Genomic imprinting of DIO3, a candidate gene for the syndrome associated with human uniparental disomy of chromosome 14

Transgenerational epigenetic self-memory of Dio3 dosage is associated with Meg3 methylation and altered growth trajectories and neonatal hormones

Intergenerational and transgenerational epigenetic effects resulting from conditions in previous generations can contribute to environmental adaptation as well as disease susceptibility. Previous studies in rodent and human models have shown that abnormal developmental exposure to thyroid hormone affects endocrine function and thyroid hormone sensitivity in later generations. Since the imprinted type 3 deiodinase gene (Dio3) regulates sensitivity to thyroid hormones, we hypothesize its epigenetic regulation is altered in descendants of thyroid hormone overexposed individuals. Using DIO3-deficient mice as a model of developmental thyrotoxicosis, we investigated Dio3 total and allelic expression and growth and endocrine phenotypes in descendants. We observed that male and female developmental overexposure to thyroid hormone altered total and allelic Dio3 expression in genetically intact descendants in a tissue-specific manner. This was associated with abnormal growth and neonatal levels of thyroid hormone and leptin. Descendant mice also exhibited molecular abnormalities in the Dlk1-Dio3 imprinted domain, including increased methylation in Meg3 and altered foetal brain expression of other genes of the Dlk1-Dio3 imprinted domain. These molecular abnormalities were also observed in the tissues and germ line of DIO3-deficient ancestors originally overexposed to thyroid hormone in utero. Our results provide a novel paradigm of epigenetic self-memory by which Dio3 gene dosage in a given individual, and its dependent developmental exposure to thyroid hormone, influences its own expression in future generations. This mechanism of epigenetic self-correction of Dio3 expression in each generation may be instrumental in descendants for their adaptive programming of developmental growth and adult endocrine function.

Gene polymorphisms and thyroid hormone signaling: implication for the treatment of hypothyroidism

INTRODUCTION

Mutations and single nucleotide polymorphisms (SNPs) in the genes encoding the network of proteins involved in thyroid hormone signaling (TH) may have implications for the effectiveness of the treatment of hypothyroidism with LT4. It is conceivable that loss-of-function mutations or SNPs impair the ability of LT4 to be activated to T3, reach its targets, and ultimately resolve symptoms of hypothyroidism. Some of these patients do benefit from therapy containing LT4 and LT3.

METHODS

Here, we reviewed the PubMed and examined gene mutations and SNPs in the TH cellular transporters, deiodinases, and TH receptors, along with their impact on TH signaling, and potential clinical implications.

RESULTS

In some mechanisms, such as the Thr92Ala-DIO2 SNP, there is a compelling rationale for reduced T4 to T3 activation that limits the effectiveness of LT4 to restore euthyroidism. In other mechanisms, a potential case can be made but more studies with a larger number of individuals are needed.

DISCUSSION/CONCLUSION

Understanding the clinical impact of the genetic makeup of LT4-treated patients may help in the preemptive identification of those individuals that would benefit from therapy containing LT3.

Epigenetic developmental programming and intergenerational effects of thyroid hormones

Mounting evidence is showing that altered signaling through the nuclear hormone receptor superfamily can cause abnormal, long-term epigenetic changes which translate into pathological modifications and susceptibility to disease. These effects seem to be more prominent if the exposure occurs early in life, when transcriptomic profiles are rapidly changing. At this time, the coordination of the complex coordinated processes of cell proliferation and differentiation that characterize mammalian development. Such exposures may also alter the epigenetic information of the germ line, potentially leading to developmental changes and abnormal outcomes in subsequent generations. Thyroid hormone (TH) signaling is mediated by specific nuclear receptors, which have the ability to markedly change chromatin structure and gene transcription, and can also regulate other determinants of epigenetic marks. TH exhibits pleiotropic effects in mammals, and during development, its action is regulated in a highly dynamic manner to suit the rapidly evolving needs of multiple tissues. Their molecular mechanisms of action, timely developmental regulation and broad biological effects place THs in a central position to play a role in the developmental epigenetic programming of adult pathophysiology and, through effects on the germ line, in inter- and trans-generational epigenetic phenomena. These areas of epigenetic research are in their infancy, and studies regarding THs are limited. In the context of their characteristics as epigenetic modifiers and their finely tuned developmental action, here we review some of the observations underscoring the role that altered TH action may play in the developmental programming of adult traits and in the phenotypes of subsequent generations via germ line transmission of altered epigenetic information. Considering the relatively high prevalence of thyroid disease and the ability of some environmental chemicals to disrupt TH action, the epigenetic effects of abnormal levels of TH action may be important contributors to the non-genetic etiology of human disease.

DIO3 protects against thyrotoxicosis-derived cranio-encephalic and cardiac congenital abnormalities

Maternal hyperthyroidism is associated with an increased incidence of congenital abnormalities at birth, but it is not clear which of these defects arise from a transient developmental excess of thyroid hormone and which depend on pregnancy stage, antithyroid drug choice, or unwanted subsequent fetal hypothyroidism. To address this issue, we studied a mouse model of comprehensive developmental thyrotoxicosis secondary to a lack of type 3 deiodinase (DIO3). Dio3-/- mice exhibited reduced neonatal viability on most genetic backgrounds and perinatal lethality on a C57BL/6 background. Dio3-/- mice exhibited severe growth retardation during the neonatal period and cartilage loss. Mice surviving after birth manifested brain and cranial dysmorphisms, severe hydrocephalus, choanal atresia, and cleft palate. These abnormalities were noticeable in C57BL/6J Dio3-/- mice at fetal stages, in addition to a thyrotoxic heart with septal defects and thin ventricular walls. Our findings stress the protecting role of DIO3 during development and support the hypothesis that human congenital abnormalities associated with hyperthyroidism during pregnancy are caused by transient thyrotoxicosis before clinical intervention. Our results also suggest thyroid hormone involvement in the etiology of idiopathic pathologies including cleft palate, choanal atresia, Chiari malformations, Kaschin-Beck disease, and Temple and other cranio-encephalic and heart syndromes.

Temple syndrome and Kagami-Ogata syndrome: clinical presentations, genotypes, models and mechanisms

Temple syndrome (TS) and Kagami-Ogata syndrome (KOS) are imprinting disorders caused by absence or overexpression of genes within a single imprinted cluster on human chromosome 14q32. TS most frequently arises from maternal UPD14 or epimutations/deletions on the paternal chromosome, whereas KOS most frequently arises from paternal UPD14 or epimutations/deletions on the maternal chromosome. In this review, we describe the clinical symptoms and genetic/epigenetic features of this imprinted region. The locus encompasses paternally expressed protein-coding genes (DLK1, RTL1 and DIO3) and maternally expressed lncRNAs (MEG3/GTL2, RTL1as and MEG8), as well as numerous miRNAs and snoRNAs. Control of expression is complex, with three differentially methylated regions regulating germline, placental and tissue-specific transcription. The strong conserved synteny between mouse chromosome 12aF1 and human chromosome 14q32 has enabled the use of mouse models to elucidate imprinting mechanisms and decipher the contribution of genes to the symptoms of TS and KOS. In this review, we describe relevant mouse models and highlight their value to better inform treatment options for long-term management of TS and KOS patients.

Thyroid Hormone Deiodinases: Dynamic Switches in Developmental Transitions

Thyroid hormones exert pleiotropic, essential actions in mammalian, including human, development. These actions depend on provision of thyroid hormones in the circulation but also to a remarkable extent on deiodinase enzymes in target tissues that amplify or deplete the local concentration of the primary active form of the hormone T3 (3,5,3'-triiodothyronine), the high affinity ligand for thyroid hormone receptors. Genetic analyses in mice have revealed key roles for activating (DIO2) and inactivating (DIO3) deiodinases in cell differentiation fates and tissue maturation, ultimately promoting neonatal viability, growth, fertility, brain development, and behavior, as well as metabolic, endocrine, and sensory functions. An emerging paradigm is how the opposing activities of DIO2 and DIO3 are coordinated, providing a dynamic switch that controls the developmental timing of a tissue response, often during neonatal and maturational transitions. A second paradigm is how cell to cell communication within a tissue determines the response to T3. Deiodinases in specific cell types, often strategically located near to blood vessels that convey thyroid hormones into the tissue, can regulate neighboring cell types, suggesting a paracrine-like layer of control of T3 action. We discuss deiodinases as switches for developmental transitions and their potential to influence tissue dysfunction in human thyroid disorders.

The Role of Thyroid Hormone in Neuronal Protection

Thyroid hormone is essential for brain development and brain function in the adult. During development, thyroid hormone acts in a spatial and temporal-specific manner to regulate the expression of genes essential for normal neural cell differentiation, migration, and myelination. In the adult brain, thyroid hormone is important for maintaining normal brain function. Thyroid hormone excess, hyperthyroidism, and thyroid hormone deficiency, hypothyroidism, are associated with disordered brain function, including depression, memory loss, impaired cognitive function, irritability, and anxiety. Adequate thyroid hormone levels are required for normal brain function. Thyroid hormone acts through a cascade of signaling components: activation and inactivation by deiodinase enzymes, thyroid hormone membrane transporters, and nuclear thyroid hormone receptors. Additionally, the hypothalamic-pituitary-thyroid axis, with negative feedback of thyroid hormone on thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion, regulates serum thyroid hormone levels in a narrow range. Animal and human studies have shown both systemic and local reduction in thyroid hormone availability in neurologic disease and after brain trauma. Treatment with thyroid hormone and selective thyroid hormone analogs has resulted in a reduction in injury and improved recovery. This article will describe the thyroid hormone signal transduction pathway in the brain and the role of thyroid hormone in the aging brain, neurologic diseases, and the protective role when administered after traumatic brain injury. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Imprinted Genes and Multiple Sclerosis: What Do We Know?

Multiple sclerosis (MS) is a chronic autoimmune neurodegenerative disease of the central nervous system that arises from interplay between non-genetic and genetic risk factors. The epigenetics functions as a link between these factors, affecting gene expression in response to external influence, and therefore should be extensively studied to improve the knowledge of MS molecular mechanisms. Among others, the epigenetic mechanisms underlie the establishment of parent-of-origin effects that appear as phenotypic differences depending on whether the allele was inherited from the mother or father. The most well described manifestation of parent-of-origin effects is genomic imprinting that causes monoallelic gene expression. It becomes more obvious that disturbances in imprinted genes at the least affecting their expression do occur in MS and may be involved in its pathogenesis. In this review we will focus on the potential role of imprinted genes in MS pathogenesis.

Decreased expression of the thyroid hormone-inactivating enzyme type 3 deiodinase is associated with lower survival rates in breast cancer

Thyroid hormones (THs) are critical regulators of cellular processes, while changes in their levels impact all the hallmarks of cancer. Disturbed expression of type 3 deiodinase (DIO3), the main TH-inactivating enzyme, occurs in several human neoplasms and has been associated with adverse outcomes. Here, we investigated the patterns of DIO3 expression and its prognostic significance in breast cancer. DIO3 expression was evaluated by immunohistochemistry in a primary cohort of patients with breast cancer and validated in a second cohort using RNA sequencing data from the TCGA database. DNA methylation data were obtained from the same database. DIO3 expression was present in normal and tumoral breast tissue. Low levels of DIO3 expression were associated with increased mortality in the primary cohort. Accordingly, low DIO3 mRNA levels were associated with an increased risk of death in a multivariate model in the validation cohort. DNA methylation analysis revealed that the DIO3 gene promoter is hypermethylated in tumors when compared to normal tissue. In conclusion, DIO3 is expressed in normal and tumoral breast tissue, while decreased expression relates to poor overall survival in breast cancer patients. Finally, loss of DIO3 expression is associated with hypermethylation of the gene promoter and might have therapeutic implications.

Physiologic Significance of Epigenetic Regulation of Thyroid Hormone Target Gene Expression

BACKGROUND

In previous publications, we have reported our findings demonstrating that exposure to high maternal levels of thyroid hormones (TH) has life-long effects on the wild-type (WT, without THRB mutation) progeny of mothers with resistance to thyroid hormone beta (RTHβ). The mechanism of this epigenetic effect remains unclear.

OBJECTIVES

We reviewed the mechanisms involved in the epigenetic regulation of TH target genes and understand how they may explain the reduced sensitivity to TH in the WT progeny of RTHβ mothers.

METHODS

The availability of a large, formerly genotyped Azorean population with many individuals harboring the THRB mutation, R243Q, provided us a model to study the influence of fetal exposure to high maternal TH levels.

RESULTS

The thyroid-stimulating hormone (TSH) response in WT adults was less suppressible following the administration of L-triiodothyronine (L-T₃). This finding suggests reduced sensitivity to TH that is induced by an epigenetic mechanism resulting from exposure to high maternal levels of TH during pregnancy. The persistence of this effect across 3 generations of WT subjects favors transgenerational epigenetic inheritance. Based on preliminary studies in mice, we identified the naturally imprinted gene encoding deiodinase type 3, i.e., DIO3, as a possible mediator of this epigenetic effect through increased inactivation of TH.

CONCLUSION

Increased D3 expression and consequently increased T₃ degradation appear to be responsible for the reduced sensitivity of the anterior pituitary to administered L-T₃. The imprinted DIO3 gene may be a candidate gene that mediates the epigenetic effect induced by exposure to high maternal levels of TH. However, we cannot exclude the role of other TH-responsive genes.

Thyroid hormone action in the developing testis: intergenerational epigenetics

Male fertility involves the successful transmission of the genetic code to the next generation. It requires appropriately timed cellular processes during testis development, adequate support of spermatogenesis by hormonal cues from the reproductive axis and cellular cross-talk between germ and somatic cells. In addition to being the vessel of the father’s genome, increasing evidence shows that the mature sperm carries valuable epigenetic information – the epigenome – that, after fecundation, influences the development of the next generation, affecting biological traits and disease susceptibility. The epigenome of the germ line is susceptible to environmental factors, including exogenous chemicals and diet, but it is also affected by endogenous molecules and pathophysiological conditions. Factors affecting testis development and the epigenetic information of the germ line are critical for fertility and of relevance to the non-genetic but heritable component in the etiology of complex conditions. Thyroid hormones are one of those factors and their action, when untimely, produces profound effects on the developing testis, affecting spermatogenesis, steroidogenesis, testis size, reproductive hormones and fertility. Altered thyroid hormone states can also change the epigenetic information of the male germ line, with phenotypic consequences for future generations. In the context of past literature concerning the consequences of altered thyroid hormone action for testis development, here we review recent findings about the pathophysiological roles of the principal determinants of testicular thyroid hormone action. We also discuss limited work on the effects of thyroid hormone on the male germ line epigenome and the implications for the intergenerational transmission of phenotypes via epigenetic mechanisms.

Reduced Sensitivity to Thyroid Hormone as a Transgenerational Epigenetic Marker Transmitted Along the Human Male Line

Background: Evidence for transgenerational epigenetic inheritance in humans is still controversial, given the requirement to demonstrate persistence of the phenotype across three generations. A previous study showed that exposure of human and mouse embryos to high maternal thyroid hormone (TH) concentrations not only affects the newborns but also subsequently reduces thyrotroph sensitivity to TH during adult life. The current investigation set out to determine if this epigenetic effect is transmitted by humans not exposed in utero to high TH levels to their offspring. Methods: The study took advantage of the high frequency of intrauterine exposure to high TH in the Azorean wild-type population born to healthy mothers with high TH levels because of a heterozygous TH receptor beta gene mutation. Wild-type individuals from F2 (second) and F3 (third) generations were studied, whose parents and grandparents, respectively, were not exposed to high maternal TH levels. Twenty-six individuals belonging to 17 nuclear families were tested for their sensitivity to TH using their thyrotropin (TSH) response to thyrotropin-releasing hormone (TRH) after administration of liothyronine (LT3). Results: Preservation of reduced sensitivity to TH (RSTH) was found in descendants of males but not of females with likewise RSTH. In F2, offspring of fathers but not of mothers exposed to high TH levels had RSTH (TRH-stimulated TSH of 6.39 ± 0.63 vs 1.58 ± 0.41 mIU/L [p < 0.001], respectively, after treatment with LT3). In F3, whose parents nor themselves were exposed to TH excess during their fetal life, descendants of fathers and not mothers had RSTH (TRH-stimulated TSH of 4.60 ± 0.61 vs 1.37 ± 0.23 mIU/L [p < 0.01], respectively, after pretreatment with LT3). Conclusions: Since intrauterine total body and gonadal exposure to elevated TH can potentially affect only the F1 and F2, respectively, the results obtained from F3 confirm a true inheritance of an epigenetic effect, scarcely observed in humans. While the exact mechanism underlying the inheritance of this epigenetic effect remains unknown, it correlates with type 3 deiodinase overexpression demonstrated in pituitary glands of mice born to dams with high TH. This enzyme inactivates TH, and is encoded by an imprinted gene with specific parent of origin expression.

The Type 3 Deiodinase: Epigenetic Control of Brain Thyroid Hormone Action and Neurological Function

Thyroid hormones (THs) influence multiple processes in the developing and adult central nervous system, and their local availability needs to be maintained at levels that are tailored to the requirements of their biological targets. The local complement of TH transporters, deiodinase enzymes, and receptors is critical to ensure specific levels of TH action in neural cells. The type 3 iodothyronine deiodinase (DIO3) inactivates THs and is highly present in the developing and adult brain, where it limits their availability and action. DIO3 deficiency in mice results in a host of neurodevelopmental and behavioral abnormalities, demonstrating the deleterious effects of TH excess, and revealing the critical role of DIO3 in the regulation of TH action in the brain. The fact the Dio3 is an imprinted gene and that its allelic expression pattern varies across brain regions and during development introduces an additional level of control to deliver specific levels of hormone action in the central nervous system (CNS). The sensitive epigenetic nature of the mechanisms controlling the genomic imprinting of Dio3 renders brain TH action particularly susceptible to disruption due to exogenous treatments and environmental exposures, with potential implications for the etiology of human neurodevelopmental disorders.

Fetal Exposure to High Maternal Thyroid Hormone Levels Causes Central Resistance to Thyroid Hormone in Adult Humans and Mice

Context

Fetuses exposed to the high thyroid hormone (TH) levels of mothers with resistance to thyroid hormone beta (RTH-β), due to mutations in the THRB gene, have low birth weight and suppressed TSH.

Objective

Determine if such exposure to high TH levels in embryonic life has a long-term effect into adulthood.

Design

Observations in humans with a parallel design on animals to obtain a preliminary information regarding mechanism.

Setting

University research centers.

Patients or other participants

Humans and mice with no RTH-β exposed during intrauterine life to high TH levels from mothers who were euthyroid due to RTH-β. Controls were humans and mice of the same genotype but born to fathers with RTH-β and mothers without RTH-β and thus, with normal serum TH levels.

Interventions

TSH responses to stimulation with thyrotropin-releasing hormone (TRH) during adult life in humans and male mice before and after treatment with triiodothyronine (T3). We also measured gene expression in anterior pituitaries, hypothalami, and cerebral cortices of mice.

Results

Adult humans and mice without RTH-β, exposed to high maternal TH in utero, showed persistent central resistance to TH, as evidenced by reduced responses of serum TSH to TRH when treated with T3. In mice, anterior pituitary TSH-β and deiodinase 3 (D3) mRNAs, but not hypothalamic and cerebral cortex D3, were increased.

Conclusions

Adult humans and mice without RTH-β exposed in utero to high maternal TH levels have persistent central resistance to TH. This is likely mediated by the increased expression of D3 in the anterior pituitary, enhancing local T3 degradation.

Decreased anxiety- and depression-like behaviors and hyperactivity in a type 3 deiodinase-deficient mouse showing brain thyrotoxicosis and peripheral hypothyroidism

Hypo- and hyperthyroid states, as well as functional abnormalities in the hypothalamic-pituitary-thyroid axis have been associated with psychiatric conditions like anxiety and depression. However, the nature of this relationship is poorly understood since it is difficult to ascertain the thyroid status of the brain in humans. Data from animal models indicate that the brain exhibits efficient homeostatic mechanisms that maintain local levels of the active thyroid hormone, triiodothyronine (T3) within a narrow range. To better understand the consequences of peripheral and central thyroid status for mood-related behaviors, we used a mouse model of type 3 deiodinase (DIO3) deficiency (Dio3 -/- mouse). This enzyme inactivates thyroid hormone and is highly expressed in the adult central nervous system. Adult Dio3 -/- mice exhibit elevated levels of T3-dependent gene expression in the brain, despite peripheral hypothyroidism as indicated by low circulating levels of thyroxine and T3. Dio3 -/- mice of both sexes exhibit hyperactivity and significantly decreased anxiety-like behavior, as measured by longer time spent in the open arms of the elevated plus maze and in the light area of the light/dark box. During the tail suspension, they stayed immobile for a significantly shorter time than their wild-type littermates, suggesting decreased depression-like behavior. These results indicate that increased thyroid hormone in the brain, not necessarily in peripheral tissues, correlates with hyperactivity and with decreases in anxiety and depression-like behaviors. Our results also underscore the importance of DIO3 as a determinant of behavior by locally regulating the brain levels of thyroid hormone.