INS-IGF2, the splice variant of preproinsulin and the untranslated ORF of the IGF2 gene, was recently reported to be an autoantigen in autoimmune (type 1) diabetes. We hypothesized that autoantibodies against human islet INS-IGF2 that contains the preproinsulin signal peptide, the B chain and 8 amino acids of the c-peptide in addition to 138 unique amino acids are associated with autoimmune (type 1) diabetes. The aim was to determine whether levels of specific INS-IGF2 autoantibodies (INS-IGF2A), specifically blocked by both cold insulin and INS-IGF2 (doubly reactive sera), were related to islet autoantibodies, HLA-DQ, or both, in newly diagnosed patients (n=676) and controls (n=363).
Patients, 0-18 years of age, diagnosed with type 1 diabetes in 1996-2005 were analyzed for specific doubly reactive INS-IGF2 autoantibodies (INS-IGF2A) after displacement with both insulin and INS-IGF2 to correct for non-specific binding. Islet autoantibodies (GADA, IA-2A, IAA, ICA, ZnT8RA, ZnT8WA, and ZnT8QA) and HLA-DQ genotypes were also determined.
The median levels of INS-IGF2A were higher in the patients compared to controls (p<0.001). Irrespective of age at diagnosis, 68% (458/676) patients had INS-IGF2A using the 75th percentile of the controls as a cut off at 586 U/ml (p<0.001). Similarly, 19 % (126/676) of the patients had INS-IGF2A when the cut-off was the 95th percentile of the controls (1198 U/ml) (p<0.001). Levels of INS-IGF2A and GADA correlated (p=0.024). The risk to be diagnosed with INS-IGF2A was increased among HLA-DQ2/8, but not 2/2 and 8/8, patients (OR=1.509; 95th CI 1.011, 2.252; p=0.045). Levels of INS-IGF2A correlated with antibodies against Ljungan virus (p=0.008).
Conclusions/interpretationDoubly reactive INS-IGF2A sera at diagnosis support the notion that INS-IGF2 is an autoantigen in type 1 diabetes. The association specifically with HLA-DQ2/8 suggests that this autoantigen primarily is presented on HLA-DQ heterodimers in trans, rather than cis, transcription.