rs3135388 — HLA-DRA Tag for DRB1*15:01

HLA-DRB1*15:01 — The Master Gateway to Multiple Sclerosis Risk

The HLA region on chromosome 6 is the most gene-dense and clinically significant stretch of the human genome, and rs3135388 sits within it as a highly efficient tag SNP¹¹ tag SNP
A "tag SNP" doesn't cause disease itself but travels with disease-causing variants due to linkage disequilibrium (r²=0.97), serving as a reliable proxy for HLA-DRB1*15:01 in European populations for the HLA-DRB1*15:01 allele. HLA-DRB1 encodes one chain of the HLA class II protein complex, which sits on the surface of antigen-presenting cells and physically binds peptide fragments to display them to CD4+ T cells. The *15:01 allele has an unusual peptide-binding groove geometry that predisposes to autoimmune attack on the central nervous system. This makes it the single strongest genetic risk factor for multiple sclerosis, with OR approximately 3.08 across independent HLA typing meta-analyses and present in ~50–60% of MS patients versus ~25–30% of population controls in European cohorts.

The Mechanism

HLA-DRB1*15:01 contributes to MS through at least three converging mechanisms. First, the peptide-binding groove²² peptide-binding groove
The unique three-dimensional structure of the DRB1*15:01 binding pocket accommodates specific myelin-derived peptide sequences that other HLA-DR variants do not bind well of HLA-DRB1*15:01 preferentially binds and presents certain myelin peptides — including those from myelin basic protein (MBP85-99) — to CD4+ T cells, triggering immune responses against the central nervous system. Second, the *15:01 haplotype contains a vitamin D response element (VDRE)³³ vitamin D response element (VDRE)
A specific DNA sequence in the promoter region that vitamin D receptor binds to, regulating gene transcription in the promoter region of HLA-DRB1*15, making expression of this MS-risk allele directly sensitive to circulating vitamin D levels. Flow cytometry experiments⁴⁴ Flow cytometry experiments
Ramagopalan et al. showed increased cell-surface HLA-DRB1 expression specifically in cells carrying DRB1*15, not other alleles, after vitamin D3 treatment confirmed this: vitamin D3 specifically upregulates DRB1*15:01 expression. Third, rs3135388 A allele carriers show dramatically elevated HLA gene expression⁵⁵ dramatically elevated HLA gene expression
AA genotype associated with 8.3-fold higher DRB1, 5.2-fold higher DRB5, and 15.7-fold higher DQB1 expression compared to GG carriers across multiple co-regulated HLA genes, flooding antigen-presenting cells with the MS-risk isoform.

The Evidence

The 2007 NEJM genome-wide association study⁶⁶ 2007 NEJM genome-wide association study
IMSGC tested 334,923 SNPs in 931 MS family trios; replication in 12,000+ subjects identified rs3135388 at HLA-DRA as the most significant association in the genome (P = 8.94 × 10⁻⁸¹, OR 1.99 for the tag SNP). Independent meta-analyses of HLA allele frequency studies across European and multi-ancestry cohorts established an average OR of approximately 3.08 for DRB1*15:01 itself, with homozygotes reaching OR of approximately 6. A Czech cohort study⁷⁷ Czech cohort study
306 MS patients and 137 controls genotyped for rs3135388 found heterozygotes (GA) and homozygotes (AA) combined had OR 4.27 (95% CI 2.64–6.92), with the effect even stronger in women (OR 5.11, 95% CI 2.86–9.15). MS affects women approximately 3 times more often than men, and HLA-DRB1*15:01 contributes to this sex bias.

The gene-environment interaction with Epstein-Barr virus (EBV)⁸⁸ Epstein-Barr virus (EBV)
EBV infects more than 95% of adults globally; prior infection is nearly universal among MS patients and is now considered a necessary environmental trigger is mechanistically striking. HLA-DRB1*15:01 protein acts as a co-receptor enabling EBV to infect B cells, meaning carriers are more easily infected and harbour higher EBV loads. Meanwhile, EBV nuclear antigen-1 (EBNA-1) contains peptide sequences that cross-react with myelin proteins — classic molecular mimicry. The combined result: individuals positive for both DRB1*15:01 and high anti-EBNA-1 antibodies⁹⁹ individuals positive for both DRB1*15:01 and high anti-EBNA-1 antibodies
Sundström et al. case-referent study of EBV-HLA interaction face up to a 24-fold higher MS risk than those with neither risk factor.

Vitamin D status modulates this risk. Population data¹⁰¹⁰ Population data
Inverse correlation between MS prevalence and hours of ultraviolet radiation across latitudes, consistent across continents shows MS prevalence tracks inversely with ultraviolet light exposure across latitudes. The VDRE in the DRB1*15:01 promoter suggests that vitamin D insufficiency during critical developmental windows may paradoxically upregulate the MS-risk allele expression in genetically susceptible individuals, amplifying autoimmune priming.

Practical Implications

HLA-DRB1*15:01 is necessary but far from sufficient for MS. Approximately 25–30% of Europeans carry at least one copy, but lifetime MS risk is only 1–3% for heterozygotes. The key leverage points are the modifiable environmental factors: vitamin D status and EBV immune control. Maintaining serum 25-hydroxyvitamin D above 40–60 ng/mL (100–150 nmol/L) is associated with reduced MS risk in epidemiological studies and may dampen pathological DRB1*15:01 upregulation driven by the VDRE. EBV is effectively unavoidable (>95% of adults are infected), but monitoring for early MS symptoms enables faster diagnosis — time to treatment matters significantly for long-term disability outcomes.

Early MS symptoms are often subtle and episodic: optic neuritis (vision blurring or pain with eye movement), unilateral limb weakness or numbness, gait unsteadiness, double vision, or bladder urgency that resolves within days to weeks. These transient neurological episodes should prompt urgent neurological evaluation, as early initiation of disease-modifying therapy dramatically reduces lesion accumulation and long-term disability.

Interactions

The interaction between HLA-DRB1*15:01 and EBV is well established and synergistic. Carriers of *15:01 who also have high EBNA-1 antibody titres face a multiplicative (not merely additive) risk elevation above either factor alone. Within the HLA region, *15:01 often co-segregates with DQB1*06:02 (also associated with MS risk) and is tagged by this SNP along with that haplotype block. The protective allele HLA-A*02:01 partially counteracts DRB1*15:01 risk within the same haplotype, explaining why some DRB1*15:01 carriers have lower risk than population-level estimates predict. Interactions with rs2187668 (HLA-DQ2.5 tag) and rs7454108 (HLA-DQ8 tag) are present in the same HLA haplotype block but concern distinct disease associations (celiac disease and type 1 diabetes respectively) rather than MS risk.