The piRNA-pathway factor FKBP6 is essential for spermatogenesis but dispensable for control of meiotic LINE-1 expression in humans

• DOI: 10.1016/j.ajhg.2022.09.002
• Publication Date: 2022-09-22T14:36:18Z
• Authors: Margot J. Wyrwoll; Channah M. Gaasbeek; Ieva Golubickaite; Rytis Stakaitis; Manon S. Oud; Liina Nagirnaja; Camille Dion; Emad B. Sindi; Harry G. Leitch; Channa N. Jayasena; Anu Sironen; Ann-Kristin Dicke; Nadja Rotte; Birgit Stallmeyer; Sabine Kliesch; Carlos H.P. Grang...; Thaís F. Araujo; Paul Lasko; Kathleen D’Hauwers; Roos M. Smits; Liliana Ramos; Miguel J. Xavier; Don F. Conrad; Kristian Almstrup; Joris A. Veltman; Frank Tüttelmann; Godfried W. van d...
• Keywords: Male; 0301 basic medicine; 570; 610; male infertility; Article; 576; Tacrolimus Binding Proteins; LINE-1; Mice; 03 medical and health sciences; Gynaecology - Radboud University Medical Center; Semen; Testis; meiosis; Animals; Humans; genetics; RNA, Small Interfering; Spermatogenesis; 11 Medical and Health Sciences; Infertility, Male; Azoospermia; Genetics & Heredity; Genetics of Male Infertility Initiative (GEMINI) consortium; Radboudumc 7: Neurodevelopmental disorders DCMN: Donders Center for Medical Neuroscience; ta1184; azoospermia; 06 Biological Sciences; spermatogenesis; oligozoospermia; FKBP6; Long Interspersed Nucleotide Elements; round spermatid arrest; Urology - Radboud University Medical Center; piRNA-pathway; Radboudumc 0: Other Research RIHS: Radboud Institute for Health Sciences; Radboudumc 10: Reconstructive and regenerative medicine RIHS: Radboud Institute for Health Sciences

Infertility affects around 7% of the male population and can be due to severe spermatogenic failure (SPGF), resulting in no or very few sperm in the ejaculate. We initially identified a homozygous frameshift variant in FKBP6 in a man with extreme oligozoospermia. Subsequently, we screened a total of 2,699 men with SPGF and detected rare bi-allelic loss-of-function variants in FKBP6 in five additional persons. All six individuals had no or extremely few sperm in the ejaculate, which were not suitable for medically assisted reproduction. Evaluation of testicular tissue revealed an arrest at the stage of round spermatids. Lack of FKBP6 expression in the testis was confirmed by RT-qPCR and immunofluorescence staining. In mice, Fkbp6 is essential for spermatogenesis and has been described as being involved in piRNA biogenesis and formation of the synaptonemal complex (SC). We did not detect FKBP6 as part of the SC in normal human spermatocytes, but small RNA sequencing revealed that loss of FKBP6 severely impacted piRNA levels, supporting a role for FKBP6 in piRNA biogenesis in humans. In contrast to findings in piRNA-pathway mouse models, we did not detect an increase in LINE-1 expression in men with pathogenic FKBP6 variants. Based on our findings, FKBP6 reaches a "strong" level of evidence for being associated with male infertility according to the ClinGen criteria, making it directly applicable for clinical diagnostics. This will improve patient care by providing a causal diagnosis and will help to predict chances for successful surgical sperm retrieval.