Infertility represents a huge psychological burden for affected men. Receiving a causative diagnosis not only aids to cope with their situation but also helps to estimate success rates for medically assisted reproduction (MAR). Having this in mind, our research group at the Institute of Reproductive Genetics, Centre of Medical Genetics, in Münster is focusing on the identification and functional characterisation of disease genes for male infertility. Of particular interest are genes in which pathogenic variants are linked to an arrest of spermatogenesis and, consequently, leading to a complete absence of sperm in the ejaculate. This condition is one of the most severe forms of male infertility, leaving affected men with minimal chances of fathering a child through MAR.
While research on monogenic causes of spermatogenic arrest has predominantly concentrated on meiosis-related genes, our recent study provides compelling evidence that genes encoding proteins involved in the production of PIWI-interacting RNAs (piRNAs) are also major contributors to human spermatogenic failure.
piRNAs represent a subgroup of small RNA molecules predominantly expressed in the mammalian testis. They bind to certain Argonaut proteins, known as PIWI proteins, and contribute to the repression of transposons through de novo methylation of genomic DNA and post-transcriptional silencing and are, thus, essential for maintaining genome integrity. In addition, in the adult testis, they also contribute to the regulation of gene expression. While the function of piRNAs has been well-characterized in mice, their role in human germ cell development is much less understood. Mouse models have shown that a disturbed production of piRNAs results in male infertility, associated with small testes and spermatogenic arrest. However, can these results be extrapolated to men?
Our journey on the piRNA pathway in the human testis started with identifying a significant association between homozygous loss-of-function variants found in exome/genome data of men with spermatogenic failure and genes involved in “piRNA processing”. Accordingly, in a subsequent screening approach of genomic data from four independent cohorts of infertile men, we identified 39 men carrying high-impact biallelic variants in 14 different piRNA-related genes. These include genes of the piRNA‐induced silencing complex (piRISC) such as PIWIL1, PIWIL2, and GTSF1, genes involved in piRNA maturation such as PLD6, GPAT2, MOV10L1, and MAEL, and genes of the TDRD scaffold protein gene family.
By characterising the reproductive and testicular phenotypes of affected men, we show concordant gene-specific findings that highlight important differences between humans and mice. Our results reveal that, for several genes, the phenotype in men differs from the arrest phenotype observed in mice, indicating that findings from mouse models are not generally valid for all mammalians.
But what about the impact of the human piRNA pathway on transposon regulation? Our research highlights that disruption of certain piRNA biogenesis factors not only reduces piRNA levels, but is also linked to the de-repression of LINE1 transposons in spermatogonia. This finding was unexpected as it is in contrast to impaired piRNA biogenesis in mice, where LINE1 de-repression was frequently observed in spermatocytes.
Returning to the men affected by infertility, this study marks a substantial advancement in the field of reproductive genetics by significantly expanding the number of candidate genes linked to male infertility and establishing a connection between genetically disrupted piRNA biogenesis and male infertility. The success of our journey is rooted in strong collaborations with research groups from the International Male Infertility Genomics Consortium (IMIGC) and the Wellcome Centre for Cell Biology that provided genomic data of infertile men, specific laboratory expertise and expertise on the piRNA biogenesis field. This underscores the value of assembling large patient cohorts for the study of rare diseases.