The neuroendocrine tyrosine phosphatase-like protein IA-2 is a transmembrane glycoprotein localized in the secretory granules of β-cells and a major autoantigen in type 1 diabetes (T1D). We previously reported that a subgroup of subjects with Ab directed to full length IA-2 exhibited a very high risk as well as rapid progression to T1D onset. Kaplan and Meier analysis revealed that a subgroup subjects with no detectable Ab against the conventional constructs (IA-2ic or ICA512bdc) exhibited Ab responses toward IA-2 (Log rank, P = 0.008) as well as the IA-2 extracellular domain (amino acid residues: 26-577).
We demonstrated that the full length IA-2 nucleotide sequence contains three validated non-synonymous SNPs (listed in SNP Consortium and Celera databases) (SNP-IA-2 construct: aa residues 27, 608, and 671) and identified SNP-IA-2 Ab reactivity in individuals who lack serologic responses against other conventional antigenic determinants of IA-2. The prevalence of Ab to SNP-IA-2 was 60% in new onset T1D and 36.2% in prediabetics respectively.
Protein sequence and folding analysis indicates that the IA-2 variants influence the overall folding of SNP-IA-2 and create potential new Ab-protein interaction sites compared to native IA-2. Three dimensional models of the native IA-2 (Locus: NM_002846.3) and SNP-IA-2 display distinctly different structural morphology within the extracellular domain, likely due to the amino acid variants found within the juxtamembrane domain of IA-2. Correspondingly, while previous studies have focused on T cell epitopes within the C-terminal region of IA-2, we identified CD4+ T cells in T1D patients that respond to novel epitopes within the extracellular domain (n=8).
In sum, the ability to identify at risk individuals due to SNP-IA-2 Ab reactivity may aid in improving T1D prediction. Furthermore, the identification of T cell responses against the extracellular domain of IA-2 may lead to advances in peptide-based immunotherapies targeted to prevent T1D.