A Novel SLC6A8 Mutation in a Large Family with X-Linked Intellectual Disability: Clinical and Proton Magnetic Resonance Spectroscopy Data of Both Hemizygous Males and Heterozygous Females

X-linked creatine transporter (SLC6A8) deficiency in females: Difficult to recognize, but a potentially treatable disease

Creatine transporter deficiency (CTD), caused by pathogenic variants in SLC6A8, is the second most common cause of X-linked intellectual disability. Symptoms include intellectual disability, epilepsy, and behavioral disorders and are caused by reduced cerebral creatine levels. Targeted treatment with oral supplementation is available, however the treatment efficacy is still being investigated. There are clinical and theoretical indications that heterozygous females with CTD respond better to supplementation treatment than hemizygous males. Unfortunately, heterozygous females with CTD often have more subtle and uncharacteristic clinical and biochemical phenotypes, rendering diagnosis more difficult. We report a new female case who presented with learning disabilities and seizures. After determining the diagnosis with molecular genetic testing confirmed by proton magnetic resonance spectroscopy (1H-MRS), the patient was treated with supplementation treatment including creatine, arginine, and glycine. After 28 months of treatment, the patient showed prominent clinical improvement and increased creatine levels in the brain. Furthermore, we provide a review of the 32 female cases reported in the current literature including a description of phenotypes, genotypes, diagnostic approaches, and effects of supplementation treatment. Based on this, we find that supplementation treatment should be tested in heterozygous female patients with CTD, and a prospective treatment underlines the importance of diagnosing these patients. The diagnosis should be suspected in a broad clinical spectrum of female patients and can only be made by molecular genetic testing. 1H-MRS of cerebral creatine levels is essential for establishing the diagnosis in females, and especially valuable when assessing variants of unknown significance.

Primary creatine deficiency syndrome as a potential missed diagnosis in children with psychomotor delay and seizure: case presentation with two novel variants and literature review

Creatine is the main source of energy for the brain. Primary creatine deficiency syndromes (PCDSs) are inborn error of metabolism of creatine synthesis. Symptoms of central nervous system involvement are the most common clinical manifestations in these disorders. We reviewed medical records of all genetically confirmed patients diagnosed by whole exome sequencing who were referred to Myelin and Neurodegenerative Disorders Clinic, Children's Medical Center, Tehran, Iran, from May 2016 to Dec 2018. A literature review was conducted on clinical and genomic variability of PCDS to compare our patients with previously reported cases. We report two patients with creatine deficiency among a cohort of 550 registered cases out of which 200 patients had a genetically confirmed neurodegenerative disorder diagnosis. The main complain in the first patient with creatine transporter (CRTR) deficiency was seizure and genetic study in this patient identified a novel hemizygote variant of "c.92 > T; p.Pro31Leu" in the first exon of SLC6A8 gene. The second patient with guanidinoacetate methyltransferase (GAMT) deficiency had an unknown motor and speech delay as the striking manifestation and molecular assay revealed a novel homozygote variant of "c.134G > A; p.Trp45*" in the first exon of GAMT gene. PCDSs usually are associated with nonspecific neurologic symptoms. The first presented case had a mean delayed diagnosis of 5 years. Therefore, in children with unexplained neurologic features including developmental delay and/or regression, mental disability and repeated seizures without any significant findings in metabolic studies, PCDSs can be considered as a differential diagnosis and molecular analysis can be helpful for the precise diagnosis and treatment.

[Clinical features and SLC6A8 gene mutations of cerebral creatine deficiency syndrome I: an analysis of two families]

This article reports the clinical and genetic features of two cases of cerebral creatine deficiency syndrome I (CCDSI) caused by SLC6A8 gene mutations. Both children were boys. Boy 1 (aged 2 years and 10 months) and Boy 2 (aged 8 years and 11 months) had the clinical manifestations of delayed mental and motor development, and convulsion. Their older brothers had the same symptoms. The mother of the boy 1 had mild intellectual disability. The genetic analysis showed two novel homozygous mutations, c.200G>A(p.Gly67Asp) and c.626_627delCT(p.Pro209Argfs*87), in the SLC6A8 gene on the X chromosome, both of which came from their mothers. These two novel mutations were rated as possible pathogenic mutations and were not reported in the literature before. This study expands the mutation spectrum of the SLC6A8 gene and has great significance in the diagnosis of boys with delayed development, and epilepsy.

Abnormal N-Glycosylation of a Novel Missense Creatine Transporter Mutant, G561R, Associated with Cerebral Creatine Deficiency Syndromes Alters Transporter Activity and Localization

Cerebral creatine deficiency syndromes (CCDSs) are caused by loss-of-function mutations in creatine transporter (CRT, SLC6A8), which transports creatine at the blood-brain barrier and into neurons of the central nervous system (CNS). This results in low cerebral creatine levels, and patients exhibit mental retardation, poor language skills and epilepsy. We identified a novel human CRT gene missense mutation (c.1681 G>C, G561R) in Japanese CCDSs patients. The purpose of the present study was to evaluate the reduction of creatine transport in G561R-mutant CRT-expressing 293 cells, and to clarify the mechanism of its functional attenuation. G561R-mutant CRT exhibited greatly reduced creatine transport activity compared to wild-type CRT (WT-CRT) when expressed in 293 cells. Also, the mutant protein is localized mainly in intracellular membrane fraction, while WT-CRT is localized in plasma membrane. Western blot analysis revealed a 68 kDa band of WT-CRT protein in plasma membrane fraction, while G561R-mutant CRT protein predominantly showed bands at 55, 110 and 165 kDa in crude membrane fraction. The bands of both WT-CRT and G561R-mutant CRT were shifted to 50 kDa by N-glycosidase treatment. Our results suggest that the functional impairment of G561R-mutant CRT was probably caused by incomplete N-linked glycosylation due to misfolding during protein maturation, leading to oligomer formation and changes of cellular localization.

Creatine in the central nervous system: From magnetic resonance spectroscopy to creatine deficiencies

Creatine (Cr) is an important organic compound acting as intracellular high-energy phosphate shuttle and in energy storage. While located in most cells where it plays its main roles in energy metabolism and cytoprotection, Cr is highly concentrated in muscle and brain tissues, in which Cr also appears to act in osmoregulation and neurotransmission. This review discusses the basis of Cr metabolism, synthesis and transport within brain cells. The importance of Cr in brain function and the consequences of its impaired metabolism in primary and secondary Cr deficiencies are also discussed. Cr and phosphocreatine (PCr) in living systems can be well characterized using in vivo magnetic resonance spectroscopy (MRS). This review describes how ¹H MRS allows the measurement of Cr and PCr, and how ³¹P MRS makes it possible to estimate the creatine kinase (CK) rate constant and so detect dynamic changes in the Cr/PCr/CK system. Absolute quantification by MRS using creatine as internal reference is also debated. The use of in vivo MRS to study brain Cr in a non-invasive way is presented, as well as its use in clinical and preclinical studies, including diagnosis and treatment follow-up in patients.

Copy Number Variants in Patients with Autism and Additional Clinical Features: Report of VIPR2 Duplication and a Novel Microduplication Syndrome

Autism is a common neurodevelopmental disorder estimated to affect 1 in 68 children. Many studies have shown the role of copy number variants (CNVs) as a major contributor in the etiology of autism with the overall detection rate of about 10-15 % and over 20 % when syndromic forms of autism exist. In this study, we used array CGH to identify CNVs in 15 Iranian patients with autism. To elevate our diagnostic yield, we selected the sporadic patients who had additional clinical features including intellectual disability (ID), craniofacial anomaly, and seizure. Six out of 15 patients showed clinically relevant CNVs including pathogenic and likely pathogenic copy number gains or losses. We report a novel gene duplication syndrome (10q21.2q21.3 microduplication) and present a new evidence for VIPR2 duplication, as a candidate gene for autism. Furthermore, we describe the first manifesting carrier female with deletion of SLC6A8 and BCAP31 genes on Xq28. Our findings suggest that there might be a higher prevalence of clinically significant CNVs in patients with autism and additional clinical manifestations. The CNV analysis in such patients could lead to the discovery of novel syndromes as well as unraveling the etiology of autism.

A novel SLC6A8 mutation associated with motor dysfunction in a child exhibiting creatine transporter deficiency

Creatine transporter (CT) deficiency is an X-linked disorder caused by mutations in the SLC6A8 gene. We describe a clinical, biochemical and molecular examination of a child with X-linked cerebral creatine deficiency. Increased urinary creatine/creatinine ratio, abnormal brain proton magnetic resonance spectroscopy and reduced creatine transport confirmed the clinical diagnosis. SLC6A8 analysis revealed a novel mutation that was hemizygous in the child and not detected in his mother. CT deficiency should be considered in children, especially males, with mental retardation.

Integrated sequence analysis pipeline provides one-stop solution for identifying disease-causing mutations

Next-generation sequencing has greatly accelerated the search for disease-causing defects, but even for experts the data analysis can be a major challenge. To facilitate the data processing in a clinical setting, we have developed a novel medical resequencing analysis pipeline (MERAP). MERAP assesses the quality of sequencing, and has optimized capacity for calling variants, including single-nucleotide variants, insertions and deletions, copy-number variation, and other structural variants. MERAP identifies polymorphic and known causal variants by filtering against public domain databases, and flags nonsynonymous and splice-site changes. MERAP uses a logistic model to estimate the causal likelihood of a given missense variant. MERAP considers the relevant information such as phenotype and interaction with known disease-causing genes. MERAP compares favorably with GATK, one of the widely used tools, because of its higher sensitivity for detecting indels, its easy installation, and its economical use of computational resources. Upon testing more than 1,200 individuals with mutations in known and novel disease genes, MERAP proved highly reliable, as illustrated here for five families with disease-causing variants. We believe that the clinical implementation of MERAP will expedite the diagnostic process of many disease-causing defects.

Heat shock alters the expression of schizophrenia and autism candidate genes in an induced pluripotent stem cell model of the human telencephalon

Schizophrenia (SZ) and autism spectrum disorders (ASD) are highly heritable neuropsychiatric disorders, although environmental factors, such as maternal immune activation (MIA), play a role as well. Cytokines mediate the effects of MIA on neurogenesis and behavior in animal models. However, MIA stimulators can also induce a febrile reaction, which could have independent effects on neurogenesis through heat shock (HS)-regulated cellular stress pathways. However, this has not been well-studied. To help understand the role of fever in MIA, we used a recently described model of human brain development in which induced pluripotent stem cells (iPSCs) differentiate into 3-dimensional neuronal aggregates that resemble a first trimester telencephalon. RNA-seq was carried out on aggregates that were heat shocked at 39°C for 24 hours, along with their control partners maintained at 37°C. 186 genes showed significant differences in expression following HS (p<0.05), including known HS-inducible genes, as expected, as well as those coding for NGFR and a number of SZ and ASD candidates, including SMARCA2, DPP10, ARNT2, AHI1 and ZNF804A. The degree to which the expression of these genes decrease or increase during HS is similar to that found in copy loss and copy gain copy number variants (CNVs), although the effects of HS are likely to be transient. The dramatic effect on the expression of some SZ and ASD genes places HS, and perhaps other cellular stressors, into a common conceptual framework with disease-causing genetic variants. The findings also suggest that some candidate genes that are assumed to have a relatively limited impact on SZ and ASD pathogenesis based on a small number of positive genetic findings, such as SMARCA2 and ARNT2, may in fact have a much more substantial role in these disorders - as targets of common environmental stressors.