rs5742909 — CTLA4 Promoter -318C/T

CTLA4 Promoter -318C/T — The Upstream Regulator

CTLA-4 (Cytotoxic T-Lymphocyte Associated Protein 4) is one of the immune system's most critical brakes. While much attention has focused on functional coding variants in CTLA4, the promoter also harbours a polymorphism that influences how much CTLA-4 is made in the first place. rs5742909, the -318C/T variant, sits 318 base pairs upstream of the CTLA4 transcription start site¹¹ CTLA4 transcription start site
Upstream variants in this position regulate transcription factor access and gene activation thresholds and modulates gene expression by altering promoter activity. The T allele is associated with higher CTLA-4 mRNA and cell-surface protein, potentially strengthening the immune checkpoint — and paradoxically this makes carriers more responsive to abatacept, a drug that mimics CTLA-4 function.

The Mechanism

The -318 position lies within a core promoter region that binds transcription factors controlling CTLA4 activation in T cells. Luciferase reporter assays have shown that the T allele produces significantly higher promoter activity²² T allele produces significantly higher promoter activity
Relative luciferase units: 8.13 ± 0.46 for T allele vs 6.87 ± 0.49 for C allele. More recent analysis identified LEF1 and TCF7 as transcription factors that bind differentially at this position³³ LEF1 and TCF7 as transcription factors that bind differentially at this position
Electrophoretic mobility shift assays show increased band intensity for the T allele, indicating stronger transcription factor binding, with the T allele enhancing TCF7-mediated transcriptional activation in Jurkat T cells.

In cellular expression studies, individuals carrying the T allele at -318 combined with the AA genotype at the exon 1 Thr17Ala position (rs231775)⁴⁴ AA genotype at the exon 1 Thr17Ala position (rs231775)
The two variants can act synergistically — the promoter variant controls transcription while the coding variant affects protein trafficking showed significantly higher CTLA-4 cell-surface expression after cellular stimulation and elevated CTLA-4 mRNA in non-stimulated cells. This suggests the -318T allele primarily upregulates basal transcription, while stimulation-induced expression is jointly regulated by both the promoter and coding regions.

The net functional result: T allele carriers have a modestly more active CTLA-4 checkpoint at baseline. Whether this constitutes protection from autoimmune disease or a subtle shift in immune tone depends strongly on the disease context and interacting genetic background.

The Evidence

Autoimmune disease associations — mixed picture: The -318C/T variant has been intensively studied across autoimmune conditions, and the overall conclusion is one of inconsistency across populations and diseases.

For Graves' disease and Hashimoto's thyroiditis, a meta-analysis of 29 independent studies including 3,614 cases and 8,839 controls⁵⁵ meta-analysis of 29 independent studies including 3,614 cases and 8,839 controls
Shi et al. 2018, published in Thyroid Research found no statistically significant association between the -318C/T polymorphism and Hashimoto's risk in any of the tested genetic models (allelic, codominant, dominant, or recessive). A large case-control study in Han Chinese⁶⁶ case-control study in Han Chinese
289 adult Graves' disease, 265 pediatric Graves', 229 pediatric Hashimoto's, 1,058 controls similarly found no association for -318C/T, while the +49A/G (rs231775) and CT60 (rs3087243) variants in the same gene showed significant risk.

For rheumatoid arthritis, a meta-analysis of 10 studies including 2,477 patients and 2,941 controls⁷⁷ meta-analysis of 10 studies including 2,477 patients and 2,941 controls
Comprehensive pooled analysis across Caucasian, Asian, and Latin American populations pooled results across multiple genetic models and found no significant association between rs5742909 and RA susceptibility in any ethnicity. The T allele OR for allelic comparison was 1.21 (95% CI 0.93–1.57, P=0.15), which did not reach significance. The -318 promoter variant is not the primary CTLA4 determinant of most autoimmune disease risk.

Abatacept pharmacogenomics — the strongest actionable signal: The most clinically relevant finding for rs5742909 comes from a retrospective cohort study of 109 RA patients⁸⁸ retrospective cohort study of 109 RA patients
Kowalska-Kępczyńska et al. 2020, MDPI Journal of Personalized Medicine treated with abatacept (a CTLA-4-Ig fusion protein used in biologic RA therapy). Patients carrying at least one copy of the T allele showed substantially better treatment outcomes: EULAR response at 12 months was associated with the T allele at OR = 5.88 (95% CI: 1.48–23.29), and achievement of low disease activity showed OR = 4.75. The same study identified rs231775 G allele as an independent predictor in the same dataset, suggesting the two CTLA4 variants may additively inform treatment selection. Remission rates were further improved in patients who initiated abatacept earlier and had fewer prior biologic failures.

This pharmacogenomic signal is biologically plausible: abatacept works by mimicking CTLA-4 and blocking CD28 co-stimulation of T cells. Patients whose T cells already express more CTLA-4 (T allele carriers) may have a disease phenotype particularly driven by co-stimulation pathways that abatacept targets, making the drug especially effective for them.

Practical Implications

For most people, the -318C/T variant is a background autoimmune susceptibility modifier rather than a primary disease determinant. The strongest single-SNP CTLA4 signals for autoimmune diseases — Graves', Hashimoto's, SLE, T1D — come from rs231775 (Thr17Ala) and rs3087243 (CT60), not from this promoter variant. The -318C/T result should be interpreted alongside those variants.

The clearest clinical application is in rheumatoid arthritis patients considering abatacept. If you carry the T allele, the available evidence — though from a single retrospective study — suggests meaningfully better odds of achieving EULAR response or low disease activity on abatacept. This is worth raising with a rheumatologist when discussing biologic treatment options, as CTLA4 genotyping is not yet routine practice but may inform treatment sequencing.

Interactions

rs5742909 is in partial linkage disequilibrium with rs231775 (Thr17Ala) and rs3087243 (CT60). The interaction between the -318T allele and rs231775 AA genotype is particularly notable: combined carriers show enhanced CTLA-4 expression above either variant alone⁹⁹ combined carriers show enhanced CTLA-4 expression above either variant alone
Studies of cell-surface CTLA-4 density show synergistic effects when both protective alleles are co-inherited. This means that interpreting rs5742909 in isolation is less informative than examining the full CTLA4 haplotype.

The CTLA4 locus also interacts epistatically with PTPN22 rs2476601, another T cell regulatory variant. In conditions like SLE and T1D, the combined CTLA4 + PTPN22 genotype profile confers a substantially different risk landscape than either gene alone.

For abatacept response in RA, co-carriage of the rs5742909 T allele and rs231775 G allele may be additive — the same study that identified rs5742909 as a predictor also found independent contributions from rs231775, consistent with these variants acting through partially complementary mechanisms (transcription vs. protein trafficking).