Bi-allelic variants in TBC1D8 result in non-obstructive azoospermia in both humans and mice

Non-obstructive azoospermia (NOA) is a severe form of male infertility. Proteins from the Tre2-Bub2-Cdc16 (TBC) family emerge as key contributors to spermatogenesis, including TBC1D20, TBC1D21, and TBC1D25. Nonetheless, the specific role of TBC1D8 in male male fertility remains unclear. Our study aims to elucidate the relationship between TBC1D8 and male infertility in humans and mice. Male Tbc1d8 ^-/- mice were completely infertile, producing no offspring. They exhibited spermiogenesis defects from steps 9 to 16, accompanied by pronounced acrosomal abnormalities during sperm maturation, ultimately preventing the formation of functional spermatozoa. Mechanistically, TBC1D8 deficiency impaired autophagic flux and enhanced apoptosis in the seminiferous epithelium through disrupted coupling with RAB8A, culminating in azoospermia and infertility. Consistent with the murine phenotype, whole-exome sequencing (WES) identified compound heterozygous missense variants in TBC1D8 (NM_001102426.3) among three patients from unrelated Chinese Han families diagnosed with NOA or cryptozoospermia: Patient 1: c.3322A>G and c.1747C>T; Patient 2: c.3322A>G and c.2566A>G; Patient 3: c.845C>T and c.2525C>T. Histological examination of patient testicular biopsies revealed severely disrupted spermatogenic architecture and markedly reduced TBC1D8 expression, suggesting a loss-of- function effect. Together, our findings establish TBC1D8 as a previously unrecognized regulator of spermatogenesis, where its deficiency leads to NOA through impaired autophagy and increased apoptosis..