rs3129934 — HLA-DRB5 DR15 haplotype tag

HLA-DRB5*01:01 — The Second MHC Gateway to Multiple Sclerosis and Narcolepsy

The major histocompatibility complex (MHC) on chromosome 6p21 is the most gene-dense and clinically consequential region of the human genome, and rs3129934 sits within it as an intronic variant in TSBP1/C6orf10¹¹ intronic variant in TSBP1/C6orf10
Physically at 6p21.32, approximately 200 kb from the HLA-DRB1 locus and in high LD with rs3135388; annotated to TSBP1 but functions as a proxy for the entire DR15 HLA haplotype block that efficiently tags HLA-DRB5*01:01 on the DR15 susceptibility haplotype. The T allele travels with the disease-associated haplotype DRB5*01:01–DRB1*15:01–DQA1*01:02–DQB1*06:02 in near-complete linkage disequilibrium (r²=0.93) with the better-known MS tag rs3135388.

Whereas rs3135388 has been more widely studied as the DRB1*15:01 proxy, rs3129934 is physically closer to the HLA-DRB5 gene and captures an overlapping but slightly different slice of the haplotype block, making it a complementary tag — particularly useful for populations where r² with rs3135388 is lower. In European cohorts, it is considered a reliable marker of the DR15 haplotype²² reliable marker of the DR15 haplotype
Wixárika subjects from Mexico who have no reported MS were found to be uniformly homozygous CC at rs3129934, consistent with the absence of the DR15 risk haplotype with identical sensitivity and specificity for DRB5*01:01 carriage as for DRB1*15:01 carriage, because the two alleles virtually always co-occur on the same ancestral chromosome segment.

The Gene: HLA-DRB5 and the DR2a Molecule

HLA-DRB5 encodes the beta chain of the HLA-DR2a heterodimer — one of two DR molecules expressed by individuals carrying the DR15 haplotype. DR2a (DRB5*01:01 + DRA) and DR2b (DRB1*15:01 + DRA) are co-expressed on the surface of antigen-presenting cells, and both contribute to immune self-surveillance. The beta chain encoded by DRB5*01:01 has a distinct peptide-binding groove³³ distinct peptide-binding groove
The groove geometry of DR2a differs from DR2b, allowing it to bind different sets of peptide fragments — at least three myelin basic protein epitopes have been identified for DR2a versus one primary epitope for DR2b that accommodates a broader range of myelin-derived peptides than DRB1*15:01 alone.

The Mechanism

rs3129934 is a non-coding intronic variant that does not alter protein sequence; its biological significance derives entirely from tagging the full DR15 haplotype through linkage disequilibrium. The disease-relevant allele is DRB5*01:01 itself, whose protein product contributes to MS pathogenesis through two documented mechanisms.

First, DRB5*01:01 presents myelin basic protein (MBP) peptide fragments⁴⁴ myelin basic protein (MBP) peptide fragments
Particularly MBP83-99, MBP131-145, and MBP76-91 — a wider repertoire of myelin epitopes than DRB1*15:01 alone, supporting more diverse autoreactive T cell activation to CD4+ T cells on the surface of antigen-presenting cells in the brain and periphery, priming myelin-reactive T cell populations. Humanized transgenic mice expressing DRB5*01:01 alongside a patient-derived MBP-specific T cell receptor develop spontaneous experimental autoimmune encephalomyelitis⁵⁵ spontaneous experimental autoimmune encephalomyelitis
EAE is the preclinical model of MS; disease developed at a rate dependent on DR2a expression level, confirming a dose effect, establishing this allele as a direct etiologic contributor to CNS autoimmunity — not merely a bystander to DRB1*15:01.

Second, the DR15 haplotype includes a vitamin D response element (VDRE)⁶⁶ vitamin D response element (VDRE)
A regulatory DNA sequence in the DRB1*15 promoter that binds the vitamin D receptor; confirmed by Ramagopalan et al. to upregulate DRB1*15:01 expression specifically in cells carrying this allele upon vitamin D3 treatment in the promoter of the co-expressed DRB1*15:01 gene; since DRB5*01:01 and DRB1*15:01 are always inherited together on the DR15 haplotype, vitamin D insufficiency that upregulates this haplotype's expression simultaneously increases both DR2a and DR2b surface density.

Third, a T cell receptor from an MS patient has been shown to cross-recognize DRB5*01:01-restricted Epstein-Barr virus (EBV) DNA polymerase peptide⁷⁷ DRB5*01:01-restricted Epstein-Barr virus (EBV) DNA polymerase peptide
Molecular mimicry: the EBV peptide resembles MBP in structure, causing T cells trained against EBV to also attack myelin and DRB1*15:01-restricted MBP, providing a direct mechanistic bridge between EBV infection, HLA class II allele presentation, and myelin autoimmunity.

For narcolepsy, the same DR15 haplotype is the strongest known genetic risk factor. Ninety-six percent of narcolepsy type 1 patients⁸⁸ Ninety-six percent of narcolepsy type 1 patients
Narcolepsy type 1 = narcolepsy with cataplexy and low CSF hypocretin; the DRB5*01:01-DRB1*15:01-DQA1*01:02-DQB1*06:02 haplotype was present in 30/31 patients (96.8%) vs 28% of population controls carry the full DR15 haplotype. T cells restricted to DRB5*01:01 and DRB1*15:01 are thought to target hypocretin-producing neurons in the lateral hypothalamus via autoimmune attack, possibly triggered by molecular mimicry with influenza or vaccine antigens in genetically susceptible individuals.

The Evidence

The pooled genome-wide scan in MS⁹⁹ pooled genome-wide scan in MS
Combined SNP chip pools from 600 MS patients and 600 controls; replication in Spanish and US independent cohorts found rs3129934 to be the single most significant HLA-region association in one of the early replicated GWASes (allele-based P=4.2×10⁻¹⁰, OR≈3 in US replication; OR=3.0 in Spanish replication), with statistical power driven by tagging the DR15 haplotype block. A multilocus analysis in three ethnically homogeneous Russian populations¹⁰¹⁰ multilocus analysis in three ethnically homogeneous Russian populations
1,049 MS cases and 816 healthy controls typed for multiple candidate SNPs across MHC and immune pathways confirmed rs3129934 T allele as the single strongest predictor (OR=2.16, 95% CI 1.85–2.74, P=2.53×10⁻¹³). In women, the TT homozygous genotype combined with a STAT3 variant conferred OR=11.87 — among the largest single-study MS risk estimates reported.

The transgenic mouse study¹¹¹¹ transgenic mouse study
Myelin-reactive CD4+ T cells bearing a patient-derived TCR restricted by DRB5*01:01 developed spontaneous EAE at 4.5% incidence in high-expressing mice, rising to 90% when disease was actively induced remains the strongest animal-model evidence that DRB5*01:01 is a causal contributor to MS pathogenesis, not simply co-inherited with the causally responsible DRB1*15:01.

Relationship to rs3135388

rs3129934 and rs3135388 are in near-complete LD (D′=0.964, R²=0.93) in Europeans and tag the same DR15 disease haplotype from slightly different physical positions — rs3135388 is near HLA-DRA while rs3129934 is in TSBP1/C6orf10, closer to HLA-DRB5. Platforms that genotype one but not the other can use either as a proxy; together they provide redundant coverage of this high-value haplotype block. The platform already holds the rs3135388 entry under HLA-DRA; this entry provides the DRB5-proximal tag and the DRB5*01:01-specific mechanistic context.

Practical Actions

For individuals carrying the T allele (the DR15 haplotype), the practical guidance centres on vitamin D optimisation, early neurological awareness, and informed monitoring for signs of narcolepsy. There is no supplement or lifestyle modification that negates the DR15 haplotype's immune configuration, but vitamin D optimisation addresses the direct transcriptional link between vitamin D levels and HLA-DR15 expression density, and represents the most evidence-supported actionable modifier available.

Interactions

rs3129934 is in near-complete LD with rs3135388 (HLA-DRA tag for DRB1*15:01). Both capture the DR15 susceptibility block; they should not be combined additively in risk calculation — they represent the same underlying haplotype. The combination with rs2300747 (CD58) is of clinical interest: CD58 regulates regulatory T cell function, and individuals carrying both the DR15 risk haplotype (reduced self-tolerance induction via DRB5*01:01 and DRB1*15:01) and reduced CD58 expression (rs2300747 AA) may face additive MS susceptibility from both antigen-presentation and immune-regulatory pathways. The rs2187668 (HLA-DQ2.5) and rs7454108 (HLA-DQ8) variants occupy the same HLA block but concern celiac disease and type 1 diabetes risk rather than MS or narcolepsy.