X-linked Creatine Transporter Defect Research Articles

EP194: Three generations of females with a heterozygous likely pathogenic variant in SLC6A8 causative of X-linked creatine transporter defect

SLC6A8-Related Disorders falls under the umbrella of cerebral creatine deficiency syndromes (CCDS). CCDS are inborn errors of creatine metabolism. SLC6A8-Related Disorders is specifically a creatine transporter deficiency. The SLC6A8 is located at Xq28 and encodes for the sodium- and chloride-dependent creatine transporter 1 that assists in transporting creatine into cells. Pathogenic variants in SLC6A8 impair the ability of the transporter protein to bring creatine into cells, resulting in a creatine shortage.

Diagnostic yield from routine metabolic screening tests in evaluation of global developmental delay and intellectual disability

The aim of this retrospective review was to evaluate the diagnostic yield from these first-tier tests in the 7 years before (2006 to 2012) and after (2013 to 2019) implementation at the community paediatrician level. Prior and postimplementation diagnostic yield of an IEM from first-tier metabolic testing was 9 out of 986 (0.91%) and 11 out of 4,345 children (0.25%), respectively. Disorders of creatine metabolism and organic acidurias were the most frequently established diagnoses in both time periods. No diagnoses were established through acylcarnitine copper/ceruloplasmin, lactate, or ammonia testing. Twenty out of 24 patients had specific neurological or other red flag signs in addition to GDD/ID. Four boys diagnosed with an x-linked creatine transporter defect (CTD) had speech-language delay as the most prominent finding. The expansion of first-tier metabolic testing to community-based paediatricians in BC did not yield an increase in IEM diagnoses. A modified first-tier test panel should be offered to patients with GDD/ID, neurologic, and/or red flag signs. Urine creatine testing in boys with speech-language delay warrants consideration to detect CTD.

Characterization of novel SLC6A8 variants with the use of splice-site analysis tools and implementation of a newly developed LOVD database

The X-linked creatine transporter defect is caused by mutations in the SLC6A8 gene. Until now, 66 synonymous and intronic variants in SLC6A8 were detected in our laboratory. To gain more insight in the effect of the detected variants, we applied five free web-based splice-site analysis tools to 25 published variants that were stratified as (non-)disease causing. All were correctly predicted to have no effect (n=18) or to cause erroneous splicing (n=7), with the exception of a pathogenic de novo 24 bp intronic deletion. Second, 41 unclassified variants, including 28 novel, were subjected to analysis by these tools. At least four splice-site analysis tools predicted that three of the variants would affect splicing as the mutations disrupted the canonical splice site. Urinary creatine/creatinine and brain MRS confirmed creatine transporter deficiency in five patients (four families), including one female. Another variant was predicted to moderately affect splicing by all five tools. However, transient transfection of a minigene containing the variant in a partial SLC6A8 segment showed no splicing errors, and thus was finally classified as non-disease causing. This study shows that splice tools are useful for the characterization of the majority of variants, but also illustrates that the actual effect can be misclassified in rare occasions. Therefore, further laboratory studies should be considered before final conclusions on the disease-causing nature are drawn. To provide an accessible database, the 109 currently known SLC6A8 variants, including 35 novel ones, are included in a newly developed LOVD DNA variation database.

X-Linked Creatine Transporter Deficiency Presenting as a Mitochondrial Disorder

X-linked creatine transporter defect is caused by mutations in SLC6A8 at Xq28, which encodes the sodium-dependent creatine transporter. Reduction in creatine uptake results in elevated urine creatine and CSF creatine deficiency, which can be detected on magnetic resonance spectroscopy. We report a patient who was initially suspected of having a mitochondrial disorder but was later found to have a creatine transporter defect. The abnormal laboratory study results seen in this patient suggesting a mitochondrial cytopathy could be due to excess mitochondrial stress as well as the mitochondrial inclusion bodies. This report looks at the mitochondrial presentation of the creatine transporter deficiency.

High frequency of creatine deficiency syndromes in patients with unexplained mental retardation

Cerebral creatine deficiency syndromes (CCDSs) are inborn errors of metabolism that include two autosomal recessive creatine biosynthesis defects (arginine–glycine amidinotransferase [AGAT; OMIM 602360] and guanidinoacetate methyl transferase [GAMT; OMIM 601240] deficiency) and an X-linked creatine transporter defect (OMIM 300036).1 The clinical phenotype is variable, associating nonspecific mental retardation, epilepsy, extrapyramidal movement disorders, and autistic behavior. The frequency of CCDS is unknown and probably underestimated. We report a high frequency of CCDS in mentally retarded children, mostly boys with an X-linked creatine transporter deficiency. Over a period of18 months, children referred to the Department of Pediatric Neurology with unexplained mild to severe mental retardation, normal karyotype, and absence of fragile X syndrome were prospectively screened for CCDS. Children were from diverse ethnic backgrounds. Children with polymalformative syndromes were excluded. Creatinine metabolism was evaluated using creatine/creatinine and guanidinoacetate (GAA)/creatinine ratios on a spot urine.2 Diagnosis was further confirmed using brain proton MR spectroscopy (H-MRS) and …

X‐linked creatine transporter defect: A report on two unrelated boys with a severe clinical phenotype

We report two unrelated boys with the X-linked creatine transporter defect (CRTR) and clinical features more severe than those previously described with this disorder. These two boys presented at ages 12 and 30 months with severe mental retardation, absent speech development, hypotonia, myopathy and extra-pyramidal movement disorder. One boy has seizures and some dysmorphic features; he also has evidence of an oxidative phosphorylation defect. They both had classical absence of creatine peak on brain magnetic resonance spectroscopy (MRS). In one, however, this critical finding was overlooked in the initial interpretation and was discovered upon subsequent review of the MRS. Molecular studies showed large genomic deletions of a large part of the 3' end of the complete open reading frame of the SLC6A8 gene. This report emphasizes the importance of MRS in evaluating neurological symptoms, broadens the phenotypic spectrum of CRTR and adds knowledge about the pathogenesis of creatine depletion in the brain and retina.

X-linked creatine transporter defect: an overview.

In 2001 we identified a new inborn error of metabolism caused by a defect in the X-linked creatine transporter SLC6A8 gene mapped at Xq28 (SLC6A8 deficiency, McKusick 300352). An X-linked creatine transporter defect was presumed because of (1) the absence of creatine in the brain as indicated by proton magnetic resonance spectroscopy (MRS); (2) the elevated creatine levels in urine and normal guanidinoacetate levels in plasma, ruling out a creatine biosynthesis defect; (3) the absence of an improvement on creatine supplementation; and (4) the fact that the pedigree suggested an X-linked disease. Our hypothesis was proved by the presence of a hemizygous nonsense mutation in the male index patient and by the impaired creatine uptake by cultured fibroblasts. Currently, at least 7 unrelated families (13 male patients and 13 carriers) with a SLC6A8 deficiency have been identified. Four families come from one metropolitan area. This suggests that SLC6A8 deficiency may have a relatively high incidence. The hallmarks of the disorder are X-linked mental retardation, expressive speech and language delay, epilepsy, developmental delay and autistic behaviour. In approximately 50% of the female carriers, learning disabilities of varying degrees have been noted.