Disease characteristics of MCT8 deficiency: an international, retrospective, multicentre 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.

Phenylbutyrate Treatment in a Boy With MCT8 Deficiency: Improvement of Thyroid Function Tests and Possible Hepatotoxicity

CONTEXT

Monocarboxylate transporter 8 (MCT8) deficiency is a rare X-chromosomal inherited disease leading to severe cognitive impairment, muscular hypotonia, and symptoms of peripheral thyrotoxicosis. Experimental approaches aiming to functionally rescue mutant MCT8 activity by the chemical chaperone phenylbutyrate (PB) demonstrated promising effects in vitro for several MCT8 missense mutations.

OBJECTIVE

The objective was to evaluate biochemical and clinical effects of PB in doses equivalent to those approved for the treatment of urea cycle disorders in a boy with MCT8 deficiency due to a novel MCT8 missense mutation c.703G>T (p.V235L).

RESULTS

During a treatment period of 13 months, PB led to a significant decrease of elevated thyrotropin and triiodothyronine (T3) serum concentrations, while free thyroxine (fT4) increased. The weight z-score of the toddler remained remarkably stable during the treatment period. Neurodevelopmental assessments (BSID-III) revealed a slight increase of gross motor skills from developmental age 4 to 6 months. However, increasing liver enzyme serum activities and accumulation of phenylacetate in urine led to treatment interruptions and dose alterations. In vitro analyses in MDCK1 cells confirmed the pathogenicity of MCT8 p.V235L. However, while PB increased expression of the mutant protein, it did not rescue T3 transport, suggesting a PB effect on thyroid function tests independent of restoring MCT8 activity.

CONCLUSION

In a clinical attempt of PB treatment in MCT8 deficiency we observed a significant improvement of thyroid hormone function tests, tendencies toward body weight stabilization and slight neurodevelopmental improvement. Hepatotoxicity of PB may be a limiting factor in MCT8 deficiency and requires further investigation.

Emerging Role of Noncovalent Interactions and Disulfide Bond Formation in the Cellular Uptake of Small Molecules and Proteins

Intracellular delivery of proteins and small molecules is an important barrier in the development of strategies to deliver functional proteins and therapeutics into the cells to realize their full potential in biotechnology, biomedicine, cell-based therapies, and gene editing protein systems. Most of the intracellular protein delivery strategies involve the conjugation of cell penetrating peptides to enable the permeability of plasma membrane of mammalian cells to allow proteins to enter cytosol. The conjugations of small molecules such as (p-methylphenyl) glycine, pyrenebutyrate and cysteines are used for the same purpose. Molecular level interactions are governed mostly by ionic (cationic/anionic), covalent and noncovalent interactions with various molecular entities of glycocalyx matrix on plasma membrane lipid bilayer. Although the role of noncovalent interactions in cellular uptake is not fully understood, several recent advances have focused on the noncovalent interaction-based strategies of intracellular delivery of small molecules and proteins into mammalian cells. These are achieved by simple modification of protein surfaces with chemical moieties which can form noncovalent interactions other than hydrogen bonding. In this review, we describe the recent advances and the mechanistic aspects of intracellular delivery and role of noncovalent interactions in the cellular uptake of proteins and small molecules.

Patients with Allan-Herndon-Dudley Syndrome (MCT8 Deficiency) Display Symptoms of Parkinsonism in Childhood and Respond to Levodopa/Carbidopa Treatment

BACKGROUND

Patients with mutations in the monocarboxylate transporter 8 (MCT8, SLC16A2) suffer from X-linked recessive Allan-Herndon-Dudley syndrome (AHDS), which is characterized by developmental delay and a severe movement disorder. Current trials using thyroid hormone derivatives to overcome the transporter defect have failed to achieve patient-oriented therapeutic goals.

OBJECTIVES

Our aim was to define the type of movement disorder in AHDS in an observational cohort study and to investigate the causative role of the dopaminergic system.

METHODS

We present longitudinal clinical data from the DEEPTYPE registry of 11 patients with video documentation, standardized phenotyping, cerebrospinal fluid (CSF) analysis, neuroimaging data, and the treatment response to levodopa/carbidopa supplementation.

RESULTS

Children presented with signs of childhood parkinsonism, including hypokinesia, hypomimia, inability to sit or stand, rigidity, dystonia, and autonomic dysfunction. CSF homovanillic acid concentrations were decreased (n = 12), suggesting an isolated dopamine pathway impairment. Seven out of 8 patients responded favorably to l-dopa/carbidopa supplementation and we did not observe any adverse drug reactions.

CONCLUSIONS

AHDS is associated with childhood parkinsonism, which is linked with biochemical abnormalities of dopamine metabolism. It can be treated with l-dopa/carbidopa supplementation. However, further research is needed to elucidate the exact effect of MCT8 deficiency on dopamine metabolism. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

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.

Identification of a novel non-coding deletion in Allan-Herndon-Dudley syndrome by long-read HiFi genome sequencing

BACKGROUND

Allan-Herndon-Dudley syndrome (AHDS) is an X-linked disorder caused by pathogenic variants in the SLC16A2 gene. Although most reported variants are found in protein-coding regions or adjacent junctions, structural variations (SVs) within non-coding regions have not been previously reported.

METHODS

We investigated two male siblings with severe neurodevelopmental disorders and spasticity, who had remained undiagnosed for over a decade and were negative from exome sequencing, utilizing long-read HiFi genome sequencing. We conducted a comprehensive analysis including short-tandem repeats (STRs) and SVs to identify the genetic cause in this familial case.

RESULTS

While coding variant and STR analyses yielded negative results, SV analysis revealed a novel hemizygous deletion in intron 1 of the SLC16A2 gene (chrX:74,460,691 - 74,463,566; 2,876 bp), inherited from their carrier mother and shared by the siblings. Determination of the breakpoints indicates that the deletion probably resulted from Alu/Alu-mediated rearrangements between homologous AluY pairs. The deleted region is predicted to include multiple transcription factor binding sites, such as Stat2, Zic1, Zic2, and FOXD3, which are crucial for the neurodevelopmental process, as well as a regulatory element including an eQTL (rs1263181) that is implicated in the tissue-specific regulation of SLC16A2 expression, notably in skeletal muscle and thyroid tissues.

CONCLUSIONS

This report, to our knowledge, is the first to describe a non-coding deletion associated with AHDS, demonstrating the potential utility of long-read sequencing for undiagnosed patients. Although interpreting variants in non-coding regions remains challenging, our study highlights this region as a high priority for future investigation and functional studies.

Newborn screening for central congenital hypothyroidism: past, present and future

Congenital hypothyroidism (CH) is defined as thyroid hormone deficiency at birth and constitutes one of the most common causes of preventable intellectual disability worldwide. Central CH is caused by insufficient pituitary or hypothalamic control of thyroid function, biochemically characterized by a low serum free thyroxine (fT4), in combination with a low, normal or mildly elevated thyroid-stimulating hormone (TSH). Central CH is less common than primary CH and is part of multiple pituitary hormone deficiencies (MPHD) in most of the cases. MPHD at birth, also known as 'congenital hypopituitarism', is a potentially life-threatening condition due to the possible co-occurrence of adrenocorticotropin hormone and growth hormone deficiency that can result in severe hypoglycemia and adrenal crisis. To date, central CH is the only pituitary hormone deficiency suitable for newborn screening (NBS), providing an opportunity for early detection of MPHD. Even though the first NBS programs utilized T4-based methods that were able to identify central CH, most countries have since transitioned to TSH-based approaches due to the high rate of false positives associated with T4-based strategies. Now, 50 years after the introduction of NBS for CH, only a few countries around the world have a screening program capable of detecting central CH. In this paper, we review the past, present and future of NBS for central CH. We will outline the importance of early detection of central CH and discuss the challenges and opportunities of screening for this condition.

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.

Increased thyroid hormone sensitivity is correlated with visceral obesity in patients with type 2 diabetes

OBJECTIVE

The study aimed to assess whether thyroid hormone (TH) sensitivity is related to visceral fat area (VFA) and visceral obesity in euthyroid subjects with type 2 diabetes (T2D).

METHODS

750 euthyroid patients with T2D were enrolled. A VFA of 80 cm2 or more was considered visceral obesity. Central TH sensitivity was conducted using thyrotrophic thyroxine resistance index (TT4RI), thyrotropin index (TSHI), and thyroid feedback quantile-based index (TFQI). Free triiodothyronine to free thyroxine (FT3/FT4) was utilized for assessing peripheral TH sensitivity.

RESULTS

The subjects had a mean age of 51.5 ± 11.1 years, and 540 (72.0%) of them were men. In multivariable regression analyses, there was a positive correlation of FT3/FT4 tertile with visceral obesity, after full adjustment for confounding variables (P < 0.05). The middle and highest FT3/FT4 tertiles were correlated with a 134% [95% CI (1.24, 4.44)] and 98% [95% CI (1.04, 3.78)] higher prevalence of visceral obesity than the lowest tertile, respectively. Conversely, elevated TFQI levels were linked to a decreased prevalence of visceral obesity. Stratified analysis revealed that these associations were particularly pronounced in participants who are neither overweight nor obese and those aged less than 60 years (all P < 0.05).

CONCLUSIONS

Higher TH sensitivity is correlated with visceral obesity and elevated VFA in euthyroid patients with T2D, particularly among those younger than 60 years and individuals who are neither overweight nor obese.

Glycerol Phenylbutyrate Treatment of 2 Patients With Monocarboxylate Transporter 8 Deficiency

CONTEXT

Monocarboxylate transporter 8 (MCT8) deficiency is a rare genetic disease that leads to severe global developmental delay. MCT8 facilitates thyroid hormone (TH) transport across the cell membrane, and the serum TH profile is characterized by high T3 and low T4 levels. Recent studies have shown that the chemical chaperone sodium phenylbutyrate (NaPB) restored mutant MCT8 function and increased TH content in patient-derived induced pluripotent stem cells, making it a potential treatment for MCT8 deficiency.

OBJECTIVE

We aimed to assess the efficacy and safety of glycerol phenylbutyrate (GPB) in MCT8 deficiency.

METHODS

We treated 2 monozygotic twins aged 14.5 years with MCT8 deficiency due to P321L mutation with escalating doses of GPB over 13 months. We recorded TH, vital signs, anthropometric measurements, and neurocognitive functions. Resting metabolic rate (RMR) was measured by indirect calorimetry. Serum metabolites of GPB were monitored as a safety measure. In vitro effects of NaPB were evaluated in MDCK1 cells stably expressing the MCT8P321L mutation. The effects of GPB were compared to the effects of DITPA and TRIAC, thyromimetic medications that the patients had received in the past.

RESULTS

NaPB restored mutant MCT8 expression in MDCK1 cells and increased T3 transport into cells carrying the P321L mutation. GPB treatment reduced high T3 and increased low T4 levels. The patients showed a significant weight gain simultaneously with a reduction in RMR. Only minor neurocognitive improvement was observed, in hyperreflexia score and in cognitive functions. Serum metabolites did not exceed the toxic range, but elevated liver transaminases were observed.

CONCLUSION

In the first report of GPB treatment in MCT8 deficiency we found an improvement in TH profile and body mass index, with minor neurodevelopmental changes.

Gene Misexpression in a Smoc2+ve/Sox2-Low Population in Juvenile Prop1-Mutant Pituitary Gland

Mutations in the pituitary-specific transcription factor Prophet of Pit-1 (PROP1) are the most common genetic etiology of combined pituitary hormone deficiency (CPHD). CPHD is associated with short stature, attributable to growth hormone deficiency and/or thyroid-stimulating hormone deficiency, as well as hypothyroidism and infertility. Pathogenic lesions impair pituitary development and differentiation of endocrine cells. We performed single-cell RNA sequencing of pituitary cells from a wild-type and a Prop1-mutant P4 female mouse to elucidate population-specific differential gene expression. We observed a Smoc2+ve population that expressed low Sox2, which trajectory analyses suggest are a transitional cell state as stem cells differentiate into endocrine cells. We also detected ectopic expression of Sox21 in these cells in the Prop1df/df mutant. Prop1-mutant mice are known to overexpress Pou3f4, which we now show to be also enriched in this Smoc2+ve population. We sought to elucidate the role of Pou3f4 during pituitary development and to determine the contributions of Pou3f4 upregulation to pituitary disease by utilizing double-mutant mice lacking both Prop1 and Pou3f4. However, our data showed that Pou3f4 is not required for normal pituitary development and function. Double mutants further demonstrated that the upregulation of Pou3f4 was not causative for the overexpression of Sox21. These data indicate loss of Pou3f4 is not a potential cause of CPHD, and further studies may investigate the functional consequence of upregulation of Pou3f4 and Sox21, if any, in the novel Smoc2+ve cell population.

Paediatric thyroid disease

The spectrum of thyroid disorders presenting to paediatricians is different to that seen by adult physicians. Referrals reflect cases detected by the neonatal screening programme for congenital hypothyroidism and many of the inherited defects of thyroid hormone generation or action will be manifest in early life. Autoimmune thyroid disease can be particularly challenging to manage in the young and the potential impact of thyroid status on neurodevelopment and schooling are key considerations throughout childhood and adolescence.

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.

Melanocortin-4 Receptor PLC Activation Is Modulated by an Interaction with the Monocarboxylate Transporter 8

The melanocortin-4 receptor (MC4R) is a key player in the hypothalamic leptin-melanocortin pathway that regulates satiety and hunger. MC4R belongs to the G protein-coupled receptors (GPCRs), which are known to form heterodimers with other membrane proteins, potentially modulating receptor function or characteristics. Like MC4R, thyroid hormones (TH) are also essential for energy homeostasis control. TH transport across membranes is facilitated by the monocarboxylate transporter 8 (MCT8), which is also known to form heterodimers with GPCRs. Based on the finding in single-cell RNA-sequencing data that both proteins are simultaneously expressed in hypothalamic neurons, we investigated a putative interplay between MC4R and MCT8. We developed a novel staining protocol utilizing a fluorophore-labeled MC4R ligand and demonstrated a co-localization of MC4R and MCT8 in human brain tissue. Using in vitro assays such as BRET, IP1, and cAMP determination, we found that MCT8 modulates MC4R-mediated phospholipase C activation but not cAMP formation via a direct interaction, an effect that does not require a functional MCT8 as it was not altered by a specific MCT8 inhibitor. This suggests an extended functional spectrum of MCT8 as a GPCR signaling modulator and argues for the investigation of further GPCR-protein interactions with hitherto underrepresented physiological functions.

Late diagnosis of the X-linked MCT8 deficiency (Allan-Herndon-Dudley syndrome) in a teenage girl with primary ovarian insufficiency

OBJECTIVES

To report an unusual case of MCT8 deficiency (Allan-Herndon-Dudley syndrome), an X-linked condition caused by pathogenic variants in the SLC16A2 gene. Defective transport of thyroid hormones (THs) in this condition leads to severe neurodevelopmental impairment in males, while heterozygous females are usually asymptomatic or have mild TH abnormalities.

CASE PRESENTATION

A girl with profound developmental delay, epilepsy, primary amenorrhea, elevated T3, low T4 and free T4 levels was diagnosed with MCT8-deficiency at age 17 years, during evaluation for primary ovarian insufficiency (POI). Cytogenetic analysis demonstrated balanced t(X;16)(q13.2;q12.1) translocation with a breakpoint disrupting SLC16A2. X-chromosome inactivation studies revealed a skewed inactivation of the normal X chromosome.

CONCLUSIONS

MCT8-deficiency can manifest clinically and phenotypically in women with SLC16A2 aberrations when nonrandom X inactivation occurs, while lack of X chromosome integrity due to translocation can cause POI.

Spatiotemporal expression of thyroid hormone transporter MCT8 and THRA mRNA in human cerebral organoids recapitulating first trimester cortex development

Thyroid hormones (TH) play critical roles during nervous system development and patients carrying coding variants of MCT8 (monocarboxylate transporter 8) or THRA (thyroid hormone receptor alpha) present a spectrum of neurological phenotypes resulting from perturbed local TH action during early brain development. Recently, human cerebral organoids (hCOs) emerged as powerful in vitro tools for disease modelling recapitulating key aspects of early human cortex development. To begin exploring prospects of this model for thyroid research, we performed a detailed characterization of the spatiotemporal expression of MCT8 and THRA in developing hCOs. Immunostaining showed MCT8 membrane expression in neuronal progenitor cell types including early neuroepithelial cells, radial glia cells (RGCs), intermediate progenitors and outer RGCs. In addition, we detected robust MCT8 protein expression in deep layer and upper layer neurons. Spatiotemporal SLC16A2 mRNA expression, detected by fluorescent in situ hybridization (FISH), was highly concordant with MCT8 protein expression across cortical cell layers. FISH detected THRA mRNA expression already in neuroepithelium before the onset of neurogenesis. THRA mRNA expression remained low in the ventricular zone, increased in the subventricular zone whereas strong THRA expression was observed in excitatory neurons. In combination with a robust up-regulation of known T3 response genes following T3 treatment, these observations show that hCOs provide a promising and experimentally tractable model to probe local TH action during human cortical neurogenesis and eventually to model the consequences of impaired TH function for early cortex development.

Approach to the Patient With Raised Thyroid Hormones and Nonsuppressed TSH

Measurement of free thyroid hormones (THs) and thyrotropin (TSH) using automated immunoassays is central to the diagnosis of thyroid dysfunction. Using illustrative cases, we describe a diagnostic approach to discordant thyroid function tests, focusing on entities causing elevated free thyroxine and/or free triiodothyronine measurements with nonsuppressed TSH levels. Different types of analytical interference (eg, abnormal thyroid hormone binding proteins, antibodies to iodothyronines or TSH, heterophile antibodies, biotin) or disorders (eg, resistance to thyroid hormone β or α, monocarboxylate transporter 8 or selenoprotein deficiency, TSH-secreting pituitary tumor) that can cause this biochemical pattern will be considered. We show that a structured approach, combining clinical assessment with additional laboratory investigations to exclude assay artifact, followed by genetic testing or specialized imaging, can establish a correct diagnosis, potentially preventing unnecessary investigation or inappropriate therapy.

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.

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.

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.

Thyroid hormone action during GABAergic neuron maturation: The quest for mechanisms

Thyroid hormone (TH) signaling plays a major role in mammalian brain development. Data obtained in the past years in animal models have pinpointed GABAergic neurons as a major target of TH signaling during development, which opens up new perspectives to further investigate the mechanisms by which TH affects brain development. The aim of the present review is to gather the available information about the involvement of TH in the maturation of GABAergic neurons. After giving an overview of the kinds of neurological disorders that may arise from disruption of TH signaling during brain development in humans, we will take a historical perspective to show how rodent models of hypothyroidism have gradually pointed to GABAergic neurons as a main target of TH signaling during brain development. The third part of this review underscores the challenges that are encountered when conducting gene expression studies to investigate the molecular mechanisms that are at play downstream of TH receptors during brain development. Unravelling the mechanisms of action of TH in the developing brain should help make progress in the prevention and treatment of several neurological disorders, including autism and epilepsy.

Sensitivity to thyroid hormone and risk of components of metabolic syndrome in a Chinese euthyroid population

INTRODUCTION

To evaluate the association of sensitivity to thyroid hormone with metabolic syndrome (MetS) and its components in a Chinese euthyroid population.

METHODS

A total of 3573 participants from Pinggu Metabolic Disease Study were analyzed. Serum-free triiodothyronine (FT3), free thyroxine (FT4), thyrotropin (TSH), total adipose tissue (TAT), visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT) area of abdominal, and lumbar skeletal muscle area (SMA) were measured. Central thyroid hormone resistance was calculated by the Thyroid Feedback Quantile-based Index (TFQI) and Chinese-referenced Parametric TFQI (PTFQI), Thyrotroph T4 Resistance Index (TT4RI) and TSH Index (TSHI). Peripheral thyroid hormone resistance was assessed by FT3/FT4 ratio.

RESULTS

Higher values of TSHI (odds ratio [OR] = 1.167, 95% confidence interval [CI]: 1.079-1.262, p < .001), TT4RI (OR = 1.115, 95% CI: 1.031-1.206, p = .006), TFQI (OR = 1.196, 95% CI: 1.106-1.294, p < .001), PTFQI (OR = 1.194, 95% CI: 1.104-1.292, p < .001), and lower values of FT3/FT4 ratio (OR = 0.914, 95% CI: 0.845-0.990, p = .026) were associated with MetS. Increased levels of TFQI and PTFQI were associated with abdominal obesity, hypertriglyceridemia, and hypertension. Increased levels of TSHI and TT4RI were associated with hypertriglyceridemia, abdominal obesity, low high-density lipoprotein cholesterol. Reduced levels of FT3/FT4 ratio were associated with hyperglycemia, hypertension, and hypertriglyceridemia. The levels of TSHI, TFQI, and PTFQI were negatively related to SMA and positively related to VAT, SAT, and TAT (all p < .05).

CONCLUSIONS

Reduced thyroid hormone sensitivity was associated with MetS and its components. Impaired thyroid hormone sensitivity might affect the distribution of adipose tissue and muscle.

Role and Clinical Significance of Monocarboxylate Transporter 8 (MCT8) During Pregnancy

The review aims to summarize the available research focusing on the importance of monocarboxylate transporter (MCT8) in thyroid hormone trafficking across the placenta and fetal development. A systematic search was carried out in PubMed; studies available in English related to "monocarboxylate transporter", "adverse pregnancy", "fetal development," and "thyroid hormone" were identified and assessed. The references within the resulting articles were manually searched. MCT8 is a highly active and selective thyroid hormone transporter that facilitates the cellular uptake of triiodothyronine (T3), thyroxine (T4), reverse triiodothyronine (rT3), and diiodothyronine (T2) in different tissues. MCT8 is expressed in the placenta from the first trimester onwards, allowing the transport of thyroid hormone from mother to fetus. Mutations in MCT8 cause an X-linked disorder known as Allan-Herndon-Dudley syndrome (AHDS), characterized by severe psychomotor impairment and peripheral thyrotoxicosis. Hence, any maternal thyroid dysfunction may cause severe consequences for the fetus and newborn. Further research regarding MCT8 gene expression, polymorphic variation, and adverse pregnancy outcomes must be done to establish that MCT8 is a novel prognostic marker for the early detection of pregnancy-related complications.

Thyroid Hormone Transporters MCT8 and OATP1C1 Are Expressed in Projection Neurons and Interneurons of Basal Ganglia and Motor Thalamus in the Adult Human and Macaque Brains

Monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptide 1C1 (OATP1C1) are thyroid hormone (TH) transmembrane transporters relevant for the availability of TH in neural cells, crucial for their proper development and function. Mutations in MCT8 or OATP1C1 result in severe disorders with dramatic movement disability related to alterations in basal ganglia motor circuits. Mapping the expression of MCT8/OATP1C1 in those circuits is necessary to explain their involvement in motor control. We studied the distribution of both transporters in the neuronal subpopulations that configure the direct and indirect basal ganglia motor circuits using immunohistochemistry and double/multiple labeling immunofluorescence for TH transporters and neuronal biomarkers. We found their expression in the medium-sized spiny neurons of the striatum (the receptor neurons of the corticostriatal pathway) and in various types of its local microcircuitry interneurons, including the cholinergic. We also demonstrate the presence of both transporters in projection neurons of intrinsic and output nuclei of the basal ganglia, motor thalamus and nucleus basalis of Meynert, suggesting an important role of MCT8/OATP1C1 for modulating the motor system. Our findings suggest that a lack of function of these transporters in the basal ganglia circuits would significantly impact motor system modulation, leading to clinically severe movement impairment.

In a zebrafish biomedical model of human Allan-Herndon-Dudley syndrome impaired MTH signaling leads to decreased neural cell diversity

Background

Maternally derived thyroid hormone (T3) is a fundamental factor for vertebrate neurodevelopment. In humans, mutations on the thyroid hormones (TH) exclusive transporter monocarboxylic acid transporter 8 (MCT8) lead to the Allan-Herndon-Dudley syndrome (AHDS). Patients with AHDS present severe underdevelopment of the central nervous system, with profound cognitive and locomotor consequences. Functional impairment of zebrafish T3 exclusive membrane transporter Mct8 phenocopies many symptoms observed in patients with AHDS, thus providing an outstanding animal model to study this human condition. In addition, it was previously shown in the zebrafish mct8 KD model that maternal T3 (MTH) acts as an integrator of different key developmental pathways during zebrafish development.

Methods

Using a zebrafish Mct8 knockdown model, with consequent inhibition of maternal thyroid hormones (MTH) uptake to the target cells, we analyzed genes modulated by MTH by qPCR in a temporal series from the start of segmentation through hatching. Survival (TUNEL) and proliferation (PH3) of neural progenitor cells (dla, her2) were determined, and the cellular distribution of neural MTH-target genes in the spinal cord during development was characterized. In addition, in-vivo live imaging was performed to access NOTCH overexpression action on cell division in this AHDS model. We determined the developmental time window when MTH is required for appropriate CNS development in the zebrafish; MTH is not involved in neuroectoderm specification but is fundamental in the early stages of neurogenesis by promoting the maintenance of specific neural progenitor populations. MTH signaling is required for developing different neural cell types and maintaining spinal cord cytoarchitecture, and modulation of NOTCH signaling in a non-autonomous cell manner is involved in this process.

Discussion

The findings show that MTH allows the enrichment of neural progenitor pools, regulating the cell diversity output observed by the end of embryogenesis and that Mct8 impairment restricts CNS development. This work contributes to the understanding of the cellular mechanisms underlying human AHDS.

Triac Treatment Prevents Neurodevelopmental and Locomotor Impairments in Thyroid Hormone Transporter Mct8/Oatp1c1 Deficient Mice

Patients with inactive thyroid hormone (TH) transporter MCT8 display intellectual disability due to compromised central TH transport and action. As a therapeutic strategy, application of thyromimetic, MCT8-independent compounds Triac (3,5,3'-triiodothyroacetic acid), and Ditpa (3,5-diiodo-thyropropionic acid) was proposed. Here, we directly compared their thyromimetic potential in Mct8/Oatp1c1 double knock-out mice (Dko) modeling human MCT8 deficiency. Dko mice received either Triac (50 ng/g or 400 ng/g) or Ditpa (400 ng/g or 4000 ng/g) daily during the first three postnatal weeks. Saline-injected Wt and Dko mice served as controls. A second cohort of Dko mice received Triac (400 ng/g) daily between postnatal weeks 3 and 6. Thyromimetic effects were assessed at different postnatal stages by immunofluorescence, ISH, qPCR, electrophysiological recordings, and behavior tests. Triac treatment (400 ng/g) induced normalized myelination, cortical GABAergic interneuron differentiation, electrophysiological parameters, and locomotor performance only when administered during the first three postnatal weeks. Ditpa (4000 ng/g) application to Dko mice during the first three postnatal weeks resulted in normal myelination and cerebellar development but only mildly improved neuronal parameters and locomotor function. Together, Triac is highly-effective and more efficient than Ditpa in promoting CNS maturation and function in Dko mice yet needs to be initiated directly after birth for the most beneficial effects.

TRIAC Treatment Improves Impaired Brain Network Function and White Matter Loss in Thyroid Hormone Transporter Mct8/Oatp1c1 Deficient Mice

Dysfunctions of the thyroid hormone (TH) transporting monocarboxylate transporter MCT8 lead to a complex X-linked syndrome with abnormal serum TH concentrations and prominent neuropsychiatric symptoms (Allan-Herndon-Dudley syndrome, AHDS). The key features of AHDS are replicated in double knockout mice lacking MCT8 and organic anion transporting protein OATP1C1 (Mct8/Oatp1c1 DKO). In this study, we characterize impairments of brain structure and function in Mct8/Oatp1c1 DKO mice using multimodal magnetic resonance imaging (MRI) and assess the potential of the TH analogue 3,3',5-triiodothyroacetic acid (TRIAC) to rescue this phenotype. Structural and functional MRI were performed in 11-weeks-old male Mct8/Oatp1c1 DKO mice (N = 10), wild type controls (N = 7) and Mct8/Oatp1c1 DKO mice (N = 13) that were injected with TRIAC (400 ng/g bw s.c.) daily during the first three postnatal weeks. Grey and white matter volume were broadly reduced in Mct8/Oatp1c1 DKO mice. TRIAC treatment could significantly improve white matter thinning but did not affect grey matter loss. Network-based statistic showed a wide-spread increase of functional connectivity, while graph analysis revealed an impairment of small-worldness and whole-brain segregation in Mct8/Oatp1c1 DKO mice. Both functional deficits could be substantially ameliorated by TRIAC treatment. Our study demonstrates prominent structural and functional brain alterations in Mct8/Oatp1c1 DKO mice that may underlie the psychomotor deficiencies in AHDS. Additionally, we provide preclinical evidence that early-life TRIAC treatment improves white matter loss and brain network dysfunctions associated with TH transporter deficiency.

Thyroid Hormone Transporters in Pregnancy and Fetal Development

Thyroid hormone is essential for fetal (brain) development. Plasma membrane transporters control the intracellular bioavailability of thyroid hormone. In the past few decades, 15 human thyroid hormone transporters have been identified, and among them, mutations in monocarboxylate transporter (MCT)8 and organic anion transporting peptide (OATP)1C1 are associated with clinical phenotypes. Different animal and human models have been employed to unravel the (patho)-physiological role of thyroid hormone transporters. However, most studies on thyroid hormone transporters focus on postnatal development. This review summarizes the research on the thyroid hormone transporters in pregnancy and fetal development, including their substrate preference, expression and tissue distribution, and physiological and pathophysiological role in thyroid homeostasis and clinical disorders. As the fetus depends on the maternal thyroid hormone supply, especially during the first half of pregnancy, the review also elaborates on thyroid hormone transport across the human placental barrier. Future studies may reveal how the different transporters contribute to thyroid hormone homeostasis in fetal tissues to properly facilitate development. Employing state-of-the-art human models will enable a better understanding of their roles in thyroid hormone homeostasis.

Tsh Induces Agrp1 Neuron Proliferation in Oatp1c1-Deficient Zebrafish

Thyroid hormones (THs), thyroxine (T4), and triiodothyronine (T3), regulate growth, metabolism, and neurodevelopment. THs secretion is controlled by the pituitary thyroid-stimulating hormone (TSH) and the hypothalamic-pituitary-thyroid (HPT) axis. The organic anion-transporting polypeptide 1C1 (OATP1C1/SLCO1C1) and the monocarboxylate transporter 8 (MCT8/SLC16A2) actively transport THs, which bind to their nuclear receptors and induce gene expression. A mutation in OATP1C1 is associated with brain hypometabolism, gradual neurodegeneration, and impaired cognitive and motor functioning in adolescent patients. To understand the role of Oatp1c1 and the mechanisms of the disease, we profiled the transcriptome of oatp1c1 mutant (oatp1c1 -/-) and mct8 -/- xoatp1c1 -/- adult male and female zebrafish brains. Among dozens of differentially expressed genes, agouti-related neuropeptide 1 (agrp1) expression increased in oatp1c1 -/- adult brains. Imaging in the hypothalamus revealed enhanced proliferation of Agrp1 neurons in oatp1c1 -/- larvae and adults, and increased food consumption in oatp1c1 -/- larvae. Similarly, feeding and the number of Agrp1 neurons increased in thyroid gland-ablated zebrafish. Pharmacological treatments showed that the T3 analog TRIAC (3,3',5-tri-iodothyroacetic acid), but not T4, normalized the number of Agrp1 neurons in oatp1c1 -/- zebrafish. Since the HPT axis is hyperactive in the oatp1c1 -/- brain, we used the CRISPR-Cas9 system to knockdown tsh in oatp1c1 -/- larvae, and inducibly enhanced the HPT axis in wild-type larvae. These manipulations showed that Tsh promotes proliferation of Agrp1 neurons and increases food consumption in zebrafish. The results revealed upregulation of both the HPT axis-Agrp1 circuitry and feeding in a zebrafish model for OATP1C1 deficiency.SIGNIFICANCE STATEMENT Mutation in the thyroid hormone (TH) transporter OATP1C1 is associated with cognitive and motor functioning disturbances in humans. Here, we used an oatp1c1 -/- zebrafish to understand the role of organic anion-transporting polypeptide 1C1 (Oatp1c1), and the characteristics of OATP1C1 deficiency. Transcriptome profiling identified upregulation of agrp1 expression in the oatp1c1 -/- brain. The oatp1c1 -/- larvae showed increased thyroid-stimulating hormone (tsh) levels, proliferation of Agrp1 neurons and food consumption. Genetic manipulations of the hypothalamic-pituitary-thyroid (HPT) axis showed that Tsh increases the number of Agrp1 neurons and food consumption. The T3 analog TRIAC (3,3',5-tri-iodothyroacetic acid) normalizes the number of Agrp1 neurons and may have potential for the treatment of Oatp1c1 deficiency. The findings demonstrate a functional interaction between the thyroid and feeding systems in the brain of zebrafish and suggest a neuroendocrinological mechanism for OATP1C1 deficiency.

A nationwide survey of monocarboxylate transporter 8 deficiency in Japan: Its incidence, clinical course, MRI and laboratory findings

BACKGROUND

Monocarboxylate transporter 8 (MCT8) deficiency is an X-linked recessive developmental disorder characterized by initially marked truncal hypotonia, later athetotic posturing, and severe intellectual disability caused by mutations in SLC16A2, which is responsible for the transport of triiodothyronine (T3) into neurons. We conducted a nationwide survey of patients with MCT8 deficiency to clarify their current status.

METHODS

Primary survey: In 2016-2017, we assessed the number of patients diagnosed with MCT8 deficiency from 1027 hospitals. Secondary survey: in 2017-2018, we sent case surveys to 31 hospitals (45 cases of genetic diagnosis), who responded in the primary survey. We asked for: 1) perinatal history, 2) developmental history, 3) head MRI findings, 4) neurophysiological findings, 5) thyroid function tests, and 5) genetic test findings.

RESULTS

We estimated the prevalence of MCT8 deficiency to be 1 in 1,890,000 and the incidence of MCT8 deficiency per million births to be 2.12 (95 % CI: 0.99-3.25). All patients showed severe psychomotor retardation, and none were able to walk or speak. The significantly higher value of the free T3/free T4 (fT3/fT4) ratio found in our study can be a simple and useful diagnostic biomarker (Our value 11.60 ± 4.14 vs control 3.03 ± 0.38). Initial white matter signal abnormalities on head MRI showed recovery, but somatosensory evoked potentials (SEP) showed no improvement, suggesting that the patient remained dysfunctional.

CONCLUSION

For early diagnosis, including in mild cases, it might be important to consider the clinical course, early head MRI, SEP, and fT3/fT4 ratio.

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.

Gene therapy targeting the blood-brain barrier improves neurological symptoms in a model of genetic MCT8 deficiency

A genetic deficiency of the solute carrier monocarboxylate transporter 8 (MCT8), termed Allan-Herndon-Dudley syndrome, is an important cause of X-linked intellectual and motor disability. MCT8 transports thyroid hormones across cell membranes. While thyroid hormone analogues improve peripheral changes of MCT8 deficiency, no treatment of the neurological symptoms is available so far. Therefore, we tested a gene replacement therapy in Mct8- and Oatp1c1-deficient mice as a well-established model of the disease. Here, we report that targeting brain endothelial cells for Mct8 expression by intravenously injecting the vector AAV-BR1-Mct8 increased tri-iodothyronine (T3) levels in the brain and ameliorated morphological and functional parameters associated with the disease. Importantly, the therapy resulted in a long-lasting improvement in motor coordination. Thus, the data support the concept that MCT8 mediates the transport of thyroid hormones into the brain and indicate that a readily accessible vascular target can help overcome the consequences of the severe disability associated with MCT8 deficiency.

IGSF1 Deficiency Leads to Reduced TSH Production Independent of Alterations in Thyroid Hormone Action in Male Mice

Loss of function mutations in IGSF1/Igsf1 cause central hypothyroidism. Igsf1 knockout mice have reduced pituitary thyrotropin-releasing hormone receptor, Trhr, expression, perhaps contributing to the phenotype. Because thyroid hormones negatively regulate Trhr, we hypothesized that IGSF1 might affect thyroid hormone availability in pituitary thyrotropes. Consistent with this idea, IGSF1 coimmunoprecipitated with the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in transfected cells. This association was impaired with IGSF1 bearing patient-derived mutations. Wild-type IGSF1 did not, however, alter MCT8-mediated thyroid hormone import into heterologous cells. IGSF1 and MCT8 are both expressed in the apical membrane of the choroid plexus. However, MCT8 protein levels and localization in the choroid plexus were unaltered in Igsf1 knockout mice, ruling out a necessary chaperone function for IGSF1. MCT8 expression was low in the pituitary and was similarly unaffected in Igsf1 knockouts. We next assessed whether IGSF1 affects thyroid hormone transport or action, by MCT8 or otherwise, in vivo. To this end, we treated hypothyroid wild-type and Igsf1 knockout mice with exogenous thyroid hormones. T4 and T3 inhibited TSH release and regulated pituitary and forebrain gene expression similarly in both genotypes. Interestingly, pituitary TSH beta subunit (Tshb) expression was consistently reduced in Igsf1 knockouts relative to wild-type regardless of experimental condition, whereas Trhr was more variably affected. Although IGSF1 and MCT8 can interact in heterologous cells, the physiological relevance of their association is not clear. Nevertheless, the results suggest that IGSF1 loss can impair TSH production independently of alterations in TRHR levels or thyroid hormone action.

AAV9-MCT8 Delivery at Juvenile Stage Ameliorates Neurological and Behavioral Deficits in a Mouse Model of MCT8-Deficiency

Background: Allan-Herndon-Dudley syndrome (AHDS) is a severe psychomotor disability disorder that also manifests characteristic abnormal thyroid hormone (TH) levels. AHDS is caused by inactivating mutations in monocarboxylate transporter 8 (MCT8), a specific TH plasma membrane transporter widely expressed in the central nervous system (CNS). MCT8 mutations cause impaired transport of TH across brain barriers, leading to insufficient neural TH supply. There is currently no successful therapy for the neurological symptoms. Earlier work has shown that intravenous (IV), but not intracerebroventricular adeno-associated virus serotype 9 (AAV9) -based gene therapy given to newborn Mct8 knockout (Mct8^-/y) male mice increased triiodothyronine (T₃) brain content and partially rescued TH-dependent gene expression, suggesting a promising approach to treat this neurological disorder. Methods: The potential of IV delivery of AAV9 carrying human MCT8 was tested in the well-established Mct8^-/y/Organic anion-transporting polypeptide 1c1 (Oatp1c1)^{-/ -} double knockout (dKO) mouse model of AHDS, which, unlike Mct8^-/y mice, displays both neurological and TH phenotype. Further, as the condition is usually diagnosed during childhood, treatment was given intravenously to P30 mice and psychomotor tests were carried out blindly at P120-P140 after which tissues were collected and analyzed. Results: Systemic IV delivery of AAV9-MCT8 at a juvenile stage led to improved locomotor and cognitive functions at P120-P140, which was accompanied by a near normalization of T₃ content and an increased response of positively regulated TH-dependent gene expression in different brain regions examined (thalamus, hippocampus, and parietal cortex). The effects on serum TH concentrations and peripheral tissues were less pronounced, showing only improvement in the serum T₃/reverse T₃ (rT₃) ratio and in liver deiodinase 1 expression. Conclusion: IV administration of AAV9, carrying the human MCT8, to juvenile dKO mice manifesting AHDS has long-term beneficial effects, predominantly on the CNS. This preclinical study indicates that this gene therapy has the potential to ameliorate the devastating neurological symptoms in patients with AHDS.

A novel frameshift mutation in Allan-Herndon-Dudley syndrome

Allan-Herndon-Dudley syndrome (AHDS) is a very rare, X-linked psychomotor disability syndrome with delayed myelination, almost exclusively affecting boys. We present a case of a 4-year-old boy with AHDS who was found cyanotic, with intermittent vomiting and paroxysmal convulsions about 4 h after his parents went out, and was then taken to the hospital, where he eventually died the next day. The autopsy revealed foreign bodies in the tiny bronchi and alveoli of the deceased, congestion, and punctate hemorrhage in multiple organs, consistent with the diagnosis of asphyxia. Compared with a normally developing 4-year-old boy, the deceased showed cerebral atrophy and cerebral edema, and Luxol Fast Blue (LFB) stain indicated delayed cerebellar, hippocampal, and basal ganglia development and myelination. A novel frameshift mutation c.584delG in the SLC16A2 gene was detected. Family lineage investigation showed that the mutation was also detected in the deceased's 8-year-old brother and biological mother. The present work enriches the profile mutations in SLC16A2 related to AHDS and emphasizes the importance of autopsy and postmortem genetic analysis in such cases.

Allan-Herndon-Dudley syndrome in a female patient and related mechanisms

Allan-Herndon-Dudley syndrome (AHDS) is characterized by neuropsychomotor developmental delay/intellectual disability, neurological impairment with a movement disorder, and an abnormal thyroid hormone profile. This disease is an X-linked disorder that mainly affects men. We described a female patient with a de novo variant in the SLC16A2 gene, a milder AHDS phenotype, and a skewed X chromosome inactivation profile. We discuss the mechanisms associated with the expression of the phenotypic characteristics in female patients, including SLC16A2 gene variants and cytogenomic alterations, as well as preferential inactivation of the normal X chromosome.

A novel variant in SLC16A2 associated with typical Allan-Herndon-Dudley syndrome: a case report

Background

Allan-Herndon-Dudley syndrome (AHDS) is an X-linked recessive neurodegenerative disorder caused by mutations in the SLC16A2 gene that encodes thyroid hormone transporter. AHDS has been rarely reported in China.

Case presentation

This study reported a novel splicing mutation in the SLC16A2 gene in an 18-month-old male patient with AHDS. The patient was born to non-consanguineous, healthy parents of Chinese origin. He passed new-born screening for hypothyroidism, but failed to reach developmental milestones. He presented with hypotonia, severe mental retardation, dysarthria and ataxia. Genetic analysis identified a novel splicing mutation, NM_006517.4: c.431-2 A > G, in the SLC16A2 gene inherited from his mother. The patient received Triac treatment, (triiodothyroacetic acid), a thyroid hormone analogue for 3 months. Triac treatment effectively reduced serum TSH concentrations and normalized serum T3 concentrations in the patient.

Conclusions

This study reported the first case of AHDS treated by Triac in China. And the study expanded the mutational spectrum of the SLC16A2 gene in AHDS patients.

Supplementary information

The online version contains supplementary material available at 10.1186/s12887-022-03259-5.

Fetal and Neonatal Thyroid Dysfunction

Fetal and neonatal dysfunctions include rare serious disorders involving abnormal thyroid function during the second half of gestation, which may persist throughout life, as for most congenital thyroid disorders, or be transient, resolving in the first few weeks of life, as in autoimmune hyperthyroidism or hypothyroidism and some cases of congenital hypothyroidism (CH) with the thyroid gland in situ. Primary CH is diagnosed by neonatal screening, which has been implemented for 40 years in developed countries and should be introduced worldwide, as early treatment prevents irreversible neurodevelopmental delay. Central CH is a rarer entity occurring mostly in association with multiple pituitary hormone deficiencies. Other rare disorders impair the action of thyroid hormones. Neonatal Graves' disease (GD) results from the passage of thyrotropin receptor antibodies (TRAbs) across the placenta, from mother to fetus. It may affect the fetuses and neonates of mothers with a history of current or past GD, but hyperthyroidism develops only in those with high levels of stimulatory TRAb activity. The presence of antibodies predominantly blocking thyroid-stimulating hormone receptors may result in transient hypothyroidism, possibly followed by neonatal hyperthyroidism, depending on the balance between the antibodies present. Antithyroid drugs taken by the mother cross the placenta, treating potential fetal hyperthyroidism, but they may also cause transient fetal and neonatal hypothyroidism. Early diagnosis and treatment are key to optimizing the child's prognosis. This review focuses on the diagnosis and management of these patients during the fetal and neonatal periods. It includes the description of a case of fetal and neonatal autoimmune hyperthyroidism.

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

CONTEXT

Patients with mutations in thyroid hormone transporter MCT8 have developmental delay and chronic thyrotoxicosis associated with being underweight and having cardiovascular dysfunction.

OBJECTIVE

Our previous trial showed improvement of key clinical and biochemical features during 1-year treatment with the T3 analogue Triac, but long-term follow-up data are needed.

METHODS

In this real-life retrospective cohort study, we investigated the efficacy of Triac in MCT8-deficient patients in 33 sites. The primary endpoint was change in serum T3 concentrations from baseline to last available measurement. Secondary endpoints were changes in other thyroid parameters, anthropometric parameters, heart rate, and biochemical markers of thyroid hormone action.

RESULTS

From October 15, 2014 to January 1, 2021, 67 patients (median baseline age 4.6 years; range, 0.5-66) were treated up to 6 years (median 2.2 years; range, 0.2-6.2). Mean T3 concentrations decreased from 4.58 (SD 1.11) to 1.66 (0.69) nmol/L (mean decrease 2.92 nmol/L; 95% CI, 2.61-3.23; P < 0.0001; target 1.4-2.5 nmol/L). Body-weight-for-age exceeded that of untreated historical controls (mean difference 0.72 SD; 95% CI, 0.36-1.09; P = 0.0002). Heart-rate-for-age decreased (mean difference 0.64 SD; 95% CI, 0.29-0.98; P = 0.0005). SHBG concentrations decreased from 245 (99) to 209 (92) nmol/L (mean decrease 36 nmol/L; 95% CI, 16-57; P = 0.0008). Mean creatinine concentrations increased from 32 (11) to 39 (13) µmol/L (mean increase 7 µmol/L; 95% CI, 6-9; P < 0.0001). Mean creatine kinase concentrations did not significantly change. No drug-related severe adverse events were reported.

CONCLUSIONS

Key features were sustainably alleviated in patients with MCT8 deficiency across all ages, highlighting the real-life potential of Triac for MCT8 deficiency.

Mathematical modeling and simulation of thyroid homeostasis: Implications for the Allan-Herndon-Dudley syndrome

INTRODUCTION

A mathematical model of the pituitary-thyroid feedback loop is extended to deepen the understanding of the Allan-Herndon-Dudley syndrome (AHDS). The AHDS is characterized by unusual thyroid hormone concentrations and a mutation in the SLC16A2 gene encoding for the monocarboxylate transporter 8 (MCT8). This mutation leads to a loss of thyroid hormone transport activity. One hypothesis to explain the unusual hormone concentrations of AHDS patients is that due to the loss of thyroid hormone transport activity, thyroxine (T ₄) is partially retained in thyroid cells.

METHODS

This hypothesis is investigated by extending a mathematical model of the pituitary-thyroid feedback loop to include a model of the net effects of membrane transporters such that the thyroid hormone transport activity can be considered. A nonlinear modeling approach based on the Michaelis-Menten kinetics and its linear approximation are employed to consider the membrane transporters. The unknown parameters are estimated through a constrained parameter optimization.

RESULTS

In dynamic simulations, damaged membrane transporters result in a retention of T ₄ in thyroid cells and ultimately in the unusual hormone concentrations of AHDS patients. The Michaelis-Menten modeling approach and its linear approximation lead to similar results.

DISCUSSION

The results support the hypothesis that a partial retention of T ₄ in thyroid cells represents one mechanism responsible for the unusual hormone concentrations of AHDS patients. Moreover, our results suggest that the retention of T ₄ in thyroid cells could be the main reason for the unusual hormone concentrations of AHDS patients.

Movement disorders in MCT8 deficiency/Allan-Herndon-Dudley Syndrome

BACKGROUND AND OBJECTIVES

MCT8 deficiency is a rare genetic leukoencephalopathy caused by a defect of thyroid hormone transport across cell membranes, particularly through blood brain barrier and into neural cells. It is characterized by a complex neurological presentation, signs of peripheral thyrotoxicosis and cerebral hypothyroidism. Movement disorders (MDs) have been frequently mentioned in this condition, but not systematically studied.

METHODS

Each patient recruited was video-recorded during a routine outpatient visit according to a predefined protocol. The presence and the type of MDs were evaluated. The type of MD was blindly scored by two child neurologists experts in inherited white matter diseases and in MD. Dystonia was scored according to Burke-Fahn-Marsden Dystonia Rating Scale (BFMDRS). When more than one MD was present, the predominant one was scored.

RESULTS

27 patients were included through a multicenter collaboration. In many cases we saw a combination of different MDs. Hypokinesia was present in 25/27 patients and was the predominant MD in 19. It was often associated with hypomimia and global hypotonia. Dystonia was observed in 25/27 patients, however, in a minority of cases (5) it was deemed the predominant MD. In eleven patients, exaggerated startle reactions and/or other paroxysmal non-epileptic events were observed.

CONCLUSION

MDs are frequent clinical features of MCT8 deficiency, possibly related to the important role of thyroid hormones in brain development and functioning of normal dopaminergic circuits of the basal ganglia. Dystonia is common, but usually mild to moderate in severity, while hypokinesia was the predominant MD in the majority of patients.