Genetic variation in thyroid hormone transporters

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.

Lactobacillus for ribosome peptide editing cancer

This study reviews newly discovered insect peptide point mutations as new possible cancer research targets. To interpret newly discovered peptide point mutations in insects as new possible cancer research targets, we focused on the numerous peptide changes found in the 'CSP' family on the sex pheromone gland of the female silkworm moth Bombyx mori. We predict that the Bombyx peptide modifications will have a significant effect on cancer CUP (cancers of unknown primary) therapy and that bacterial peptide editing techniques, specifically Lactobacillus combined to CRISPR, will be used to regulate ribosomes and treat cancer in humans.

The relevance of T3 in the management of hypothyroidism

Levothyroxine monotherapy has been the standard of care for treatment of hypothyroidism for more than 40 years. However, patients treated with levothyroxine have relatively lower serum tri-iodothyronine (T₃) concentrations than the general population, and symptoms of hypothyroidism persist for some patients despite normalisation of thyroid-stimulating hormone (TSH) concentrations. The understanding that maintenance of normal T₃ concentrations is the priority for the thyroid axis has redirected the clinical focus to serum T₃ concentrations in patients with hypothyroidism. This Personal View explores whether it is currently feasible to identify patients who could be considered for liothyronine supplementation in combination with levothyroxine. Genetic profiling stands out as a potential future tool to identify patients who do not respond well to levothyroxine due to suboptimal peripheral thyroxine (T₄) activation. Moreover, new slow-release liothyronine preparations are being developed to be trialled in these symptomatic patients, in an attempt to restore T₃ concentrations and provide conclusive results for the use of T₄ plus T₃ combination therapy.

Monocarboxylate Transporters (SLC16): Function, Regulation, and Role in Health and Disease

The solute carrier family 16 (SLC16) is comprised of 14 members of the monocarboxylate transporter (MCT) family that play an essential role in the transport of important cell nutrients and for cellular metabolism and pH regulation. MCTs 1-4 have been extensively studied and are involved in the proton-dependent transport of L-lactate, pyruvate, short-chain fatty acids, and monocarboxylate drugs in a wide variety of tissues. MCTs 1 and 4 are overexpressed in a number of cancers, and current investigations have focused on transporter inhibition as a novel therapeutic strategy in cancers. MCT1 has also been used in strategies aimed at enhancing drug absorption due to its high expression in the intestine. Other MCT isoforms are less well characterized, but ongoing studies indicate that MCT6 transports xenobiotics such as bumetanide, nateglinide, and probenecid, whereas MCT7 has been characterized as a transporter of ketone bodies. MCT8 and MCT10 transport thyroid hormones, and recently, MCT9 has been characterized as a carnitine efflux transporter and MCT12 as a creatine transporter. Expressed at the blood brain barrier, MCT8 mutations have been associated with an X-linked intellectual disability, known as Allan-Herndon-Dudley syndrome. Many MCT isoforms are associated with hormone, lipid, and glucose homeostasis, and recent research has focused on their potential roles in disease, with MCTs representing promising novel therapeutic targets. This review will provide a summary of the current literature focusing on the characterization, function, and regulation of the MCT family isoforms and on their roles in drug disposition and in health and disease. SIGNIFICANCE STATEMENT: The 14-member solute carrier family 16 of monocarboxylate transporters (MCTs) plays a fundamental role in maintaining intracellular concentrations of a broad range of important endogenous molecules in health and disease. MCTs 1, 2, and 4 (L-lactate transporters) are overexpressed in cancers and represent a novel therapeutic target in cancer. Recent studies have highlighted the importance of MCTs in glucose, lipid, and hormone homeostasis, including MCT8 in thyroid hormone brain uptake, MCT12 in carnitine transport, and MCT11 in type 2 diabetes.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

Hypothyroid Patients Encoding Combined MCT10 and DIO2 Gene Polymorphisms May Prefer L-T3 + L-T4 Combination Treatment - Data Using a Blind, Randomized, Clinical Study

OBJECTIVES

In previous studies, around half of all hypothyroid patients preferred levo-thyroxine (L-T4) + levo-triiodothyronine (L-T3) combination therapy, 25% preferred T4, and 25% had no preference. The reason for this is yet to be explored.

METHODS

A total of 45 overtly autoimmune, hypothyroid patients - now euthyroid on ≥6 months' L-T4 therapy - participated in a prospective, double-blind, cross-over study. The patients were randomized into 2 groups of either 3 continuous months' L-T4 therapy followed by 3 months' combination therapy or vice versa. In all periods, 50 μg L-T4 was blindly replaced by either (identical) 50 μg L-T4 or by 20 μg T3. L-T4 was hereafter adjusted to obtain normal serum TSH values. We investigated 3 single nucleotide polymorphisms (SNPs) on the type II iodothyronine deiodinase (DIO2) gene (rs225014 (Thr92Ala), rs225015, and rs12885300 (ORFa-Gly3Asp)) and 1 SNP on the cellular membrane transport-facilitating monocarboxylate transporter (MCT10) gene (rs17606253), and asked in which of the 2 treatment periods patients felt better (i.e., which treatment was preferred).

RESULTS

27 out of 45 patients (60%) preferred the combination therapy. Two polymorphisms (rs225014 (DIO2, Thr92Ala) and rs17606253 (MCT10)) were combined yielding 3 groups: none vs. 1 of 2 vs. both SNPs present, and 42 vs. 63 vs. 100% of our patients in the 3 groups preferred the combined treatment (Jongheere-Terpstra trend test, p = 0.009).

CONCLUSION

The present study indicates that the combination of polymorphisms in DIO2 (rs225014) and MCT10 (rs17606253) enhances hypothyroid patients' preference for L-T4 + L-T3 replacement therapy. In the future, combination therapy may be restricted or may be even recommended to individuals harbouring certain polymorphisms.

Genetics of thyroid function

Recent studies show that subtle variations in thyroid function, including subclinical thyroid dysfunction, and even variation in thyroid function within the normal range, are associated with morbidity and mortality. It is estimated that 40-65% of the inter-individual variation in serum TSH and FT4 levels is determined by genetic factors. To identify these factors, various linkage and candidate gene studies have been performed in the past, which have identified only a few genes. In the last decade, genome-wide association studies identified many new genes, while recent whole-genome sequencing efforts have also been proven to be effective. In the current review, we provide a systematic overview of these studies, including strengths and limitations. We discuss new techniques which will further clarify the genetic basis of thyroid function in the near future, as well as the potential use of these genetic markers in personalizing the management of thyroid disease patients.

Genetic determination of the hypothalamic-pituitary-thyroid axis: where do we stand?

For a long time it has been known that both hypo- and hyperthyroidism are associated with an increased risk of morbidity and mortality. In recent years, it has also become clear that minor variations in thyroid function, including subclinical dysfunction and variation in thyroid function within the reference range, can have important effects on clinical endpoints, such as bone mineral density, depression, metabolic syndrome, and cardiovascular mortality. Serum thyroid parameters show substantial interindividual variability, whereas the intraindividual variability lies within a narrow range. This suggests that every individual has a unique hypothalamus-pituitary-thyroid axis setpoint that is mainly determined by genetic factors, and this heritability has been estimated to be 40-60%. Various mutations in thyroid hormone pathway genes have been identified in persons with thyroid dysfunction or altered thyroid function tests. Because these causes are rare, many candidate gene and linkage studies have been performed over the years to identify more common variants (polymorphisms) associated with thyroid (dys)function, but only a limited number of consistent associations have been found. However, in the past 5 years, advances in genetic research have led to the identification of a large number of new candidate genes. In this review, we provide an overview of the current knowledge about the polygenic basis of thyroid (dys)function. This includes new candidate genes identified by genome-wide approaches, what insights these genes provide into the genetic basis of thyroid (dys)function, and which new techniques will help to further decipher the genetic basis of thyroid (dys)function in the near future.

THYROID FUNCTION IN PREGNANCY: MATERNAL AND FETAL OUTCOMES WITH HYPOTHYROIDISM AND SUBCLINICAL THYROID DYSFUNCTION

Thyroid hormones are important in the development of the fetus and the placenta as well as in maintaining maternal wellbeing. Thyroid disorders are common in the population as a whole, particularly in women, and therefore are common during pregnancy and the puerperium. Biochemical derangement of thyroid function tests are present in approximately 2.5–5% of pregnant women.

Minireview: thyroid hormone transporters: the knowns and the unknowns

The effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field.

Retinoic acid induces expression of the thyroid hormone transporter, monocarboxylate transporter 8 (Mct8)

Retinoic acid (RA) and thyroid hormone are critical for differentiation and organogenesis in the embryo. Mct8 (monocarboxylate transporter 8), expressed predominantly in the brain and placenta, mediates thyroid hormone uptake from the circulation and is required for normal neural development. RA induces differentiation of F9 mouse teratocarcinoma cells toward neurons as well as extraembryonal endoderm. We hypothesized that Mct8 is functionally expressed in F9 cells and induced by RA. All-trans-RA (tRA) and other RA receptor (RAR) agonists dramatically (>300-fold) induced Mct8. tRA treatment significantly increased uptake of triiodothyronine and thyroxine (4.1- and 4.3-fold, respectively), which was abolished by a selective Mct8 inhibitor, bromosulfophthalein. Sequence inspection of the Mct8 promoter region and 5'-rapid amplification of cDNA ends PCR analysis in F9 cells identified 11 transcription start sites and a proximal Sp1 site but no TATA box. tRA significantly enhanced Mct8 promoter activity through a consensus RA-responsive element located 6.6 kilobases upstream of the coding region. A chromatin immunoprecipitation assay demonstrated binding of RAR and retinoid X receptor to the RA response element. The promotion of thyroid hormone uptake through the transcriptional up-regulation of Mct8 by RAR is likely to be important for extraembryonic endoderm development and neural differentiation. This finding demonstrates cross-talk between RA signaling and thyroid hormone signaling in early development at the level of the thyroid hormone transporter.

Prevalence and functional analysis of the S107P polymorphism (rs6647476) of the monocarboxylate transporter 8 (SLC16A2) gene in the male population of north-west Spain (Galicia)

OBJECTIVE

Mutations in SLC16A2, the gene encoding the thyroid hormone (TH)-specific transporter monocarboxylate transporter 8 (MCT8), result in a thyroid phenotype and severe mental retardation caused by neuronal TH deficiency. These mutational effects raise the question of whether polymorphic variation in SLC16A2 may also be associated with differences in serum levels of TH and/or TSH.

DESIGN

This is the first major study of the frequency of the SLC16A2 rs6647476 single nucleotide polymorphism (SNP) (amino acid change Ser107Pro). We also studied the relationships of SLC16A2 genetic variants with serum levels of TSH, T4 and T3, with their mRNA expression and with expression of the TH-responsive genes ZAKI-4 and BTEB in white blood cells. Experiments in cultured fibroblasts were carried out to ascertain the dynamics of the T3 response.

METHODS

A total of 276 men were studied. Genotyping of the S107P SNP was carried out using polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP); serum hormone levels were determined by chemiluminescence; expression of mRNA was quantified by real-time PCR.

RESULTS

The SLC16A2 S107P SNP was found in 36% of Galician males. With the present sample size we did not find any association of this polymorphism with variability in serum levels of TSH, free T4 (fT4) or fT3, or with basal expression of mRNA for SLC16A2 or the two TH-responsive genes ZAKI-4 and BTEB, either in white blood cells or in cultured human fibroblasts from either Ser107 or Pro107 genotypes under T3 stimulation.

CONCLUSIONS

The S107P change in MCT8 is frequent in the male population in Galicia. In the population studied in this report an association with a thyroid phenotype was not demonstrated, even though the S107P SNP causes an important amino acid change.

Novel insights into thyroid hormones from the study of common genetic variation

Effects of thyroid hormones in individual tissues are determined by many factors beyond their serum levels, including local deiodination and expression and activity of thyroid hormone transporters. These effects are difficult to examine by traditional techniques, but a novel approach that exploits the existence of common genetic variants has yielded new and surprising insights. Convincing evidence indicates a role of type 1 iodothyronine deiodinase (D1) in determining the serum T(4):T(3) ratio and a role of phosphodiesterase 8B in determining TSH levels. In addition, studies of type 2 iodothyronine deiodinase (D2) variants have shown that thyroid hormones contribute to osteoarthritis and these variants influence Intelligence quotient alterations associated with iodine deficiency. Preliminary evidence suggests associations between TSH-receptor variants and fasting glucose level, D1 variants and insulin-like growth factor I production, and D2 variants and hypertension, psychological well-being and response to T(3) or T(4) treatment. Intriguingly, most of these associations are independent of serum thyroid hormone levels, which highlights the importance of local regulation of thyroid hormones in tissues. Future research might reveal novel roles for thyroid hormones in obesity, cardiovascular disease, osteoporosis and depression and could have implications for interpretation of thyroid function tests and individualization of thyroid hormone replacement therapy.

Identification and consequences of polymorphisms in the thyroid hormone receptor alpha and beta genes

OBJECTIVE

Genetic factors exert considerable influence on thyroid function variables. Single nucleotide polymorphisms (SNPs) in thyroid hormone pathway genes have been associated with serum thyroid parameters implying small alterations in the hypothalamus-pituitary-thyroid axis. However, little is known about SNPs in the THRA (17q11.2) and THRB (3p24.2) genes. The aim of this study was to map THRA and THRB for the occurrence and frequencies of SNPs and relate these to thyroid parameters.

DESIGN AND METHODS

SNPs were identified by sequencing all THRA and THRB exons and flanking regions in 52 randomly selected subjects. SNPs were genotyped in 1116 healthy Danish twins by TaqMan assays and related to thyroid parameters. One SNP in THRB was additionally genotyped in the elderly population of the Rotterdam Scan Study (n = 940).

MAIN OUTCOME

15 SNPs (7 novel) in THRA and THRB were identified. Two SNPs in the 3' untranslated region of THRA were genotyped: a novel SNP (2390A/G) and 1895C/A (rs12939700). In THRB, a synonymous (735C/T; rs3752874) and an intronic SNP (in9-G/A; rs13063628) were genotyped. No associations between SNPs and thyroid hormone levels (total and free 3,3',5-triiodo-L-thyronine [T3] and thyroxine, reverse T3) were found. THRB-in9-G/A was significantly associated with higher serum thyroid stimulating hormone (TSH) (p(lnTSH) = 0.01) in the Danish twins, but not in subjects of the Rotterdam Scan Study, although it showed a similar trend.

CONCLUSIONS

Analysis of the T3 receptor genes revealed 15 SNPs, including 7 novel. Only THRB-in9-G/A was associated with higher serum TSH in a large population of Danish twins.

Genetic loci linked to pituitary-thyroid axis set points: a genome-wide scan of a large twin cohort

OBJECTIVE

Previous studies have shown that circulating concentrations of TSH, free T4, and free T3 are genetically regulated, but the genes responsible remain largely unknown. The aim of this study was to identify genetic loci associated with these parameters.

DESIGN

We performed a multipoint, nonparametric genome-wide linkage scan of 613 female dizygotic twin pairs. All subjects were euthyroid (TSH 0.4-4.0 mU/liter) with negative thyroid peroxidase antibodies and no history of thyroid disease. The genome scan comprised 737 microsatellite markers supplemented with dinucleotide markers. Data were analyzed using residualized thyroid hormone data after adjustment for age, smoking, and body mass index.

RESULTS

Multipoint linkage analysis gave linkage peaks for free T4 on chromosome 14q13 and 18q21 [logarithm of odds (LOD) 2.4-3.2]; TSH on chromosomes 2q36, 4q32, and 9q34 (LOD 2.1-3.2); and free T3 on chromosomes 7q36, 8q22, and 18q21 (LOD 2.0-2.3).

CONCLUSIONS

This study has identified eight genomic locations with linkage of LOD of 2.0 or greater. These results should enable targeted positional candidate and positional cloning studies to advance our understanding of genetic control of the pituitary-thyroid axis.

A common variation in deiodinase 1 gene DIO1 is associated with the relative levels of free thyroxine and triiodothyronine

INTRODUCTION

Genetic factors influence circulating thyroid hormone levels, but the common gene variants involved have not been conclusively identified. The genes encoding the iodothyronine deiodinases are good candidates because they alter the balance of thyroid hormones. We aimed to thoroughly examine the role of common variation across the three deiodinase genes in relation to thyroid hormones.

METHODS

We used HapMap data to select single-nucleotide polymorphisms (SNPs) that captured a large proportion of the common genetic variation across the three deiodinase genes. We analyzed these initially in a cohort of 552 people on T(4) replacement. Suggestive findings were taken forward into three additional studies in people not on T(4) (total n = 2513) and metaanalyzed for confirmation.

RESULTS

A SNP in the DIO1 gene, rs2235544, was associated with the free T(3) to free T(4) ratio with genome-wide levels of significance (P = 3.6 x 10(-13)). The C-allele of this SNP was associated with increased deiodinase 1 (D1) function with resulting increase in free T(3)/T(4) ratio and free T(3) and decrease in free T(4) and rT(3). There was no effect on serum TSH levels. None of the SNPs in the genes coding for D2 or D3 had any influence on hormone levels.

CONCLUSIONS

This study provides convincing evidence that common genetic variation in DIO1 alters deiodinase function, resulting in an alteration in the balance of circulating free T(3) to free T(4). This should prove a valuable tool to assess the relative effects of circulating free T(3) vs. free T(4) on a wide range of biological parameters.

Thyroid hormone transport in and out of cells

Thyroid hormone (TH) is essential for the proper development of numerous tissues, notably the brain. TH acts mostly intracellularly, which requires transport by TH transporters across the plasma membrane. Although several transporter families have been identified, only monocarboxylate transporter (MCT)8, MCT10 and organic anion-transporting polypeptide (OATP)1C1 demonstrate a high degree of specificity towards TH. Recently, the biological importance of MCT8 has been elucidated. Mutations in MCT8 are associated with elevated serum T(3) levels and severe psychomotor retardation, indicating a pivotal role for MCT8 in brain development. MCT8 knockout mice lack neurological damage, but mimic TH abnormalities of MCT8 patients. The exact pathophysiological mechanisms in MCT8 patients remain to be elucidated fully. Future research will probably identify novel TH transporters and disorders based on TH transporter defects.