Novel biallelic FSIP2 variants cause male infertility with multiple morphological abnormalities of sperm flagella in humans

Biallelic variants in fibrous sheath-interacting protein 2 (FSIP2) gene are a known cause of multiple morphological abnormalities of the sperm flagella (MMAF). This study aimed to identify novel FSIP2 variants and evaluate their impact on sperm ultrastructure and intracytoplasmic sperm injection (ICSI) outcomes. Whole-exome sequencing (WES) was employed to screen a cohort of 92 MMAF patients, with candidate variants validated via Sanger sequencing and third-generation sequencing. We identified one homozygous variant in a proband from a consanguineous family and two pairs of compound heterozygous variants in two unrelated, non-consanguineous families. Routine semen analysis demonstrated markedly reduced motility across all probands. Detailed morphological and ultrastructural assessments using Papanicolaou staining, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) demonstrated that approximately 80.0% of spermatozoa exhibited pathological elongation of the mitochondrial sheath in the midpiece. Furthermore, 50.0%-70.0% of spermatozoa displayed fibrous sheath dysplasia or loss in the principal piece. Immunofluorescence assays and Western blotting confirmed that FSIP2 protein localization was disrupted, and the expression of key axonemal assembly factors was dysregulated. Notably, successful pregnancies were achieved via ICSI in the partners of two probands. This study expands the mutational spectrum of FSIP2 in both consanguineous and non-consanguineous populations. Ultrastructural abnormalities, such as mitochondrial sheath elongation and fibrous sheath disassembly, highlight FSIP2's critical role in flagellar assembly. Clinical results further support ICSI as an effective therapeutic intervention for affected individuals.