SPEM2 deficiency disrupts spermiation leading to oligoasthenoteratozoospermia and male infertility

A single gene defect leaves mouse and human sperm wrapped in leftover cytoplasm, misshapen, and infertile

Journal: Biology of Reproduction | Published: 2026-07-15 | Type: Journal Article | PMID: 42028965 Authors: Li Yong, Lu Wenqing, Meng Lanlan, et al. (NHC Key Laboratory of Human Stem Cell and Reproductive Engineering, Xiangya School of Basic Medical Sciences, Central South University; Center for Reproduction and Genetics, First Affiliated Hospital) Funding/COI: National Key Research and Development Program of China, National Natural Science Foundation of China, Hunan Provincial Natural Science Foundation, and a graduate research grant from Central South University. No conflicts of interest listed.

Summary

Using a mouse knockout model, the authors found that the gene SPEM2 is needed for the final step of sperm production, called spermiation, when maturing sperm shed excess cytoplasm and detach from the testis wall. Without SPEM2, sperm failed to clear that cytoplasm properly, producing bent heads, cytoplasmic debris clinging to the sperm head, and clumps of sperm stuck together, a pattern that matches oligoasthenoteratozoospermia (OAT), meaning low count, poor motility, and abnormal shape. The team then found three new SPEM2 mutations in four infertile men with OAT, linking the mouse mechanism to a human disease.

Claims

• Spem2-deficient mice produced structurally abnormal sperm (bent heads, retained cytoplasmic droplets, sperm clumping) despite normal early spermatogenesis and normal elongated spermatid formation
• SPEM2 interacts with cell-polarity proteins VANGL2, PRICKLE3, and DVL3, and its transmembrane domain is the region responsible for binding VANGL2
• Spem2-deficient mouse sperm, despite the defects, could still fertilize eggs when injected directly (ICSI), suggesting the sperm's core genetic material remains usable
• Whole-exome sequencing identified three novel heterozygous SPEM2 variants across four human OAT patients

Study Quality

This is a mechanistic mouse-knockout study paired with a small human genetic correlation, not a clinical trial, so questions about "does this treatment work" don't apply. The mouse work looks methodologically solid on its face: genetic knockout, histological and protein-interaction characterization (co-immunoprecipitation, domain-mapping), and a functional ICSI fertilization test to check whether the sperm defect was compatible with reproduction. That combination, phenotype plus mechanism plus functional readout, is the standard toolkit for this kind of reproductive genetics paper.

The human arm is much thinner: four OAT patients with three variants, found via exome sequencing. The abstract doesn't say whether these are heterozygous carriers with a dominant-negative or gain-of-function effect, or whether compound heterozygosity or other genetic contributions were ruled out, and heterozygous variants in a gene whose mouse phenotype was studied as a full knockout raises a real dosage question the abstract doesn't resolve. Four patients is not enough to establish penetrance, and there's no mention of functional validation (e.g., testing whether the human variants disrupt VANGL2 binding the way the mouse model's mechanism predicts) or comparison against a matched fertile control cohort for variant frequency.

Red Flags

• Human cohort is only four patients, too small to establish how often SPEM2 variants explain OAT in the general infertile population
• Variants are heterozygous, but the mouse model characterized appears to be a full knockout; the paper doesn't spell out how the human variants are thought to act to reduce protein function
• No stated validation that the human SPEM2 variants actually disrupt VANGL2 binding or reproduce the mouse cytoplasmic-retention phenotype in patient cells
• COI section is empty rather than an explicit disclosure statement

Strengths

• Combines a knockout animal model with human patient sequencing, giving both mechanism and a candidate clinical correlate
• Identifies a specific molecular interaction (SPEM2-VANGL2 via the transmembrane domain) rather than just describing a phenotype
• Functional ICSI experiment addresses a genuinely useful question for genetic counseling: whether affected sperm remain viable for assisted reproduction
• Multiple government funding sources with no indication of industry money shaping the result

Verdict

Solid basic-science mechanism paper that plausibly identifies a new genetic contributor to OAT and ties it to a specific cell-polarity pathway, but the human genetics is a preliminary case series, not a validated diagnostic finding. Worth attention for researchers tracking the genetic causes of male infertility; not yet strong enough evidence to treat SPEM2 variants as an established or common cause of OAT.