Ferdy S. van Geest

Background: Patients with resistance to thyroid hormone β (RTHβ) show elevated thyroid hormone concentrations with non-suppressed thyroid-stimulating hormone (TSH) concentrations and large phenotypic variability. Triac therapy has been successfully applied in some patients. Mosaic mutations causing mild RTHβ have been reported three times so far. Patient: We present a case of severe RTHβ caused by a mosaic frameshift mutation in the thyroid hormone receptor β (p.R438Lfs445X). Methimazole and Triac combination therapy was commenced at the age of 8 years, resulting in a substantial decrease in free T4 concentrations and an increase in TSH concentrations (follow-up duration of >18 months). His extreme agitation, motor restlessness, weight, and some motor and communication skills improved. The mutation was fully unresponsive to stimulation with T3 in in vitro and ex vivo analyses. Conclusion: The p.R438Lfs445X mutation leads to a severe phenotype of RTHβ. Mosaicism might underlie a subset of patients with the clinical phenotype of RTHβ. Combined methimazole/Triac therapy had beneficial effects on several thyrotoxic features.

Context: Monocarboxylate transporter (MCT) 8 facilitates thyroid hormone (TH) transport across the blood-brain barrier. Pathogenic variants in SLC16A2 cause MCT8 deficiency (Allan-Herndon-Dudley syndrome), characterized by intellectual and motor disability and abnormal thyroid function tests. MCT8 deficiency typically affects males due to its X-linked inheritance. Objective: Here, we report 8 female patients with heterozygous pathogenic variants in SLC16A2 who presented with variable neurocognitive impairment, behavioral problems, and TH function abnormalities. Methods: We performed X-chromosome inactivation studies in female patients in whom heterozygous pathogenic variants in SLC16A2 were identified. The effect of SLC16A2 variants on TH transport was assessed in transfected cells and patient-derived fibroblasts. Results: In all patients (mean age 8.6 years; range, 2.3-25 years) routine care genetic analyses identified heterozygous variants in SLC16A2 (p.(R445C), p.(N193I), p.(G276R), t(X;20), resulting in a breakpoint in intron 1, t(X;19), resulting in a breakpoint in SLC16A2, p.(I562Sfs566*), p.(G221R)). All missense variants showed substantially reduced MCT8-mediated TH uptake in transiently transfected cells. X-chromosome inactivation studies in patient cells showed skewed X-inactivation in all 7 evaluated individuals. In 5 out of 7 evaluated cases, MCT8-mediated 3,5,3′-triiodothyronine (T3) uptake in patient-derived fibroblasts was impaired to a similar degree as in fibroblasts derived from male patients with MCT8 deficiency. Conclusion: Female patients with heterozygous pathogenic variants in SLC16A2 and skewed X-chromosome inactivation may present with variable neuro(psycho)logical, behavioral, and thyroid function test abnormalities. Female patients presenting with neurocognitive impairment and abnormal TH function tests (low free thyroxine and/or high total T3 concentrations) should be tested for genetic variants in SLC16A2.

Thyroid hormones (the common name for prohormone thyroxine and the bioactive form triiodothyronine) control major developmental and metabolic processes. Release of thyroid hormones from the thyroid gland into the bloodstream and their transport into target cells is facilitated by plasma membrane transporters, including monocarboxylate transporter (MCT)8 and the highly homologous MCT10. However, the molecular mechanism underlying thyroid hormone transport is unknown. The relevance of such transporters is illustrated in patients with MCT8 deficiency, a severe neurodevelopmental and metabolic disorder. Using cryogenic-sample electron microscopy (cryo-EM), we determined the ligand-free and thyroxine-bound human MCT8 structures in the outward-facing state and the thyroxine-bound human MCT10 in the inward-facing state. Our structural analysis revealed a network of conserved gate residues involved in conformational changes upon thyroxine binding, triggering ligand release in the opposite compartment. We then determined the structure of a folded but inactive patient-derived MCT8 mutant, indicating a subtle conformational change which explains its reduced transport activity. Finally, we report a structure of MCT8 bound to its inhibitor silychristin, locked in the outward-facing state, revealing the molecular basis of its action and specificity. Taken together, this study advances mechanistic understanding of normal and disordered thyroid hormone transport.

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.

Thyroid hormones (the common name for prohormone thyroxine and the bioactive form triiodothyronine) control major developmental and metabolic processes. Release of thyroid hormones from the thyroid gland into the bloodstream and their transport into target cells is facilitated by plasma membrane transporters, of which monocarboxylate transporter (MCT)8 and the highly homologous MCT10 are most important. Patients with MCT8 mutations suffer from a severe neurodevelopmental and metabolic disorder, however, the molecular mechanism underlying thyroid hormone transport is unknown. Using cryogenic-sample electron microscopy (cryo-EM), we determined the ligand-free and thyroxine-bound human MCT8 structures in the outward-open state and the thyroxine-bound human MCT10 in the inward-facing state. Our structural analysis revealed a network of conserved gate residues involved in conformational changes upon thyroxine binding, triggering ligand release on the opposite compartment. We then determined the structure of a folded, but inactive patient-derived MCT8 mutant, indicating a subtle conformational change which explains its reduced transport activity. In addition, we determined the structure of MCT8 bound to its inhibitor silychristin, revealing an interaction with residues essential to drive transition to the inward-facing state, thereby locking the protein in the outward-facing state. This study provides molecular and structural insights into normal and disordered thyroid hormone transport.

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.

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.

Defective thyroid hormone transport due to deficiency in thyroid hormone transporter monocarboxylate transporter 8 (MCT8) results in severe neurodevelopmental delay due to cerebral hypothyroidism and in clinical negative sequelae following a chronic thyrotoxic state in peripheral tissues. The life expectancy of patients with MCT8 deficiency is severely impaired. Increased mortality is associated with lack of head control and being underweight at young age. Treatment options are available to alleviate the thyrotoxic state; particularly, treatment with the thyroid hormone analogue triiodothyroacetic acid seems a promising therapy. This review provides an overview of key clinical features and treatment options available and under development for this rare disorder.

Context Genetic variants in SLC16A2, encoding the thyroid hormone transporter MCT8, can cause intellectual and motor disability and abnormal serum thyroid function tests, known as MCT8 deficiency. The C-terminal domain of MCT8 is poorly conserved, which complicates prediction of the deleteriousness of variants in this region. We studied the functional consequences of 5 novel variants within this domain and their relation to the clinical phenotypes. Methods We enrolled male subjects with intellectual disability in whom genetic variants were identified in exon 6 of SLC16A2. The impact of identified variants was evaluated in transiently transfected cell lines and patient-derived fibroblasts. Results Seven individuals from 5 families harbored potentially deleterious variants affecting the C-terminal domain of MCT8. Two boys with clinical features considered atypical for MCT8 deficiency had a missense variant [c.1724A>G;p.(His575Arg) or c.1796A>G;p.(Asn599Ser)] that did not affect MCT8 function in transfected cells or patient-derived fibroblasts, challenging a causal relationship. Two brothers with classical MCT8 deficiency had a truncating c.1695delT;p.(Val566*) variant that completely inactivated MCT8 in vitro. The 3 other boys had relatively less-severe clinical features and harbored frameshift variants that elongate the MCT8 protein [c.1805delT;p.(Leu602HisfsTer680) and c.del1826-1835;p.(Pro609GlnfsTer676)] and retained ~50% residual activity. Additional truncating variants within transmembrane domain 12 were fully inactivating, whereas those within the intracellular C-terminal tail were tolerated. Conclusions Variants affecting the intracellular C-terminal tail of MCT8 are likely benign unless they cause frameshifts that elongate the MCT8 protein. These findings provide clinical guidance in the assessment of the pathogenicity of variants within the C-terminal domain of MCT8.