rs231806 — CTLA4

CTLA4 MH30 — The Upstream Sentinel for Adult-Onset Autoimmune Diabetes

CTLA-4 is one of the immune system's most critical brakes — a checkpoint receptor expressed on activated T cells that prevents them from attacking the body's own tissues. While much attention has focused on coding and 3'UTR variants in CTLA4, the far-upstream regulatory landscape also influences how much CTLA-4 the immune system produces. The MH30 variant (rs231806), located approximately 23 kilobases upstream of the CTLA4 coding sequence, sits within a long-range regulatory element¹¹ regulatory element
Distal regulatory elements can loop physically to gene promoters and modulate transcription factor access across tens of kilobases that influences CTLA4 gene expression. This variant has been specifically implicated in latent autoimmune diabetes in adults (LADA) — a slowly progressing form of autoimmune diabetes that is frequently misdiagnosed as type 2 diabetes at onset.

The Mechanism

The MH30 region lies within a chromatin-accessible, distal regulatory domain upstream of CTLA4. Regulatory variants at this distance can influence transcription factor binding and long-range chromatin looping that brings upstream elements into contact with the gene's core promoter. The G allele at rs231806 is the population-major allele globally (~60% frequency) and is part of a disease-associated haplotype (GCC — combining the G allele at MH30 with specific alleles at the -1147 position (rs16840252) and -318 position (rs5742909)²² -1147 position (rs16840252) and -318 position (rs5742909)
These three promoter-region SNPs were studied together in the Estonian LADA cohort as part of CTLA4 haplotype analysis).

The functional consequence appears to be reduced production of the soluble isoform of CTLA-4 (sCTLA-4)³³ soluble isoform of CTLA-4 (sCTLA-4)
Soluble CTLA-4 is secreted into the extracellular space and acts as a decoy receptor that blocks B7 costimulatory ligands, suppressing T cell activation independently of cell surface CTLA-4. In a study of newly diagnosed type 1 diabetes patients, sCTLA4 mRNA expression was significantly lower in individuals homozygous for the G allele (GG genotype, p=0.039). Less soluble CTLA-4 means a less effective extracellular dampening signal on T cell activation — potentially lowering the threshold for autoreactive T cells to escape regulation and attack pancreatic beta cells.

The Evidence

The primary association between rs231806 and LADA comes from a study of 61 LADA patients and 230 controls⁴⁴ study of 61 LADA patients and 230 controls
Douroudis, Prans, and Uibo, Human Immunology, 2009 in an Estonian population. The MH30 GG genotype was significantly more frequent in LADA patients (p=0.0051), and the G allele was overrepresented among cases (p=0.0023). Haplotype analysis reinforced the finding: the GCC haplotype (spanning rs231806, rs16840252, and rs5742909) was significantly elevated in LADA cases (p=0.000073), while the CCC haplotype was protective (p=0.0019).

A follow-up population study comparing LADA, T1D, T2D, and controls⁵⁵ follow-up population study comparing LADA, T1D, T2D, and controls
Kisand and Uibo, Gene, 2012; 65 LADA, 154 T1D, 260 T2D, 229 controls found that LADA had a distinct genetic risk profile from classical T1D: LADA was associated with protective HLA haplotypes and with CTLA4 haplotypes (including rs231806) rather than the classical T1D HLA risk alleles. A model using CTLA4 region and HLA-DQB1 predicted LADA with AUC 0.693, confirming the CTLA4 region as a genuine LADA susceptibility locus.

Expression data from a study of B7/CD28 family genes in 49 newly diagnosed T1D children⁶⁶ study of B7/CD28 family genes in 49 newly diagnosed T1D children
Pruul et al., Human Immunology, 2013 showed that sCTLA4 mRNA was lowest in GG individuals at rs231806 (p=0.039), providing a mechanistic link between genotype and reduced immune regulation.

Importantly, the -318C/T (rs5742909) and -1147 (rs16840252) SNPs studied alongside rs231806 in the same Estonian LADA cohort showed no individual associations — only the MH30 (rs231806) G allele reached significance on its own, while the three-SNP GCC haplotype provided the strongest combined signal.

The evidence level is emerging — the LADA association comes from a single case-control study with a small sample (n=61 LADA). This is a real and biologically plausible signal with mechanistic support, but independent replication in larger cohorts is needed to confirm the effect size and establish the risk quantitatively.

Practical Implications

The GG genotype is the most common globally (~36% of people) but carries the highest MH30-associated LADA risk. LADA is frequently misdiagnosed as type 2 diabetes at onset because patients are often adults and not immediately insulin-dependent — but unlike T2D, LADA involves progressive beta cell destruction by autoreactive T cells, requiring insulin therapy within a few years. Awareness of CTLA4 risk genotypes may support earlier antibody testing (GAD65, IA-2) in adults presenting with apparent type 2 diabetes who don't respond well to oral agents.

For individuals who already have LADA or are being evaluated for possible LADA, CTLA4 genotype is one of several risk markers alongside islet autoantibodies, HLA type, and clinical presentation. No CTLA4-genotype-based intervention currently alters LADA management, but abatacept (a CTLA-4-Ig fusion protein)⁷⁷ abatacept (a CTLA-4-Ig fusion protein)
Abatacept mimics CTLA-4 function and is being studied in preserving beta cell function in early autoimmune diabetes is under investigation as a beta cell-preserving agent in early T1D and LADA.

Interactions

rs231806 is part of a three-SNP CTLA4 haplotype studied together with rs16840252 (-1147) and rs5742909 (-318C/T). The GCC haplotype combining risk alleles across all three upstream variants showed the strongest LADA signal (p=0.000073), much stronger than any single variant alone. This suggests epistatic cooperation within the upstream CTLA4 regulatory architecture.

The other two established CTLA4 variants — rs3087243 (CT60, 3'UTR) and rs231775 (+49A/G, coding) — provide complementary and stronger associations with autoimmune thyroid disease, T1D, and rheumatoid arthritis. Together, rs231806 + rs3087243 + rs231775 form a fuller picture of CTLA4 autoimmune susceptibility spanning promoter, coding, and 3'UTR regions. LADA risk is particularly sensitive to the upstream regulatory haplotype (rs231806), while thyroid and rheumatoid disease are more strongly predicted by the downstream variants.