Homozygous C12orf40 Variant Contributes to Severe Oligoasthenoteratozoospermia and Sperm Sex Chromosome Aneuploidy

A single inherited frameshift variant left 18.8% of one man's sperm missing sex chromosomes entirely — and drove repeated miscarriages

Journal: Journal of Assisted Reproduction and Genetics | Published: 2026-01-25 | Type: Case Report | PMID: 41580510 Authors: Chen Yixin et al. (Reproductive Medicine Center, The First Affiliated Hospital, Sun Yat-Sen University, Guangzhou, China) Funding/COI: National Natural Science Foundation of China; National Key Research and Development Program of China; Key Clinical Technique of Guangzhou. No competing interests declared.

Summary

Two brothers carrying a homozygous frameshift variant in C12orf40 — a gene previously linked only to complete meiotic arrest and azoospermia — turned out to produce sperm, but catastrophically error-prone sperm. One brother had nearly one-in-five sperm with missing sex chromosomes; both brothers fathered embryos that were predominantly aneuploid, with the sex chromosome abnormalities tracing back to the sperm. This is the first reported case connecting this variant to oligoasthenoteratozoospermia (OAT) and recurrent pregnancy loss (RPL).

Claims

• Whole-exome sequencing identified a homozygous C12orf40 frameshift variant (c.232_233insTT) in both brothers, confirmed by Sanger sequencing
• Sperm FISH showed the proband's nullisomy rate at 18.8% (sex chromosome absent entirely); the affected brother's disomy rate at 10.3% — both far above normal background rates
• Across three PGT-A cycles: 5 of 12 blastocysts (41.7%) were euploid
• Of the 7 aneuploid embryos, 6 (85.7%) had sex chromosome abnormalities
• Parental origin analysis attributed 72.7% of embryo aneuploidies to the paternal side
• The variant expands the known C12orf40 phenotypic spectrum: prior reports described complete meiotic arrest (azoospermia); these cases show incomplete arrest with residual but severely error-prone spermatogenesis

Study Quality

This is a two-person case report — the smallest possible sample. The strength of the methodology partially compensates: WES with Sanger validation, sperm FISH for direct aneuploidy quantification, PGT-A with parental origin tracing across three IVF cycles. The sibling concordance (two unrelated men would be essentially impossible to share this exact homozygous variant by chance) adds biological plausibility that a single case would lack. That said, no functional studies are reported — the mechanism linking C12orf40 loss to meiotic chromosome segregation errors remains uncharacterized.

Because this is a case report, the study cannot establish the prevalence of this variant in infertile men broadly, and the findings may not generalize beyond this family.

Red Flags

• n=2; both subjects are brothers from the same family — no independent replication
• No functional/mechanistic experiments explaining how C12orf40 loss causes meiotic segregation errors
• No control group for FISH analysis reported in the abstract (normal reference ranges cited, but not generated in-house)
• RPL history is from the proband's partner only; the brother's reproductive outcome is not fully described
• All authors share the same single institution; no external validation

Strengths

• Sibling pair strengthens the genetic causal argument considerably
• Multiple orthogonal methods: WES, Sanger, FISH, PGT-A, and parental origin tracing
• Three IVF cycles worth of embryo data gives reasonable confidence in the 41.7% euploidy figure
• First phenotypic expansion of C12orf40 variants from azoospermia to OAT — clinically relevant distinction since these men can produce sperm
• Clearly defined variant (frameshift, not a SNP with ambiguous effect), which aids future literature comparison

Verdict

A well-executed case report that punches above its weight because the sibling pair effectively functions as independent replication within the same family. The finding that C12orf40 loss can produce sperm (not just silence spermatogenesis entirely) is genuinely useful for genetic counselors and andrologists: it means men with this variant may present to fertility clinics rather than just azoospermia workups. That said, two patients are two patients — this is hypothesis-generating, not hypothesis-confirming. Worth reading for anyone doing male-factor genetic infertility research; premature to cite as established clinical evidence.