A Novel Homozygous ZPBP Frameshift Variant Is Associated With Severe Acrosomal Structural Abnormalities in Asthenoteratozoospermia

BACKGROUND: Experimental disruption of Zpbp homologs in mice causes defective acrosomal biogenesis and male infertility. Human evidence remains limited to a small number of association-based or case-level reports linking ZPBP alterations to abnormal sperm-head or acrosomal phenotypes, without integrated characterization across protein abundance, ultrastructure, sperm chromatin integrity, and proteome-wide changes. OBJECTIVES: Our study aimed to characterize the genetic, structural, and molecular sperm phenotype associated with a novel homozygous frameshift variant in ZPBP. MATERIALS AND METHODS: Whole-exome sequencing (WES) and Sanger sequencing were performed in the proband. Semen parameters, Papanicolaou-stained morphology, and FITC-PSA-defined acrosomal staining patterns were assessed. ZPBP protein abundance was evaluated by Western blotting. Sperm morphology and ultrastructure were examined by light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Sperm chromatin structure assay, IZUMO1 immunofluorescence, and label-free proteomic profiling were also performed. RESULTS: A man born to consanguineous parents carried a novel homozygous frameshift variant in ZPBP (NM_007009.3:c.421_422del; p.Phe141Profs*4), classified as a variant of uncertain significance; parental samples were unavailable for segregation analysis. The proband had normal sperm concentration but markedly reduced motility and complete absence of morphologically normal spermatozoa, with predominantly head abnormalities, consistent with severe asthenoteratozoospermia. Head, midpiece, and principal-piece abnormalities occurred in 99.33% ± 0.47%, 14.67% ± 2.62%, and 22.67% ± 3.40% of spermatozoa, respectively. Abnormal, fragmented, or absent FITC-PSA staining was observed in 84.33% ± 1.70%. Western blotting demonstrated absence of ZPBP protein in patient sperm. SEM and TEM showed complete or partial acrosomal loss, membrane and matrix fragmentation, widening of the subacrosomal space, and nuclear-acrosomal detachment. The DNA fragmentation index was 52.25% and high DNA stainability was 5.71%. IZUMO1 staining was markedly reduced or absent in the proband's sperm. Proteomic profiling showed coordinated lower abundance of multiple acrosome-related proteins and negative enrichment of acrosomal pathways. DISCUSSION AND CONCLUSION: The findings associate the novel homozygous ZPBP frameshift variant with severe acrosomal structural abnormalities, impaired acrosomal integrity, and broader sperm-head molecular disruption. The abnormal FITC-PSA pattern is not interpreted as direct evidence of spontaneous or premature acrosome exocytosis. Detailed acrosomal and sperm-morphology phenotyping may support genetic diagnosis and counseling, but this case provides no direct evidence regarding fertilization competence or the optimal assisted-reproduction strategy for the proband's own sperm.