Long-Term Efficacy of T3 Analogue Triac in Children and Adults With MCT8 Deficiency: A Real-Life Retrospective Cohort Study

Novel SLC16A2 Frameshift Mutation as a Cause of Allan-Herndon-Dudley Syndrome and its Implications for Carrier Screening

Background

Allan-Herndon-Dudley syndrome (AHDS) is a rare X-linked neurodevelopmental disorder caused by mutations in the solute carrier family 16-member 2 (SLC16A2) gene. This syndrome leads to significant psychomotor disabilities, thyroid dysfunction, and abnormal brain development. This case report describes the genetic cause of AHDS in a male proband and to expanding the mutation spectrum of the SLC16A2 gene.

Methods

A blood specimen was collected from a one-year-old child with delayed development and abnormal thyroid function and this was followed by whole-exome sequencing (WES) was performed on the proband to identify potential genetic mutations. Sanger sequencing was subsequently used to confirm the findings and determine the inheritance pattern of the mutation within the family.

Results

The proband, who presented with developmental delay, thyroid dysfunction, and abnormal brain development, was found to have a novel hemizygous frameshift mutation, c.513_538del (p.Ile172Cysfs*60), in the SLC16A2 gene (NM_006517.5). This mutation was inherited from his asymptomatic mother, confirming the X-linked inheritance pattern. The mutation is classified as likely pathogenic, contributing to the clinical presentation observed in the proband.

Conclusion

This study identified a novel frameshift mutation in the SLC16A2 gene associated with AHDS, thereby expanding the known mutation spectrum of this gene. Given the significant impact of AHDS on neural development and hormone secretion, it is recommended that this gene be included in carrier screening panels in China, particularly for families with a history of related neurodevelopmental disorders.

Allan-Herndon-Dudley Syndrome

X-linked MCT 8 mutations cause Allan-Herndon-Dudley syndrome (AHDS), characterized by severe developmental delay and specific thyroid function abnormality. The report describes a 2-y-old boy who presented with severe developmental delay, generalized hypotonia and thyroid function abnormality (high FT3, low FT4 and normal TSH) suggesting a form of impaired thyroid hormone sensitivity. Whole exome sequencing (WES) analysis revealed mutation in exon 3 of MCT 8 gene. Improvement of neurodevelopmental delay and clinical and biochemical features of thyrotoxicosis occurs with use of tri iodoacetic acid.

Mapping variants in thyroid hormone transporter MCT8 to disease severity by genomic, phenotypic, functional, structural and deep learning integration

Predicting and quantifying phenotypic consequences of genetic variants in rare disorders is a major challenge, particularly pertinent for 'actionable' genes such as thyroid hormone transporter MCT8 (encoded by the X-linked SLC16A2 gene), where loss-of-function (LoF) variants cause a rare neurodevelopmental and (treatable) metabolic disorder in males. The combination of deep phenotyping data with functional and computational tests and with outcomes in population cohorts, enabled us to: (i) identify the genetic aetiology of divergent clinical phenotypes of MCT8 deficiency with genotype-phenotype relationships present across survival and 24 out of 32 disease features; (ii) demonstrate a mild phenocopy in ~400,000 individuals with common genetic variants in MCT8; (iii) assess therapeutic effectiveness, which did not differ among LoF-categories; (iv) advance structural insights in normal and mutated MCT8 by delineating seven critical functional domains; (v) create a pathogenicity-severity MCT8 variant classifier that accurately predicted pathogenicity (AUC:0.91) and severity (AUC:0.86) for 8151 variants. Our information-dense mapping provides a generalizable approach to advance multiple dimensions of rare genetic disorders.

Swallowing Assessment in a Pediatric Case of Allan-Herndon-Dudley Syndrome (MCT8 Deficiency): Advanced Insights into Dysphagia via Flexible Endoscopic Evaluation of Swallowing

Patients with MCT8 deficiency often present with underweight and are prone to frequent pulmonary infections, including aspiration pneumonia. Despite commonly reported swallowing difficulties in this population, specific dysphagia symptoms have not been well-documented. We conducted a flexible endoscopic evaluation of swallowing (FEES) on a young boy diagnosed with MCT8 deficiency, who exhibited recurrent pulmonary infections and failed to achieve substantial weight gain despite an oral energy intake appropriate for his age and height. The FEES revealed generally weakened swallowing mechanisms, characterized by prolonged swallow and cough sequences, along with penetration and aspiration of both fluid and semi-solid test boluses. Given the considerable effort associated with oral intake, we hypothesize that dysphagia contributes to his underweight status, alongside peripheral thyrotoxicosis. In conclusion, FEES proved to be an invaluable tool in identifying underlying swallowing impairments and assessing the need for gastrostomy in this patient. For MCT8 deficiency, patients presenting with underweight, frequent pulmonary infections, and swallowing difficulties, it is recommended that diagnostic evaluations include FEES to thoroughly assess their swallowing function and airway protection.

Clinical and genetic characteristics of patients with monocarboxylate transporter-8 deficiency: a multicentre retrospective study

Allan-Herndon-Dudley syndrome is a neurodevelopmental disorder characterized by motor and intellectual disabilities. Despite its rarity, there has been a rise in interest due to ongoing research and emerging therapy suggestions. In this multicenter, retrospective, cross-sectional study, the genetic characteristics and clinical data of twenty-one cases of genetically confirmed MCT8 deficiency were evaluated. The median age at the diagnosis was 2.4 (1.29; 5.9) years, which ranged from 0.5 to 14.0 years. The median follow-up period was 2.34 years, ranging from four months to 7.9 years. In 21 patients, 17 different variants were detected in the SLC16A2 gene. Eleven of these variants (c.1456delC, c.439G > T, c.949C > A, c.1392dupC, c.1612C > T, c.407dup, c.781del, c.589C > A, c.712G > A, c.311 T > A, c.1461del) have not been previously reported. In this study, with the exception of three cases with fT3/fT4 ratios of 4.95, 3.58, and 4.52, all cases exhibited fT3/fT4 ratios higher than five (9.9 (7.9; 12.0)).

CONCLUSION

MCT8 deficiency is a rare and devastating disorder characterized by central hypothyroidism and peripheral thyrotoxicosis. The fT3/fT4 ratio can be used as a useful diagnostic indicator of MCT8 deficiency in males with mental and motor retardation. There is a need to raise clinicians' awareness of this potentially treatable condition with the emergence of new and promising treatments.

WHAT IS KNOWN

• Allan-Herndon-Dudley syndrome, also known as MCT8 deficiency is a rare and devastating disorder characterized by central hypothyroidism and peripheral thyrotoxicosis.

WHAT IS NEW

• In this study, seventeen different variants were detected in the SLC16A2 gene, eleven of which (c.1456delC; c.439G>T; c.949C>A; c.1392dupC; c.1612C>T; c.407dup; c.781del; c.589C>A; c.712G>A; c.311T>A; c.1461del) have not been reported before. • The fT3/fT4 ratio can be used as a useful diagnostic indicator of MCT8 deficiency in males with mental and motor retardation.

Combined Levothyroxine and Propylthiouracil Treatment in Children with Monocarboxylate Transporter 8 Deficiency: A Multicenter Case Series of 12 Patients

Objective: To evaluate the combined administration of propylthiouracil (PTU) and levothyroxine (LT4) in managing monocarboxylate transporter 8 (MCT8) deficiency and identify optimal therapeutic dosages. Methods: This multicenter case series involved 12 male patients with MCT8 deficiency whose parents/guardians consented to PTU and LT4 treatment. Data were collected from January 2008 to June 24, 2024. The study focused on treatment safety and outcomes, analyzing baseline and last encounter biochemical, metabolic, and anthropometric parameters. Statistical analyses included Wilcoxon signed ranks tests and generalized estimated equations to assess effects on thyroid and metabolic markers, and receiver operating characteristics curves to predict optimal dose. Results: Patients showed a significant reduction in serum total triiodothyronine (TT3) concentration and TT3/TT4 ratio, with increased serum TT4 and free T4 (fT4) concentrations. The use of PTU effectively reduced TT3 concentration by 25% at an average dose of 6.8 mg/kg/day, while LT4 increased fT4 concentration by 40% from baseline at an average dose of 4.3 µg/kg/day. Thyrotropin concentration was undetectable on treatment. No statistical differences were observed in metabolic and physical parameters between baseline and last encounter overall for the group, but six of eight patients for whom these data were available had an increase in weight (z-score). There were no adverse effects on liver function or granulocyte numbers noted throughout the period of observation. Conclusion: Combined treatment with PTU and LT4 normalized serum T3, fT4, and TT4 in patients with MCT8 deficiency. Individualized dose adjustments were crucial for achieving therapeutic goals, indicating the need for personalized treatment plans.

3,3',5-Triiodothyroacetic Acid Transporters

Introduction: Thyroid hormone transporters are essential for thyroid hormones to enter target cells. Monocarboxylate transporter (MCT) 8 is a key transporter and is expressed at the blood-brain barrier (BBB), in neural cells and many other tissues. Patients with MCT8 deficiency have severe neurodevelopmental delays because of cerebral hypothyroidism and chronic sequelae of peripheral thyrotoxicosis. The T3 analog 3,3',5-triiodothyroacetic acid (TRIAC) rescued neurodevelopmental features in animal models mimicking MCT8 deficiency and improved key metabolic features in patients with MCT8 deficiency. However, the identity of the transporter(s) that facilitate TRIAC transport are unknown. Here, we screened candidate transporters that are expressed at the human BBB and/or brain-cerebrospinal fluid barrier and known thyroid hormone transporters for TRIAC transport. Materials and Methods: Plasma membrane expression was determined by cell surface biotinylation assays. Intracellular accumulation of 1 nM TRIAC was assessed in COS-1 cells expressing candidate transporters in Dulbecco's phosphate-buffered saline (DPBS)/0.1% glucose or Dulbecco's modified Eagle's medium (DMEM) with or without 0.1% bovine serum albumin (BSA). Expression of Slc22a8 was determined by fluorescent in situ hybridization in brain sections from wild-type and Mct8/Oatp1c1 knockout mice at postnatal days 12, 21, and 120. Results: In total, 59 plasma membrane transporters were selected for screening of TRIAC accumulation (n = 40 based on expression at the human BBB and/or brain-cerebrospinal fluid barrier and having small organic molecules as substrates; n = 19 known thyroid hormone transporters). Screening of the selected transporter panel showed that 18 transporters facilitated significant intracellular accumulation of TRIAC in DPBS/0.1% glucose or DMEM in the absence of BSA. In the presence of BSA, substantial transport was noted for SLCO1B1 and SLC22A8 (in DPBS/0.1% glucose and DMEM) and SLC10A1, SLC22A6, and SLC22A24 (in DMEM). The zebrafish and mouse orthologs of these transporters similarly facilitated intracellular accumulation of TRIAC. Highest Slc22a8 mRNA expression was detected in mouse brain capillary endothelial cells and choroid plexus epithelial cells at early postnatal time points, but was reduced at P120. Conclusions: Human SLC10A1, SLCO1B1, SLC22A6, SLC22A8, and SLC22A24 as well as their mouse and zebrafish orthologs are efficient TRIAC transporters. These findings contribute to the understanding of TRIAC treatment in patients with MCT8 deficiency and animal models thereof.

Unmet patient needs in monocarboxylate transporter 8 (MCT8) deficiency: a review

Monocarboxylate transporter 8 (MCT8) deficiency is a rare, X-linked disorder arising from mutations in the SLC16A2 gene and resulting from dysfunctional thyroid hormone transport. This disorder is characterized by profound neurodevelopmental delay and motor disability due to a lack of thyroid hormone in the brain, and coexisting endocrinological symptoms, due to chronic thyrotoxicosis, resulting from elevated thyroid hormone outside the central nervous system (CNS). In February 2024, we reviewed the published literature to identify relevant articles reporting on the current unmet needs of patients with MCT8 deficiency. There are several main challenges in the diagnosis and treatment of MCT8 deficiency, with decreased awareness and recognition of MCT8 deficiency among healthcare professionals (HCPs) associated with misdiagnosis and delays in diagnosis. Diagnostic delay may also be attributed to other factors, including the complex symptomology of MCT8 deficiency only becoming apparent several months after birth and pathognomonic serum triiodothyronine (T3) testing not being routinely performed. For patients with MCT8 deficiency, multidisciplinary team care is vital to optimize the support provided to patients and their caregivers. Although there are currently no approved treatments specifically for MCT8 deficiency, earlier identification and diagnosis of this disorder enables earlier access to supportive care and developing treatments focused on improving outcomes and quality of life for both patients and caregivers.

Identification of a novel nonsense SLC16A2 gene mutation in an infant with severe neurologic phenotype: A case report

RATIONALE

Allan-Herndon-Dudley syndrome (AHDS) results from a pathogenic variant in the hemizygous subunit of the SLC16A2 gene, which encodes monocarboxylate transporter 8 and follows an X-linked recessive pattern. AHDS manifests as neuropsychomotor developmental delay, intellectual disability, movement disorders, and thyroid hormone abnormalities. It is frequently misdiagnosed as cerebral palsy or hypothyroidism.

PATIENT CONCERNS

A 9-month-old male infant exhibited poor head control, hypodynamia, motor retardation, hypertonic limbs, and thyroid abnormalities. Despite levothyroxine supplementation and rehabilitation therapy, no improvements were observed. Whole-exome sequencing identified a novel nonsense mutation in SLC16A2 (c.124G > T, p.E42X), which unequivocally established the diagnosis.

DIAGNOSES

AHDS was confirmed.

INTERVENTIONS

Levothyroxine treatment commenced early in infancy, followed by 3 months of rehabilitation therapy, starting at 5 months of age. The combined administration of levothyroxine and methimazole was initiated at 1 year and 10 months of age, respectively.

OUTCOMES

While improvements were noted in thyroid hormone levels, neurological developmental delays persisted.

LESSONS

AHDS should be considered in patients presenting with atypical neurological features and thyroid hormone abnormalities such as elevated triiodothyronine and decreased thyroxine levels. The early utilization of exome sequencing aids in prompt diagnosis. The identified SLC16A2 nonsense mutation correlates with severe neurological phenotypes and adds to the spectrum of genetic variations associated with AHDS.

Identification of Human TRIAC Transmembrane Transporters

Background: 3,5,3'-Triiodothyroacetic acid (TRIAC) is a T3-receptor agonist pharmacologically used in patients to mitigate T3 resistance. It is additionally explored to treat some symptoms of patients with inactivating mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8, SLC16A2). MCT8 is expressed along the blood-brain barrier, on neurons, astrocytes, and oligodendrocytes. Hence, pathogenic variants in MCT8 limit the access of TH into and their functions within the brain. TRIAC was shown to enter the brain independently of MCT8 and to modulate expression of TH-dependent genes. The aim of the study was to identify transporters that facilitate TRIAC uptake into cells. Methods: We performed a whole-genome RNAi screen in HepG2 cells stably expressing a T3-receptor-dependent luciferase reporter gene. Validation of hits from the primary and confirmatory secondary screen involved a counter screen with siRNAs and compared the cellular response to TRIAC to the effect of T3, in order to exclude siRNAs targeting the gene expression machinery. MDCK1 cells were stably transfected with cDNA encoding C-terminally myc-tagged versions of the identified TRIAC-preferring transporters. Several individual clones were selected after immunocytochemical characterization for biochemical characterization of their 125I-TRIAC transport activities. Results: We identified SLC22A9 and SLC29A2 as transporters mediating cellular uptake of TRIAC. SLC22A9 encodes the organic anion transporter 7 (OAT7), a sodium-independent organic anion transporter expressed in the plasma membrane in brain, pituitary, liver, and other organs. Competition with the SLC22A9/OAT7 substrate estrone-3-sulfate reduced 125I-TRIAC uptake. SLC29A2 encodes the equilibrative nucleoside transporter 2 (ENT2), which is ubiquitously expressed, including pituitary and brain. Coincubation with the SLC29A2/ENT2 inhibitor nitrobenzyl-6-thioinosine reduced 125I-TRIAC uptake. Moreover, ABCD1, an ATP-dependent peroxisomal pump, was identified as a 125I-TRIAC exporter in transfected MDCK1 cells. Conclusions: Knowledge of TRIAC transporter expression patterns, also during brain development, may thus in the future help to interpret observations on TRIAC effects, as well as understand why TRIAC may not show a desirable effect on cells or organs not expressing appropriate transporters. The identification of ABCD1 highlights the sensitivity of our established screening assay, but it may not hold significant relevance for patients undergoing TRIAC treatment.

Impact of Early Intervention with Triiodothyroacetic Acid on Peripheral and Neurodevelopmental Findings in a Boy with MCT8 Deficiency

Monocarboxylate transporter 8 (MCT8) deficiency is a rare genetic disorder characterized by peripheral thyrotoxicosis and severe cognitive and motor disability due to cerebral hypothyroidism. 3,3’,5-triiodothyroacetic acid (Triac) was shown to improve peripheral thyrotoxicosis but data on neurodevelopmental outcome are scarce. We present a case of MCT8 deficiency and the experience with Triac focusing on change in neurodevelopmental and peripheral features. A five-month-old boy was referred because of feeding difficulty, central hypotonia and global developmental delay. Despite six months of physiotherapy, physical developmental milestones did not improve, and distal muscle tone was increased. A hemizygous pathogenic variant in SLC16A2 was found and MCT8 deficiency was confirmed at 19-months. Thyroid stimulating hormone was 2.83 mIU/mL, free thyroxine 6.24 pmol/L (N=12-22) and free triiodothyronine (FT3) 15.65pmol/L (N=3.1-6.8). He had tachycardia, blood pressure and transaminases were elevated. Triac was started at 21-months. Two weeks after treatment, FT3 dramatically decreased, steady normal serum FT3 was achieved at 28-months. Assessment of neurodevelopmental milestones and signs of hyperthyroidism were evaluated at baseline, 6 months and 12 months after treatment. Signs of hyperthyroidism were improved by 6 months. Developmental composite scores of Bayley Scales of Infant Developmental 3rd Edition remained the same but important developmental milestones (head control, recognition of caregiver, response to his name) were attained, regression in the attained milestones were not observed. Initial dose, management protocol for Triac and research into its efficacy on neurodevelopmental signs in MCT8 deficiency are progressing. This case presents evidence that Triac may resolve peripheral thyrotoxicosis successfully and may slow neurodevelopmental regression, while some developmental milestones were achieved after one year of treatment.

Factors and Mechanisms of Thyroid Hormone Activity in the Brain: Possible Role in Recovery and Protection

Thyroid hormones (THs) are essential in normal brain development, and cognitive and emotional functions. THs act through a cascade of events including uptake by the target cells by specific cell membrane transporters, activation or inactivation by deiodinase enzymes, and interaction with nuclear thyroid hormone receptors. Several thyroid responsive genes have been described in the developing and in the adult brain and many studies have demonstrated a systemic or local reduction in TH availability in neurologic disease and after brain injury. In this review, the main factors and mechanisms associated with the THs in the normal and damaged brain will be evaluated in different regions and cellular contexts. Furthermore, the most common animal models used to study the role of THs in brain damage and cognitive impairment will be described and the use of THs as a potential recovery strategy from neuropathological conditions will be evaluated. Finally, particular attention will be given to the link observed between TH alterations and increased risk of Alzheimer's Disease (AD), the most prevalent neurodegenerative and dementing condition worldwide.

Parent Perspectives on Complex Needs in Patients With MCT8 Deficiency: An International, Prospective, Registry Study

CONTEXT

Monocarboxylate transporter 8 (MCT8) deficiency is a rare neurodevelopmental and metabolic disorder, with daily care posing a heavy burden on caregivers. A comprehensive overview of these complex needs and daily care challenges is lacking.

DESIGN

We established an international prospective registry to systemically capture data from parents and physicians caring for patients with MCT8 deficiency. Parent-reported data on complex needs and daily care challenges were extracted.

RESULTS

Between July 17, 2018, and May 16, 2022, 51 patients were registered. Difficulties in daily life care were mostly related to feeding and nutritional status (17/33 patients), limited motor skills (12/33 patients), and sleeping (11/33 patients). Dietary advice was provided for 11/36 patients. Two of 32 patients were under care of a cardiologist. Common difficulties in the diagnostic trajectory included late diagnosis (20/35 patients) and visiting a multitude of specialists (15/35 patients). Median diagnostic delay was significantly shorter in patients born in or after 2017 vs before 2017 (8 vs 19 months, P < .0001).

CONCLUSIONS

Feeding and sleeping problems and limited motor skills mostly contribute to difficulties in daily care. The majority of patients did not receive professional dietary advice, although being underweight is a key disease feature, strongly linked with poor survival. Despite sudden death being a prominent cause of death, potentially related to the cardiovascular abnormalities frequently observed, patients were hardly seen by cardiologists. These findings can directly improve patient-centered multidisciplinary care and define patient-centered outcome measures for intervention studies in patients with MCT8 deficiency.

Generation of iPSC lines with SLC16A2:G401R or SLC16A2 knock out

The X-linked Allan-Herndon-Dudley syndrome (AHDS) is characterized by severely impaired psychomotor development and is caused by mutations in the SLC16A2 gene encoding the thyroid hormone transporter MCT8 (monocarboxylate transporter 8). By targeting exon 3 of SLC16A2 using CRISPR/Cas9 with single-stranded oligodeoxynucleotides as homology-directed repair templates, we introduced the AHDS patient missense variant G401R and a novel knock-out deletion variant (F400Sfs*17) into the male healthy donor hiPSC line BIHi001-B. We successfully generated cerebral organoids from these genome-edited lines, demonstrating the utility of the novel lines for modelling the effects of MCT8-deficency on human neurodevelopment.

Allan-Herndon-Dudley syndrome in Hong Kong: Implication for newborn screening

BACKGROUND

Allan-Herndon-Dudley syndrome (MCT 8 deficiency) is an X-linked recessive condition caused by hemizygous pathogenic variants in SLC16A2 encoding the monocarboxylate transporter 8 (MCT8). Patients present with global developmental delay and neurological impairment, and abnormal serum thyroid function tests. The drug, 3,3',5 triiodothyroacetic acid (TRIAC), was recently demonstrated to improve the endocrinological profile. Improvement in diagnostic approach is key to earlier start of treatment.

PATIENT FINDINGS

We described four Chinese patients with MCT8 deficiency undergoing different diagnostic odysseys. Their initial presentation included global developmental delay and dystonia. Patient 2 also had epilepsy. Patients 1 and 2 presented with two novel variants: (1)hemizygous NM_006517.4(SLC16A2):c.1170 + 2 T > A; p.(?), and (2)hemizygous NM_006517.4(SLC16A2):c.305dupT; p.(Val103GlyfsTer17) respectively. Patients 3 and 4 were biological brothers harboring hemizygous NM_006517.4(SLC16A2):c.305dupT; p.(Val103GlyfsTer17), which was first reported in 2004. We obtained the measurement of triiodothyronine (T3) and reverse T3 (rT3) from dried blood spot samples collected on Day 1 of life from Patient 1 and studied the biomarkers (rT3 and T3/rT3 ratio) proposed by Iwayama et al. for the detection of MCT8 deficiency at birth. Our data verified the significantly reduced rT3 level in Patient 1, compared with healthy newborns, although low T3 level and comparable T3/rT3 ratio with controls were detected.

SUMMARY

Patients with MCT8 deficiency often undergo diagnostic odysseys. An early diagnosis could be missed by a normal newborn thyroid function screening result based on biochemical measurement of TSH and/or T4/fT4. Early detection of rT3 is key to improving current diagnostic approach.

CONCLUSION

We recommend that full thyroid function profile (TSH, T4/fT4, T3/fT3, rT3) be considered early for all pediatric patients presenting with unexplained developmental delay and/or dystonia. The potential inclusion of rT3 measurement in newborn screening may prove promising.

Establishing Patient-Centered Outcomes for MCT8 Deficiency: Stakeholder Engagement and Systematic Literature Review

Introduction

The SCL16A2 gene encodes the thyroid hormone (TH) transporter MCT8. Pathogenic variants result in a reduced TH uptake into the CNS despite high serum T3 concentrations. Patients suffer from severe neurodevelopmental delay and require multidisciplinary care. Since a first compassionate use study in 2008, the development of therapies has recently gained momentum. Treatment strategies range from symptom-based approaches, supplementation with TH or TH-analogs, to gene therapy. All these studies have mainly used surrogate endpoints and clinical outcomes. However, the EMA and FDA strongly encourage researchers to involve patients and their advocacy groups in the design of clinical trials. This should strengthen the patients' perspective and identify clinical endpoints that are clinically relevant to their daily life.

Methods

We involved patient families to define patient-relevant outcomes for MCT8 deficiency. In close collaboration with patient families, we designed a questionnaire asking for their five most preferred therapeutic goals, which, if achieved at least, make a difference in their lives. In addition, we performed a systematic review according to Cochrane recommendations of the published treatment trials.

Results

We obtained results from 15 families with completed questionnaires from 14 mothers and 8 fathers. Improvement in development, especially in gross motor skills, was most important to the parents. 59% wished for head control and 50% for sitting ability. Another 36% wished for weight gain, 32% for improvement of expressive language skills, and 18% for a reduction of dystonia/spasticity, less dysphagia, and reflux. Paraclinical aspects were least important (5-9%). In a treatment trial (n=46) and compassionate use cases (n=83), the results were mainly inconclusive, partly due to a lack of predefined patient-centered clinical endpoints.

Discussion

We recommend that future trials should define a relevant improvement in "development" and/or other patient-relevant outcomes compared to natural history as treatment goals.

Nongenomic roles of thyroid hormones and their derivatives in adult brain: are these compounds putative neurotransmitters?

We review the evidence regarding the nongenomic (or non-canonical) actions of thyroid hormones (thyronines) and their derivatives (including thyronamines and thyroacetic acids) in the adult brain. The paper seeks to evaluate these compounds for consideration as candidate neurotransmitters. Neurotransmitters are defined by their (a) presence in the neural tissue, (b) release from neural tissue or cell, (c) binding to high-affinity and saturable recognition sites, (d) triggering of a specific effector mechanism and (e) inactivation mechanism. Thyronines and thyronamines are concentrated in brain tissue and show distinctive patterns of distribution within the brain. Nerve terminals accumulate a large amount of thyroid hormones in mature brain, suggesting a synaptic function. However, surprisingly little is known about the potential release of thyroid hormones at synapses. There are specific binding sites for thyroid hormones in nerve-terminal fractions (synaptosomes). A notable cell-membrane binding site for thyroid hormones is integrin αvβ3. Furthermore, thyronines bind specifically to other defined neurotransmitter receptors, including GABAergic, catecholaminergic, glutamatergic, serotonergic and cholinergic systems. Here, the thyronines tend to bind to sites other than the primary sites and have allosteric effects. Thyronamines also bind to specific membrane receptors, including the trace amine associated receptors (TAARs), especially TAAR1. The thyronines and thyronamines activate specific effector mechanisms that are short in latency and often occur in subcellular fractions lacking nuclei, suggesting nongenomic actions. Some of the effector mechanisms for thyronines include effects on protein phosphorylation, Na+/K+ ATPase, and behavioral measures such as sleep regulation and measures of memory retention. Thyronamines promptly regulate body temperature. Lastly, there are numerous inactivation mechanisms for the hormones, including decarboxylation, deiodination, oxidative deamination, glucuronidation, sulfation and acetylation. Therefore, at the current state of the research field, thyroid hormones and their derivatives satisfy most, but not all, of the criteria for definition as neurotransmitters.

TRIAC disrupts cerebral thyroid hormone action via negative feedback and heterogenous distribution among organs

As 3,3',5-triiodothyroacetic acid (TRIAC), a metabolite of thyroid hormones (THs), was previously detected in sewage effluent, we aimed to investigate exogenous TRIAC's potential for endocrine disruption. We administered either TRIAC or 3,3',5-triiodo-L-thyronine (LT3) to euthyroid mice and 6-propyl-2-thiouracil-induced hypothyroid mice. In hypothyroid mice, TRIAC administration suppressed the hypothalamus-pituitary-thyroid (HPT) axis and upregulated TH-responsive genes in the pituitary gland, the liver, and the heart. We observed that, unlike LT3, TRIAC administration did not upregulate cerebral TH-responsive genes. Measurement of TRIAC contents suggested that TRIAC was not efficiently trafficked into the cerebrum. By analyzing euthyroid mice, we found that cerebral TRIAC content did not increase despite TRIAC administration at higher concentrations, whereas serum levels and cerebral contents of THs were substantially decreased. Disruption by TRIAC is due to the additive effects of circulating endogenous THs being depleted via a negative feedback loop involving the HPT axis and heterogeneous distribution of TRIAC among different organs.

Maternal Administration of the CNS-Selective Sobetirome Prodrug Sob-AM2 Exerts Thyromimetic Effects in Murine MCT8-Deficient Fetuses

Background: Monocarboxylate transporter 8 (MCT8) deficiency is a rare X-linked disease where patients exhibit peripheral hyperthyroidism and cerebral hypothyroidism, which results in severe neurological impairments. These brain defects arise from a lack of thyroid hormones (TH) during critical stages of human brain development. Treatment options for MCT8-deficient patients are limited and none have been able to prevent or ameliorate effectively the neurological impairments. This study explored the effects of the TH agonist sobetirome and its CNS-selective amide prodrug, Sob-AM2, in the treatment of pregnant dams carrying fetuses lacking Mct8 and deiodinase type 2 (Mct8/Dio2 KO), as a murine model for MCT8 deficiency. Methods: Pregnant dams carrying Mct8/Dio2 KO fetuses were treated with 1 mg of sobetirome/kg body weight/day, or 0.3 mg of Sob-AM2/kg body weight/day for 7 days, starting at embryonic day 12.5 (E12.5). As controls, pregnant dams carrying wild-type and pregnant dams carrying Mct8/Dio2 KO fetuses were treated with daily subcutaneous injections of vehicle. Dams TH levels were measured by enzyme-linked immunosorbent assay (ELISA). Samples were extracted at E18.5 and the effect of treatments on the expression of triiodothyronine (T3)-dependent genes was measured in the placenta, fetal liver, and fetal cerebral cortex by real-time polymerase chain reaction. Results: Maternal sobetirome treatment led to spontaneous abortions. Sob-AM2 treatment, however, was able to cross the placental as well as the brain barriers and exert thyromimetic effects in Mct8/Dio2 KO fetal tissues. Sob-AM2 treatment did not affect the expression of the T3-target genes analyzed in the placenta, but it mediated thyromimetic effects in the fetal liver by increasing the expression of Dio1 and Dio3 genes. Interestingly, Sob-AM2 treatment increased the expression of several T3-dependent genes in the brain such as Hr, Shh, Dio3, Kcnj10, Klf9, and Faah in Mct8/Dio2 KO fetuses. Conclusions: Maternal administration of Sob-AM2 can cross the placental barrier and access the fetal tissues, including the brain, in the absence of MCT8, to exert thyromimetic actions by modulating the expression of T3-dependent genes. Therefore, Sob-AM2 has the potential to address the cerebral hypothyroidism characteristic of MCT8 deficiency from fetal stages and to prevent neurodevelopmental alterations in the MCT8-deficient fetal brain.

Genetic disorders of thyroid development, hormone biosynthesis and signalling

Development and differentiation of the thyroid gland is directed by expression of specific transcription factors in the thyroid follicular cell which mediates hormone biosynthesis. Membrane transporters are rate-limiting for cellular entry of thyroid hormones (TH) (T4 and T3) into some tissues, with selenocysteine-containing, deiodinase enzymes (DIO1 and DIO2) converting T4 to the biologically active hormone T3. TH regulate expression of target genes via hormone-inducible nuclear receptors (TRα and TRβ) to exert their physiological effects. Primary congenital hypothyroidism (CH) due to thyroid dysgenesis may be mediated by defects in thyroid transcription factors or impaired thyroid stimulating hormone receptor function. Dyshormonogenic CH is usually due to mutations in genes mediating thyroidal iodide transport, organification or iodotyrosine synthesis and recycling. Disorders of TH signalling encompass conditions due to defects in membrane TH transporters, impaired hormone metabolism due to deficiency of deiodinases and syndromes of Resistance to thyroid hormone due to pathogenic variants in either TRα or TRβ. Here, we review the genetic basis, pathogenesis and clinical features of congenital, dysgenetic or dyshormonogenic hypothyroidism and disorders of TH transport, metabolism and action.