ZP1 mutations are associated with empty follicle syndrome: evidence for the existence of an intact oocyte and a zona pellucida in follicles up to the early antral stage. A case report

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.

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.

Effect of In Vitro Maturation of Human Oocytes Obtained After Controlled Ovarian Hormonal Stimulation on the Expression of Development- and Zona Pellucida-Related Genes and Their Interactions

In an in vitro fertilization program, approximately 10-15% of oocytes obtained after controlled ovarian stimulation are immature, with germinal vesicles (GVs). These oocytes are usually discarded in clinical practice; however, an in vitro maturation (IVM) procedure can be applied to mature them. There are scarce data in the literature on the effect of IVM on the expression of important development- and zona pellucida (ZP)-related genes in human oocytes; therefore, we wanted to determine this. One hundred nine human oocytes were collected from patients enrolled in an intracytoplasmic sperm injection program. The expression of the BMP4, GDF9, ZP1, ZP2, ZP3, and ZP4 genes was analyzed using RT-qPCR in oocytes matured in vitro with different reproductive hormones in the IVM medium (AMH, FSH + hCG, FSH + hCG + AMH), in in vivo matured oocytes and in immature oocytes with GVs. No statistically significant differences in the expression of selected genes in oocytes were observed among groups with different reproductive hormones in IVM medium. However, several interesting significant correlations were found between BMP4 and GDF9, and ZP1 and ZP4; between GDF9 and ZP1, and ZP2 and ZP4; and between ZP1 and ZP3 and ZP4 in the in vitro matured oocytes, while no such correlations were present in other groups of oocytes. The type of reproductive hormone in the maturation medium does not affect the expression of the analyzed genes in oocytes during the maturation process. However, the in vitro maturation procedure itself generated correlations among analyzed genes that were otherwise not present in in vivo matured and immature oocytes.

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.

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.

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.

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.

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.

Heterozygous mutations in ZP1 and ZP3 cause formation disorder of ZP and female infertility in human

The human zona pellucida (ZP) is a highly organized glycoprotein matrix that encircles oocytes and plays an essential role in successful reproduction. Previous studies have reported that mutations in human ZP1, ZP2 and ZP3 influence their functions and result in a lack of ZP or in an abnormal oocytes and empty follicle syndrome, which leads to female infertility. Here, we performed whole‐exome sequencing in two probands with primary infertility whose oocytes lacked a ZP, and we identified a heterozygous mutation in ZP1 (NM_207341:c.326G>A p.Arg109His), which is situated in the N‐terminus, and a heterozygous mutation in ZP3 (NM_001110354:c.400G>A p.Ala134Thr), which is situated in the ZP domain. The effects of the mutations were investigated through structure prediction and in vitro studies in HeLa cells. The results, which were in line with the phenotype, suggested that these mutations might impede the function of cross‐linking and secretion of ZP proteins. Our study showed that the two mutations in ZP1 and ZP3 influenced the formation of the ZP, causing female infertility. Meanwhile, these data highlight the importance of the ZP1 N‐terminus in addition to the conserved domains for ZP1 function and ZP formation. Additionally, the patient with the ZP1 mutation delivered a baby following intracytoplasmic sperm injection (ICSI); thus, we suggest the targeted genetic diagnosis of ZP genes to choose appropriate fertilization methods and improve the success rate of assisted reproductive technology (ART) treatments.

Novel biallelic loss-of-function variants in ZP1 identified in an infertile female with empty follicle syndrome

PURPOSE

Empty follicle syndrome (EFS) refers to the inability to obtain mature oocytes after appropriate ovarian stimulation during the process of in vitro fertilization (IVF). However, the specific cause and mechanism of action underlying EFS remain to be further explored. Herein we aimed to investigate the clinical and genetic characteristics of EFS.

METHODS

After data were collected in an infertile family, we performed whole-exome sequencing (WES) on the patient and confirmed the pathogenic mutations through Sanger sequencing. Western immunoblotting, immunofluorescence staining, and minigene assay were further used to investigate the negative effects of these mutations.

RESULTS

Absence of oocytes was observed over 2 cycles of IVF in the patient, and we evaluated the novel compound heterozygous mutations c.2T>A (p. M1K) and c.1112+1G>T of the zona pellucida glycoprotein 1 gene (ZP1, MIM# 195000) by WES. For the family under study, EFS was classified as an autosomal recessive inheritance pattern. The results of western blotting and immunofluorescence staining analyses confirmed that the missense mutation of c.2T>A [p. M1K] resulted in almost missing protein production. Additionally, using a minigene assay, we demonstrated the deleterious effect on the ZP1 gene of the splice site mutation c.1112+1G>T, which caused truncation of ZP1 protein.

CONCLUSIONS

The compound heterozygous mutations of ZP1 gene identified in this study by genetic and functional experiments constituted a novel genetic cause of EFS, and further study will expand its use in the clinical and molecular diagnoses of EFS.

A novel homozygous nonsense ZP1 variant causes human female infertility associated with empty follicle syndrome (EFS)

Background

Empty follicle syndrome (EFS) is a rare but severe condition in which no oocyte is recovered in female patients undergoing in vitro fertilization (IVF) after sufficient ovarian response to hormonal trigger. Accumulating evidence highlights the genetic basis of EFS occurrence.

Methods

In this study, we report a patient with primary infertility showing the characteristics of EFS from a consanguineous family. Under the treatment of assisted reproductive technique (ART), no oocyte was retrieved following the aspiration of mature follicles. Through whole‐exome sequencing (WES), we discovered a novel recessively transmitted mutation in ZP1 (c.769 C>T, p. Q257*).

Results

In vitro Co‐immunoprecipitation assays showed that mutant ZP1 protein failed to interact with either ZP2 or ZP3, which explains the degenerated oocytes in the patient with EFS.

Conclusion

Together, our data further expand the spectrum of ZP1 mutations that are associated with human EFS and thus provide novel insight into the diagnosis of EFS patients.

Homozygous Splice Site Mutation in ZP1 Causes Familial Oocyte Maturation Defect

In vitro fertilization (IVF) involves controlled ovarian hyperstimulation using hormones to produce large numbers of oocytes. The success of IVF is tightly linked to the availability of mature oocytes. In most cases, about 70% to 80% of the oocytes are mature at the time of retrieval, however, in rare instances, all of them may be immature, implying that they were not able to reach the metaphase II (MII) stage. The failure to obtain any mature oocytes, despite a well conducted ovarian stimulation in repeated cycles is a very rare cause of primary female infertility, for which the underlying suspected genetic factors are still largely unknown. In this study, we present the whole exome sequencing analysis of a consanguineous Turkish family comprising three sisters with a recurrent oocyte maturation defect. Analysis of the data reveals a homozygous splice site mutation (c.1775-3C>A) in the zona pellucida glycoprotein 1 (ZP1) gene. Minigene experiments show that the mutation causes the retention of the intron 11 sequence between exon 11 and exon 12, resulting in a frameshift and the likely production of a truncated protein.

A genomics approach to females with infertility and recurrent pregnancy loss

Infertility affects 10% of reproductive-age women and is extremely heterogeneous in etiology. The genetic contribution to female infertility is incompletely understood, and involves chromosomal and single-gene defects. Our aim in this study is to decipher single-gene causes in infertile women in whom endocrinological, anatomical, and chromosomal causes have been excluded. Our cohort comprises women with recurrent pregnancy loss and no offspring from spontaneous pregnancies (RPL, n = 61) and those who never achieved clinical pregnancy and were referred for in vitro fertilization [primary infertility (PI), n = 14]. Whole-exome sequencing revealed candidate variants in 14, which represents 43% of those with PI and 13% of those with RPL. These include variants in previously established female infertility-related genes (TLE6, NLRP7, FSHR, and ZP1) as well as genes with only tentative links in the literature (NLRP5). Candidate variants in genes linked to primary ciliary dyskinesia (DNAH11 and CCNO) were identified in individuals with and without systemic features of the disease. We also identified variants in genes not previously linked to female infertility. These include one homozygous variant each in CCDC68, CBX3, CENPH, PABPC1L, PIF1, PLK1, and REXO4, which we propose as candidate genes for infertility based on their established biology or compatible animal models. Our study expands the contribution of single genes to the etiology of PI and RPL, improves the precision of disease classification at the molecular level, and offers the potential for future treatment and development of human genetics-inspired fertility regulators.