rs3129878 — HLA-DRA HLA-DRA Variant

HLA-DRA — When the Immune System Turns Against Spermatogenesis

The human leukocyte antigen (HLA) region¹¹ human leukocyte antigen (HLA) region
The most gene-dense and polymorphic stretch of the human genome, encoding proteins that display peptide fragments to immune cells. Class II HLA proteins sit on antigen-presenting cells and control which immune responses the body mounts on chromosome 6 is the master regulator of adaptive immunity — and rs3129878, an intronic variant in HLA-DRA, sits within one of its most functionally dense segments. HLA-DRA encodes the alpha chain of the HLA-DR heterodimer, the class II antigen-presenting molecule central to CD4+ T cell activation. The C allele of this variant was identified in a genome-wide association study as a reproducible genetic risk factor for nonobstructive azoospermia (NOA), the most severe form of male infertility.

The Mechanism

Nonobstructive azoospermia²² Nonobstructive azoospermia
Complete absence of sperm in the ejaculate due to spermatogenic failure rather than a physical blockage — affects approximately 1% of men and 10% of infertile males is a complex condition with genetic, hormonal, and environmental contributors. The HLA-DRA association points to an immune-mediated pathway: the testis is an immune-privileged organ, protected by the blood-testis barrier and specialised local immune regulation. When immune surveillance breaks down — for instance, through aberrant class II antigen presentation — the body's immune cells can recognise spermatogenic cells as foreign and mount inflammatory responses that disrupt the delicate process of sperm maturation.

Zhao et al. (2012)³³ Zhao et al. (2012)
First GWAS for NOA in Han Chinese subjects; 3-stage design with genome-wide discovery and two independent replication cohorts proposed that "variations in this region might mediate the response to testicular microenvironmental antigens and cause testicular azoospermia through autoimmune inflammatory responses," noting that "inflammation in testicular tissues might cause the failure of interactions between immune, germ, and somatic testicular cells, disrupting the process of spermatogenesis." The nearby BTNL2 gene (also associated in the same GWAS at rs498422, OR=1.42) encodes a B7 family molecule involved in regulating T cell activation, consistent with an immune-checkpoint mechanism.

The Evidence

The rs3129878 association is among the best-replicated genetic findings in male infertility research. The discovery GWAS⁴⁴ discovery GWAS
Three-stage design: 802 NOA cases + 1,863 controls in discovery; 818 + 1,755 in northern China replication; 606 + 958 in central/southern China replication identified the C allele at a combined p-value of 3.70×10⁻¹⁶ (OR=1.37), a stringent genome-wide significance threshold. The C allele frequency was approximately 0.40 in NOA cases compared to 0.29 in controls across all cohorts — a meaningfully elevated risk enrichment for a common variant.

An independent replication study⁵⁵ independent replication study
545 NOA patients and 632 normozoospermic controls from Han Chinese males, examining four GWAS-identified NOA loci confirmed the rs3129878 association in a separate population and added an important nuance: normozoospermic men carrying risk alleles did not show reduced sperm counts, suggesting the variant's effect may act through mechanisms other than directly impairing sperm production capacity — pointing further toward an immune or testicular microenvironmental disruption rather than a primary spermatogenic defect.

Practical Implications

The variant confers an approximately 37% increase in odds of NOA per C allele (additive model), not a guaranteed outcome — most C carriers will have normal or near-normal fertility. NOA affects roughly 1% of men overall, so even with elevated genetic risk, most carriers will not develop the condition. However, for men experiencing difficulty conceiving, this genotype adds important context: early semen analysis is warranted rather than assuming infertility is unlikely. For CC homozygotes, the risk is higher and semen evaluation is the decisive first step. Hormonal evaluation (FSH, LH, testosterone, inhibin B) alongside semen analysis provides a complete picture of testicular function.

Interactions

rs3129878 sits within the same HLA haplotype block as rs3135388 (a tag for HLA-DRB1*15:01, the principal genetic risk factor for multiple sclerosis) and rs3129934 (a DRB5-proximal tag associated with narcolepsy). These variants are not in high linkage disequilibrium with each other and represent distinct functional associations within a gene-dense region. The NOA association of rs3129878 is independent of the MS and narcolepsy associations of these neighbouring variants, confirmed by conditional analysis in the discovery GWAS. The nearby rs498422 (BTNL2/C6orf10), identified in the same GWAS (OR=1.42, p=2.43×10⁻¹²), may act through a complementary immune-checkpoint pathway and could represent a compound interaction target — both variants in the risk configuration may confer higher cumulative NOA susceptibility than either alone, though formal compound heterozygosity analysis across these two loci has not been published.