations.

RESULTS

A total of 29 publications were included in the final analysis. Sixty-nine mutations of the ZP genes were reported in 87 patients with different clinical phenotypes, including empty follicle syndrome (EFS), ZP-free oocytes (ZFO), ZP-thin oocytes (ZTO), degenerated and immature oocytes. The phenotypes of patients were influenced by the types and location of the mutations. The most common effects of ZP mutations are protein truncation and dysfunction. Three patients with ZP1 mutations, two with ZP2 mutations, and three with ZP4 mutations had successful pregnancies through Intracytoplasmic sperm injection (ICSI) from ZFO or ZTO. A prediction model of pregnancy outcome in patients with ZP mutation was constructed to assess the chance of pregnancy with the area under the curve (AUC) of 0.898. The normalized confusion matrix showed the true positive rate was 1.00 and the true negative rate was 0.38.

CONCLUSION

Phenotypes in patients with ZP mutations might be associated with mutation sites or the degree of protein dysfunction. Successful pregnancy outcomes could be achieved in some patients with identified ZP mutations. Clinical pregnancy prediction model based on ZP mutations and clinical characteristics will be helpful to precisely evaluate pregnancy chance and provide references and guidance for the clinical treatment of relevant patients.

Common dysmorphic oocytes and embryos in assisted reproductive technology laboratory in association with gene alternations

Amorphic or defected oocytes and embryos are commonly observed in assisted reproductive technology (ART) laboratories. It is believed that a proper gene expression at each stage of embryo development contributes to the possibility of a decent-quality embryo leading to successful implantation. Many studies reported that several defects in embryo morphology are associated with gene expressions during in vitro fertilization (IVF) treatment. There is lacking literature review on summarizing common morphological defects about gene alternations. In this review, we summarized the current literature. We selected 64 genes that have been reported to be involved in embryo morphological abnormalities in animals and humans, 30 of which were identified in humans and might be the causes of embryonic changes. Five papers focusing on associations of multiple gene expressions and embryo abnormalities using RNA transcriptomes were also included during the search. We have also reviewed our time-lapse image database with over 3000 oocytes/embryos to show morphological defects possibly related to gene alternations reported previously in the literature. This holistic review can better understand the associations between gene alternations and morphological changes. It is also beneficial to select important biomarkers with strong evidence in IVF practice and reveal their potential application in embryo selection. Also, identifying genes may help patients with genetic disorders avoid unnecessary treatments by providing preimplantation genetic testing for monogenic/single gene defects (PGT-M), reduce embryo replacements by less potential, and help scientists develop new methods for oocyte/embryo research in the near future.

Heterozygous loss-of-function variants in LHX8 cause female infertility characterized by oocyte maturation arrest

PURPOSE

The genetic causes of oocyte maturation arrest leading to female infertility are largely unknown, and no population-based genetic analysis has been applied in cohorts of patients with infertility. We aimed to identify novel pathogenic genes causing oocyte maturation arrest by using a gene-based burden test.

METHODS

Through comparison of exome sequencing data from 716 females with infertility characterized by oocyte maturation arrest and 3539 controls, we performed a gene-based burden test and identified a novel pathogenic gene LHX8. Splicing event was evaluated using a minigene assay, expression of LHX8 protein was assessed in HeLa cells, and nuclear subcellular localization was determined in both HeLa cells and mouse oocytes.

RESULTS

A total of 5 heterozygous loss-of-function LHX8 variants were identified from 6 independent families (c.389+1G>T, c.412C>T [p.Arg138∗], c.282C>A [p.Cys94∗]; c.257dup [p.Tyr86∗]; and c.180del, [p.Ser61Profs∗30]). All the identified variants in LHX8 produced truncated LHX8 protein and resulted in loss of LHX8 nuclear localization in both HeLa cells and mouse oocytes.

CONCLUSION

By combining genetic evidence and functional evaluations, we identified a novel pathogenic gene LHX8 and established the causative relationship between LHX8 haploinsufficiency and female infertility characterized by oocyte maturation arrest.

Gene mutations impede oocyte maturation, fertilization, and early embryonic development

Reproductive diseases are a long-standing problem and have become more common in the world. Currently, 15% of the world's population suffers from infertility, and half of them are women. Maturation of oocytes, successful fertilization, and high-quality embryos are prerequisites for pregnancy. With the development of assisted reproductive technology and advanced genetic assays, we have found that infertility in many young female patients is caused by mutations in various developmental regulators. These pathogenic factors may result in impediment of oocyte maturation, failure of fertilization or early embryonic development arrest. In this review, we categorize these clinically-identified, mutated genetic factors by their molecular characteristics: nuclear factors (PALT2, TRIP13, WEE2, TBPL2, REC114, MEI1 and CDC20), cytoplasmic factors (TLE6, PADI6, NLRP2/5, FBXO43, MOS and BTG4), a factor unique to primates (TUBB8), cell membrane factor (PANX1), and zona pellucida factors (ZP1-3). We compared discrepancies observed in phenotypes between human and mouse models to provide clues for clinical diagnosis and treatment of related reproductive diseases.

Advances in studying human gametogenesis and embryonic development in China

Reproductive medicine in China has developed rapidly since 1988 due to the support from the government and scientific exploration. However, the success rate of assisted reproduction technology (ART) is around 30-40% and many unknown "black boxes" in gametogenesis and embryo development are still present. With the development of single-cell and low-input sequencing technologies, the network of transcriptome and epigenetic regulation (DNA methylation, chromatin accessibility, and histone modifications) during the development of human primordial germ cells (PGCs), gametes and embryos has been investigated in depth. Furthermore, pre-implantation genetic testing (PGT) has also rapidly developed. In this review, we summarize and analyze China's outstanding progress in these fields.

Progress in modern reproductive biology research in China

Reproductive biology is closely associated with human health and social progress. Modern reproductive biology research in China began in the 1930s. Advances in science, technology, government support and international collaborations spawned the rapid growth of reproductive biology research in China. While the development of reproductive biology has provided both theoretical knowledge and applicable technologies, it has also generated new social and ethical concerns. This review summarizes and highlights the contributions of modern reproductive biology research in China, with a specific focus on aspects that are most related to human reproduction and health.

A recurrent ZP1 variant is responsible for Oocyte Maturation Defect with degenerated oocytes in infertile females

A female factor is present in approximately 70% of couple infertility, often due to ovulatory disorders. In oocyte maturation defect (OMD), affected patients have a primary infertility with normal menstrual cycles but produce no oocyte, degenerated (atretic) or abnormal oocytes blocked at different stages of maturation. Four genes have so far been associated with OMD: PATL2, TUBB8, WEE2, and ZP1. In our initial study, 6 out of 23 OMD subjects were shown to carry the same PATL2 homozygous loss of function variant and one patient had a TUBB8 truncating variant. Here, we included four additional OMD patients and reanalyzed all 27 subjects. In addition to the seven patients with a previously identified defect, five carried the same deleterious homozygous ZP1 variant (c.1097G>A; p.Arg366Gln). All the oocytes from ZP1‐associated patients appeared shriveled and dark indicating that the abnormal ZP1 protein induced oocyte death and degeneration. Overall ZP1‐associated patients had degenerated or absent oocytes contrary to PATL2‐associated subjects who had immature oocytes blocked mainly at the germinal vesicle stage. In this cohort of North African OMD patients, whole exome sequencing permitted to diagnose 44% of the patients studied and to identify a new frequent ZP1 variant.

Novel mutations in ZP2 and ZP3 cause female infertility in three patients

PURPOSE

The aim of this study was to identify the disease-causing mutations found in three infertile female patients who were diagnosed with abnormal zona pellucida (ZP) and empty follicle syndrome (EFS).

METHODS

We performed whole-exome sequencing and Sanger sequencing to identify and verify the disease-causing mutations. Additionally, we performed Western blotting and mini-gene splicing assay to assess the effects of the mutations.

RESULTS

We identified two novel compound heterozygous mutations in the ZP2 gene, a patient with an abnormal ZP carrying a novel compound heterozygous mutation (c.1695-2A>G and c.1831G>T, p.V611F) and a patient with EFS carrying a novel compound heterozygous mutation (c.1695-2A>G and c.1924 C>T, p.R642*). Furthermore, we identified a patient with typical abnormal ZP carrying a novel heterozygous mutation (c.400G>T, p.A134S) in the ZP3 gene. The splice site mutation (c.1695-2A>G) can cause abnormal pre-mRNA splicing that inserts an extra sequence of 61 bp in the mRNA of ZP2, and the missense mutation (c.1831G>T) can cause a decrease of ZP2 protein in HEK293 cells.

CONCLUSION

We identified three novel mutations in the ZP2 gene and the ZP3 gene in three Chinese female patients with infertility. Our study expands the spectrum of ZP gene mutations and phenotypes and thus is beneficial in the genetic diagnosis of infertility in females.

Causal and Candidate Gene Variants in a Large Cohort of Women With Primary Ovarian Insufficiency

CONTEXT

A genetic etiology likely accounts for the majority of unexplained primary ovarian insufficiency (POI).

OBJECTIVE

We hypothesized that heterozygous rare variants and variants in enhanced categories are associated with POI.

DESIGN

The study was an observational study.

SETTING

Subjects were recruited at academic institutions.

PATIENTS

Subjects from Boston (n = 98), the National Institutes of Health and Washington University (n = 98), Pittsburgh (n = 20), Italy (n = 43), and France (n = 32) were diagnosed with POI (amenorrhea with an elevated follicle-stimulating hormone level). Controls were recruited for health in old age or were from the 1000 Genomes Project (total n = 233).

INTERVENTION

We performed whole exome sequencing (WES), and data were analyzed using a rare variant scoring method and a Bayes factor-based framework for identifying genes harboring pathogenic variants. We performed functional studies on identified genes that were not previously implicated in POI in a D. melanogaster model.

MAIN OUTCOME

Genes with rare pathogenic variants and gene sets with increased burden of deleterious variants were identified.

RESULTS

Candidate heterozygous variants were identified in known genes and genes with functional evidence. Gene sets with increased burden of deleterious alleles included the categories transcription and translation, DNA damage and repair, meiosis and cell division. Variants were found in novel genes from the enhanced categories. Functional evidence supported 7 new risk genes for POI (USP36, VCP, WDR33, PIWIL3, NPM2, LLGL1, and BOD1L1).

CONCLUSIONS

Candidate causative variants were identified through WES in women with POI. Aggregating clinical data and genetic risk with a categorical approach may expand the genetic architecture of heterozygous rare gene variants causing risk for POI.

Molecular tools for the genomic assessment of oocyte’s reproductive competence

The most important factor associated with oocytes' developmental competence has been widely identified as the presence of chromosomal abnormalities. However, growing application of genome-wide sequencing (GS) in population diagnostics has enabled the identification of multifactorial genetic predispositions to sub-lethal pathologies, including those affecting IVF outcomes and reproductive fitness. Indeed, GS analysis in families with history of isolated infertility has recently led to the discovery of new genes and variants involved in specific human infertility endophenotypes that impact the availability and the functionality of female gametes by altering unique mechanisms necessary for oocyte maturation and early embryo development. Ongoing advancements in analytical and bioinformatic pipelines for the study of the genetic determinants of oocyte competence may provide the biological evidence required not only for improving the diagnosis of isolated female infertility but also for the development of novel preventive and therapeutic approaches for reproductive failure. Here, we provide an updated discussion and review of the progresses made in preconception genomic medicine in the identification of genetic factors associated with oocyte availability, function, and competence.

Female fertility and the zona pellucida

Fertility in female mammals, including mice and humans, is dependent on the presence of a zona pellucida (ZP) around growing oocytes and unfertilized eggs. A ZP is required to stabilize contacts between oocyte microvilli and follicle cell projections that traverse the ZP to form gap junctions that support the health of growing oocytes and developing follicles. In the absence of a ZP, due to inactivation or mutation of genes encoding ZP proteins, there is a loss of contacts between growing oocytes and neighboring follicle cells and a concomitant reduction in the production of ovulated eggs that results in female infertility.

Gene Spectrum and Clinical Traits of Nine Patients With Oocyte Maturation Arrest

Background: Oocyte maturation arrest is a disease that produces immature oocytes and cannot be mature after culturing in vitro, which leads to female primary infertility. We aimed to summarize nine representative patients in our center to retrospectively analyze the genetic variants and clinical characteristics of oocyte maturation arrest.

Methods: This study examined and analyzed nine families with oocyte maturation arrest. Whole-exome sequencing (WES) of the probands was performed to detect the pathogenic variants. Sanger sequencing verified the WES findings in patients and available parents. ExAC database was used to search the variant frequency. The variants were assessed by pathogenicity and conservational property prediction analysis and according to the American College of Medical Genetics and Genomics (ACMG). Phenotypes of oocytes were evaluated by a light microscopy, and the phenotype-genotype correlation was also evaluated.

Results: Nine pathogenic variants in five genes were detected in nine patients, of which three were novel variants, including PATL2 [c.1374A > G (p. Ile458Met)] and [1289-1291del TCC (p. Leu430del)] and ZP2 [c.1543C > T (p. Pro515Ser)]. Nine variants were predicted to be pathogenic, resulting in different types of oocyte maturation arrest and clinical phenotypes.

Conclusion: Three novel pathogenic variants were identified, enabling the expansion of the gene variant spectrum. The related pathogenic mutations of the PATL2, TUBB8, and ZP1∼3 genes were highly suggestive of being causative of oocyte maturation arrest.

Identification and in silico Characterization of Deleterious Single Nucleotide Variations in Human ZP2 Gene

ZP2, an important component of the zona matrix, surrounds mammalian oocytes and facilitates fertilization. Recently, some studies have documented the association of mutations in genes encoding the zona matrix with the infertile status of human females. Single nucleotide polymorphisms are the most common type of genetic variations observed in a population and as per the dbSNP database, around 5,152 SNPs are reported to exist in the human ZP2 (hZP2) gene. Although a wide range of computational tools are publicly available, yet no computational studies have been done to date to identify and analyze structural and functional effects of deleterious SNPs on hZP2. In this study, we conducted a comprehensive in silico analysis of all the SNPs found in hZP2. Six different computational tools including SIFT and PolyPhen-2 predicted 18 common nsSNPs as deleterious of which 12 were predicted to most likely affect the structure/functional properties. These were either present in the N-term region crucial for sperm-zona interaction or in the zona domain. 31 additional SNPs in both coding and non-coding regions were also identified. Interestingly, some of these SNPs have been found to be present in infertile females in some recent studies.

Empty Zona Pellucida Only Case: A Critical Review of the Literature

The presence of empty zona pellucida (EZP) in oocytes following oocyte retrieval (OR) during an in vitro fertilization (IVF) cycle presents a major clinical and laboratory challenge in assisted reproduction. It has been attributed to several factors such as the ovarian stimulation protocol employed, the damaging of the follicles during oocyte retrieval (OR) mainly through the high aspiration pressure, during the denudation technique, and the degeneration of oolemma within the zona pellucida (ZP) through apoptosis. The role of ZP is pivotal from the early stages of follicular development up to the preimplantation embryo development and embryo hatching. Polymorphisms or alterations on the genes that encode ZP proteins may contribute to EZP. We present a critical review of the published literature hitherto on EZP and available options when encountered with the phenomenon of EZP. Concerning the former, we found that there is rare data on this phenomenon that merits documentation. The latter includes technical, genetic, and pathophysiological perspectives, along with specific treatment options. In conclusion, we identify the lack of a definitive management proposal for couples presenting with this phenomenon, we underline the need for an algorithm, and indicate the questions raised that point towards our goal for a strategy when addressing a previous finding of EZP.

Zona Pellucida Genes and Proteins: Essential Players in Mammalian Oogenesis and Fertility

All mammalian oocytes and eggs are surrounded by a relatively thick extracellular matrix (ECM), the zona pellucida (ZP), that plays vital roles during oogenesis, fertilization, and preimplantation development. Unlike ECM surrounding somatic cells, the ZP is composed of only a few glycosylated proteins, ZP1–4, that are unique to oocytes and eggs. ZP1–4 have a large region of polypeptide, the ZP domain (ZPD), consisting of two subdomains, ZP-N and ZP-C, separated by a short linker region, that plays an essential role in polymerization of nascent ZP proteins into crosslinked fibrils. Both subdomains adopt immunoglobulin (Ig)-like folds for their 3-dimensional structure. Mouse and human ZP genes are encoded by single-copy genes located on different chromosomes and are highly expressed in the ovary by growing oocytes during late stages of oogenesis. Genes encoding ZP proteins are conserved among mammals, and their expression is regulated by cis-acting sequences located close to the transcription start-site and by the same/similar trans-acting factors. Nascent ZP proteins are synthesized, packaged into vesicles, secreted into the extracellular space, and assembled into long, crosslinked fibrils that have a structural repeat, a ZP2-ZP3 dimer, and constitute the ZP matrix. Fibrils are oriented differently with respect to the oolemma in the inner and outer layers of the ZP. Sequence elements in the ZPD and the carboxy-terminal propeptide of ZP1–4 regulate secretion and assembly of nascent ZP proteins. The presence of both ZP2 and ZP3 is required to assemble ZP fibrils and ZP1 and ZP4 are used to crosslink the fibrils. Inactivation of mouse ZP genes by gene targeting has a detrimental effect on ZP formation around growing oocytes and female fertility. Gene sequence variations in human ZP genes due to point, missense, or frameshift mutations also have a detrimental effect on ZP formation and female fertility. The latter mutations provide additional support for the role of ZPD subdomains and other regions of ZP polypeptide in polymerization of human ZP proteins into fibrils and matrix.

Case Report: A Novel Heterozygous ZP3 Deletion Associated With Empty Follicle Syndrome and Abnormal Follicular Development

Background: Empty follicle syndrome (EFS) is defined as the complete failure to retrieve oocytes after ovarian stimulation. Although several mutations in ZP1, ZP2, ZP3, and LHCGR have been identified as genetic causes of EFS, its pathogenesis is still not well-understood.

Methods: Whole-exome sequencing (WES) was employed to identify the candidate pathogenic mutations, which were then verified by Sanger sequencing. A study in CHO-K1 cells was performed to analyze the effect of the mutation on protein expression. Additionally, immunohistochemistry (IHC) staining was used to examine follicular development and zona pellucida (ZP) assembly in the ovary of an EFS patient.

Results: A novel heterozygous deletion in ZP3 (c.565_579del[p.Thr189_Gly193del]) was identified in the EFS patient. It was inherited dominantly and resulted in significant degradation of the ZP3 protein. Oocytes with degenerated cytoplasm and abnormal ZP assembly were observed in follicles up to the secondary stage, and many empty follicle-like structures were present.

Conclusion: We identified a novel ZP3 mutation that expands the mutational spectrum associated with human EFS. We also showed the abnormal follicular development and ZP assembly of the EFS patient with the heterozygous ZP3 mutation, which provides new insights into the pathogenesis of EFS.

Genetic factors as potential molecular markers of human oocyte and embryo quality

Successful human reproduction requires gamete maturation, fertilization, and early embryonic development. Human oocyte maturation includes nuclear and cytoplasmic maturation, and abnormalities in the process will lead to infertility and recurrent failure of IVF/ICSI attempts. In addition, the quality of oocytes/embryos in the clinic can only be determined by morphological markers, and there is currently a lack of molecular markers for determining oocyte quality. As the number of patients undergoing IVF/ICSI has increased, many patients have been identified with recurrent IVF/ICSI failure. However, the genetic basis behind this phenotype remains largely unknown. In recent years, a few mutant genes have been identified by us and others, which provide potential molecular markers for determining the quality of oocytes/embryos. In this review, we outline the genetic determinants of abnormalities in the processes of oocyte maturation, fertilization, and early embryonic development. Currently, 16 genes (PATL2, TUBB8, TRIP13, ZP1, ZP2, ZP3, PANX1, TLE6, WEE2, CDC20, BTG4, PADI6, NLRP2, NLRP5, KHDC3L, and REC114) have been reported to be the causes of oocyte maturation arrest, fertilization failure, embryonic arrest, and preimplantation embryonic lethality. These abnormalities mainly have Mendelian inheritance patterns, including both dominant inheritance and recessive inheritance, although in some cases de novo mutations have also appeared. In this review, we will introduce the effects of each gene in the specific processes of human early reproduction and will summarize all known variants in these genes and their corresponding phenotypes. Variants in some genes have specific effects on certain steps in the early human reproductive processes, while other variants result in a spectrum of phenotypes. These variants and genetic markers will lay the foundation for individualized genetic counseling and potential treatments for patients and will be the target for precision treatments in reproductive medicine.

New Insights into the Mammalian Egg Zona Pellucida

Mammalian oocytes are surrounded by an extracellular coat called the zona pellucida (ZP), which, from an evolutionary point of view, is the most ancient of the coats that envelope vertebrate oocytes and conceptuses. This matrix separates the oocyte from cumulus cells and is responsible for species-specific recognition between gametes, preventing polyspermy and protecting the preimplantation embryo. The ZP is a dynamic structure that shows different properties before and after fertilization. Until very recently, mammalian ZP was believed to be composed of only three glycoproteins, ZP1, ZP2 and ZP3, as first described in mouse. However, studies have revealed that this composition is not necessarily applicable to other mammals. Such differences can be explained by an analysis of the molecular evolution of the ZP gene family, during which ZP genes have suffered pseudogenization and duplication events that have resulted in differing models of ZP protein composition. The many discoveries made in recent years related to ZP composition and evolution suggest that a compilation would be useful. Moreover, this review analyses ZP biosynthesis, the role of each ZP protein in different mammalian species and how these proteins may interact among themselves and with other proteins present in the oviductal lumen.

Zp4 is completely dispensable for fertility in female rats†

Zona pellucida (ZP), which is composed of at most four extracellular glycoproteins (ZP1, ZP2, ZP3, and ZP4) in mammals, shelters the oocytes and is vital in female fertility. Several studies have identified the indispensable roles of ZP1-3 in maintaining normal female fertility. However, the understanding of ZP4 is still very poor because only one study on ZP4-associated infertility performed in rabbits has been reported up to date. Here we investigated the function of mammalian Zp4 by creating a knockout (KO) rat strain (Zp4-/- rat) using CRISPR-Cas9-mediated DNA-editing method. The influence of Zp4 KO on ZP morphology and some pivotal processes of reproduction, including oogenesis, ovulation, fertilization, and pup production, were studied using periodic acid-Schiff's staining, superovulation, in vitro fertilization, and natural mating. The ZP morphology in Zp4-/- rats was normal, and none of these pivotal processes was affected. This study renewed the knowledge of mammalian Zp4 by suggesting that Zp4 was completely dispensable for female fertility.

A novel homozygous nonsense mutation in zona pellucida 1 (ZP1) causes human female empty follicle syndrome

PURPOSE

To identify a pathogenic gene mutation in a female infertility proband characterized by empty follicle syndrome (EFS) and explore the genetic cause of EFS.

METHODS

Whole exome sequencing (WES) was performed to identify the candidate pathogenic mutation. Sanger sequencing was used to validate the mutation in family members. The pathogenicity of the identified variant and its possible effects on the protein were evaluated with in silico tools. Immunofluorescence staining was used to study the possible mechanism of the mutation on affected oocyte.

RESULTS

We identified a family with a novel homozygous nonsense mutation in zona pellucida 1 (ZP1) (c.199G > T [p.Glu67Ter]). Based on bioinformatics analysis, the mutation was predicted to be pathogenic. This variant generates a premature stop codon in exon 2 at the 199th nucleotide, and was inferred to result in a truncated ZP1 protein of 67 amino acids at the ZP-N1 domain. An in vitro study showed that the oocyte of the EFS proband was degenerated and the zona pellucida was absent. Additionally, the mutant ZP1 proteins were localized in the cytoplasm of the degenerated oocyte but not at the surface.

CONCLUSIONS

The novel mutation in ZP1 is a genetic cause of female infertility characterized by EFS. Our finding expands the genetic spectrum for EFS and will help justify the EFS diagnosis in patients.

Mutant Zp1 impedes incorporation of ZP3 and ZP4 in the zona pellucida, resulting in zona absence and female infertility in rats†

The zona pellucida (ZP) plays vital roles in reproductive processes including oogenesis, fertilization, and preimplantation development. Both human and rat ZP consist of four glycoproteins, called ZP1, ZP2, ZP3, and ZP4. Our previous research reported a novel Zp1 mutation in cases of human infertility, associated with an abnormal phenotype involving the absence of the ZP. Here, we developed a homologous rat strain to investigate the pathogenic effect. The ovaries of homozygous (Zp1MT/MT) females possessed both growing and fully grown oocytes; the oocytes completely lacked a ZP, but ZP1 was detectable inside the cytoplasm. Only 1-2 eggs were recovered from oviducts of superovulated Zp1MT/MT females, while an average of 21 eggs were recovered from superovulated Zp1WT/WT per female. The eggs of Zp1MT/MT females were not surrounded by a ZP and lost their fertilization capacity in vitro. Zp1MT/MT females mated with wild-type males failed to become pregnant. Studies in 293T cells showed that mutant Zp1 resulted in a truncated ZP1 protein, which might be intracellularly sequestered and interacted with wild-type ZP3 or ZP4. Our results suggest that the Zp1 point mutation led to infertility and loss of the ZP in oocytes in rats.

A novel homozygous variant in ZP2 causes abnormal zona pellucida formation and female infertility

PURPOSE

We aimed to identify pathogenic variants in two infertile sisters in a family with a thin zona pellucida (ZP) phenotype.

METHODS

Whole-exome sequencing was performed in the two affected sisters, and Sanger sequencing was used to confirm the identified variants. The effects of the identified variant were further investigated in mouse oocytes and Chinese hamster ovary (CHO) cells.

RESULTS

We identified a novel homozygous frameshift variant in ZP2 (c.1235_1236del, p.Q412Rfs*17) in the two affected individuals. Immunoblotting demonstrated that the variant produced a truncated ZP2 protein that was expressed at low levels in CHO cells. Immunofluorescence in mouse oocytes confirmed the decreased protein level of mutant ZP2, although the subcellular localization was not affected. In addition, immunoprecipitation showed that the pathogenic variant reduced the interaction between ZP2 and ZP3.

CONCLUSION

This study identified a novel pathogenic variant in ZP2 that produces a truncated ZP2 protein. The variant might disrupt the assembly of ZP2-ZP3 dimers, thus resulting in a thin ZP and female infertility.

Human Zona Pellucida Glycoproteins: Binding Characteristics With Human Spermatozoa and Induction of Acrosome Reaction

Human zona pellucida (ZP) matrix is composed of four glycoproteins designated as ZP glycoprotein -1 (ZP1), -2 (ZP2), -3 (ZP3), and -4 (ZP4). Mutations in the genes encoding human ZP glycoproteins are one of the causative factors leading to abnormal ZP matrix and infertility in women. Relevance of the human ZP glycoproteins in 'sperm-oocyte' binding has been delineated by using either transgenic animal models expressing human zona proteins or purified native/recombinant human zona proteins. Studies based on the purified native/recombinant human zona proteins revealed that ZP1, ZP3, and ZP4 primarily bind to the capacitated acrosome-intact human spermatozoa whereas ZP2 binds to acrosome-reacted spermatozoa. On the contrary, human spermatozoa binds to the eggs obtained from transgenic mouse lines expressing human ZP2 but not to those expressing human ZP1, ZP3, and ZP4 suggesting that ZP2 has an important role in human 'sperm-oocyte' binding. Further studies using transgenic mouse lines showed that the N-terminus of human ZP2 mediate the taxon-specific human sperm-oocyte binding. Both glycans and protein-protein interactions have a role in human gamete interaction. Further studies have revealed that the purified native/recombinant human ZP1, ZP3, and ZP4 are competent to induce acrosome reaction. Human sperm binds to the mouse transgenic eggs expressing human ZP1-4 instead of mouse ZP1-3 proteins, penetrated the ZP matrix and accumulated in the perivitelline space, which were acrosome-reacted suggesting that human ZP2 in transgenic mouse model also induce acrosome reaction. In humans N-linked glycosylation of zona proteins have been shown to play an important role in induction of the acrosome reaction. Hence in humans, based on studies using transgenic mouse model as well as purified native/recombinant zona proteins, it is likely that more than one zona protein is involved in the 'sperm-oocyte' binding and induction of the acrosome reaction.

Novel mutations in ZP1: Expanding the mutational spectrum associated with empty follicle syndrome in infertile women

Empty follicle syndrome (EFS) is a serious and complex reproductive complication for infertile women suffering from the recurrent failure of oocyte retrieval in an in vitro fertilization procedure, and its pathogenesis remains obscure. Increasing evidence highlights the genetic basis of EFS occurrence. In this study, we identified two novel missense mutations (c.1127G > A, p.C376Y and c.325C > T, p.R109C), two novel frameshift mutations (c.800_801delAG, p.E267Gfs*80 and c.1815_1825delGGTCCTTTTGC, p.V606Afs*42), one novel nonsense mutation (c.199G > T, p.E67Ter), and three reported mutations (c.769C > T, p.Q257Ter; c.1430 + 1G > T, p.C478Ter and c.1169_1176delTTTTCCCA, p.I390Tfs*16) in five unrelated probands, showing similar EFS manifestations, which expands the mutational spectrum of individuals with autosomal recessive ZP1. Current research will provide a better understanding of the biological functions of ZP1, and some insight into the determination of ZP1 variation as an additional rule for assessing the EFS disease.