Genetic analysis of adult-onset autoimmune diabetes

OBJECTIVE

In contrast with childhood-onset type 1 diabetes, the genetics of autoimmune diabetes in adults are not well understood. We have therefore investigated the genetics of diabetes diagnosed in adults positive for autoantibodies.

RESEARCH DESIGN AND METHODS

GAD autoantibodies (GADAs), insulinoma-associated antigen-2 antibodies (IA-2As), and islet cell autoantibodies were measured at time of diagnosis. Autoantibody-positive diabetic subjects (n = 1,384) and population-based control subjects (n = 2,235) were genotyped at 20 childhood-onset type 1 diabetes loci and FCRL3, GAD2, TCF7L2, and FTO.

RESULTS

PTPN22 (1p13.2), STAT4 (2q32.2), CTLA4 (2q33.2), HLA (6p21), IL2RA (10p15.1), INS (11p15.5), ERBB3 (12q13.2), SH2B3 (12q24.12), and CLEC16A (16p13.13) were convincingly associated with autoimmune diabetes in adults (P ≤ 0.002), with consistent directions of effect as reported for pediatric type 1 diabetes. No evidence of an HLA-DRB1*03/HLA-DRB1*04 (DR3/4) genotype effect was obtained (P = 0.55), but it remained highly predisposing (odds ratio 26.22). DR3/4 was associated with a lower age at diagnosis of disease, as was DR4 (P = 4.67 × 10⁻⁶) but not DR3. DR3 was associated with GADA positivity (P = 6.03 × 10⁻⁶) but absence of IA-2A (P = 3.22 × 10⁻⁷). DR4 was associated with IA-2A positivity (P = 5.45 × 10⁻⁶).

CONCLUSIONS

Our results are consistent with the hypothesis that the genetics of autoimmune diabetes in adults and children are differentiated by only relatively few age-dependent genetic effects. The slower progression toward autoimmune insulin deficiency in adults is probably due to a lower genetic load overall combined with subtle variation in the HLA class II gene associations and autoreactivity.The genetics of type 1 diabetes in children under 17 years of age (which is characterized by autoimmune destruction of the insulin-producing β-cells in the pancreatic islets and insulin deficiency) has been comprehensively studied, with over 50 susceptibility loci reported to date (1) (www.t1dbase.org). Investigation of the genetics of autoimmune diabetes in subjects who develop the disease as adults can elucidate the etiology of this late-onset autoimmunity and could impact its future treatment. However, most studies in adults have been confined to the study of a condition diagnosed as latent autoimmune diabetes in adults (LADA), a classification that is variable (2–4) (www.actionlada.org). In general, these case subjects are aged between 30 and 50 years and noninsulin dependent/not insulin treated at diagnosis and remain so for at least 3–6 months postdiagnosis and must be positive for GAD autoantibodies (GADAs), which are associated with type 1 diabetes (www.actionlada.org). A number of studies have investigated the association of the major histocompatibility complex (MHC) region, specifically the HLA class II genes HLA-DRB1 and HLA-DQB1, with LADA (5–7). They reported an age-at-diagnosis effect for the HLA-DRB1*03/HLA-DRB1*04 (DR3/4) genotype, with the frequency of DR3/4 lower in the older age-groups compared with that in the youngest age-groups, a finding that is also characteristic of pediatric type 1 diabetes. HLA class II susceptibility alone cannot account for type 1 diabetes development (8,9). However, in LADA cases to date, only one non-MHC gene has been shown to be convincingly associated: the −23 HphI INS (rs689) single nucleotide polymorphism (SNP) (10). This SNP is a near perfect proxy for the disease causal variable number tandem repeat (VNTR) polymorphism in the insulin (INS) gene in European ancestry populations. In that study, the 400 case subjects used were diagnosed with type 2 diabetes at an age at diagnosis between 25 and 68 years, were positive for GADA and/or insulinoma-associated antigen-2 autoantibodies (IA-2As), and did not require insulin during the first 3 months following diagnosis. The T allele had an odds ratio (OR) of 0.42 (95% CI 0.31–0.58), which is consistent with reports in childhood-onset type 1 diabetes and is associated with increased immune tolerance to insulin and its precursors (11). The authors concluded that the INS VNTR locus does not distinguish LADA and pediatric type 1 diabetes.In the current study, we have taken a simpler, clinic-based approach to defining adult-onset autoimmune diabetes. To be included in the study, all diabetic case subjects were positive for one or more islet autoantibody, specifically, islet cell antibody (ICA), GADA, IA-2A, or insulin-specific autoantibodies (IAAs), and had glucose levels, oral glucose tolerance test, or HbA_1c at a level diagnostic for diabetes. With these specific criteria for inclusion, the complexities associated with classification of diabetes in adults is avoided (2) and, as advocated by a number of authors in recent commentaries (3,4), the study of autoimmune diabetes may be more informative. Twenty type 1 diabetes–associated loci, selected based on effect size (OR ≥1.15, leading to power ≥0.7 at α = 0.05), potential age-at-diagnosis effects (IL2RA, IL2, and RNLS), and putative associations with thyroid peroxidize autoantibodies (STAT4, UBASH3A, IL2, BACH2, and CTLA4 [12,13]) were tested for association with autoimmune diabetes diagnosed in adults. The obesity gene, FTO, has been shown to be associated with LADA (14) and so was also tested for association in the autoimmune diabetic case subjects, as was the gene TCF7L2, which has the strongest effect of any locus in type 2 diabetes (15). Finally, FCRL3 is thought to be a general autoimmunity locus because of its association with several autoimmune diseases (16–19), so both FCRL3 and the gene GAD2 encoding the GAD antigen itself were also tested for association in these 1,212 adult-onset autoimmune diabetic case subjects. Hence, we have tested more loci in more adult-onset autoimmune diabetic case subjects than any other study has reported on to date.