Novel mutations in FSIP2 cause male infertility through multiple morphological abnormalities of the sperm flagella

Whole-exome sequencing in two infertile men pinpoints new FSIP2 mutations that gut sperm flagella structure

Journal: Asian Journal of Andrology | Published: 2025-09-19 | Type: Case Report | PMID: 40968718 Authors: Hussain Mujahid et al. (Institute of Health and Medicine, Hefei Comprehensive National Science Center; multiple Pakistani institutions) Funding/COI: Not disclosed

Summary

Two men with oligoasthenoteratozoospermia (OAT) and multiple morphological abnormalities of the sperm flagella (MMAF) were found via whole-exome sequencing to carry novel loss-of-function mutations in FSIP2, a gene critical for flagellar architecture. Transmission electron microscopy confirmed structural collapse: missing microtubule doublets, disorganized mitochondrial sheaths, and fibrous sheath dysplasia. Immunofluorescence showed complete absence not just of FSIP2 protein but of four downstream flagellar proteins, suggesting these mutations propagate a cascade of structural failure.

Claims

• Patient 1 carried two compound homozygous FSIP2 mutations: a missense variant (c.262C>A, p.P88T) and a frameshift deletion (c.10948_10951del, p.N3653Nfs*22)
• Patient 2 carried a distinct homozygous frameshift deletion (c.15982_15982del, p.I5328Lfs*33)
• Bioinformatics tools predicted the missense mutation (c.262C>A) as rare and deleterious
• TEM revealed: absent central pair microtubules, missing outer doublets, disorganized mitochondrial sheath, and fibrous sheath dysplasia in affected sperm
• RT-PCR showed significantly reduced FSIP2 mRNA in sperm lysate from both patients
• Immunofluorescence confirmed complete absence of FSIP2, AKAP4, SPAG6, IFT20, and ACTL7A proteins in patient spermatozoa

Study Quality

This is a case report — two patients. That is not a study design that supports broad conclusions; it is a mechanistic snapshot. What it does well: the authors applied five independent methods (WES, bioinformatics, TEM, RT-PCR, immunofluorescence) to characterize the same phenotype, and the convergent evidence is internally coherent. The use of whole-exome sequencing rather than targeted panels is appropriate given the genetic heterogeneity of MMAF. The ultrastructural TEM data is the most compelling piece, providing direct visual evidence of flagellar breakdown consistent with the predicted protein function.

Bioinformatics pathogenicity prediction (the basis for calling the missense variant "deleterious") is hypothesis-generating, not confirmatory. No functional rescue experiments — reintroducing wild-type FSIP2 to restore flagellar structure — were performed, which would have been the gold standard for establishing causality rather than association.

Red Flags

• n=2. Two patients. This is the entire evidence base.
• Funding and conflicts of interest are not disclosed — a basic reporting failure in any journal published after 2010
• Pathogenicity of the missense variant rests partly on computational prediction, not functional validation
• No control sperm samples from fertile men with FSIP2 protein staining are shown alongside patient samples
• "Valuable insights for genetic counseling" is stated as a conclusion but cannot be supported from two cases without population-level carrier frequency data

Strengths

• Three novel FSIP2 mutations not previously reported in the literature
• Methodological triangulation: structural (TEM), molecular (RT-PCR), and protein-level (immunofluorescence) evidence all point the same direction
• Downstream protein cascade — AKAP4, SPAG6, IFT20, ACTL7A all absent — adds mechanistic depth beyond simple gene-disease association
• Adds to the growing catalog of MMAF-causing genes, which has diagnostic relevance for a clinically recognizable but genetically heterogeneous phenotype

Verdict

Two patients is barely a sample, but the layered methodology prevents this from being pure anecdote. The real value here is cataloguing novel variants: FSIP2 mutations are already an established cause of MMAF, and this paper extends the known mutation spectrum with three new variants and unusually thorough mechanistic characterization for a case report. The missing funding and COI disclosures are sloppy. The conclusion that these findings are "valuable for genetic counseling" oversells what two cases can tell you about carrier frequency or recurrence risk. Read it as what it is — a well-documented case series expanding a mutation database — not as evidence that changes clinical practice.