SLC16A2 Mutations Research Articles

Development and metamorphosis in frogs deficient in the thyroid hormone transporter MCT8

Precisely regulated thyroid hormone (TH) signaling within tissues during frog metamorphosis gives rise to the organism-wide coordination of developmental events among organs required for survival. This TH signaling is controlled by multiple cellular mechanisms, including TH transport across the plasma membrane. A highly specific TH transporter has been identified, namely monocarboxylate transporter 8 (MCT8), which facilitates uptake and efflux of TH and is differentially and dynamically expressed among tissues during metamorphosis. We hypothesized that loss of MCT8 would alter tissue sensitivity to TH and affect the timing of tissue transformation. To address this, we used CRISPR/Cas9 to introduce frameshift mutations inslc16a2, the gene encoding MCT8, inXenopus laevis. We produced homozygous mutant tadpoles with a 29-bp mutation in the l-chromosome and a 20-bp mutation in the S-chromosome. We found that MCT8 mutants survive metamorphosis with normal growth and development of external morphology throughout the larval period. Consistent with this result, the expression of the pituitary hormone regulating TH plasma levels (tshb) was similar among genotypes as was TH response gene expression in brain at metamorphic climax. Further, delayed initiation of limb outgrowth during natural metamorphosis and reduced hindlimb and tail TH sensitivity were not observed in MCT8 mutants. In sum, we did not observe an effect on TH-dependent development in MCT8 mutants, suggesting compensatory TH transport occurs in tadpole tissues, as seen in most tissues in all model organisms examined.

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

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

Heterogeneous Clinical Characteristics of Allan-Herndon-Dudley Syndrome with SLC16A2 Mutations

Purpose: The purpose of this study was to expand our understanding of phenotypic and genetic variation in Allan-Herndon-Dudley syndrome (AHDS), which is a rare X-linked mental retardation syndrome characterized by hypotonia, generalized spasticity, and moderate-to-severe psychomotor retardation. AHDS is caused by a mutation of solute carrier family 16 member 2 (SLC16A2), which encodes monocarboxylate transporter 8 (MCT8), the transporter of triiodothyronine (T3) into neurons. Methods: We enrolled nine patients with AHDS from unrelated families, except for two patients who were cousins, through a retrospective chart review. Clinical features, brain imaging, electroencephalograms, thyroid hormone profiles, and genetic data were reviewed retrospectively and compared with previously reported cases. Results: We found three novel and five previously reported pathogenic variants in nine patients from eight families. All patients presented with hypotonia, spasticity, severe developmental delay, and elevated serum T3 levels. Cataplexy, which is a previously unreported phenotype, was found in two patients with the same mutation. In our cohort, seizures were uncommon (n=1) but intractable. Conclusion: This study broadens the known phenotypic variations of AHDS, ranging from relatively mild global developmental delay to a severe form of encephalopathy with hypotonia, spasticity, and no acquisition of independent sitting. The syndromic classification or genetic etiology of global developmental delay is extremely heterogeneous; therefore, early clinical suspicion is challenging for clinicians. However, severe mental retardation with hypotonia, spasticity, and elevated serum T3 levels in male patients is a highly suspicious clinical clue for the early diagnosis of AHDS.

Variable Clinical Characteristics and Molecular Spectrum of Patients with Syndromes of Reduced Sensitivity to Thyroid Hormone: Genetic Defects in the THRB and SLC16A2 Genes

Background/Aims: Syndromes of reduced sensitivity to thyroid hormone can be caused by innate resistance to thyroid hormone (RTH), thyroid hormone cell transporter defects, or thyroid hormone metabolism defects. This study was performed to describe clinical, endocrinological, and molecular characteristics of patients with disorders associated with impaired sensitivity to thyroid hormone due to THRB or SLC16A2 mutations. Methods: This study included 5 probands (1 male and 4 females) with RTH and 6 patients with Allan-Herndon-Dudley syndrome (AHDS). Clinical features and endocrine findings were reviewed retrospectively. Molecular analysis of two candidate genes, THRB or SLC16A2, confirmed the diagnosis. Results: Among RTH patients, median age at diagnosis was 5.6 years. Three patients were classified as having generalized RTH, whereas the other 2 patients were regarded as having isolated pituitary RTH. Three novel heterozygous mutations and 2 known mutations in THRB were identified from 5 independent pedigrees. All mutations were located in the major ligand-binding domain. In AHDS patients, delayed development was apparent between 3 and 6 months of age. Direct sequencing of SLC16A2 identified 6 hemizygous missense mutations in each patient: p.I188N, p.G221R, p.A224V, p.G276R, p.W398R, and p.G401R. Conclusions: This study identified 3 novel mutations in THRB in RTH patients and 1 novel mutation in SLC16A2 in AHDS patients. Routine neonatal screening based on the TSH assay has a limited role in detecting RTH or AHDS. Therefore, genetic testing of the candidate genes THRB and SLC16A2 should be performed for diagnosis of RTH and AHDS in patients with the suggestive clinical phenotype.

Genome-Scale Analysis Identifies Novel Genetic Vulnerabilities in Allan Herndon Dudley Syndrome

Allan Herndon Dudley syndrome (AHDS), an X-linked condition, constitutes a rare, progressive, clinically heterogeneous disorder involving severe psychomotor retardation and peripheral thyrotoxicosis, consequent to thyroid hormone transporter SLC16A2 mutations. However, direct targeted screening for SLC16A2 mutations in patients with AHDS partly predicts phenotypic heterogeneity although similar mutations are associated with differences in clinical phenotype severity, suggesting the existence of additional gene-related AHDS subtypes as in other types of mental retardation. Specifically, we report a family with AHDS comprising five individuals, including three male children (of whom two were monozygotic twins). In this pedigree, we identified a linkage of two X-linked inherited genes including SLC16A2 (p. G453R) and KIF4A (p. A491V) as a putative AHDS etiology. Additionally, we identified an independent cluster of structural variations (SVs)-enriched potential AHDS susceptibility genes, which might be as the neuronal genetic vulnerabilities likely contributing to AHDS. Our observations suggest that AHDS phenotype heterogeneity may arise from multi-genetic contributions. These familial cases, involving likely novel AHDS subtypes, provide an opportunity to explain neuropsychiatric retardation. An integrated next generation mapping and whole genome sequencing strategy for refining AHDS diagnosis and clinical categorization is recommended inclusive of cases with the related overlapping phenotypes in psychomotor disabilities. Funding Statement: This work was supported by the Stem Cell Clinical Trial Project of the National Health and Family Planning Commission of China and the China Food and Drug Administration (Registration No. CMR-201611-1003), (Medical Research Registry and Management Systemhttp://114.255.123.14), and the National Nature Science Foundation of China (Grant No. 81471308). Declaration of Interests: We declare no conflict of interest. Ethics Approval Statement: Informed consent was obtained from the parents, and the study was approved by the Research Ethics Boards at the First Affiliated Hospital of Dalian Medical University (approval number: YJ-KY-FB-2017-27)

Clinical and genetic features of five patients with Allan-Herndon-Dudley syndrome

To delineate the clinical and genetic characteristics of patients with Allan-Herndon-Dudley syndrome (AHDS). Genetic testing was carried out by next generation sequencing on 117 patients featuring intellectual disability and developmental delay. Clinical information including clinical manifestation, brain magnetic resonance imaging(MRI）, thyroid hormone levels, and electrocardiogram was collected for those with SLC16A2 mutations. Five male patients with SLC16A2 gene mutations were identified, including 2 affected brothers and 3 sporadic cases. The ages of the patients ranged from 8 months to 8 years. All patients presented with severe intellectual disability and developmental delay including poor head control, inability to sit independently, no speech, and poor response to external stimuli. All patients presented with hypotonia, dystonia, and positive pyramidal signs. Three patients had sinus tachycardia. All patients had abnormal thyroid hormone levels with elevated free triiodothyronine (FT3), decreased free tetraiodothyronine(FT4), and normal thyroid stimulating hormone (TSH). Brain MRI on 3 patients showed delayed myelination. Among the 3 sporadic patients, 2 carried de novo mutations including c.61G to T(p.E21X) and c.695_699delATGGT(p.N232SfsX7), respectively, 1 carried a c.42delC(p.W15GfsX69)mutation, which was inherited from his heterozygous mother. A nonsense mutation (c.916C to T, p.Q306X) was discovered in the two brothers, for which their mother was heterozygous. AHDS is characterized by severe psychomotor developmental delay as well as congenital hypotonia, dystonia and positive pyramidal signs. Affected males may present with distinctive thyroid hormone abnormalities including increased FT3 and low FT4 accompanied by normal TSH. Delayed meylination of white matter is common. It is an X-linked mental retardation caused by SLC16A2 gene mutations.

Novel SLC16A2 mutations in Chinese patients with Allan-Herndon-Dudley Syndrome

Novel SLC16A2 mutations in Chinese patients with Allan-Herndon-Dudley Syndrome

Deafness and loss of cochlear hair cells in the absence of thyroid hormone transporters Slc16a2 (Mct8) and Slc16a10 (Mct10)

Transmembrane proteins that mediate the cellular uptake or efflux of thyroid hormone potentially provide a key level of control over neurodevelopment. In humans, defects in one such protein, solute carrier SLC16A2 (MCT8) are associated with psychomotor retardation. Other proteins that transport the active form of thyroid hormone triiodothyronine (T3) or its precursor thyroxine (T4) have been identified in vitro but the wider significance of such transporters in vivo is unclear. The development of the auditory system requires thyroid hormone and the cochlea is a primary target tissue. We have proposed that the compartmental anatomy of the cochlea would necessitate transport mechanisms to convey blood-borne hormone to target tissues. We report hearing loss in mice with mutations in Slc16a2 and a related gene Slc16a10 (Mct10, Tat1). Deficiency of both transporters results in retarded development of the sensory epithelium similar to impairment caused by hypothyroidism, compounded with a progressive degeneration of cochlear hair cells and loss of endocochlear potential. Administration of T3 largely restores the development of the sensory epithelium and limited auditory function, indicating the T3-sensitivity of defects in the sensory epithelium. The results indicate a necessity for thyroid hormone transporters in cochlear development and function.

Novel SLC16A2 mutations in patients with Allan-Herndon-Dudley syndrome.

Allan-Herndon-Dudley syndrome (AHDS) is an X-linked disorder caused by impaired thyroid hormone transporter. Patients with AHDS usually exhibit severe motor developmental delay, delayed myelination of the brain white matter, and elevated T3 levels in thyroid tests. Neurological examination of two patients with neurodevelopmental delay revealed generalized hypotonia, and not paresis, as the main neurological finding. Nystagmus and dyskinesia were not observed. Brain magnetic resonance imaging demonstrated delayed myelination in early childhood in both patients. Nevertheless, matured myelination was observed at 6 years of age in one patient. Although the key finding for AHDS is elevated free T3, one of the patients showed a normal T3 level in childhood, misleading the diagnosis of AHDS. Genetic analysis revealed two novel SLC16A2 mutations, p.(Gly122Val) and p.(Gly221Ser), confirming the AHDS diagnosis. These results indicate that AHDS diagnosis is sometimes challenging owing to clinical variability among patients.

Thyroid hormone transporters--functions and clinical implications.

The cellular influx and efflux of thyroid hormones are facilitated by transmembrane protein transporters. Of these transporters, monocarboxylate transporter 8 (MCT8) is the only one specific for the transport of thyroid hormones and some of their derivatives. Mutations in SLC16A2, the gene that encodes MCT8, lead to an X-linked syndrome with severe neurological impairment and altered concentrations of thyroid hormones. Histopathological analysis of brain tissue from patients who have impaired MCT8 function indicates that brain lesions start prenatally, and are most probably the result of cerebral hypothyroidism. A Slc16a2 knockout mouse model has revealed that Mct8 is an important mediator of thyroid hormone transport, especially T3, through the blood-brain barrier. However, unlike humans with an MCT8 deficiency, these mice do not have neurological impairment. One explanation for this discrepancy could be differences in expression of the T4 transporter OATP1C1 in the blood-brain barrier; OATP1C1 is more abundant in rodents than in primates and permits the passage of T4 in the absence of T3 transport, thus preventing full cerebral hypothyroidism. In this Review, we discuss the relevance of thyroid hormone transporters in health and disease, with a particular focus on the pathophysiology of MCT8 mutations.

SLC16A2 mutations in two Japanese patients with Allan-Herndon-Dudley syndrome.

Allan–Herndon–Dudley syndrome (AHDS) is a neurodevelopmental disorder that manifests as intellectual disability and motor developmental delay. Thyroid hormone transporter dysfunction due to SLC16A2 mutation is the underlying cause of this disorder. We identified a novel (P537del) and a recurrent (A150V) SLC16A2 mutation in Japanese AHDS patients from two different families. A150V co-segregated with S33P. Both patients showed similar clinical features including severe neurological features and delayed myelination. Thyroid function showed a common finding of elevated T3 levels. No clear genotype–phenotype correlation was observed in patients with SLC16A2 alterations.

A Novel Deletion Mutation of SLC16A2 Encoding Monocarboxylate Transporter (MCT) 8 in a 26-year-old Japanese Patient with Allan-Herndon-Dudley Syndrome.

Allan-Herndon-Dudley Syndrome (AHDS), an X linked condition, is characterized by congenital hypotonia that progresses to spasticity with severe psychomotor delays, in combination with altered thyroid hormone levels, in particular, high serum T3 levels. Recently, this disease was proved to be caused by mutations in SLC16A2 coding for the monocarboxylate thyroid hormone transporter 8 (MCT8). Here we describe a 26-year -old Japanese patient with AHDS who had deletion of exon 3 of SLC16A2.

A Novel Deletion Mutation of <i>SLC16A2</i> Encoding Monocarboxylate Transporter (MCT) 8 in a 26-year-old Japanese Patient with Allan-Herndon-Dudley Syndrome

Allan-Herndon-Dudley Syndrome (AHDS), an X linked condition, is characterized by congenital hypotonia that progresses to spasticity with severe psychomotor delays, in combination with altered thyroid hormone levels, in particular, high serum T3 levels. Recently, this disease was proved to be caused by mutations in SLC16A2 coding for the monocarboxylate thyroid hormone transporter 8 (MCT8). Here we describe a 26-year -old Japanese patient with AHDS who had deletion of exon 3 of SLC16A2.

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

Mutations in SLC16A2, the gene encoding the thyroid hormone (TH)-specific transporter monocarboxylate transporter 8 (MCT8), result in a thyroid phenotype and severe mental retardation caused by neuronal TH deficiency. These mutational effects raise the question of whether polymorphic variation in SLC16A2 may also be associated with differences in serum levels of TH and/or TSH. This is the first major study of the frequency of the SLC16A2 rs6647476 single nucleotide polymorphism (SNP) (amino acid change Ser107Pro). We also studied the relationships of SLC16A2 genetic variants with serum levels of TSH, T4 and T3, with their mRNA expression and with expression of the TH-responsive genes ZAKI-4 and BTEB in white blood cells. Experiments in cultured fibroblasts were carried out to ascertain the dynamics of the T3 response. A total of 276 men were studied. Genotyping of the S107P SNP was carried out using polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP); serum hormone levels were determined by chemiluminescence; expression of mRNA was quantified by real-time PCR. The SLC16A2 S107P SNP was found in 36% of Galician males. With the present sample size we did not find any association of this polymorphism with variability in serum levels of TSH, free T4 (fT4) or fT3, or with basal expression of mRNA for SLC16A2 or the two TH-responsive genes ZAKI-4 and BTEB, either in white blood cells or in cultured human fibroblasts from either Ser107 or Pro107 genotypes under T3 stimulation. The S107P change in MCT8 is frequent in the male population in Galicia. In the population studied in this report an association with a thyroid phenotype was not demonstrated, even though the S107P SNP causes an important amino acid change.