Is the CTLA4 rs231775 AG genotype linked to altered immune checkpoint function and higher autoimmune thyroid disease risk?

The CTLA-4 receptor is a critical immune checkpoint that downregulates T-cell activation to maintain immunological self-tolerance. The non-synonymous CTLA4 rs231775 (+49A>G, Thr17Ala) polymorphism directly influences this inhibitory pathway, modulating susceptibility to autoimmune thyroid diseases (AITD).

Mechanistic impact on checkpoint function

• Impaired receptor processing: The rs231775 G allele causes a threonine-to-alanine substitution at codon 17 in the signal peptide. This alters early post-translational processing, leading to incomplete glycosylation in the endoplasmic reticulum, premature intracellular degradation, and RAPP pathway-mediated mRNA degradation in the cytosol.
• Altered surface expression: Heterozygous AG carriers exhibit an intermediate-to-reduced density of CTLA-4 receptors on the T-cell surface compared to those with the protective AA genotype.
• Unrestrained T-cell activation: Lower cell-surface inhibitory receptor density weakens the checkpoint's capacity to compete with CD28 for ligand binding. This lowers the threshold for co-stimulation, facilitating heightened T-cell proliferation and bypassing normal self-tolerance mechanisms.

Clinical susceptibility to autoimmune thyroid disease

• Elevated risk profile: Carrying the rs231775 AG genotype correlates with a moderately increased risk of developing AITD compared to the AA genotype, with typical odds ratios ranging from approximately 1.3 to 1.5.
• Strong correlation with Graves' disease: The genetic association is highly robust and consistent for Graves' disease across both pediatric and adult cohorts.
• Variable risk in Hashimoto's thyroiditis: The association with Hashimoto's thyroiditis is more heterogeneous and population-dependent, showing a significant predisposition in some case-control studies but failing to reach statistical significance in other independent cohorts.

Bottom line

• The CTLA4 rs231775 AG genotype impairs immune checkpoint function by reducing CTLA-4 surface expression, driving unrestrained T-cell activation and conferring a moderately increased risk (OR ~1.3–1.5) for autoimmune thyroid diseases, with the most consistent association observed in Graves' disease.