Mutations in PLCZ1 induce male infertility associated with polyspermy and fertilization failure

High rate of detected variants in male PLCZ1 and ACTL7A genes causing failed fertilization after ICSI

STUDY QUESTION

What is the frequency of PLCZ1, ACTL7A, and ACTL9 variants in male patients showing fertilization failure after ICSI, and how effective is assisted oocyte activation (AOA) for them?

SUMMARY ANSWER

Male patients with fertilization failure after ICSI manifest variants in PLCZ1 (29.09%), ACTL7A (14.81%), and ACTL9 (3.70%), which can be efficiently overcome by AOA treatment with ionomycin.

WHAT IS KNOWN ALREADY

Genetic variants in PLCZ1, and more recently, in ACTL7A, and ACTL9 male genes, have been associated with total fertilization failure or low fertilization after ICSI. A larger patient cohort is required to understand the frequency at which these variants occur, and to assess their effect on the calcium ion (Ca2+) release during oocyte activation. AOA, using ionomycin, can restore fertilization and pregnancy rates in patients with PLCZ1 variants, but it remains unknown how efficient this is for patients with ACTL7A and ACTL9 variants.

STUDY DESIGN SIZE DURATION

This prospective study involved two patient cohorts. In the first setting, group 1 (N = 28, 2006-2020) underwent only PLCZ1 genetic screening, while group 2 (N = 27, 2020-2023) underwent PLCZ1, ACTL7A, and ACTL9 genetic screening. Patients were only recruited when they had a mean fertilization rate of ≤33.33% in at least one ICSI cycle with at least four MII oocytes. Patients underwent a mouse oocyte activation test (MOAT) and at least one ICSI-AOA cycle using calcium chloride (CaCl2) injection and double ionomycin exposure at our centre. All patients donated a saliva sample for genetic screening and a sperm sample for further diagnostic tests, including Ca2+ imaging.

PARTICIPANTS/MATERIALS SETTING METHODS

Genetic screening was performed via targeted next-generation sequencing. Identified variants were classified by applying the revised ACMG guidelines into a Bayesian framework and were confirmed by bidirectional Sanger sequencing. If variants of uncertain significance or likely pathogenic or pathogenic variants were found, patients underwent additional determination of the sperm Ca2+-releasing pattern in mouse (MOCA) and in IVM human (HOCA) oocytes. Additionally, ACTL7A immunofluorescence and acrosome ultrastructure analyses by transmission electron microscopy (TEM) were performed for patients with ACTL7A and/or ACTL9 variants.

MAIN RESULTS AND THE ROLE OF CHANCE

Overall, the frequency rate of PLCZ1 variants was 29.09%. Moreover, 14.81% of patients carried ACTL7A variants and 3.70% carried ACTL9 variants. Seven different PLCZ1 variants were identified (p.Ile74Thr, p.Gln94*, p.Arg141His, p.His233Leu, p.Lys322*, p.Ile379Thr, and p.Ser500Leu), five of which are novel. Interestingly, PLCZ1 variants p.Ser500Leu and p.His233Leu occurred in 14.55% and 9.09% of cases. Five different variants were found in ACTL7A (p.Tyr183His, p.Gly214Ser, p.Val340Met, p.Ser364Glnfs*9, p.Arg373Cys), four of them being identified for the first time. A novel variant in ACTL9 (p.Arg271Pro) was also described. Notably, both heterozygous and homozygous variants were identified.The MOCA and HOCA tests revealed abnormal or absent Ca2+ release during fertilization in all except one patient, including patients with PLCZ1 heterozygous variants. TEM analysis revealed abnormal acrosome ultrastructure in three patients with ACTL7A variants, but only patients with homozygous ACTL7A variants showed reduced fluorescence intensity in comparison to the control.AOA treatment significantly increased the fertilization rate in the 19 patients with detected variants (from 11.24% after conventional ICSI to 61.80% after ICSI-AOA), as well as positive hCG rate (from 10.64% to 60.00%) and live birth rate (from 6.38% to 37.14%), resulting in 13 healthy newborns. In particular, four live births and two ongoing pregnancies were produced using sperm from patients with ACTL7A variants.

LIMITATIONS REASONS FOR CAUTION

Genetic screening included exonic and outflanking intronic regions, which implies that deep intronic variants were missed. In addition, other male genes or possible female-related factors affecting the fertilization process remain to be investigated.

WIDER IMPLICATIONS OF THE FINDINGS

Genetic screening of PLCZ1, ACTL7A, and ACTL9 offers a fast, cost-efficient, and easily implementable diagnostic test for total fertilization failure or low fertilization after ICSI, eliminating the need for complex diagnostic tests like MOAT or Ca2+ analysis. Nonetheless, HOCA remains the most sensitive functional test to reveal causality of uncertain significance variants. Interestingly, heterozygous PLCZ1 variants are sufficient to cause inadequate Ca2+ release during ICSI. Most importantly, AOA treatment using CaCl2 injection followed by double ionomycin exposure is highly effective for this patient group, including those with ACTL7A variants, who also display a Ca2+-release deficiency.

STUDY FUNDING/COMPETING INTERESTS

This study was supported by the Flemish Fund for Scientific Research (FWO) (TBM-project grant T002223N awarded to B.H.) and by the Special Research Fund (BOF) (starting grant BOF.STG.2021.0042.01 awarded to B.H.). A.C.B., R.R.G., C.C., E.V.D.V., A.R., D.S., L.L., P.C., S.S., A.B., and F.V.M. have nothing to disclose. B.H. reports a research grant from FWO and BOF, and reports being a board member of the Belgian Ethical Committee on embryo research.

TRIAL REGISTRATION NUMBER

N/A.

Novel PLCZ1 mutation caused polyspermy during in vitro fertilization

Failure of oocyte activation, including polyspermy and defects in pronuclear (PN) formation, triggers early embryonic developmental arrest. Many studies have shown that phospholipase C zeta 1 ( PLCZ1 ) mutations cause failure of PN formation following intracytoplasmic sperm injection (ICSI); however, whether PLCZ1 mutation is associated with polyspermy during in vitro fertilization (IVF) remains unknown. Whole-exome sequencing (WES) was performed to identify candidate mutations in couples with primary infertility. Sanger sequencing was used to validate the mutations. Multiple PLCZ1 -mutated sperm were injected into human and mouse oocytes to explore whether PN formation was induced. Assisted oocyte activation (AOA) after ICSI was performed to overcome the failure of oocyte activation. We identified three PLCZ1 mutations in three patients who experienced polyspermy during IVF cycles, including a novel missense mutation c.1154C>T, p.R385Q. PN formation failure was observed during the ICSI cycle. However, injection of multiple PLCZ1- mutated sperm induced PN formation, suggesting that the Ca 2+ oscillations induced by the sperm exceeded the necessary threshold for PN formation. AOA after ICSI enabled normal fertilization, and all patients achieved successful pregnancies. These findings expand the mutational spectrum of PLCZ1 and suggest an important role for PLCZ1 in terms of blocking polyspermy. Furthermore, this study may benefit genetic diagnoses in cases of abnormal fertilization and provide potential appropriate therapeutic measures for these patients with sperm-derived polyspermy.

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.

Overdue Calcium Oscillation Causes Polyspermy but Possibly Permits Normal Development in Mouse Eggs

In some non-mammalian eggs, the fusion of one egg and multiple sperm (polyspermy) induces a robust rise in intracellular calcium ion (Ca2+) concentration due to a shortage of inducers carried by a single sperm. Instead, one of the sperm nuclei is selected inside the egg for normal embryogenesis. Polyspermy also occurs during the in vitro fertilization of human eggs; however, the fate of such eggs is still under debate. Hence, the relationship between polyspermy and repetitive Ca2+ increases (Ca2+ oscillation) in mammals remains unknown. To address this issue, we used mouse sperm lacking extramitochondrial citrate synthase (eCS), which functions as a Ca2+ oscillation inducer; its lack causes retarded Ca2+ oscillation initiation (eCs-KO sperm). Elevated sperm concentrations normalize Ca2+ oscillation initiation. As expected, eCS deficiency enhanced polyspermy in both zona pellucida (ZP)-free and ZP-intact eggs despite producing the next generation of eCs-KO males. In conclusion, similarly to non-mammalian eggs, mouse eggs may develop normally under polyspermy conditions caused by problematic Ca2+ oscillation.

Novel biallelic ASTL variants are associated with polyspermy and female infertility: A successful live birth following ICSI treatment

Fertilization of the egg by the sperm is the first vital stage of embryogenesis. In mammals, only one sperm is incorporated into the oocyte. Polyspermy is a key anomaly of fertilization that is generally lethal to the embryo. To date, only a few causative genes for polyspermy have been reported. In a recent study, a homozygous variant in astacin-like metalloendopeptidase (ASTL), which encodes the ovastacin enzyme that cleaves ZP2 to prevent polyspermy, was found to be associated with female infertility characterized by polyspermy in vitro. Herein, we identified two ASTL variants in a Chinese woman likely responsible for her primary infertility and polyspermy in in vitro fertilization. Both variants were located within the key catalytic domain and predicted to alter hydrogen bonds, potentially impairing protein stability. Moreover, expression and immunoblot analyses in CHO-K1 cells indicated abnormal ovastacin zymogen activation or decreased enzyme stability. Intracytoplasmic sperm injection treatment successfully bypassed the defect in polyspermy blocking and resulted in a live birth. Our study associates ASTL variants with human infertility and further supports the contribution of this gene to blocking polyspermy in humans. Our findings expand the spectrum of ASTL mutations and should facilitate the diagnosis of oocyte-borne polyspermy.

Phospholipase C Zeta 1 (PLCZ1): The Function and Potential for Fertility Assessment and In Vitro Embryo Production in Cattle and Horses

Phospholipase C Zeta 1 (PLCZ1) is considered a major sperm-borne oocyte activation factor. After gamete fusion, PLCZ1 triggers calcium oscillations in the oocyte, resulting in oocyte activation. In assisted fertilization, oocyte activation failure is a major cause of low fertility. Most cases of oocyte activation failures in humans related to male infertility are associated with gene mutations and/or altered PLCZ1. Consequently, PLCZ1 evaluation could be an effective diagnostic marker and predictor of sperm fertilizing potential for in vivo and in vitro embryo production. The characterization of PLCZ1 has been principally investigated in men and mice, with less known about the PLCZ1 impact on assisted reproduction in other species, such as cattle and horses. In horses, sperm PLCZ1 varies among stallions, and sperm populations with high PLCZ1 are associated with cleavage after intracytoplasmic sperm injection (ICSI). In contrast, bull sperm is less able to initiate calcium oscillations and undergo nuclear remodeling, resulting in poor cleavage after ICSI. Advantageously, injections of PLCZ1 are able to rescue oocyte failure in mouse oocytes after ICSI, promoting full development and birth. However, further research is needed to optimize PLCZ1 diagnostic tests for consistent association with fertility and to determine whether PLCZ1 as an oocyte-activating treatment is a physiological, efficient, and safe method for improving assisted fertilization in cattle and horses.

Novel mutations in PLCZ1 lead to early embryonic arrest as a male factor

Early embryonic arrest is one of the causes of assist reproduction technology (ART) failure. We have previously reported that the first sperm-derived genetic factor, ACTL7a mutations, could lead to early embryonic arrest. However, whether there are other male genetic factors associated with early embryonic arrest remains elusive. Here, we reported bi-allelic mutations in PLCZ1, a well-known causal gene of total fertilization failure, in four infertile males. Among these mutations, p.403_404del, p.I489S, and p.W536X were newly reported in this study. Histological and Western blotting analysis of the patients' sperm indicated these variants as loss-of-function mutations. These patients manifested normal conventional semen parameters and ultra-structures in sperm heads. However, among four in vitro fertilization (IVF) cycles, 81.8% (18/22) of the oocytes were polyspermic fertilized, which was rarely reported in PLCZ1-related male patients. In the following six ICSI cycles, artificial oocyte activation (AOA) was applied and successfully rescued the fertilization failure and polyspermy phenotypes, with 31.3% (15/48) of the MII oocytes normally fertilized. However, 60.0% (9/15) of these normally fertilized zygotes were arrested at 2-5-cell stage, with one failing to cleave, indicating that PLCZ1 was not only necessary for fertilization, but also crucial for early embryonic development. However, these rescued zygotes showed a lower potential in developing into blastocysts when cultured in vitro. Thus, fresh cleavage transfer was tried and two live births were successfully achieved thereafter. In conclusion, this study provided novel mutations in PLCZ1 gene to expand the pathogenic mutational spectrum in male infertility and demonstrated that PLCZ1 was a crucial sperm-related genetic factor for early embryonic arrest. We also proposed that cleavage transfer after ICSI and AOA treatment could be a potential treatment method for male patients carrying bi-allelic mutations in PLCZ1.