Coexistent autoimmunity in familial type 1 diabetes: increased susceptibility in sib-pairs?

A new, all-encompassing aetiology of type 1 diabetes

The aetiology of type 1 diabetes (T1D) is considered multifactorial with the contribution of the MHC on chromosome 6 being most important. Multiple factors also contribute to the aetiology of colorectal neoplasia, but the final event causing the change from normal mucosa to polyp and from polyp to cancer is due to a single somatic mutation event. Repeated formation of colorectal neoplasia within an at-risk population results in a predictable, tapering, exponential neoplasia distribution. Critical mutations driving colorectal neoplasia formation occur in mutation-prone DNA. These observations led to three hypotheses related to T1D. First, a single somatic mutation within the MHC of antigen presenting cells results in a change in phenotype from normal to T1D. Second, the distribution of additional autoimmune diseases (AAIDs) among persons with T1D adheres to a predictable, tapering, exponential distribution. And third, critical mutations driving development of T1D occur in mutation-prone DNA. To address the hypotheses in an orderly fashion, a new analytical method called genome-wide aetiology analysis (GWEA) consisting of nine steps is presented. All data required for GWEA of T1D are obtained from peer-reviewed publications or publicly available genome and proteome databases. Critical GWEA steps include AAID distribution among persons with T1D, analysis of at-risk HLA loci for mutation-prone DNA, determination of the role of non-MHC genes on GWAS, and verification of human data by cell culture or animal experiments. GWEA results show that distribution of AAID among persons with T1D adheres to a predictable, tapering, exponential distribution. A single, critical, somatic mutation within the epitope-binding groove of at-risk HLA loci alters HLA-insulin-peptide-T-cell-receptor (TCR) complex binding affinity and creates a new pathway that leads to loss of self-tolerance. The at-risk HLA loci, in particular binding pockets P1, P4 and P9, are encoded by mutation-prone DNA: GC-rich DNA sequence and somatic hypermutation hotspots. All other genes on GWAS can but do not have to amplify the new autoimmune pathway by facilitating DNA mutations, changing peptide binding affinity, reducing signal inhibition or augmenting signal intensity. Animal experiments agree with human studies. In conclusion, T1D is caused by a somatic mutation within the epitope-binding groove of an at-risk HLA gene that affects HLA-insulin-peptide-TCR complex binding affinity and initiates an autoimmune pathway. The nature of the peptide that binds to a mutated epitope-binding groove of an at-risk HLA gene determines the type of autoimmune disease that develops, that is, one at-risk HLA locus, multiple autoimmune diseases. Thus, T1D and AAIDs, and therefore common autoimmune diseases, share a similar somatic mutation-based aetiology.

Risk for Coexistent Autoimmune Diseases in Familial and Sporadic Type 1 Diabetes is Related to Age at Diabetes Onset

OBJECTIVE

Type 1 diabetes (T1D) is frequently associated with other autoimmune diseases (AIDs). Although most of T1D patients are sporadic cases (S-T1D), 10% to 15% have a familial form (F-T1D) involving 2 or more first-degree relatives. This study evaluated the effect of T1D family aggregation and age onset on AIDs occurrence.

METHODS

In this observational, cross-sectional, case-control, single center study, we enrolled 115 F-T1D and 115 S-T1D patients matched for gender, age, T1D age onset, and duration. With respect to T1D age onset (before or after 18 years), both groups were further subdivided into young- or adult-onset F-T1D and young- or adult-onset S-T1D. The presence of organ-specific antibodies and/or overt AIDs was evaluated.

RESULTS

The F-T1D group had a higher percentage of AIDs (29.8% vs 18.4%, P = .04) and a significant earlier onset of AIDs at Cox regression analysis (P = .04) than the S-T1D group. Based on multivariate analysis, the adult-onset F-T1D subgroup had the highest prevalence of both additional organ-specific antibodies (60.5%) and overt AIDs (34.9%), whereas the adult S-T1D subgroup was the least frequently involved (29.1% and 12.7%, respectively). In F-T1D patients, offsprings develop T1D and AIDs earlier than their parents do.

CONCLUSIONS

In T1D patients, familial aggregation and adult-onset of T1D increase the risk for coexistent AIDs. These clinical predictors could guide clinicians to address T1D patients for the screening of T1D-related AIDs.

Extended family history of autoimmune diseases and phenotype and genotype of children with newly diagnosed type 1 diabetes

OBJECTIVE

Based on the concept of clustering autoimmunity, children with a positive family history of autoimmunity could be expected to have a different pathogenetic form of type 1 diabetes (T1D) and thus a stronger autoimmune reactivity against β-cells and an increased prevalence of the HLA-DR3-DQ2 haplotype.

DESIGN AND METHODS

We tested this hypothesis in a cross-sectional observational study from the Finnish Pediatric Diabetes Register. HLA class II genotypes and β-cell autoantibodies were analyzed, and data on the extended family history of autoimmunity and clinical markers at diagnosis were collected with a structured questionnaire from 1488 children diagnosed with T1D under the age of 15 years (57% males).

RESULTS

Only 23 children (1.5%) had another autoimmune disease (AID) known at diagnosis, and they had a milder metabolic decompensation at diabetes presentation. One-third (31.4%) had at least one relative with an AID other than T1D with affected mothers being overrepresented (8.2%) compared with fathers (2.8%). The children with a positive family history of other AIDs had higher levels of islet cell antibodies (P=0.003), and the HLA-DR3-DQ2 haplotype in the children was associated with celiac disease in the extended family (P<0.001), but not with an increased frequency of autoimmune disorders, in general.

CONCLUSIONS

Approximately one-third of children with newly diagnosed T1D have a first- and/or second-degree relative affected by an AID. Our data do not consistently support the hypothesis of differential pathogenetic mechanisms in such children.

How do autoimmune diseases cluster in families? A systematic review and meta-analysis

BACKGROUND

A primary characteristic of complex genetic diseases is that affected individuals tend to cluster in families (that is, familial aggregation). Aggregation of the same autoimmune condition, also referred to as familial autoimmune disease, has been extensively evaluated. However, aggregation of diverse autoimmune diseases, also known as familial autoimmunity, has been overlooked. Therefore, a systematic review and meta-analysis were performed aimed at gathering evidence about this topic.

METHODS

Familial autoimmunity was investigated in five major autoimmune diseases, namely, rheumatoid arthritis, systemic lupus erythematosus, autoimmune thyroid disease, multiple sclerosis and type 1 diabetes mellitus. Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines were followed. Articles were searched in Pubmed and Embase databases.

RESULTS

Out of a total of 61 articles, 44 were selected for final analysis. Familial autoimmunity was found in all the autoimmune diseases investigated. Aggregation of autoimmune thyroid disease, followed by systemic lupus erythematosus and rheumatoid arthritis, was the most encountered.

CONCLUSIONS

Familial autoimmunity is a frequently seen condition. Further study of familial autoimmunity will help to decipher the common mechanisms of autoimmunity.