We found that the distribution of HLA genotype changed in youth with type 1 diabetes diagnosed 27 years apart. Having type 1 diabetes at age 5–17 years in 2002–2004 was associated with an 11% difference or 0.6-fold lower odds for carrying the high-risk HLA genotype (DRB1*03-DQB1*02/DRB1*04-DQB1*03) compared with having type 1 diabetes in 1978–1988. The largest proportional difference (19%) over time in the high-risk genotype was found in 5- to 9-year-olds, suggesting that environmental pressures may have a greater impact on type 1 diabetes risk at earlier ages. This is the first documentation of such a trend in a representative biracial sample of youth with type 1 diabetes in the U.S.
Our study supports previous findings from Finland (9
) and the U.K (8
), both including Caucasian adults with childhood-onset type 1 diabetes diagnosed up to 80 years apart. The U.K. study showed that the frequency of the high-risk genotype was 12% lower in adults diagnosed up to 15 years of age in 1985–2002 compared with those diagnosed between 1922–1946 and 21% lower in those diagnosed under 5 years of age (8
). The Finnish study showed that the frequency of the high-risk genotype was 7.1% lower and that of protective genotypes was 7.2% higher in adults with childhood-onset type 1 diabetes diagnosed after 1990 compared with those diagnosed before 1965 (9
). Two older studies conducted in Finland in the 1980s reported a decrease in HLA-DR3 by 25% for those diagnosed with type 1 diabetes in the 1980s compared with those diagnosed in the 1960s (6
). However, the major limitation of these studies was the potential for selection bias. Neither of these studies provided information on the size or representativeness of the referent population. More importantly, two studies (8
) measured HLA in adults who were diagnosed with type 1 diabetes as children in the first half of the 20th century, when early mortality associated with type 1 diabetes was high. Therefore, the frequency and distribution of HLA genotypes in this sample may be significantly influenced by factors associated with survival of type 1 diabetes. An underestimate of the proportional distribution of high-risk HLA genotypes in earlier time periods (due to a potential association of high-risk HLA genes with increased mortality) may have resulted in underestimating the true temporal trends in the proportion of type 1 diabetes cases with high-risk HLA genotypes in these earlier studies. In contrast, our study genotyped youth with type 1 diabetes relatively close to their disease onset and thus is not affected by survivor bias. In addition, we were able to provide evidence that the sample included in the analysis was representative of the Colorado population of youth with type 1 diabetes in both time periods.
Nevertheless, our study has several limitations. Importantly, although the same method was used for typing, due to the specific time periods in which they were performed, two slightly different assays were used, namely, OneLambda LabType Luminex SSO in 2002–2004 and OneLambda SSO in 1978–1988. We were unable to validate the results obtained with the earlier SSO assay due to the lack of stored DNA samples and inability to recontact participants. However, previous studies (19
) suggest comparable agreement in serologic specificity with an 85% allele agreement. Due to this and the difference in the number of allele SSOs available for typing over time, only the gene locus was used in this analysis. By not using expanded allele information, we potentially introduced some misclassification, likely increasing the proportion with the high-risk genotype. However, misclassification would be similar for both time periods and therefore unlikely to influence our analysis of trends. Because for 2002–2004 we did have allele information on 100 DQB1 SSOs, we estimated the magnitude of the potential misclassification, and only four (1.5%) cases were misclassified as having the highest-risk genotype in 2002–2004 using the expanded allele information only. In addition, there were 12 youth from 1978–1988 that did not have complete genotypes. These youth were included in the study and classified as haplotype unknown. This categorization could have led to some misclassification. Specifically, if the unknown haplotype was DRB1*03-DQB1*02 or DRB1*04-DQB1*03, youth would have been classified as high risk based on the typed haplotype or as heterozygous for the above haplotypes. For example, in the extreme situation where all youth with the unknown haplotype in the first time period would have been in fact at “high-risk,” the proportion of youth with the highest risk genotype in 1978–1988 would have been 51% (instead of 39%). This would have made our trend estimates even more substantial (i.e., a 23% decrease in the proportion with high-risk genotype). Consequently, our estimated change of 11% is conservative.
There are several hypotheses proposed to explain the increase in type 1 diabetes incidence in youth and the earlier age at onset. The “accelerator” and “overload” hypotheses suggest that an increase in body mass in genetically susceptible individuals accelerates β-cell decline or stresses (overloads) the β-cells, leading to an early age at type 1 diabetes diagnosis (22
). The “hygiene hypothesis” postulates that the decreasing early life exposure to infectious agents in Westernized societies has led to an impairment in the maturation of the immune system, thus permitting an increased occurrence of immune-mediated disorders, such as asthma and type 1 diabetes (24
). Further testing is required for all of these hypotheses.
Given that there is an increasing incidence of type 1 diabetes in Colorado youth (2
), our findings support the hypothesis that environmental pressures may be increasing the disease penetrance despite a shift in the distribution of HLA genotypes from high to moderate-low risk. Although this and previous studies (6
) evaluating the distribution and frequency of HLA susceptibility genes over time have provided evidence in support of this hypothesis, the nature of environmental factors responsible for the increasing incidence of type 1 diabetes remains unknown.