Biallelic variants in TBC1D8 are potentially associated with male infertility due to non-obstructive azoospermia or cryptozoospermia

Non-obstructive azoospermia (NOA) and cryptozoospermia are two significant conditions contributing to male infertility. However, the underlying genetic factors in most cases remain unknown. In our study, whole exome sequencing identified novel biallelic variants in TBC1 domain family member 8 (TBC1D8) in two patients. Patient P1 with NOA harbored c.890C>T (p.A297V) and c.2461G>A (p.V821I), and patient P2 with cryptozoospermia carried c.854C>T (p.P285L) and c.1912G>A (p.D638N). Bioinformatic analyses predicted that all identified TBC1D8 variants were likely pathogenic. Compared with a normal control, patient P1 showed reduced expression of TBC1D8 in testicular tissue. Subsequently, hematoxylin-eosin staining and immunofluorescence analysis of testicular sections showed defective acrosome formation and the absence of elongated spermatids in patient P1, resulting from abnormal autophagy. Additionally, intracytoplasmic sperm injection treatment was beneficial for patient P2 with cryptozoospermia. In conclusion, our results suggest that TBC1D8 is a potentially novel candidate gene for male infertility associated with NOA or cryptozoospermia in humans.