Biomarkers of susceptibility to type 1 diabetes with special reference to the Indian population

Abstract

Type 1 Diabetes (T1D) is a polygenic autoimmune disease. Susceptibility to T1D is strongly linked to a major genetic locus that is the Major Histocompatibility Complex (MHC) and several other minor loci including insulin, CTLA4 that contribute to diabetes risk in an epistatic way. MHC harbours genes whose primary function is to govern immune responsiveness. Being the most polymorphic genomic region known in humans, MHC serves as a very exciting minigenome model for studying susceptibility to T1D. We have observed enormous diversity in HLA class I and class II genes in the north Indian population and identified several ‘novel alleles’ and ‘unique haplotypes’. For example, multiple DR3+ve autoimmunity favouring haplotypes have been identified, some of which are unique to the Asian north Indian T1D patients. Our molecular studies have revealed that (i) the classical Caucasian autoimmunity favouring AH8.1 (HLA-A1 B8 DR3) is rare in the Indian population and has been replaced by a variant AH8.1v that differs from the Caucasian AH8.1 at several gene loci, (ii) AH8.2 (HLA-A26 B8 DR3) is the most common DR3 positive haplotype in this population and resembles the Indian AH8.1v rather than Caucasian AH8.1, and (iii) there are additional HLA-DR3 haplotypes HLA-A24 B8 DR3 (AH8.3), A3 B8 DR3 (AH8.4) and A31 B8 DR 3 (AH 8.5) that occur in the Indian population. The studies have led to a hypothesis that AH8.1 and AH8.1v might have co-evolved from a common ancestor but preferential divergence of AH8.2 over AH8.1 leading to survival advantage might have been driven by vigorous pathogenic challenges encountered by the Indian population. These studies have important implications in our understanding of disease pathogenesis, identification of high risk individuals, disease diagnosis, disease management and immunological therapeutic approaches.