Article

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

Department of Internal Medicine, Erasmus Medical Center, Room Ee502, Dr Molewaterplein 50, Rotterdam, The Netherlands.
Journal of Clinical Endocrinology &amp Metabolism (Impact Factor: 6.31). 07/2007; 92(6):2378-81. DOI: 10.1210/jc.2006-2570
Source: PubMed

ABSTRACT 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.
The objective of this study was to determine the functional consequences of MCT8 mutations regarding transport of T(3).
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.
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.
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.

0 Followers
 · 
72 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Background: Allan-Herndon-Dudley syndrome (AHDS) is an X-linked type of mental retardation resulting from hindered thyroid hormone access to neurons. Clustered non-recurrent deletions of SLC16A2 exon 1 have been described in three patients with AHDS. We report a fourth patient with such a deletion, and discuss possible mechanisms leading to these rearrangements. Case Presentation: A three-and-a-half year old male with clinical and biochemical AHDS phenotype and a history of normal neonatal screening for hypothyroidism underwent SLC16A2 molecular analysis. Unexpectedly, he showed skeletal signs of hypothyroidism. Methods and Results: The exons of the SLC16A2 (MCT8) gene and the sequences surrounding exon 1 were amplified using PCR. The patient had a 36 kb deletion affecting exon 1 of SLC16A2. The deletion junction was subjected to bioinformatic analyses, along with two other reported exon 1 deletion junctions, identifying possible sequence features and mechanisms responsible for such genomic rearrangements. Discussion/Conclusion: This patient had a classic AHDS phenotype with an unexpectedly large anterior fontanel and delayed bone age and dentition. Bioinformatic analyses suggested that exon 1 deletions in patients with AHDS are caused by microhomology-mediated replicative-based and non-homologous end-joining mechanisms. Rearrangement susceptibility may be due to the size of intron 1 and the percentage of repeat sequences.
    Thyroid: official journal of the American Thyroid Association 12/2014; 25(3). DOI:10.1089/thy.2014.0284 · 3.84 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Monocarboxylate transporter 8 (MCT8) transports thyroid hormone (TH) across the plasma membrane. Mutations in MCT8 result in the Allan-Herndon-Dudley syndrome, comprising severe psychomotor retardation and elevated serum T3 levels. Because the neurological symptoms are most likely caused by a lack of TH transport into the central nervous system, the administration of a TH analog that does not require MCT8 for cellular uptake may represent a therapeutic strategy. Here, we investigated the therapeutic potential of the biologically active T3 metabolite Triac (TA3) by studying TA3 transport, metabolism, and action both in vitro and in vivo. Incubation of SH-SY5Y neuroblastoma cells and MO3.13 oligodendrocytes with labeled substrates showed a time-dependent uptake of T3 and TA3. In intact SH-SY5Y cells, both T3 and TA3 were degraded by endogenous type 3 deiodinase, and they influenced gene expression to a similar extent. Fibroblasts from MCT8 patients showed an impaired T3 uptake compared with controls, whereas TA3 uptake was similar in patient and control fibroblasts. In transfected cells, TA3 did not show significant transport by MCT8. Most importantly, treatment of athyroid Pax8-knockout mice and Mct8/Oatp1c1-double knockout mice between postnatal days 1 and 12 with TA3 restored T3-dependent neural differentiation in the cerebral and cerebellar cortex, indicating that TA3 can replace T3 in promoting brain development. In conclusion, we demonstrated uptake of TA3 in neuronal cells and in fibroblasts of MCT8 patients and similar gene responses to T3 and TA3. This indicates that TA3 bypasses MCT8 and may be used to improve the neural status of MCT8 patients.
    Molecular Endocrinology 11/2014; 28(12):me20141135. DOI:10.1210/me.2014-1135 · 4.20 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Thyroid hormones (TH) cross the plasma membrane with the help of transporter proteins. As charged amino acid derivatives, TH cannot simply diffuse across a lipid bilayer membrane, despite their notorious hydrophobicity. The identification of monocarboxylate transporter 8 (MCT8, SLC16A2) as a specific and very active TH transporter paved the way to the finding that mutations in the MCT8 gene cause a syndrome of psychomotor retardation in humans. The purpose of this review is to introduce the current model of transmembrane transport and highlight the diversity of TH transmembrane transporters. The interactions of TH with plasma transfer proteins, T3 receptors, and deiodinase are summarized. It is shown that proteins may bind TH owing to their hydrophobic character in hydrophobic cavities and/or by specific polar interaction with the phenolic hydroxyl, the aminopropionic acid moiety, and by weak polar interactions with the iodine atoms. These findings are compared with our understanding of how TH transporters interact with substrate. The presumed effects of mutations in MCT8 on protein folding and transport function are explained in light of the available homology model.
    09/2014; 3(3):143-53. DOI:10.1159/000367858

Full-text (2 Sources)

Download
29 Downloads
Available from
May 16, 2014