CTLA4 Thr17Ala

CTLA4 Thr17Ala — The Immune Brake Variant

CTLA-4 (Cytotoxic T-Lymphocyte Associated Protein 4) is one of the immune system's most powerful
brakes. Expressed on activated T cells, it competes with CD28 to bind CD80 and CD86 on antigen-
presenting cells — and when CTLA-4 wins, T cell activation is dampened. The rs231775 variant
(also called +49A>G or Thr17Ala) sits in the leader peptide | The signal peptide is a short
N-terminal sequence that guides newly synthesised protein into the secretory pathway; it is cleaved
from the mature protein of the CTLA-4 protein.
This single amino acid change — threonine to alanine at position 17 — affects how efficiently
CTLA-4 traffics to the T cell surface, subtly weakening the immune checkpoint and increasing
susceptibility to a broad range of autoimmune conditions. With 539 published studies in the
literature | Ensembl VEP annotates 539 publications for rs231775,
this is among the most intensively studied common autoimmune susceptibility variants.

The Mechanism

CTLA-4 is synthesized with an N-terminal signal peptide that guides it into the endoplasmic
reticulum and ultimately to the cell surface. The Thr17Ala substitution changes the glycosylation
profile of the leader peptide, altering the efficiency with which the protein is
processed and trafficked | ClinVar expert panel notes flow cytometry data showing Thr17Ala
produces cell surface staining comparable to wild-type, while population-level studies document
functional immune differences
to the plasma membrane. The G allele (Ala17) is associated with reduced CTLA-4 surface density
on T cells, meaning fewer inhibitory signals reach the T cell receptor complex. With less CTLA-4
at the surface, the molecular brake on T cell activation is partially released, allowing T cells
to respond more vigorously — and potentially to self-antigens they should ignore.

This variant is in partial linkage disequilibrium with the CT60 variant (rs3087243) in the
CTLA4 3'UTR, which has an independent effect on mRNA stability. Together, the two variants
can compound to lower CTLA-4 expression through complementary mechanisms: reduced surface
trafficking (rs231775) and reduced mRNA stability (rs3087243).

The Evidence

The G allele of rs231775 is one of the most replicated common autoimmune susceptibility variants
in the human genome. A meta-analysis of 63 published studies | Tang et al. 2012, Gene, reviewing
data through December 2011 confirmed that the G allele
increases type 1 diabetes susceptibility with an odds ratio of 1.47 (95% CI 1.36–1.60, P<0.001),
a remarkably consistent signal that held across both Caucasian and Asian populations and all
age groups.

For Graves' disease and autoimmune thyroid disease, the association is similarly robust. A
meta-analysis of 42 case-control studies | Si et al. 2013, 8,288 cases and 9,372 controls,
predominantly Asian and Caucasian populations found
G allele risk across multiple genetic models: additive OR = 1.44 (95% CI 1.32–1.58), dominant
OR = 1.62 (95% CI 1.43–1.84), and recessive OR = 1.59 (95% CI 1.40–1.81). The association
was significant and consistent across all models tested.

In systemic lupus erythematosus, an updated meta-analysis of 18 studies | Chang et al. 2012,
1,806 SLE cases and 2,490 controls found GG versus
AA OR = 1.53 overall, rising to OR = 1.89 in Asian populations. A broader autoimmune meta-analysis
| Yu et al. 2021, 47 studies involving 11,893 cases and 12,032 controls across AS, RA, and SLE confirmed the G allele significantly elevates risk
for rheumatoid arthritis in Caucasian and Mongolian populations.

Practical Implications

Carrying one or two copies of the G allele does not mean you will develop an autoimmune disease —
most carriers never do. However, the G allele is a consistent signal of reduced immune checkpoint
function, which means your immune system has a somewhat lower threshold for mounting inflammatory
responses against self-tissues. The conditions most strongly linked to this variant are Graves'
disease, Hashimoto's thyroiditis, type 1 diabetes, and systemic lupus erythematosus.

For those already diagnosed with autoimmune disease, rs231775 G allele status has a practical
clinical implication: abatacept (Orencia), a biologic medication that is literally a
recombinant CTLA-4 fusion protein | Abatacept mimics CTLA-4 by binding CD80/CD86 and blocking
T cell costimulation, compensating for the reduced endogenous CTLA-4 activity
used in rheumatoid arthritis and other autoimmune conditions, may work especially well for G allele
carriers. A retrospective cohort of 109 RA patients found the G allele was associated with
an OR of 3.48 for achieving EULAR response at 12 months and OR = 4.68 for low disease activity,
suggesting that restoring what the variant reduces — CTLA-4 activity — is particularly effective
treatment for those with this genotype.

Women with G allele genotypes should be aware that thyroid autoimmunity (Graves' disease,
Hashimoto's) is considerably more common in women and is further elevated by this variant. Annual
thyroid function testing (TSH, free T4) is a low-cost, high-yield monitoring strategy for
G allele carriers with personal or family history of thyroid disease.

Interactions

The strongest documented epistatic interaction for rs231775 is with MSH5 rs3131379 in systemic
lupus erythematosus. A gene-gene epistasis study of 4,248 SLE cases and 3,818 controls | Hughes et al. 2012,
identified gene-gene interactions across SLE susceptibility loci and documented epistasis between rs3131379 and rs231775 found the combination of rs3131379 risk allele
and rs231775 G allele shows an interaction OR of 1.19 (P=7.8×10⁻⁵), the strongest pairwise
epistasis signal in that dataset. This means carriers of both risk alleles face disproportionately
higher SLE risk than expected from either variant alone.

Within the CTLA4 locus, the CT60 variant rs3087243 (a 3'UTR regulatory variant) interacts
with rs231775 to compound autoimmune susceptibility through independent mechanisms: rs231775
reduces surface trafficking efficiency while rs3087243 reduces mRNA stability. Combined risk
alleles at both positions are most common in populations with high autoimmune thyroid disease
prevalence.

CTLA4 rs231775 also interacts with PTPN22 rs2476601 (the R620W variant) in determining risk
for seropositive autoimmune conditions including RA and type 1 diabetes. Both variants impair
the braking mechanisms that prevent T cell activation against self-antigens, and their
co-occurrence increases absolute disease risk beyond what either confers individually.