Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0025362 (mental retardation)
 15,878 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We restudied a family with X-linked mental retardation (XLMR) originally reported in abstract form by Davis et al. [1981]. All 8 living affected males were examined. Characteristics included severe mental retardation, spastic paraplegia, dysarthria, muscle wasting, scoliosis, broad shallow pectus excavatum, long face, large ears with minor modeling anomalies, foot deformities, joint contractures, and neck drop. Stature, OFC, testicular volume, high resolution chromosome and fragile X studies, and plasma amino acids were all normal. Their manifestations closely resemble those of a large family with XLMR originally reported by Allan et al. [1944] and restudied by Stevenson et al. [1990]. This condition has been termed the Allan-Herndon-Dudley syndrome (AHDS). As AHDS has been mapped to Xq21, mapping studies were undertaken to determine if this family maps to the same location. These studies demonstrate tight linkage to Xq21, with a maximum lod score of 2.88 obtained with probe pX65H7 (DXS72). Multipoint analysis located the mutant gene quite close to pX65H7 (multipoint Z = 4.14), slightly more proximal in Xq21 than was suggested by the data from the original AHDS family. It appears likely that this family is the second reported family with AHDS.
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PMID:Allan-Herndon-Dudley syndrome: clinical and linkage studies on a second family. 160 31

Genetic studies in families with X linked mental retardation have suggested the location of several MR genes in the human q21 region. Since the establishment of cloned resources is an essential step towards the cloning of genes involved in inherited diseases, we built a yeast artificial chromosome (YAC) contig and an STS map of this part of the X chromosome. The contig, which extends from PGK1 in Xq13.3 to DXS1002 in Xq21.2, consists of 30 YACs mapped with 21 markers and spans about 6 Mb. The YAC contig was used as a framework to localise several previously known genes and CEPH/Genethon polymorphic markers, as well as to construct a physical map of the region surrounding one of these genes. We recently localised a presumed MR locus to the region flanked by DXS233 (proximal) and CHM (distal). In the present work, the zinc finger gene, ZNF6, has been shown to lie within this region and to be highly expressed in brain, making it a good candidate MR gene. Similarly the VDAC1 gene has been mapped between DXS986 and DXS72 and its candidate gene status for the Allan-Herndon-Dudley syndrome is discussed.
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PMID:Localisation of two candidate genes for mental retardation using a YAC physical map of the Xq21.1-21.2 subbands. 873 41

The fragile X syndrome, an X-linked disease, is the most frequent cause of inherited mental retardation. The syndrome results from the absence of expression of the FMR1 gene (fragile mental retardation 1) owing to the expansion of a CGG trinucleotide repeat located in the 5' untranslated region of the gene and the subsequent methylation of its CpG island. The FMR1 gene product (FMRP) is a cytoplasmic protein that contains two KH domains and one RGG box, characteristics of RNA-binding proteins. FMRP is associated with mRNP complexes containing poly(A)+mRNA within actively translating polyribosomes and contains nuclear localization and export signals making it a putative transporter (chaperone) of mRNA from the nucleus to the cytoplasm. FMRP is the archetype of a novel family of cytoplasmic RNA-binding proteins that includes FXR1P and FXR2P. Both of these proteins are very similar in overall structure to FMRP and are also associated with cytoplasmic mRNPs. Members of the FMR family are widely expressed in mouse and human tissues, albeit at various levels, and seem to play a subtle choreography of expression. FMRP is most abundant in neurons and is absent in muscle. FXR1P is strongly expressed in muscle and low levels are detected in neurons. The complex expression patterns of the FMR1 gene family in different cells and tissues suggest that independent, however similar, functions for each of the three FMR-related proteins might be expected in the selection and metabolism of tissue-specific classes of mRNA. The molecular mechanisms altered in cells lacking FMRP still remain to be elucidated as well as the putative role(s) of FXR1P and FXR2P as compensatory molecules.
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PMID:Biology of the fragile X mental retardation protein, an RNA-binding protein. 1054 96

Allan-Herndon-Dudley syndrome was among the first of the X-linked mental retardation syndromes to be described (in 1944) and among the first to be regionally mapped on the X chromosome (in 1990). Six large families with the syndrome have been identified, and linkage studies have placed the gene locus in Xq13.2. Mutations in the monocarboxylate transporter 8 gene (MCT8) have been found in each of the six families. One essential function of the protein encoded by this gene appears to be the transport of triiodothyronine into neurons. Abnormal transporter function is reflected in elevated free triiodothyronine and lowered free thyroxine levels in the blood. Infancy and childhood in the Allan-Herndon-Dudley syndrome are marked by hypotonia, weakness, reduced muscle mass, and delay of developmental milestones. Facial manifestations are not distinctive, but the face tends to be elongated with bifrontal narrowing, and the ears are often simply formed or cupped. Some patients have myopathic facies. Generalized weakness is manifested by excessive drooling, forward positioning of the head and neck, failure to ambulate independently, or ataxia in those who do ambulate. Speech is dysarthric or absent altogether. Hypotonia gives way in adult life to spasticity. The hands exhibit dystonic and athetoid posturing and fisting. Cognitive development is severely impaired. No major malformations occur, intrauterine growth is not impaired, and head circumference and genital development are usually normal. Behavior tends to be passive, with little evidence of aggressive or disruptive behavior. Although clinical signs of thyroid dysfunction are usually absent in affected males, the disturbances in blood levels of thyroid hormones suggest the possibility of systematic detection through screening of high-risk populations.
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PMID:Allan-Herndon-Dudley syndrome and the monocarboxylate transporter 8 (MCT8) gene. 1588 50

We report a family with X-linked mental retardation that has a novel mutation in the monocarboxylate transporter 8 (MCT8) gene associated with a characteristic neurodevelopmental phenotype with early childhood hypotonia that progresses to spasticity and global developmental delays. Affected patients experience moderate to severe psychomotor delays and congenital hypotonia, develop a myopathic facies, have diminished muscle bulk and generalized muscle weakness, develop progressive spasticity and movement disorders, and have limited speech but alert, affable personalities. Acquired microcephaly and abnormal myelination on brain magnetic resonance imaging can be present. Normal monocarboxylate transporter 8 gene functioning appears to be necessary for normal thyroid-associated metabolism in neurons. Abnormal thyroid function tests appear to be a consistent finding in the absence of typical signs of thyroid dysfunction. Although the phenotype appears to be consistent, and although the neurotoxic effects of abnormal central and peripheral neuromuscular system thyroid metabolism might be partly or wholly responsible for the neurologic phenotype reported, the exact mechanism remains unclear.
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PMID:X-linked MCT8 gene mutations: characterization of the pediatric neurologic phenotype. 1641 86

Mutations in the thyroid monocarboxylate transporter 8 gene (MCT8/SLC16A2) have been reported to result in X-linked mental retardation (XLMR) in patients with clinical features of the Allan-Herndon-Dudley syndrome (AHDS). We performed MCT8 mutation analysis including 13 XLMR families with LOD scores >2.0, 401 male MR sibships and 47 sporadic male patients with AHDS-like clinical features. One nonsense mutation (c.629insA) and two missense changes (c.1A>T and c.1673G>A) were identified. Consistent with previous reports on MCT8 missense changes, the patient with c.1673G>A showed elevated serum T3 level. The c.1A>T change in another patient affects a putative translation start codon, but the same change was present in his healthy brother. In addition normal serum T3 levels were present, suggesting that the c.1A>T (NM_006517) variation is not responsible for the MR phenotype but indicates that MCT8 translation likely starts with a methionine at position p.75. Moreover, we characterized a de novo translocation t(X;9)(q13.2;p24) in a female patient with full blown AHDS clinical features including elevated serum T3 levels. The MCT8 gene was disrupted at the X-breakpoint. A complete loss of MCT8 expression was observed in a fibroblast cell-line derived from this patient because of unfavorable nonrandom X-inactivation. Taken together, these data indicate that MCT8 mutations are not common in non-AHDS MR patients yet they support that elevated serum T3 levels can be indicative for AHDS and that AHDS clinical features can be present in female MCT8 mutation carriers whenever there is unfavorable nonrandom X-inactivation.
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PMID:MCT8 mutation analysis and identification of the first female with Allan-Herndon-Dudley syndrome due to loss of MCT8 expression. 1839 36

Thyroid hormone transport into cells requires plasma membrane transport proteins. Mutations in one of these, monocarboxylate transporter 8 (MCT8), have been identified as underlying cause for the Allan-Herndon-Dudley syndrome, an X-linked mental retardation in which the patients also present with abnormally high 3',3,5-triiodothyronine (T(3)) plasma levels. Mice deficient in Mct8 replicate the thyroid hormone abnormalities observed in the human condition. However, no neurological deficits have been described in mice lacking Mct8. Therefore, we subjected Mct8-deficient mice to a comprehensive immunohistochemical, neurological, and behavioral screen. Several behavioral abnormalities were found in the mutants. Interestingly, some of these behavioral changes are compatible with hypothyroidism, whereas others rather indicate hyperthyroidism. We thus hypothesized that neurons exclusively dependent on Mct8 are in a hypothyroid state, whereas neurons expressing other T(3) transporters become hyperthyroid, if they are exposed directly to the high plasma T(3). The majority of T(3) uptake in primary cortical neurons is mediated by Mct8, but pharmacological inhibition suggested functional expression of additional T(3) transporter classes. mRNAs encoding six T(3) transporters, including L-type amino acid transporters (LATs), were coexpressed with Mct8 in isolated neurons. We then demonstrated Lat2 expression in cultured neurons and throughout murine brain development. In contrast, LAT2 is expressed in microglia in the developing human brain during gestation, but not in neurons. We suggest that lack of functional complementation by alternative thyroid hormone transporters in developing human neurons precipitates the devastating neurodevelopmental phenotype in MCT8-deficient patients, whereas Mct8-deficient mouse neurons are functionally complemented by other transporters, for possibly Lat2.
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PMID:Neuronal 3',3,5-triiodothyronine (T3) uptake and behavioral phenotype of mice deficient in Mct8, the neuronal T3 transporter mutated in Allan-Herndon-Dudley syndrome. 1964 Nov 7

The Allan-Herndon-Dudley syndrome (AHDS;MIM 300523) of X-linked mental retardation and hypotonia is caused by mutations in a thyroid hormone transporter gene--the monocarboxylate transporter 8 (MCT8 also known as SLC16A2) gene. A 23-month-old boy with severe developmental delay, hypotonia, recurrent emesis, and irritability is described. He was diagnosed with hypothyroidism at the age of 4 months. However, T3 level was elevated. Molecular analysis of the MCT8 gene detected a single base duplication in exon 5 c.1614dupC (p.Ile539fs), consistent with a diagnosis of AHDS. While T3 is the best marker for this disorder, elevations in TSH should alert to the diagnosis.
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PMID:Elevated TSH levels in a mentally retarded boy. 1993 87

Thyroid hormone (TH) is crucial for the development of different organs, in particular the brain, as disturbances in TH supply cause severe neurological abnormalities. TH transporters are necessary for the intracellular availability of TH to have access to the deiodinases and nuclear receptors inside the cell. The clinical importance of TH transporters is dramatically shown in patients with mutations in MCT8, suffering from severe X-linked psychomotor retardation in combination with disturbed TH levels, especially high serum T(3) levels, now referred as Allan-Herndon-Dudley Syndrome (AHDS). Worldwide >45 families have now been identified with MCT8 mutations. Most MCT8 mutations result in a complete loss of TH transport function when tested in vitro, but some mutations show significant residual activity and are associated with a somewhat milder clinical phenotype. It is difficult to identify MCT8 patients only on the basis of the clinical characteristics of X-linked mental retardation. Therefore, the criterion for MCT8 mutation screening in these patients is the profile of increased T(3) and low-normal to low FT(4) serum levels.
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PMID:Genetics and phenomics of thyroid hormone transport by MCT8. 2008 55

Thyroid hormones are known to be essential for growth, development and metabolism. Recently mutations in the SLC16A2 gene coding for the monocarboxylate thyroid hormone transporter 8, MCT8, have been associated with Allan-Herndon-Dudley syndrome (AHDS), an X-linked condition characterized by severe mental retardation, dysarthria, athetoid movements, muscle hypoplasia and spastic paraplegia. Here we describe in detail the clinical and biochemical features in a boy affected by AHDS with severe neurological abnormalities and a novel de novo SLC16A2 gene insertion, 1343-1344insGCCC, resulting in a truncated protein lacking the last four transmembrane domains (TMDs) as well as the carboxyl cytoplasmic end. He presents mental retardation, axial hypotonia, hypertonia of arms and legs, paroxysmal dyskinesias, seizures. The endocrine phenotype showed low serum total and free thyroxine (T4), very elevated total and free triiodothyronine (T3) and normal thyrotropin (TSH) with blunted response to thyrotropin-releasing hormone (TRH). The latter finding was unexpected and suggested that the lack of functional MCT8 was counterbalanced at the thyrotrope cell level by high serum T3 concentration and/or by increased intrapituitary type 2 deiodinase (D2) activity. Our case constitutes a relevant contribution to better characterize this disorder and to elucidate the functional consequences of SLC16A2 gene mutations.
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PMID:Allan-Herndon-Dudley syndrome (AHDS) caused by a novel SLC16A2 gene mutation showing severe neurologic features and unexpectedly low TRH-stimulated serum TSH. 2071 92


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