The −590CC Genotype in the IL4 Gene as a Strong Predictive Factor for the Development of Hypothyroidism in Hashimoto Disease

The severity of Hashimoto disease (HD) varies among patients and is difficult to predict when the disease is in the subclinical state and diagnosed by the presence of thyroid-specific autoantibody. Likewise, the intractability of Graves disease (GD) is difficult to predict. Autoimmune thyroid destruction that underlies both diseases is strongly determined by T-cell cytotoxicity, which is activated by interferon (IFN)-γ (1), and the T allele in +874A/T polymorphism of the interferon gamma (IFNG) gene, which promotes increased IFN-γ production, has been noted more frequently among patients with severe HD (2). Cytokine balance between T-helper 1 (Th1) cytokines, such as IFN-γ, and Th2 cytokines is important in immune regulation (3). Therefore, it is possible that Th2 cytokines may also affect the severity of HD and the intractability of GD. Interleukin (IL)-4, one of the key Th2 cytokines, stimulates humoral immunity and suppresses the production of inflammatory Th1 cytokines, including IFN-γ (3). Individuals who carry the T allele in −590C/T polymorphism of the interleukin 4 (IL4) gene (rs2243250) have a higher proportion of IL-4–producing T-helper cells (4). Individuals with the Ile/Ile genotype in the Ile50Val polymorphism (rs1805010) of the interleukin 4 receptor, alpha (IL4RA) gene also have higher IL-4Rα activity compared to individuals with the Ile/Val or Val/Val genotype (5). To clarify the association of these polymorphisms of the IL4 and IL4RA genes with HD severity and GD intractability, we performed genotyping using a direct sequencing reaction for −590C/T and TaqMan® single-nucleotide polymorphism genotyping assays for Ile50Val in the following individuals: 35 patients <50 years old with HD who developed permanent hypothyroidism and were treated with L-thyroxine (rapid destruction type; severe HD), 39 euthyroid patients >50 years old with HD who were untreated (slow destruction type; mild HD), 50 euthyroid patients with GD who remained positive for antithyrotropin receptor antibody (TRAb) despite being on antithyroid drug treatment for >5 years (intractable GD), 26 patients with GD in remission who had been euthyroid and negative for TRAb for >2 years after cessation of antithyroid drug treatment, and 47 controls. All patients with HD were positive for antithyroid microsomal antibodies. All patients with GD showed increased TRAb concentrations and thyrotoxicosis when first diagnosed. All study participants were Japanese and unrelated. Written informed consent was obtained from all patients and controls, and the study protocol was approved by the Ethics Committee of Osaka University. The frequency of the −590CC genotype in the IL4 gene among the patients with severe HD was 20.0%, whereas the frequency among the patients with mild HD was 0.0% (P = 0.0037) (Table 1 ). If this finding is confirmed prospectively, a high percentage of HD patients with the IL-4 −590CC genotype would be expected to develop severe disease (hypothyroidism) before 50 years of age. A possible mechanism to explain this phenomenon is that in HD patients with the −590CC genotype, the proportion of IL-4–producing T-helper cells is lower than that in HD patients with the −590TT or −590CT genotype (4), which results in higher activity of inflammatory Th1 cytokines and more rapid progression of thyroid destruction, followed by early development of hypothyroidism. Conversely, we found no association between the Ile50Val polymorphism in the IL4RA gene and HD severity (Table 1 ) despite the receptor activity being different among individuals with this polymorphism. Furthermore, we found no association between these IL4 and IL4RA gene polymorphisms and GD intractability (Table 1 ); however, GD is caused by TRAb and tends to be more intractable in patients with higher TRAb concentrations. In this context, we also found no relationship between these polymorphisms and serum TRAb concentrations (data not shown). Genotype frequencies of IL-4 −590C/T and IL-4Rα Ile50Val polymorphisms in patients with autoimmune thyroid disease and in controls. Hashimoto disease patients <50 years old who developed hypothyroidism and were treated with L-thyroxine. Untreated and euthyroid Hashimoto disease patients >50 years old. Graves disease patients who remained positive for TRAb despite being treated by antithyroid drug for more than 5 years. Graves disease patients in remission who have been euthyroid and negative for TRAb for more than 2 years after cessation of anti-thyroid drug treatment. Analyzed by χ2 test. NS, not significant; ND, not determined; T4, thyroxine; T3, triiodothyronine; TSH, thyrotropin; TRAb, antithyrotropin receptor antibody; McAb, antithyroid microsomal antibody; TgAb, antithyroglobulin antibody; PTU, Propylthiouracil. Analyzed by Fisher exact test. Results of clinical characteristics are expressed as mean (SD). Genotype frequencies of IL-4 −590C/T and IL-4Rα Ile50Val polymorphisms in patients with autoimmune thyroid disease and in controls. Hashimoto disease patients <50 years old who developed hypothyroidism and were treated with L-thyroxine. Untreated and euthyroid Hashimoto disease patients >50 years old. Graves disease patients who remained positive for TRAb despite being treated by antithyroid drug for more than 5 years. Graves disease patients in remission who have been euthyroid and negative for TRAb for more than 2 years after cessation of anti-thyroid drug treatment. Analyzed by χ2 test. NS, not significant; ND, not determined; T4, thyroxine; T3, triiodothyronine; TSH, thyrotropin; TRAb, antithyrotropin receptor antibody; McAb, antithyroid microsomal antibody; TgAb, antithyroglobulin antibody; PTU, Propylthiouracil. Analyzed by Fisher exact test. Results of clinical characteristics are expressed as mean (SD). In conclusion, the −590CC genotype in the IL4 gene appears to be a strong predictive factor for the development of hypothyroidism in HD. Grant/funding Support: This work was supported by KAKENHI (17390164). Financial Disclosures: None declared.