The rs1990760 polymorphism within the IFIH1 locus is not associated with Graves' disease, Hashimoto's thyroiditis and Addison's disease

Autoimmune diseases are characterised by the failure to maintain self-tolerance, which results from the interaction between genetic and enviromental factors. The Major Histocompatibility Complex, the cytotoxic T lymphocyte antigen 4 locus and the protein tyrosine phosphatase non-receptor 22 gene are involved in the pathogenesis of autoimmune endocrine diseases [1]. Recently, a novel locus called Interferon-induced helicard (IFIH1), rs1990760, was identified to be associated with type 1 diabetes (T1D) and Graves' disease [2, 3]. IFIH1 gene, also referred to as melanoma differentiation-associated 5 or Helicard, is located on the chromosome 2q24 and encodes an early type I interferon beta responsive gene. The helicase detects cytoplasmatic viral double strand RNA (dsRNA), leading to interferon beta production via activation of interferon regulatory factor-3 (IRF-3) and nuclear factor kappa-B (NFkB). The inappropriate activation of these virus-sensitive proteins by self-derived, intracellular nucleic acids might have an effect in autoimmunity [4]. Therefore and in order to determine whether IFIH1 rs1990760 polymorphism is involved in the genetic background of other endocrine autoimmune disease we compared genotype and allele frequencies in German patients with Graves' disease (GD), Hashimoto's thyroiditis (HT), Addison's disease (AD) and healthy controls (HC) and performed transmission disequilibrium tests (TDT) for that polymorphism in families with GD (German) and families with HT (Italian). Furthermore, we analyzed the distribution of IFIH1 rs1990760 polymorphism in families and case-controls according to the presence or absence of the human leukocyte antigen (HLA) risk haplotype DQ2 (DQA*0501-DQB*0201), risk haploypes DQ2/DQ8 (DQA*0301-DQB*0302)] and thyroid antibodies status.

Allele and genotype frequencies for IFIH1 rs1990760 polymorphism in all subjects did not deviate from Hardy-Weinberg equilibrium (HWE; p > 0.05).

However, the associations obtained in the family analyses did not remain statistically significant after correction of the p-values. Because TRAb titers of the parents in the GD families were not available, the transmission analysis was not performed. Additionally, we compared the transmission of the IFIH1 rs1990760 polymorphism in parents, in either mothers and fathers in GD and HT families with the haplotype DQ2 and without DQ2 by TDT. In Italian HT families as well as the in German GD families the interaction analysis between IFIH1 rs1990760 polymorphism and HLA DQ2 haplotype revealed no difference in the transmission (data not shown).

The innate immune system detects dsRNA, a replication product of most viruses, within infected cells by Toll-like receptor 3 and cytoplasmic RNA helicases such as retinoic acid inducible gene I and interferon-induced helicase. IFIH1, a 1025 amino acid cytoplasmic protein, protects the host from viral infection by triggering a cellular antiviral and apoptotic response. In this process IFIH1 signaling activates transcription factors, IRFs and NFkB pathways through its caspase recruitment domain (CARD) that induce IFN-α/β and interferon-inducible genes [7–9]. IFIH1 transcripts were found to be expressed in lymphoid and other tissues, suggesting a role in autoimmune diseases. A large scale association study of candidate (nonsynonymous, i.e. amino acid changing) polymorphism identified IFIH1, rs1990760, as a new locus for T1D [2]. The rs1990760 polymorphism is located in HNF-3b binding site within exon 15, encodes an alanine to threonine amino acid change at codon 946 (Ala946Thr), which does not reside in either the CARD or helicase domain of the protein. However, this region of the protein is conserved between mammals and may have other, unknown functions or may influence the active domains through effects on tertiary structure. Ala946 is the fully conserved amino acid, suggesting that this is the functional allele. The association between IFIH1 rs1990760 polymorphism and T1D was not only confirmed by other authors [10–12] but also reported in other autoimmune diseases [3, 13, 14]. Studies that reported associations between viral infection and susceptibility to development T1D as well as autoimmune thyroid disease (ATD) [15–17], reinforce the IFIH1 gene as a good functional candidate for these autoimmune disorders. On the basis on this finding we investigated the role of IFIH1 rs1990760 polymorphism in ATD including GD, HT and Addison's disease (all Germans) and enlarge our analysis in the transmission of this polymorphism in families with offspring affected with GD and HT. Similar to other studies, that investigated IFIH1 rs1990760 polymorphism in diverse autoimmune disease we could not find a association between IFIH1 rs1990760 polymorphism and our case-control study population [18, 19]. In contrast, Sutherland et al. (2007) tested the same coding variant for association to ATD (646 cases;446 controls) and autoimmune Addison's disease (204 cases) from Caucasian United Kingdom (UK) and found that the A allele increased the odds of Graves' disease 1.47-fold. Since in our study the distribution of rs1990760 polymorphism in controls (Germans) was not different from that in UK controls and the fact that Sutherland et al. (2007) analogue to our findings did not detect an association in AD patients, the smaller number of patients could be an explanation for the lack of an association between endocrine autoimmune diseases and the rs1990760 polymorphism observed in the present study. However, the trend associations obtained in the HT family in which mothers and fathers were both positive for TPOAb/TgAb, overtransmitted the allele A of IFIH1 rs1990760 to their HT affected offspring (61.8% vs 38.2%;p = 0.05; corrected p [pc] = 0.1), did not remain statistically significant after correction of the p-values. This may reflect that the selection of the patient collectives could influence the detection of an association.

Also the rare polymorphisms, rs35667974 and rs35337543 which reside within the IFIH1 gene were found to be associated with T1D [20] but their effect can only be studied in large population samples. Nevertheless these data point to a possible role of IFIH1 in the pathogenesis of T1D and marke the investigations of large cohorts necessary.

The IFIH1 rs1990760 polymorphism is in linkage disequilibrium (LD) with other IFIH1 variants for instance rs2068330, rs2111485 and rs984971 and could be different due to population heterogeneity [12]. Therefore the different patterns of LD across studies and populations could be a further explanation for the discrepancy of the results.

We conclude from the results of the present case-control and families study that there is no significant role of the IFIH1 rs1990760 polymorphism in the genetic susceptibility to GD, HT and AD in Germans and HT in Italians. However in order to exclude a possible influence of the rs1990760 polymorphism in specified subgroups within patients with ATD further investigations in larger samples are required. Also other polymorphisms e.g. rs2068330, rs2111485, rs984971 within the IFIH1 LD block are recommended to be investigated in future studies, in order to obtain more suitable markers to analyse the susceptibility for the development ATD and AD in Germans.