Novel Variant of Thyroglobulin Promoter Triggers Thyroid Autoimmunity through an Epigenetic Interferon α-modulated Mechanism

Abstract

Autoimmune thyroid diseases (AITD) arise from complex interactions between genetic, epigenetic, and environmental factors. Whole genome linkage scans and association studies have established thyroglobulin (TG) as a major AITD susceptibility gene. However, the causative TG variants and the pathogenic mechanisms are unknown. Here, we describe a genetic/ epigenetic mechanism by which a newly identified TG promoter single-nucleotide polymorphism (SNP) variant predisposes to AITD. Sequencing analyses followed by case control and familybased association studies identified an SNP (؊1623A3 G) that was associated with AITD in the Caucasian population (p ‫؍‬ 0.006). We show that the nucleotide substitution introduced by SNP (؊1623A/G) modified a binding site for interferon regulatory factor-1 (IRF-1), a major interferon-induced transcription factor. Using chromatin immunoprecipitation, we demonstrated that IRF-1 binds to the 5 TG promoter motif, and the transcription factor binding correlates with active chromatin structure and is marked by enrichment of mono-methylated Lys-4 residue of histone H3, a signature of active transcriptional enhancers. Using reporter mutations and siRNA approaches, we demonstrate that the disease-associated allele (G) conferred increased TG promoter activity through IRF-1 binding. Finally, treatment of thyroid cells with interferon ␣, a known trigger of AITD, increased TG promoter activity only when it interacted with the disease-associated variant through IRF-1 binding. These results reveal a new mechanism of interaction between environmental (IFN␣) and genetic (TG) factors to trigger AITD.Autoimmune thyroid diseases (AITD), 4 including Graves disease (GD) and Hashimoto thyroiditis (HT), are characterized by infiltration of the thyroid by T and B cells that react with local antigens leading to immune destruction of the thyroid in HT and production of thyroid-stimulating hormone receptor (TSHR) antibodies in GD. These result in the clinical manifestations of hypothyroidism in HT and hyperthyroidism in GD (1, 2). There is solid evidence that interactions between susceptibility genes and environmental triggers activate the sequence of cellular and humoral immune responses to thyroid antigens that cause AITD (1, 3, 4). Several environmental factors, including exposure to excess iodine, selenium deficiency, various infectious diseases, certain drugs, and pollutants have been associated with AITD (5, 6). Among these factors, interferon ␣ (INF␣), a therapeutic agent widely used for the treatment of chronic hepatitis C infection, has recently emerged as a major factor that triggers AITD (7,8).To date, several gene loci have been associated with AITD, including immune genes (HLA-DR, CTLA-4, CD40, FOXP3, and CD25) and thyroid-specific genes (TSHR and TG). Whole genome linkage screens, performed by our group (9) and others (10), have shown that the thyroglobulin (TG) locus on chromosome 8q24 is strongly linked with AITD. Moreover, TG has emerged as the only thyroid-specific gene that confers susceptibili...

Genetic–epigenetic dysregulation of thymic TSH receptor gene expression triggers thyroid autoimmunity

“…Our reporter assays showed a more efficient effect of PLZF when both binding sites were intact and the diseaseassociated allele was present at rs12101261. These results are consistent with previous data by us and others demonstrating the importance of disease-associated SNPs in noncoding regions in impacting transcriptional mechanisms by interrupting TF binding sites (6,29) or by disturbing long-range interactions (30). (B) Correlation between TSHR and PLZF mRNA levels in thymus tissues from individuals who were homozygous TT (n = 9), homozygous CC (n = 11), and heterozygous CT (n = 19) for the rs12101261 SNP.…”

“…Moreover, epigenetic changes in response to the environment can occur at sites of disease-associated polymorphisms, resulting in profound effects on gene expression through modulation of chromatin accessibility to transcriptional regulators (5). Indeed, we have recently shown that chromatin modifications induced by interferon alpha (IFNα) at a Tg gene variant associated with thyroid autoimmunity resulted in significant changes in Tg expression levels (6).…”

Genetic Profiling in Graves' Disease: Further Evidence for Lack of a Distinct Genetic Contribution to Graves' Ophthalmopathy

“…We now know that GD is a complex genetic disease with significant genetic and environmental influences that may exert their effects via epigenetic mechanisms (26,(33)(34)(35). The susceptibility genes have been identified both by the candidate gene approach and by whole genome linkage and association studies (36).…”

“…These include HLA-DR3 (7,8), CTLA4 (9,10), PTPN22 (11,12), CD40 (13,14), IL2RA (CD25) (15,16), FCRL3 (17,18), and the IL23R (19). The second group includes genes that are specific to the thyroid gland: the TSHR (20-24) and thyroglobulin (TG) (25,26). However, the distinct factors predisposing GD patients to the development of GO remain unclear (27).…”

Hepatitis C virus infection and thyroid autoimmune disorders: A model of interactions between the host and the environment

“…The absence of HCV clearance from thyrocytes perpetuates the chronic inflammation and autoimmunity. a-IFN triggers AITD through an epigenetic mechanism involving variant of Tg and TSHr gene promoter [101,102] . Moreover, a-IFN locally enhances the expression of TSH-r, Tg, TPO and HLA class Ⅰ molecules on thyrocytes and the secretion of the potent proinflammatory IL-2 cytokine [11] .…”