MON-454 - Fetal exposure to high thyroid hormone results in epigenetic reprogramming of the deiodinase 3 imprinted locus

Background: Unaffected infants born to mothers with resistance to thyroid hormone beta (RTHβ) caused by a R243Q mutation in the ligand binding domain of the thyroid hormone (TH) receptor beta (THRB) gene, are exposed to high maternal TH levels in utero, and have low birth weight and suppressed TSH. These individuals and their descendants along male but not female line, have persistent central reduced sensitivity to TH (RSTH) in adulthood demonstrated by a reduced TSH suppression during administration of L-T3. Based on studies in an RTHβ mouse model, the putative mechanism for this RSTH is an increase in expression of pituitary TH inactivating deiodinase 3, however, this mechanism has not been previously demonstrated in humans. The parent dependent imprinted DIO3 gene is located in humans on chromosome 14q32.

Methods: To study the effect of the fetal exposure to TH excess on epigenetic reprogramming at the DIO3 locus we studied 20 individuals each from generation F2 and F3 born to unaffected progeny of grandmothers and great grandmothers with RTHβ, respectively. Those born to male line had RSTH while those born to female line had no RSTH and served as controls. Mapping of the DIO3 promoter-exon region, -151 to +93bp from the transcription start site, identified 27 methylation sensitive CpGs. The methylation status of DIO3 promoter was assessed using bisulfite sequencing of blood gDNA.

Results: RSTH phenotype was confirmed in the unaffected and unexposed to TH F2 and F3 descendants only born to fathers with RSTH and was absent in those born to mothers with RSTH who served as controls. This is in accordance with transgenerational epigenetic inheritance exclusively through the male germ line. The following thyroid function tests showed significant (p < 0.05) differences between individuals with RSTH vs controls. Higher rT3 20.9 vs 16.8 ng/dl in F2 and 30.2 vs 23.8 ng/dl in F3, with high rT3/T3 of 0.22 vs 0.16 ratio and rT3/T4 3.3 vs 2.3 ratio. Bisulfite sequencing of gDNA from RSTH individuals in F2 demonstrated significantly decreased methylation across the DIO3 promoter region (20.1 vs 28.3%) containing critical transcription factors binding sites including FIGLA, EGR/WT1, TFAP2A, and RXR. Methylation status in F3 is in process. Decreased DIO3 promoter methylation leads to putative increased DIO3 expression and enzymatic activity resulting in TH inactivation.

Conclusion: Fetal exposure to TH excess induces a persistent RSTH phenotype that is successfully transmitted across subsequent unexposed generations via the male germline. DIO3 is the mechanistic driver of this phenomenon, functioning both as a potent TH-inactivating enzyme and as a parent-of-origin imprinted gene capable of harboring transgenerational epigenetic markers. These findings illustrate a novel paradigm where early maternal thyroid axis programming permanently intersects with imprinting remodeling during fetal development.

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