Novel Biallelic TLE6 Variants Induce Preimplantation Embryonic Lethality That Cannot be Rescued by IVF or ICSI

Background: Preimplantation embryonic lethality is a rare cause of primary female infertility. Transducin-like enhancer of split 6 (TLE6) is a maternal effect gene which encodes a component of the subcortical maternal complex located in the oocytes and early embryos. It has been reported that biallelic variants in TLE6 can lead to preimplantation embryonic lethality. However, the incidence of TLE6 variants in patients with preimplantation embryonic lethality is not fully understood.Methods: Whole-exome sequencing and bioinformatics analyses were used to analyze a cohort of 28 patients with preimplantation embryonic lethality. We retrospectively analyzed the process and outcome of their attempts at in vitro fertilization and intracytoplasmic sperm injection. Results: In this study, four patients (14.29 %, 4/28) from three unrelated families in a cohort of 28 individuals with preimplantation embryonic lethality were identified as carrying biallelic TLE6 variants, including two homozygous variants and one compound heterozygous variant. These novel frameshift variants in TLE6 caused truncation of the TLE6 protein likely impairing its function. Immunofluorescence staining of oocytes for TLE6 indicated that it is localized in the oocyte cytoplasm, and that TLE6 was almost absent in the oocytes of the patients carrying biallelic TLE6 variants compared with normal control oocytes. A retrospective analysis showed that the four affected individuals underwent a total of nine in vitro fertilization and intracytoplasmic sperm injection attempts, in which a total of 119 metaphase II oocytes were retrieved, but none of them obtained high-quality blastocysts or obtained a successful pregnancy. However, one of these patients became pregnant on the first attempt using donated oocytes.Conclusions: Biallelic TLE6 variants cause preimplantation embryonic lethality that cannot be rescued by in vitro fertilization or intracytoplasmic sperm injection. Thus, oocyte donation may be the preferred treatment for patients harboring biallelic TLE6 variants.

Smchd1 is a maternal effect gene required for genomic imprinting

Genomic imprinting establishes parental allele-biased expression of a suite of mammalian genes based on parent-of-origin specific epigenetic marks. These marks are under the control of maternal effect proteins supplied in the oocyte. Here we report epigenetic repressor Smchd1 as a novel maternal effect gene that regulates the imprinted expression of ten genes in mice. We also found zygotic SMCHD1 had a dose-dependent effect on the imprinted expression of seven genes. Together, zygotic and maternal SMCHD1 regulate three classic imprinted clusters and eight other genes, including non-canonical imprinted genes. Interestingly, the loss of maternal SMCHD1 does not alter germline DNA methylation imprints pre-implantation or later in gestation. Instead, what appears to unite most imprinted genes sensitive to SMCHD1 is their reliance on polycomb-mediated methylation as germline or secondary imprints, therefore we propose that SMCHD1 acts downstream of polycomb imprints to mediate its function.

Homozygous RRM2 Variant Might Lead to Early Embryo Developmental Arrest: A Case Report

Background: Early embryonic developmental stagnation is one of the reasons that affect the outcome of in vitro fertilization-embryo transfer, leading to the depletion of available embryos or failure after transplantation. It has been shown that defects in the ribonucleotide-reductase lead to cell cycle arrest, developmental delay, and high mutation rates.Case presentation: Two female patients, who were siblings from an inbreeding family, suffered primary infertility of unknown causes and early embryonic developmental arrest during IVF treatment. A total of 39 oocytes were obtained from the patients in collectively 5 IVF/ICSI cycles, of which 37 were mature eggs, only 2 transplantable embryos were formed, and no pregnancy was achieved. Whole genome sequencing and Sanger sequencing were adopted to identify and confirm variations that might cause early embryo developmental stagnation in this family. We identified a homozygous variant c.262C>T:p.His88Tyr in ribonucleotide-diphosphate reductase subunit M2 (RRM2) in both patients and their parents each carried a heterozygous allele. Pedigree analysis showed an autosomal recessive inheritance pattern. Function of this variant was predicted by online databases, which indicated it to be a potential pathogenic mutation. Conclusions: We identified RRM2 as a potential causative gene for early embryonic developmental stagnation. It was also suggested that RRM2 might be a maternal effect gene.

Smchd1 is a maternal effect gene required for autosomal imprinting

Genomic imprinting establishes parental allele-biased expression of a suite of mammalian genes based on parent-of-origin specific epigenetic marks. These marks are under the control of maternal effect proteins supplied in the oocyte. Here we report the epigenetic repressor Smchd1 as a novel maternal effect gene that regulates imprinted expression of 16 genes. Most Smchd1-sensitive genes only show loss of imprinting post-implantation, indicating maternal Smchd1’s long-lived epigenetic effect. Sm-chd1-sensitive genes include both those controlled by germline polycomb marks and germline DNA methylation imprints; however, Smchd1 differs to other maternal effect genes that regulate the latter group, as Smchd1 does not affect germline DNA methylation imprints. Instead, Smchd1-sensitive genes are united by their reliance on polycomb-mediated histone methylation marks as germline or secondary imprints. We propose that Smchd1 translates these imprints to establish a heritable chromatin state required for imprinted expression later in development, revealing a new mechanism for maternal effect genes.

SMCHD1 terminates the first embryonic genome activation event in mouse two-cell embryos and contributes to a transcriptionally repressive state

Embryonic genome activation (EGA) in mammals begins with transient expression of a large group of genes (EGA1). Importantly, entry into and exit from the 2C/EGA state is essential for viability. Dux family member genes play an integral role in EGA1 by activating other EGA marker genes such as Zscan4 family members. We previously reported that structural maintenance of chromosomes flexible hinge domain-containing protein 1 ( Smchd1) is expressed at the mRNA and protein levels in mouse oocytes and early embryos and that elimination of Smchd1 expression inhibits inner cell mass formation, blastocyst formation and hatching, and term development. We extend these observations here by showing that siRNA knockdown of Smchd1 in zygotes results in overexpression of Dux and Zscan4 in two-cell embryos, with continued overexpression of Dux at least through the eight-cell stage as well as prolonged expression of Zscan4. These results are consistent with a role for SMCHD1 in promoting exit from the EGA1 state and establishing SMCHD1 as a maternal effect gene and the first chromatin regulatory factor identified with this role. Additionally, bioinformatics analysis reveals that SMCHD1 also contributes to the creation of a transcriptionally repressive state to allow correct gene regulation.

Genetic diagnosis of subfertility: the impact of meiosis and maternal effects

During reproductive age, approximately one in seven couples are confronted with fertility problems. While the aetiology is diverse, including infections, metabolic diseases, hormonal imbalances and iatrogenic effects, it is becoming increasingly clear that genetic factors have a significant contribution. Due to the complex nature of infertility that often hints at a multifactorial cause, the search for potentially causal gene mutations in idiopathic infertile couples has remained difficult. Idiopathic infertility patients with a suspicion of an underlying genetic cause can be expected to have mutations in genes that do not readily affect general health but are only essential in certain processes connected to fertility. In this review, we specifically focus on genes involved in meiosis and maternal-effect processes, which are of critical importance for reproduction and initial embryonic development. We give an overview of genes that have already been linked to infertility in human, as well as good candidates which have been described in other organisms. Finally, we propose a phenotypic range in which we expect an optimal diagnostic yield of a meiotic/maternal-effect gene panel.