[A toddler with status dystonicus due to medication]. / Een peuter met een status dystonicus door medicatie.

BACKGROUND: Status dystonicus (SD) is a severe episode of generalized dystonia, potentially complicated by respiratory and metabolic disruption. Triggers can be infection, medication, or metabolic disturbance. The prognosis is variable and mortality is approximately 10%. CASE DESCRIPTION: An 18 month old girl presented to the ER with clinical suspicion of a febrile status epilepticus and was evaluated according to APLS principles. Eventually, a SD became apparent, with generalized dystonic features at examination. Most likely, the episode was provoked by a single dose of metoclopramide. Her clinical state improved rapidly, possibly aided by administration of biperiden. CONCLUSION: Treatment of SD encompasses elimination or treatment of the trigger, stabilization of vital functions, possible administration of sedatives and dystonia specific medication. Metoclopramide holds a relatively high risk for extrapyramidal complications (1-10%) and dystonia (0.1-1.5%), even within therapeutic range. The use of anti-emetics with less alarming side effect profiles, for example ondansetron, is recommended.

Effectiveness and safety of the tri-iodothyronine analogue Triac in children and adults with MCT8 deficiency: an international, single-arm, open-label, phase 2 trial.

BACKGROUND: Deficiency of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) causes severe intellectual and motor disability and high serum tri-iodothyronine (T3) concentrations (Allan-Herndon-Dudley syndrome). This chronic thyrotoxicosis leads to progressive deterioration in bodyweight, tachycardia, and muscle wasting, predisposing affected individuals to substantial morbidity and mortality. Treatment that safely alleviates peripheral thyrotoxicosis and reverses cerebral hypothyroidism is not yet available. We aimed to investigate the effects of treatment with the T3 analogue Triac (3,3',5-tri-iodothyroacetic acid, or tiratricol), in patients with MCT8 deficiency. METHODS: In this investigator-initiated, multicentre, open-label, single-arm, phase 2, pragmatic trial, we investigated the effectiveness and safety of oral Triac in male paediatric and adult patients with MCT8 deficiency in eight countries in Europe and one site in South Africa. Triac was administered in a predefined escalating dose schedule-after the initial dose of once-daily 350 µg Triac, the daily dose was increased progressively in 350 µg increments, with the goal of attaining serum total T3 concentrations within the target range of 1·4-2·5 nmol/L. We assessed changes in several clinical and biochemical signs of hyperthyroidism between baseline and 12 months of treatment. The prespecified primary endpoint was the change in serum T3 concentrations from baseline to month 12. The co-primary endpoints were changes in concentrations of serum thyroid-stimulating hormone (TSH), free and total thyroxine (T4), and total reverse T3 from baseline to month 12. These analyses were done in patients who received at least one dose of Triac and had at least one post-baseline evaluation of serum throid function. This trial is registered with ClinicalTrials.gov, number NCT02060474. FINDINGS: Between Oct 15, 2014, and June 1, 2017, we screened 50 patients, all of whom were eligible. Of these patients, four (8%) patients decided not to participate because of travel commitments. 46 (92%) patients were therefore enrolled in the trial to receive Triac (median age 7·1 years [range 0·8-66·8]). 45 (98%) participants received Triac and had at least one follow-up measurement of thyroid function and thus were included in the analyses of the primary endpoints. Of these 45 patients, five did not complete the trial (two patients withdrew [travel burden, severe pre-existing comorbidity], one was lost to follow-up, one developed of Graves disease, and one died of sepsis). Patients required a mean dose of 38.3 µg/kg of bodyweight (range 6·4-84·3) to attain T3 concentrations within the target range. Serum T3 concentration decreased from 4·97 nmol/L (SD 1·55) at baseline to 1·82 nmol/L (0·69) at month 12 (mean decrease 3·15 nmol/L, 95% CI 2·68-3·62; p<0·0001), while serum TSH concentrations decreased from 2·91 mU/L (SD 1·68) to 1·02 mU/L (1·14; mean decrease 1·89 mU/L, 1·39-2·39; p<0·0001) and serum free T4 concentrations decreased from 9·5 pmol/L (SD 2·5) to 3·4 (1·6; mean decrease 6·1 pmol/L (5·4-6·8; p<0·0001). Additionally, serum total T4 concentrations decreased by 31·6 nmol/L (28·0-35·2; p<0·0001) and reverse T3 by 0·08 nmol/L (0·05-0·10; p<0·0001). Seven treatment-related adverse events (transiently increased perspiration or irritability) occurred in six (13%) patients. 26 serious adverse events that were considered unrelated to treatment occurred in 18 (39%) patients (mostly hospital admissions because of infections). One patient died from pulmonary sepsis leading to multi-organ failure, which was unrelated to Triac treatment. INTERPRETATION: Key features of peripheral thyrotoxicosis were alleviated in paediatric and adult patients with MCT8 deficiency who were treated with Triac. Triac seems a reasonable treatment strategy to ameliorate the consequences of untreated peripheral thyrotoxicosis in patients with MCT8 deficiency. FUNDING: Dutch Scientific Organization, Sherman Foundation, NeMO Foundation, Wellcome Trust, UK National Institute for Health Research Cambridge Biomedical Centre, Toulouse University Hospital, and Una Vita Rara ONLUS.

The thyroid hormone transporters MCT8 and MCT10 transport the affinity-label N-bromoacetyl-[(125)I]T3 but are not modified by it.

Thyroid hormone (TH) transporter proteins mediate transport of TH across the plasma membrane, thereby facilitating its intracellular bioavailability. As only a few transporters have been identified which are relatively specific for TH, including monocarboxylate transporter (MCT) 8 and MCT10, the need for identification of novel specific TH transporters is obvious. A possible strategy to identify TH transporters is their modification with a ligand-derived affinity-label and subsequent identification by mass spectrometry. Previously, N-bromoacetyl (BrAc)-iodothyronines have been reported as useful affinity-labels for human (h) MCT8. In the present study we reinvestigated possible BrAc[(125)I]T3-labeling of hMCT8 and hMCT10. The present study demonstrates that hMCT8 and hMCT10 both facilitate BrAc[(125)I]T3 transport, but are not labeled by BrAc[(125)I]T3. We provide evidence that human protein disulfide isomerase, which molecular mass is similar to hMCT8, is labeled by BrAc[(125)I]T3. In addition, differential inhibitory effects were observed of iodothyronines derivatives with different side chains on T3 transport by hMCT8 and hMCT10. In conclusion, we demonstrated that not hMCT8 and hMCT10, but human protein disulfide isomerase, is labeled by BrAc[(125)I]T3. The usefulness of BrAc[(125)I]T3 as a tool for the identification of novel TH transporters remains to be explored.

Evidence for a homodimeric structure of human monocarboxylate transporter 8.

The human monocarboxylate transporter 8 (hMCT8) protein mediates transport of thyroid hormone across the plasma membrane. Association of hMCT8 mutations with severe psychomotor retardation and disturbed thyroid hormone levels has established its physiological relevance, but little is still known about the basic properties of hMCT8. In this study we present evidence that hMCT8 does not form heterodimers with the ancillary proteins basigin, embigin, or neuroplastin, unlike other MCTs. In contrast, it is suggested that MCT8 exists as monomer and homodimer in transiently and stably transfected cells. Apparently hMCT8 forms stable dimers because the complex is resistant to denaturing conditions and dithiothreitol. Cotransfection of wild-type hMCT8 with a mutant lacking amino acids 267-360 resulted in formation of homo-and heterodimers of the variants, indicating that transmembrane domains 4-6 are not involved in the dimerization process. Furthermore, we explored the structural and functional role of the 10 Cys residues in hMCT8. All possible Cys>Ala mutants did not behave differently from wild-type hMCT8 in protein expression, cross-linking experiments with HgCl(2) and transport function. Our findings indicate that individual Cys residues are not important for the function of hMCT8 or suggest that hMCT8 has other yet-undiscovered functions in which cysteines play an essential role.

Novel pathogenic mechanism suggested by ex vivo analysis of MCT8 (SLC16A2) mutations.

Monocarboxylate transporter 8 (MCT8; approved symbol SLC16A2) facilitates cellular uptake and efflux of 3,3',5-triiodothyronine (T3). Mutations in MCT8 are associated with severe psychomotor retardation, high serum T3 and low 3,3',5'-triiodothyronine (rT3) levels. Here we report three novel MCT8 mutations. Two subjects with the F501del mutation have mild psychomotor retardation with slightly elevated T3 and normal rT3 levels. T3 uptake was mildly affected in F501del fibroblasts and strongly decreased in fibroblasts from other MCT8 patients, while T3 efflux was always strongly reduced. Moreover, type 3 deiodinase activity was highly elevated in F501del fibroblasts, whereas it was reduced in fibroblasts from other MCT8 patients, probably reflecting parallel variation in cellular T3 content. Additionally, T3-responsive genes were markedly upregulated by T3 treatment in F501del fibroblasts but not in fibroblasts with other MCT8 mutations. In conclusion, mutations in MCT8 result in a decreased T3 uptake in skin fibroblasts. The much milder clinical phenotype of patients with the F501del mutation may be correlated with the relatively small decrease in T3 uptake combined with an even greater decrease in T3 efflux. If fibroblasts are representative of central neurons, abnormal brain development associated with MCT8 mutations may be the consequence of either decreased or increased intracellular T3 concentrations.

Effective cellular uptake and efflux of thyroid hormone by human monocarboxylate transporter 10.

Cellular entry of thyroid hormone is mediated by plasma membrane transporters, among others a T-type (aromatic) amino acid transporter. Monocarboxylate transporter 10 (MCT10) has been reported to transport aromatic amino acids but not iodothyronines. Within the MCT family, MCT10 is most homologous to MCT8, which is a very important iodothyronine transporter but does not transport amino acids. In view of this paradox, we decided to reinvestigate the possible transport of thyroid hormone by human (h) MCT10 in comparison with hMCT8. Transfection of COS1 cells with hMCT10 cDNA resulted in 1) the production of an approximately 55 kDa protein located to the plasma membrane as shown by immunoblotting and confocal microscopy, 2) a strong increase in the affinity labeling of intracellular type I deiodinase by N-bromoacetyl-[(125)I]T(3), 3) a marked stimulation of cellular T(4) and, particularly, T(3) uptake, 4) a significant inhibition of T(3) uptake by phenylalanine, tyrosine, and tryptophan of 12.5%, 22.2%, and 51.4%, respectively, and 5) a marked increase in the intracellular deiodination of T(4) and T(3) by different deiodinases. Cotransfection studies using the cytosolic thyroid hormone-binding protein micro-crystallin (CRYM) indicated that hMCT10 facilitates both cellular uptake and efflux of T(4) and T(3). In the absence of CRYM, hMCT10 and hMCT8 increased T(3) uptake after 5 min incubation up to 4.0- and 1.9-fold, and in the presence of CRYM up to 6.9- and 5.8-fold, respectively. hMCT10 was less active toward T(4) than hMCT8. These findings establish that hMCT10 is at least as active a thyroid hormone transporter as hMCT8, and that both transporters facilitate iodothyronine uptake as well as efflux.

Thyroid hormone transport in and out of cells.

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

Genotype-phenotype relationship in patients with mutations in thyroid hormone transporter MCT8.

Loss-of-function mutations in thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to severe X-linked psychomotor retardation and elevated serum T(3) levels. Most patients, for example those with mutations V235M, S448X, insI189, or delF230, cannot stand, walk, or speak. Patients with mutations L434W, L568P, and S194F, however, walk independently and/or develop some dysarthric speech. To study the relationship between mutation and phenotype, we transfected JEG3 and COS1 cells with wild-type or mutant MCT8. Expression and function of the transporter were studied by analyzing T(3) and T(4) uptake, T(3) metabolism (by cotransfected type 3 deiodinase), Western blotting, affinity labeling with N-bromoacetyl-T(3), immunocytochemistry, and quantitative RT-PCR. Wild-type MCT8 increased T(3) uptake and metabolism about 5-fold compared with empty vector controls. Mutants V235M, S448X, insI189, and delF230 did not significantly increase transport. However, S194F, L568P, and L434W showed about 20, 23, and 37% of wild-type activity. RT-PCR did not show significant differences in mRNA expression between wild-type and mutant MCT8. Immunocytochemistry detected the nonfunctional mutants V235M, insI189, and delF230 mostly in the cytoplasm, whereas mutants with residual function were expressed at the plasma membrane. Mutants S194F and L434W showed high protein expression but low affinity for N-bromoacetyl-T(3); L568P was detected in low amounts but showed relatively high affinity. Mutations in MCT8 cause loss of function through reduced protein expression, impaired trafficking to the plasma membrane, or reduced substrate affinity. Mutants L434W, L568P, and S194F showed significant residual transport capacity, which may underlie the more advanced psychomotor development observed in patients with these mutations.

Thyroid hormone transport by monocarboxylate transporters.

Thyroid hormone (TH) is essential for the normal development and metabolism of different tissues. TH action and metabolism take place intracellularly, which requires cellular uptake via transporters. Several transporter families have been identified, of which the monocarboxylate transporter (MCT) family deserves special attention. So far, only MCT1, MCT2, MCT3, MCT4 and MCT6 have been demonstrated to transport monocarboxylates; MCT8 has been identified as a specific TH transporter. MCT8 mutations in humans are associated with severe psychomotor retardation and elevated 3,3',5-triiodothyronine (T(3)) levels. Recently, MCT8 knockout mice have been shown to perfectly imitate the thyroid state in patients with MCT8 mutations; however, they lack the neurological defects. Although it was long hypothesized that a T-type amino acid transporter also transports iodothyronines, it only recently became clear that MCT10 is involved in the bidirectional transport of aromatic amino acids and iodothyronines. MCT10 preferentially transports T(3) even more effectively than does MCT8. However, its precise function in the human body is poorly understood.

Functional analysis of monocarboxylate transporter 8 mutations identified in patients with X-linked psychomotor retardation and elevated serum triiodothyronine.

CONTEXT: T(3) action in neurons is essential for brain development. Recent evidence indicates that monocarboxylate transporter 8 (MCT8) is important for neuronal T(3) uptake. Hemizygous mutations have been identified in the X-linked MCT8 gene in boys with severe psychomotor retardation and elevated serum T(3) levels. OBJECTIVE: The objective of this study was to determine the functional consequences of MCT8 mutations regarding transport of T(3). DESIGN: MCT8 function was studied in wild-type or mutant MCT8-transfected JEG3 cells by analyzing: 1) T(3) uptake, 2) T(3) metabolism in cells cotransfected with human type 3 deiodinase, 3) immunoblotting, and 4) immunocytochemistry. RESULTS: The mutations identified in MCT8 comprise four deletions (24.5 kb, 2.4 kb, 14 bp, and 3 bp), three missense mutations (Ala224Val, Arg271His, and Leu471Pro), a nonsense mutation (Arg245stop), and a splice site mutation (94 amino acid deletion). All tested mutants were inactive in uptake and metabolism assays, except MCT8 Arg271His, which showed approximately 20% activity vs. wild-type MCT8. CONCLUSION: These findings support the hypothesis that the severe psychomotor retardation and elevated serum T(3) levels in these patients are caused by inactivation of the MCT8 transporter, preventing action and metabolism of T(3) in central neurons.

Mechanisms of disease: psychomotor retardation and high T3 levels caused by mutations in monocarboxylate transporter 8.

The actions and the metabolism of thyroid hormone are intracellular events that require the transport of iodothyronines across the plasma membrane. It is increasingly clear that this process does not occur by simple diffusion, but is facilitated by transport proteins. Only recently have iodothyronine transporters been identified at the molecular level, of which organic anion transporting polypeptide 1C1 and monocarboxylate transporter 8 (MCT8) deserve special mention, because of their high activity and specificity for iodothyronines. Organic anion transporting polypeptide 1C1 is almost exclusively expressed in brain capillaries, and may be crucial for the transport of the prohormone T4 across the blood-brain barrier. MCT8 is also expressed in the brain--in particular, in neurons--but also in other tissues. MCT8 seems to be especially important for the uptake of active hormone T3 into neurons, which is essential for optimal brain development. T3 is produced from T4 by type 2 deiodinase in neighboring astrocytes. Neurons express type 3 deiodinase, the enzyme that terminates T3 activity. The SLC16A2 (formerly MCT8) gene is located on chromosome Xq13.2 and has recently been associated with a syndrome combining severe, X-linked, psychomotor retardation and high serum T3 levels. In over 20 families, where affected males have developed this syndrome, several mutations in MCT8 have been identified. The disease mechanism is thought to involve a defect in the neuronal entry of T3 and, therefore, in the action and metabolism of T3 in these cells. This defect results in impaired neurological development and a decrease in T3 clearance.

Thyroid hormone transport by the human monocarboxylate transporter 8 and its rate-limiting role in intracellular metabolism.

Cellular entry of thyroid hormone is mediated by plasma membrane transporters. We have identified rat monocarboxylate transporter 8 (MCT8) as an active and specific thyroid hormone transporter. The MCT8 gene is located on the X-chromosome. The physiological relevance of MCT8 has been demonstrated by the identification of hemizygous mutations in this gene in males with severe psychomotor retardation and elevated serum T(3) levels. We have characterized human (h) MCT8 by analysis of iodothyronine uptake and metabolism in cell lines transiently transfected with hMCT8 cDNA alone or together with cDNA coding for iodothyronine deiodinase D1, D2, or D3. MCT8 mRNA was detected by RT-PCR in a number of human cell lines as well as in COS1 cells but was low to undetectable in other cell lines, including JEG3 cells. MCT8 protein was not detected in nontransfected cell lines tested by immunoblotting using a polyclonal C-terminal hMCT8 antibody but was detectable in transfected cells at the expected size (61 kDa). Transfection of COS1 and JEG3 cells with hMCT8 cDNA resulted in 2- to 3-fold increases in uptake of T(3) and T(4) but little or no increase in rT(3) or 3,3'-diiodothyronine (3,3'-T(2)) uptake. MCT8 expression produced large increases in T(4) metabolism by cotransfected D2 or D3, T(3) metabolism by D3, rT(3) metabolism by D1 or D2, and 3,3'-T(2) metabolism by D3. Affinity labeling of hMCT8 protein was observed after incubation of intact transfected cells with N-bromoacetyl-[(125)I]T(3). hMCT8 also facilitated affinity labeling of cotransfected D1 by bromoacetyl-T(3). Our findings indicate that hMCT8 mediates plasma membrane transport of iodothyronines, thus increasing their intracellular availability.

Cervicogenic headache. Smith/Robinson approach in bilateral cases.

The aim was to follow the postoperative fate of cervicogenic headache (CEH) patients with a hard-to-treat, bilateral headache, operated upon by the Smith/Robinson procedure, a stabilization and decompression operation in the cervical spine. CEH is a typically unilateral headache, but in this study, bilateral cases were deliberately selected (n=28). The patients were, otherwise, diagnosed according to the Cervicogenic Headache International Study Group (CHISG) CEH criteria. In most cases, the discs C(4-5), C(5-6) and C(6-7) were affected, and one or two discs were removed by anterior approach; an interbody fusion was carried out. Immediately postoperatively - up to 2-3 months - there was pain freedom. Secondary deterioration was reported to us in 10 patients, in nine of whom it occurred within three years. The remaining 18 patients were followed up for 2-100 months; the mean duration of improvement was 22.7 months. Bilateral, Smith/Robinson operated CEH patients seemed to fare as well as unilateral ones.

Thyroid hormone transporters in health and disease.

Cellular entry is required for conversion of thyroid hormone by the intracellular deiodinases and for binding of 3,3',5-triiodothyronine (T(3)) to its nuclear receptors. Recently, several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have demonstrated that organic anion transporters (e.g., OATPs), and L-type amino acid transporters (LATs) facilitate thyroid hormone uptake. Among these, OATP1C1 has a high affinity and specificity for thyroxine (T(4)). OATP1C1 is expressed in capillaries throughout the brain, suggesting it is critical for transport of T(4) over the blood-brain barrier. We have also characterized a member of the monocarboxylate transporter family, MCT8, as a very active and specific thyroid hormone transporter. Human MCT8 shows preference for T(3) as the ligand. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. The MCT8 gene is located on the X chromosome. Recently, mutations in MCT8 have been found to be associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels.

Thyroid hormone transporters.

Thyroid hormone is essential for the development of the brain and the nervous system. Cellular entry is required for conversion of thyroid hormones by the intracellular deiodinases and for binding of T(3) to its nuclear receptors. Several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have so far identified two categories of transporters involved in thyroid hormone uptake (i.e., organic anion transporters and amino acid transporters). Among the organic anion transporters, both Na(+) taurocholate cotransporting polypeptide (NTCP) and various members of the organic anion transporting polypeptide (OATP) family mediate transport of iodothyronines. Because iodothyronines are a particular class of amino acids derived from tyrosine residues, it is no surprise that some amino acid transporters have been shown to be involved in thyroid hormone transport. We have characterized monocarboxylate transporter 8 (MCT8) as a very active and specific thyroid hormone transporter, the gene of which is located on the X chromosome. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. MCT8 shows 50% amino acid identity with a system T amino acid transporter 1 (TAT1). TAT1, also called MCT10, has been characterized to transport aromatic amino acids but no iodothyronines. We have also found that mutations in MCT8 are associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels in young boys.