Homozygous deleterious variants in the C‐terminal of TDRD5 impair spermiogenesis, causing severe oligoasthenoteratozoospermia in humans

BACKGROUND: PiRNA pathway factors, including evolutionarily conserved Tudor domain-containing proteins, play crucial roles in suppressing transposons and regulating post-meiotic gene expression. TDRD5 is essential for retrotransposon silencing and pachytene piRNA biogenesis; however, a causal link between TDRD5 variants and human infertility has not yet been established. OBJECTIVE: To identify the likely pathogenic variants of TDRD5 in infertile men, characterised by azoospermia or severe oligozoospermia. MATERIAL AND METHODS: Potential candidate variants were identified and confirmed using whole-exome and Sanger sequencing. Haematoxylin and eosin staining, immunofluorescence, and ultrastructural analyses were performed to investigate the structural and functional abnormalities of spermatozoa. The pathogenicity of the identified TDRD5 variants was verified using in vitro experiments. Functional effects of the C-terminal nonsense variant were assessed via histology, immunofluorescence staining, and small-RNA sequencing. Intracytoplasmic sperm injection (ICSI) was also performed to evaluate the efficacy of the clinical treatment. RESULTS: We identified a homozygous missense variant (c.3043G > A, p.A1015T) and a homozygous nonsense variant (c.2293G > T, p.E765*) of TDRD5 in two unrelated infertile men. Both patients exhibited severe oligoasthenoteratozoospermia, characterised by the presence of spermatozoa with multiple heads and/or flagella, as well as acrosomal hypoplasia. In vitro experiments revealed that the p.A1015T variant caused a diffuse distribution of TDRD5 granules, whereas the p.E765* variant led to the production of a C-terminal truncated protein with nuclear localisation, instead of the typical cytoplasmic localisation observed for the wild-type protein. Functional investigations also revealed that truncation of the C-terminal region of TDRD5 could potentially lead to a decline in the expression levels of intermitochondrial cement and chromatoid body components, such as MIWI (PIWIL1) and UPF1, and a slight decrease in the abundance of pachytene piRNA, ultimately resulting in compromised spermiogenesis. ICSI may be an effective treatment for these deficiencies. DISCUSSION AND CONCLUSION: This study implicates TDRD5 as a novel candidate gene in the pathogenesis of human male infertility, emphasising the contribution of piRNA pathway genes to male infertility. In addition, our data suggest that ICSI could be a promising treatment for infertile men harbouring TDRD5 variants.