SLC6A8 creatine transporter deficiency can be detected by plasma creatine and creatinine concentrations.

Creatine transporter deficiency has been described with normal or uninformative levels of creatine and creatinine in plasma, while urine has been the preferred specimen type for biochemical diagnosis. We report a cohort of untreated patients with creatine transporter deficiency and abnormal plasma creatine panel results, characterized mainly by markedly decreased plasma creatinine. We conclude that plasma should be considered a viable specimen type for the biochemical diagnosis of this disorder, and abnormal results should be followed up with further confirmatory testing.

Long-Term Efficacy of T3 Analogue Triac in Children and Adults With MCT8 Deficiency: A Real-Life Retrospective Cohort Study.

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.

Measurement of Reverse Triiodothyronine Level and the Triiodothyronine to Reverse Triiodothyronine Ratio in Dried Blood Spot Samples at Birth May Facilitate Early Detection of Monocarboxylate Transporter 8 Deficiency.

Background: Monocarboxylate transporter 8 (MCT8) deficiency is an X-chromosome-linked neurodevelopmental disorder resulting from impaired thyroid hormone transport across the cell membrane. The diagnosis of MCT8 deficiency is typically delayed owing to the late appearance of signs and symptoms as well as the inability of standard biomarkers of neonatal screening to provide early detection. In this study, we report, for the first time, the ability to detect MCT8 deficiency at birth using dried blood spot (DBS) samples. Methods: We retrospectively measured triiodothyronine (T3), thyroxine (T4), and reverse T3 (rT3) levels in DBS samples obtained at 4-5 days of life from 6 infants with genetically confirmed MCT8 deficiency and from 110 controls. The latter consisted of 58 healthy term neonates obtained at the same time, 16 were stored for more than 1 year before measurement to match samples from the MCT8-deficient infants. Ten DBS samples were collected at day 1 of life and 42 samples were from prematurely born neonates. Measurements were carried out in extract from eight millimeters diameter DBS using liquid chromatography-tandem mass spectrometry. Results: Contrary to characteristic iodothyronine abnormalities of MCT8 deficiency during later life, T3 and T4 values were not discriminatory from those of other study groups. In contrast, rT3 was significantly lower. The T3/rT3 ratio was higher in the DBS samples from the MCT8-deficient infants compared with all other groups with no overlap (p < 0.0001). Conclusions: rT3 and T3/rT3 ratio in DBS samples obtained from neonates can serve as biomarkers to detect MCT8 deficiency at birth.

Downregulation of the GHRH/GH/IGF1 axis in a mouse model of Börjeson-Forssman-Lehman syndrome.

Börjeson-Forssman-Lehmann syndrome (BFLS) is an intellectual disability and endocrine disorder caused by plant homeodomain finger 6 (PHF6) mutations. Individuals with BFLS present with short stature. We report a mouse model of BFLS, in which deletion of Phf6 causes a proportional reduction in body size compared with control mice. Growth hormone (GH) levels were reduced in the absence of PHF6. Phf6-/Y animals displayed a reduction in the expression of the genes encoding GH-releasing hormone (GHRH) in the brain, GH in the pituitary gland and insulin-like growth factor 1 (IGF1) in the liver. Phf6 deletion specifically in the nervous system caused a proportional growth defect, indicating a neuroendocrine contribution to the phenotype. Loss of suppressor of cytokine signaling 2 (SOCS2), a negative regulator of growth hormone signaling partially rescued body size, supporting a reversible deficiency in GH signaling. These results demonstrate that PHF6 regulates the GHRH/GH/IGF1 axis.

Expanding the phenotypic spectrum of Allan-Herndon-Dudley syndrome in patients with SLC16A2 mutations.

The aim of the study was to redefine the phenotype of Allan-Herndon-Dudley syndrome (AHDS), which is caused by mutations in the SLC16A2 gene that encodes the brain transporter of thyroid hormones. Clinical phenotypes, brain imaging, thyroid hormone profiles, and genetic data were compared to the existing literature. Twenty-four males aged 11 months to 29 years had a mutation in SLC16A2, including 12 novel mutations and five previously described mutations. Sixteen patients presented with profound developmental delay, three had severe intellectual disability with poor language and walking with an aid, four had moderate intellectual disability with language and walking abilities, and one had mild intellectual disability with hypotonia. Overall, eight had learned to walk, all had hypotonia, 17 had spasticity, 18 had dystonia, 12 had choreoathetosis, 19 had hypomyelination, and 10 had brain atrophy. Kyphoscoliosis (n=12), seizures (n=7), and pneumopathies (n=5) were the most severe complications. This study extends the phenotypic spectrum of AHDS to a mild intellectual disability with hypotonia. Developmental delay, hypotonia, hypomyelination, and thyroid hormone profile help to diagnose patients. Clinical course depends on initial severity, with stable acquisition after infancy; this may be adversely affected by neuro-orthopaedic, pulmonary, and epileptic complications. WHAT THIS PAPER ADDS: Mild intellectual disability is associated with SLC16A2 mutations. A thyroid hormone profile with a free T3 /T4 ratio higher than 0.75 can help diagnose patients. Patients with SLC16A2 mutations present a broad spectrum of neurological phenotypes that are also observed in other hypomyelinating disorders. Axial hypotonia is a consistent feature of Allan-Herndon-Dudley syndrome and leads to specific complications.

Laboratory diagnosis of creatine deficiency syndromes: a technical standard and guideline of the American College of Medical Genetics and Genomics.

Disclaimer: These ACMG Standards and Guidelines are intended as an educational resource for clinical laboratory geneticists to help them provide quality clinical laboratory genetic services. Adherence to these standards and guidelines is voluntary and does not necessarily assure a successful medical outcome. These Standards and Guidelines should not be considered inclusive of all proper procedures and tests or exclusive of others that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, clinical laboratory geneticists should apply their professional judgment to the specific circumstances presented by the patient or specimen. Clinical laboratory geneticists are encouraged to document in the patient's record the rationale for the use of a particular procedure or test, whether or not it is in conformance with these Standards and Guidelines. They also are advised to take notice of the date any particular guideline was adopted, and to consider other relevant medical and scientific information that becomes available after that date. It also would be prudent to consider whether intellectual property interests may restrict the performance of certain tests and other procedures.Cerebral creatine deficiency syndromes are neurometabolic conditions characterized by intellectual disability, seizures, speech delay, and behavioral abnormalities. Several laboratory methods are available for preliminary and confirmatory diagnosis of these conditions, including measurement of creatine and related metabolites in biofluids using liquid chromatography-tandem mass spectrometry or gas chromatography-mass spectrometry, enzyme activity assays in cultured cells, and DNA sequence analysis. These guidelines are intended to standardize these procedures to help optimize the diagnosis of creatine deficiency syndromes. While biochemical methods are emphasized, considerations for confirmatory molecular testing are also discussed, along with variables that influence test results and interpretation.Genet Med 19 2, 256-263.

Variable White Matter Atrophy and Intellectual Development in a Family With X-linked Creatine Transporter Deficiency Despite Genotypic Homogeneity.

BACKGROUND: The X-linked creatine transporter deficiency (CRTD) caused by an SLC6A8 mutation represents the second most common cause of X-linked intellectual disability. The clinical phenotype ranges from mild to severe intellectual disability, epilepsy, short stature, poor language skills, and autism spectrum disorders. The objective of this study was to investigate phenotypic variability in the context of genotype, cerebral creatine concentration, and volumetric analysis in a family with CRTD. PATIENTS AND METHODS: The clinical phenotype and manifestations of epilepsy were assessed in a Caucasian family with CRTD. DNA sequencing and creatine metabolism analysis confirmed the diagnosis. Cerebral magnetic resonance imaging (cMRI) with voxel-based morphometry and magnetic resonance spectroscopy was performed in all family members. RESULTS: An SLC6A8 missense mutation (c.1169C>T; p.Pro390Leu, exon 8) was detected in four of five individuals. Both male siblings were hemizygous, the mother and the affected sister heterozygous for the mutation. Structural cMRI was normal, whereas voxel-based morphometry analysis showed reduced white matter volume below the first percentile of the reference population of 290 subjects in the more severely affected boy compared with family members and controls. Normalized creatine concentration differed significantly between the individuals (P < 0.005). CONCLUSIONS: There is a broad phenotypic variability in CRTD even in family members with the same mutation. Differences in mental development could be related to atrophy of the subcortical white matter.

A Case of MECP2 Duplication Syndrome with Gonadotropin-Dependent Precocious Puberty.

BACKGROUND: MECP2 duplication syndrome, which is caused by duplication of part of the Xq28 region containing the MECP2 gene, causes intellectual disability and mild dysmorphic features in males. To date, precocious puberty has not been reported as a clinical feature of MECP2 duplication syndrome. METHODS: A 6-year-old male with severe intellectual disability was referred because of growth acceleration and precocious puberty. We checked his hormonal profile and conducted imaging studies and an array comparative genomic hybridization analysis. RESULTS: His bone age (9 years and 6 months) was accelerated, and the basal level of testosterone was 8.99 ng/ml. In a luteinizing hormone (LH)-releasing hormone (LHRH) stimulation test, LH increased from 3.69 to 9.32 IU/l, and follicle-stimulating hormone increased from 0.65 to 0.90 IU/l. Chest and abdominal CTs and a brain MRI did not reveal any abnormalities. Treatment with an LHRH analogue effectively suppressed the level of testosterone to <0.03 ng/ml, consistent with the diagnosis of gonadotropin-dependent precocious puberty (GDPP). We identified a duplication of the Xq28 locus including MECP2 in the patient. CONCLUSION: Precocious puberty is often a benign central process in girls, but it is rarely idiopathic in boys. The present case raises the possibility that GDPP is a novel clinical feature of MECP2 duplication syndrome.

N(8)-acetylspermidine as a potential plasma biomarker for Snyder-Robinson syndrome identified by clinical metabolomics.

Clinical metabolomics has emerged as a powerful tool to study human metabolism in health and disease. Comparative statistical analysis of untargeted metabolic profiles can reveal perturbations of metabolite levels in diseases and thus has the potential to identify novel biomarkers. Here we have applied a simultaneous genetic-metabolomic approach in twin boys with epileptic encephalopathy of unclear etiology. Clinical exome sequencing identified a novel missense mutation in the spermine synthase gene (SMS) that causes Snyder-Robinson syndrome (SRS). Untargeted plasma metabolome analysis revealed significantly elevated levels of N(8)-acetylspermidine, a precursor derivative of spermine biosynthesis, as a potential novel plasma biomarker for SRS. This result was verified in a third patient with genetically confirmed SRS. This study illustrates the potential of metabolomics as a translational technique to support exome data on a functional and clinical level.

The Thyroid Hormone Analog DITPA Ameliorates Metabolic Parameters of Male Mice With Mct8 Deficiency.

Mutations in the gene encoding the thyroid hormone (TH) transporter, monocarboxylate transporter 8 (MCT8), cause mental retardation in humans associated with a specific thyroid hormone phenotype manifesting high serum T3 and low T4 and rT3 levels. Moreover, these patients have failure to thrive, and physiological changes compatible with thyrotoxicosis. Recent studies in Mct8-deficient (Mct8KO) mice revealed that the high serum T3 causes increased energy expenditure. The TH analog, diiodothyropropionic acid (DITPA), enters cells independently of Mct8 transport and shows thyromimetic action but with a lower metabolic activity than TH. In this study DITPA was given daily ip to adult Mct8KO mice to determine its effect on thyroid tests in serum and metabolism (total energy expenditure, respiratory exchange rate, and food and water intake). In addition, we measured the expression of TH-responsive genes in the brain, liver, and muscles to assess the thyromimetic effects of DITPA. Administration of 0.3 mg DITPA per 100 g body weight to Mct8KO mice brought serum T3 levels and the metabolic parameters studied to levels observed in untreated Wt animals. Analysis of TH target genes revealed amelioration of the thyrotoxic state in liver, somewhat in the soleus, but there was no amelioration of the brain hypothyroidism. In conclusion, at the dose used, DITPA mainly ameliorated the hypermetabolism of Mct8KO mice. This thyroid hormone analog is suitable for the treatment of the hypermetabolism in patients with MCT8 deficiency, as suggested in limited preliminary human trials.

A 7-month-old male with Allan-Herndon-Dudley syndrome and the power of T3.

Allan-Herndon-Dudley syndrome (AHDS, MIM 300523) is an X-linked neurodegenerative disorder characterized by intellectual disability, severe hypotonia, diminished muscle mass, and progressive spastic paraplegia. All affected males have pathognomonic thyroid profiles with an elevated T3 , low-normal free T4 , and normal TSH. Mutations in the monocarboxylate transporter 8 (MCT8) gene, SLC16A2, have been found to be causative. Here, we describe a proband whose extensive evaluation and ultimate diagnosis of AHDS unmasked three previously undiagnosed generations of affected individuals in one family. This case illustrates the need for clinicians to consider obtaining full thyroid studies on individuals with the non-specific findings of severe hypotonia, failure to thrive, and gross motor delay.

Diiodothyropropionic acid (DITPA) in the treatment of MCT8 deficiency.

CONTEXT: Monocarboxylate transporter 8 (MCT8) is a thyroid hormone-specific cell membrane transporter. MCT8 deficiency causes severe psychomotor retardation and abnormal thyroid tests. The great majority of affected children cannot walk or talk, and all have elevated serum T(3) levels, causing peripheral tissue hypermetabolism and inability to maintain weight. Treatment with thyroid hormone is ineffective. In Mct8-deficient mice, the thyroid hormone analog, diiodothyropropionic acid (DITPA), does not require MCT8 to enter tissues and could be an effective alternative to thyroid hormone treatment in humans. OBJECTIVE: The objective of the study was to evaluate the effect and efficacy of DITPA in children with MCT8 deficiency. METHODS: This was a multicenter report of four affected children given DITPA on compassionate grounds for 26-40 months. Treatment was initiated at ages 8.5-25 months, beginning with a small dose of 1.8 mg, increasing to a maximal 30 mg/d (2.1-2.4 mg/kg · d), given in three divided doses. RESULTS: DITPA normalized the elevated serum T(3) and TSH when the dose reached 1 mg/kg · d and T(4) and rT(3) increased to the lower normal range. The following significant changes were also observed: decline in SHBG (in all subjects), heart rate (in three of four), and ferritin (in one of four). Cholesterol increased in two subjects. There was no weight loss and weight gain occurred in two. None of the treated children required a gastric feeding tube or developed seizures. No adverse effects were observed. CONCLUSION: DITPA (1-2 mg/kg · d) almost completely normalizes thyroid tests and reduces the hypermetabolism and the tendency for weight loss. The effects of earlier commencement and long-term therapy remain to be determined.

Elevated TSH levels in a mentally retarded boy.

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.

MCT8 mutation analysis and identification of the first female with Allan-Herndon-Dudley syndrome due to loss of MCT8 expression.

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.

Detection of low-level somatic and germline mosaicism by denaturing high-performance liquid chromatography in a EURO-MRX family with SLC6A8 deficiency.

Creatine transporter deficiency is an X-linked mental retardation disorder caused by mutations in the creatine transporter gene, SLC6A8. In a European Mental Retardation Consortium panel of 66 patients, we identified a male with mental retardation, caused by a c.1059_1061delCTT; p.Phe354del mutation in the SLC6A8 gene. With the use of direct DNA sequencing, the mutation was also found in the brother of the proband, but not in their mother. However, by analyzing EDTA blood of the mother with denaturing high-performance liquid chromatography (DHPLC), we could show that the mother displays low-level somatic mosaicism for the three base-pair deletion. This study indicates DHPLC as an important tool in the detection of low-level mosaicism, as does it illustrate the importance of considering somatic and germline mosaicism in the case of apparent de novo mutation.

X-linked mental retardation: a comprehensive molecular screen of 47 candidate genes from a 7.4 Mb interval in Xp11.

About 30% of the mutations causing nonsyndromic X-linked mental retardation (MRX) are thought to be located in Xp11 and in the pericentromeric region, with a particular clustering of gene defects in a 7.4 Mb interval flanked by the genes ELK1 and ALAS2. To search for these mutations, 47 brain-expressed candidate genes located in this interval have been screened for mutations in up to 22 mental retardation (MR) families linked to this region. In total, we have identified 57 sequence variants in exons and splice sites of 27 genes. Based on these data, four novel MR genes were identified, but most of the sequence variants observed during this study have not yet been described. The purpose of this article is to present a comprehensive overview of this work and its outcome. It describes all sequence variants detected in 548 exons and their flanking sequences, including disease-causing mutations as well as possibly relevant polymorphic and silent sequence changes. We show that many of the studied genes are unlikely to play a major role in MRX. This information will help to avoid duplication of efforts in the ongoing endeavor to unravel the molecular causes of MRX.

A third MRX family (MRX68) is the result of mutation in the long chain fatty acid-CoA ligase 4 (FACL4) gene: proposal of a rapid enzymatic assay for screening mentally retarded patients.

BACKGROUND: The gene encoding fatty acid CoA ligase 4 (FACL4) is mutated in families with non-specific X linked mental retardation (MRX) and is responsible for cognitive impairment in the contiguous gene syndrome ATS-MR (Alport syndrome and mental retardation), mapped to Xq22.3. This finding makes this gene a good candidate for other mental retardation disorders mapping in this region. METHODS: We have screened the FACL4 gene in eight families, two MRX and six syndromic X linked mental retardation (MRXS), mapping in a large interval encompassing Xq22.3. RESULTS: We have found a missense mutation in MRX68. The mutation (c.1001C>T in the brain isoform) cosegregates with the disease and changes a highly conserved proline into a leucine (p.P375L) in the first luciferase domain, which markedly reduces the enzymatic activity. Furthermore, all heterozygous females showed completely skewed X inactivation in blood leucocytes, as happens in all reported females with other FACL4 point mutations or deletions. CONCLUSIONS: Since the FACL4 gene is highly expressed in brain, where it encodes a brain specific isoform, and is located in hippocampal and cerebellar neurones, a role for this gene in cognitive processes can be expected. Here we report the third MRX family with a FACL4 mutation and describe the development of a rapid enzymatic assay on peripheral blood that we propose as a sensitive, robust, and efficient diagnostic tool in mentally retarded males.