Graves' disease, autoimmune hypothyroidism (AIH) and type 1 diabetes (T1D) — what's the connection? These autoimmune diseases commonly occur in family clusters, and susceptibility to them is known to be associated with the same region on human chromosome 2q33. Similarly, the syntenic region on mouse chromosome 1 (the Idd5.1 locus) is associated with susceptibility to T1D in nonobese diabetic (NOD) mice. Now, Ueda et al. have narrowed down the suspected region to a mutation of the gene encoding the inhibitory co-stimulatory molecule cytotoxic T-lymphocyte antigen 4 (CTLA4), and have described a possible mechanism for the effects of this mutation.

CD28 , CTLA4 and immune co-stimulator ( ICOS ) are the only three functional genes in this region of human chromosome 2q33, but until now, the association with autoimmune-disease susceptibility has not been refined any further. A 100-kb section of this region (comprising CTLA4 and the 5′ region of ICOS) was found to contain a cluster of single nucleotide polymorphisms (SNPs) that are more common in patients with Graves' disease than in control individuals. This disease-associated cluster was then refined further to a non-coding region 3′ of the end of the CTLA4 transcript. The SNP with the strongest association with Graves' disease in this region is known as CT60, with the A allele being protective and the G allele increasing susceptibility. CT60 was also shown to be associated with AIH to a similar extent, although it had a weaker effect in determining susceptibility to T1D.

As nucleotide variation at CT60, or its neighbouring SNPs, does not result in an amino-acid change in the CTLA4 protein, the authors looked for differences in the level of expression of CTLA4. There are two main isoforms of CTLA4 in humans — the full-length isoform (flCTLA4) and a soluble isoform (sCTLA4). The ratio of sCTLA4 to flCTLA4 messenger RNA was 50% lower in T cells from disease-susceptible CT60 G/G individuals than from disease-resistant CT60 A/A individuals.

In support of the results from humans, it was shown that there is also a difference in the level of expression of a Ctla4 splice variant between congenic NOD mice having the C57BL/10 Idd5.1 locus and non-congenic NOD mice, which have haplotypes of the Idd5.1 region that confer protection against or susceptibility to T1D, respectively. The newly identified ligand-independent isoform of Ctla4 (liCtla4; a transmembrane isoform that lacks the CD80/CD86-binding domain) had a fourfold increase in expression by T cells from Idd5.1-congenic mice compared with NOD mice.

So, these results indicate that differential expression of alternatively spliced forms of CTLA4 might have an important role in determining susceptibility to autoimmune disease. For example, the authors propose that reduced levels of sCTLA4 could increase T-cell activation by reducing binding of sCTLA4 to its ligands CD80/CD86 and allowing increased T-cell activation through CD28.