Biallelic Truncating Variant in <scp> <i>LRGUK</i> </scp> Is Associated With Severe Multiple Morphological Abnormalities of the Sperm Flagella and Sperm Nuclear Defects in Humans

Multiple morphological abnormalities of the flagella (MMAF) is a severe form of male infertility characterized by immotile spermatozoa with absent, short, coiled, or irregular flagella. Despite advances in whole-exome sequencing, many cases remain genetically unexplained. Here, we identify a homozygous truncating variant in LRGUK (c.1063C>T; p.Arg355Ter) in an infertile man presenting with a typical MMAF phenotype. The variant, identified by whole-exome sequencing in a cohort of 168 MMAF patients and confirmed by Sanger sequencing, is predicted to result in loss of the C-terminal guanylate kinase-like domain. Immunofluorescence showed absence of LRGUK protein in patient spermatozoa, supporting a loss-of-function effect. Semen analysis revealed impaired motility and complete teratozoospermia. Morphological and ultrastructural analyzes demonstrated severe defects affecting both sperm head and flagellum, including disorganized axonemal architecture and central pair abnormalities. Nuclear analyzes showed increased nuclear size, defective chromatin compaction, and elevated DNA fragmentation. Immunostaining further indicated alterations of central apparatus-associated proteins, supporting a role for LRGUK in C1b projection organization. These findings identify LRGUK as a novel gene involved in male infertility, essential for spermatid morphogenesis and flagellum assembly.