ClinGen variant curation expert panel recommendations for classification of variants in GAMT, GATM and SLC6A8 for cerebral creatine deficiency syndromes

Cerebral creatine deficiency syndromes (CCDS) are inherited metabolic phenotypes of creatine synthesis and transport. There are two enzyme deficiencies, guanidinoacetate methyltransferase (GAMT), encoded by GAMT and arginine-glycine amidinotransferase (AGAT), encoded by GATM, which are involved in the synthesis of creatine. After synthesis, creatine is taken up by a sodium-dependent membrane bound creatine transporter (CRTR), encoded by SLC6A8, into all organs. Creatine uptake is very important especially in high energy demanding organs such as the brain, and muscle. To classify the pathogenicity of variants in GAMT, GATM, and SLC6A8, we developed the CCDS Variant Curation Expert Panel (VCEP) in 2018, supported by The Clinical Genome Resource (ClinGen), a National Institutes of Health (NIH)-funded resource. We developed disease-specific variant classification guidelines for GAMT-, GATM-, and SLC6A8-related CCDS, adapted from the American College of Medical Genetics/Association of Molecular Pathology (ACMG/AMP) variant interpretation guidelines. We applied specific variant classification guidelines to 30 pilot variants in each of the three genes that have variants associated with CCDS. Our CCDS VCEP was approved by the ClinGen Sequence Variant Interpretation Working Group (SVI WG) and Clinical Domain Oversight Committee in July 2022. We curated 181 variants including 72 variants in GAMT, 45 variants in GATM, and 64 variants in SLC6A8 and submitted these classifications to ClinVar, a public variant database supported by the National Center for Biotechnology Information. Missense variants were the most common variant type in all three genes. We submitted 32 new variants and reclassified 34 variants with conflicting interpretations. We report specific phenotype (PP4) using a points system based on the urine and plasma guanidinoacetate and creatine levels, brain magnetic resonance spectroscopy (MRS) creatine level, and enzyme activity or creatine uptake in fibroblasts ranging from PP4, PP4_Moderate and PP4_Strong. Our CCDS VCEP is one of the first panels applying disease specific variant classification algorithms for an X-linked disease. The availability of these guidelines and classifications can guide molecular genetics and genomic laboratories and health care providers to assess the molecular diagnosis of individuals with a CCDS phenotype.

Electro-clinical features and long-term outcomes in guanidinoacetate methyltransferase (GAMT) deficiency

OBJECTIVE

To evaluate clinical characteristics and long-term outcomes in patients with guanidinoacetate methyltransferase (GAMT) deficiency with a special emphasis on seizures and electroencephalography (EEG) findings.

METHODS

We retrospectively analyzed the clinical and molecular characteristics, seizure types, EEG findings, neuroimaging features, clinical severity scores, and treatment outcomes in six patients diagnosed with GAMT deficiency.

RESULTS

Median age at presentation and diagnosis were 11.5 months (8-12 months) and 63 months (18 months -11 years), respectively. Median duration of follow-up was 14 years. Global developmental delay (6/6) and seizures (5/6) were the most common symptoms. Four patients presented with febrile seizures. The age at seizure-onset ranged between 8 months and 4 years. Most common seizure types were generalized tonic seizures (n = 4) and motor seizures resulting in drop attacks (n = 3). Slow background activity (n = 5) and generalized irregular sharp and slow waves (n = 3) were the most common EEG findings. Burst-suppression and electrical status epilepticus during slow-wave sleep (ESES) pattern was present in one patient. Three of six patients had drug-resistant epilepsy. Post-treatment clinical severity scores showed improvement regarding movement disorders and epilepsy. All patients were seizure-free in the follow-up.

CONCLUSIONS

Epilepsy is one of the main symptoms in GAMT deficiency with various seizure types and non-specific EEG findings. Early diagnosis and initiation of treatment are crucial for better seizure and cognitive outcomes. This long-term follow up study highlights to include cerebral creatine deficiency syndromes in the differential diagnosis of patients with global developmental delay and epilepsy and describes the course under treatment.

Method modification to reduce false positives for newborn screening of guanidinoacetate methyltransferase deficiency

Guanidinoacetate methyltransferase (GAMT) deficiency is an autosomal recessive disorder that results in reduced activity of guanidinoacetate methyltransferase, an accumulation of guanidinoacetate (GUAC), and a lack of cerebral creatine (CRE). Lack of CRE in the brain can cause intellectual disability, autistic-like behavior, seizures, and movement disorders. Identification at birth and immediate therapy can prevent intellectual disability and seizures. If started early in life, treatment with creatine supplements is highly effective. Because there are reliable biomarkers for GAMT deficiency, GUAC and CRE, and because the disorder is readily treatable with a significant improvement in outcomes, GAMT deficiency is an excellent candidate for newborn screening. Several programs have conducted pilot programs or started screening. An isobaric interferant of the GUAC marker has been reported which may cause false positive results. To reduce the number of false positives, a second-tier HPLC test to separate GUAC from unknown, isobaric interferants may be incorporated into the screening algorithm. New York State began screening for GAMT deficiency in October 2018 using a three-tiered screening approach. Quantification of GUAC and CRE were incorporated into routine screening for amino acids and acylcarnitines. In the first year of screening a total of 263,739 samples were tested for GAMT deficiency. Of these, 3382 required second tier testing. After second tier testing, 210 repeat specimens were requested for borderline results and 10 referrals were made to specialty care centers for confirmatory testing. In the first year of screening there were no confirmed cases of GAMT deficiency detected. To reduce the number of samples needing second tier testing and the number false positives we explored the use of a second MS transition to confirm the identity of the GUAC marker. GUAC and its internal standard are detected as butylated esters after sample preparation and derivatization. The original method used transition of the GUAC molecular ion of m/z 174.1 to a reactant ion of m/z 101.1. To confirm the identity of the GUAC marker we selected a qualifier ion of 174.1 > 73. The alternative product ion results were found to agree more closely with the second tier HPLC-MS/MS results for GUAC. It was found that the alternative transition may be used for quantification of the GUAC marker with acceptable analytical performance (linearity, accuracy, and precision). On March 5, 2020, the method of analysis for GUAC was modified to use the alternative product ion. For a comparable 6-month period, the modified method reduced the number of samples requiring second tier testing by 98%, reduced the number of borderline results requiring a repeat sample by 87.5%, and reduced the number of referrals to specialty care centers by 85%. Using the modified method, the correlation (r-squared) of the first and second tier screening results for GUAC is greater than 0.95. Since the first-tier results correlate well with the second-tier results, the second-tier screening is no longer necessary with the modified method.

Prospective identification by neonatal screening of patients with guanidinoacetate methyltransferase deficiency

INTRODUCTION

Guanidinoacetate methyltransferase (GAMT) deficiency is an inherited metabolic disorder that impairs the synthesis of creatine (CRE). Lack of CRE in the brain can cause intellectual disability, autistic-like behavior, seizures, and movement disorders. Identification at birth and immediate therapy can prevent intellectual disability and seizures. Here we report the first two cases of GAMT deficiency identified at birth by newborn screening (NBS) in Utah and New York.

METHODS

NBS dried blood spots were analyzed by tandem mass spectrometry (MS/MS) using either derivatized or non-derivatized assays to detect guanidinoacetate (GUAC) and CRE. For any positive samples, a second-tier test using a more selective method, ultra-performance liquid chromatography (UPLC) combined with MS/MS, was performed to separate GUAC from potential isobaric interferences.

RESULTS

NBS for GAMT deficiency began in Utah on June 1, 2015 using a derivatized method for the detection of GUAC and CRE. In May 2019, the laboratory and method transitioned to a non-derivatized method. GAMT screening was added to the New York State NBS panel on October 1, 2018 using a derivatized method. In New York, a total of 537,408 babies were screened, 23 infants were referred and one newborn was identified with GAMT deficiency. In Utah, a total of 273,902 infants were screened (195,425 with the derivatized method, 78,477 with the non-derivatized method), three infants referred and one was identified with GAMT deficiency. Mean levels of GUAC and CRE were similar between methods (Utah derivatized: GUAC = 1.20 ± 0.43 μmol/L, CRE = 238 ± 96 μmol/L; Utah non-derivatized: GUAC = 1.23 ± 0.61 μmol/L, CRE = 344 ± 150 μmol/L, New York derivatized: GUAC = 1.34 ± 0.57 μmol/L, CRE = 569 ± 155 μmol/L). With either Utah method, similar concentrations of GUAC are observed in first (collected around 1 day of age) and the second NBS specimens (routinely collected at 7-16 days of age), while CRE concentrations decreased in the second NBS specimens. Both infants identified with GAMT deficiency started therapy by 2 weeks of age and are growing and developing normally at 7 (Utah) and 4 (New York) months of age.

CONCLUSIONS

Newborn screening allows for the prospective identification of GAMT deficiency utilizing elevated GUAC concentration as a marker. First-tier screening may be incorporated into existing methods for amino acids and acylcarnitines without the need for new equipment or staff. Newborn screening performed by either derivatized or non-derivatized methods and coupled with second-tier testing, has a very low false positive rate and can prospectively identify affected children. SummaryCerebral creatine deficiency syndromes caused by defects in creatine synthesis can result in intellectual disability, and are preventable if therapy is initiated early in life. This manuscript reports the identification of two infants with GAMT deficiency (one of the cerebral creatine deficiency syndromes) by newborn screening and demonstrates NBS feasibility using a variety of methods.

Cardiac expression and location of hexokinase changes in a mouse model of pure creatine deficiency

Creatine kinase (CK) is considered the main phosphotransfer system in the heart, important for overcoming diffusion restrictions and regulating mitochondrial respiration. It is substrate limited in creatine-deficient mice lacking l-arginine:glycine amidinotransferase (AGAT) or guanidinoacetate N-methyltranferase (GAMT). Our aim was to determine the expression, activity, and mitochondrial coupling of hexokinase (HK) and adenylate kinase (AK), as these represent alternative energy transfer systems. In permeabilized cardiomyocytes, we assessed how much endogenous ADP generated by HK, AK, or CK stimulated mitochondrial respiration and how much was channeled to mitochondria. In whole heart homogenates, and cytosolic and mitochondrial fractions, we measured the activities of AK, CK, and HK. Lastly, we assessed the expression of the major HK, AK, and CK isoforms. Overall, respiration stimulated by HK, AK, and CK was ∼25, 90, and 80%, respectively, of the maximal respiration rate, and ∼20, 0, and 25%, respectively, was channeled to the mitochondria. The activity, distribution, and expression of HK, AK, and CK did not change in GAMT knockout (KO) mice. In AGAT KO mice, we found no changes in AK, but we found a higher HK activity in the mitochondrial fraction, greater expression of HK I, but a lower stimulation of respiration by HK. Our findings suggest that mouse hearts depend less on phosphotransfer systems to facilitate ADP flux across the mitochondrial membrane. In AGAT KO mice, which are a model of pure creatine deficiency, the changes in HK may reflect changes in metabolism as well as influence mitochondrial regulation and reactive oxygen species production.NEW & NOTEWORTHY In creatine-deficient AGAT-/- and GAMT-/- mice, the myocardial creatine kinase system is substrate limited. It is unknown whether subcellular localization and mitochondrial ADP channeling by hexokinase and adenylate kinase may compensate as alternative phosphotransfer systems. Our results show no changes in adenylate kinase, which is the main alternative to creatine kinase in heart. However, we found increased expression and activity of hexokinase I in AGAT-/- cardiomyocytes. This could affect mitochondrial regulation and reactive oxygen species production.

Magnetic resonance imaging reveals specific anatomical changes in the brain of Agat- and Gamt-mice attributed to creatine depletion and guanidinoacetate alteration

Arginine:glycine amidinotransferase- and guanidinoacetate methyltransferase deficiency are severe neurodevelopmental disorders. It is not known whether mouse models of disease express a neuroanatomical phenotype. High-resolution magnetic resonance imaging (MRI) with advanced image analysis was performed in perfused, fixed mouse brains encapsulated with the skull from male, 10-12 week old Agat ^-exc and B6J.Cg-Gamt ^tm1Isb mice (n = 48; n = 8 per genotype, strain). T2-weighted MRI scans were nonlinearly aligned to a 3D atlas of the mouse brain with 62 structures identified. Local differences in brain shape related to genotype were assessed by analysis of deformation fields. Creatine (Cr) and guanidinoacetate (GAA) were measured with high-performance liquid chromatography (HPLC) in brain homogenates (n = 24; n = 4 per genotype, strain) after whole-body perfusion. Cr was decreased in the brain of Agat- and Gamt mutant mice. GAA was decreased in Agat^-/- and increased in Gamt^-/- . Body weight and brain volume were lower in Agat^-/- than in Gamt^-/- . The analysis of entire brain structures revealed corpus callosum, internal capsule, fimbria and hypothalamus being different between the genotypes in both strains. Eighteen and fourteen significant peaks (local areas of difference in relative size) were found in Agat- and Gamt mutants, respectively. Comparing Agat^-/- with Gamt^-/- , we found changes in three brain regions, lateral septum, amygdala, and medulla. Intra-strain differences in four brain structures can be associated with Cr deficiency, while the inter-strain differences in three brain structures of the mutant mice may relate to GAA. Correlating these neuroanatomical findings with gene expression data implies the role of Cr metabolism in the developing brain and the importance of early intervention in patients with Cr deficiency syndromes.

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.

First reported Chinese case of guanidinoacetate methyltransferase deficiency in a 4-year-old child

Guanidinoacetate methyltransferase (GAMT) deficiency is a rare inherited disorder characterized by creatine (Cr) depletion and guanidinoacetate (GAA) accumulation in body fluids. We report the first identified Chinese case, diagnosed in a 4-year-old girl with onset of global developmental. Low Cr and high GAA levels were detected in her serum and urine, and low Cr level in her brain. Compound heterozygous variants in GAMT gene were found, including a previously reported variant at c.491dupG which was inherited from her mother and a novel variant at c.564G>T, which was inherited from her father. The Cr and GAA levels returned back to normal after 3 months of treatment. After one year of treatment, the patient stopped taking antiepileptic drugs and her electroencephalogram (EEG) was also back to normal. The girl was followed up for five years and exhibited good results beyond our expectation. The results have shown that protein restriction with high-dose ornithine and creatine supplements have strong therapeutic potential for our patient.

Creatine Deficiency Syndrome could be Missed Easily: A Case Report of Guanidinoacetate Methyltransferase Deficiency Presented with Neurodevelopmental Delay, Seizures, and Behavioral Changes, but Normal Structural MRI

A case with GAMT deficiency (homozygous c.64dupG mutation) presented with neurodevelopmental delay, rare seizures, behavioral disturbances, and mild hypotonia, posing diagnostic challenges. Metabolic investigations showed low creatinine in plasma and urine (guanidinoacetate couldn't be investigated) and slightly elevated lactate. MRI was normal. Correct diagnosis was possible only after MR spectroscopy was performed at age 5½ years. A homozygous c.64dupG mutation of the GAMT gene was identified in the proband. In conclusion, every case with neurodevelopmental delay or arrest, especially when accompanied by seizures, behavioral impairment, muscle hypotonia or extrapyramidal symptoms should undergo MRI with MR spectroscopy. Normal structural MRI doesn't exclude a creatine deficiency syndrome. Biochemical investigations of guanidinoacetate, creatine, and creatinine in body fluid should be done to diagnose cerebral creatine deficiency syndromes and to specify the deficient enzyme. Thus, a treatable disease will not be missed.

A three-tier algorithm for guanidinoacetate methyltransferase (GAMT) deficiency newborn screening

BACKGROUND

Guanidinoacetate methyltransferase (GAMT) deficiency is a rare disorder of creatine biosynthesis presenting with epilepsy and developmental delay in infancy. Excellent developmental outcomes have been reported for infants treated from birth due to a family history. The BC Newborn Screening Program initiated a 3year pilot screening study for GAMT deficiency to evaluate the performance of a novel three-tiered screening approach.

METHODS

Over 36months all bloodspots submitted for routine newborn screening were included in the pilot study (de-identified). Initial GAA measurement was integrated into the standard acylcarnitine/amino acid first-tier assay. All samples with elevated GAA were subjected to second-tier GAA analysis by LC-MS/MS integrated into an existing branched-chain amino acid (MSUD) method. GAMT gene sequencing was completed on the original bloodspot for all specimens with elevated GAA on the second-tier test. The protocol allowed for re-identification for treatment of any specimen with one or two likely pathogenic GAMT mutations.

RESULTS

Over the study period 135,372 specimens were tested with 259 (0.19%) over the first-tier GAA cut-off. The second-tier assay removed an interference falsely elevating GAA levels, and only 3 samples required genotyping. No mutations were identified in any samples, all were deemed negative screens and no follow-up was initiated.

CONCLUSIONS

A three-tier algorithm for GAMT newborn screening showed excellent test performance with zero false positives. No cases were detected, supporting a low incidence for this disorder. Given the low incremental costs and evidence of positive outcomes with early intervention, GAMT deficiency remains an excellent candidate for newborn screening.

Creatine synthesis and exchanges between brain cells: What can be learned from human creatine deficiencies and various experimental models?

While it has long been thought that most of cerebral creatine is of peripheral origin, the last 20 years has provided evidence that the creatine synthetic pathway (AGAT and GAMT enzymes) is expressed in the brain together with the creatine transporter (SLC6A8). It has also been shown that SLC6A8 is expressed by microcapillary endothelial cells at the blood-brain barrier, but is absent from surrounding astrocytes, raising the concept that the blood-brain barrier has a limited permeability for peripheral creatine. The first creatine deficiency syndrome in humans was also discovered 20 years ago (GAMT deficiency), followed later by AGAT and SLC6A8 deficiencies, all three diseases being characterized by creatine deficiency in the CNS and essentially affecting the brain. By reviewing the numerous and latest experimental studies addressing creatine transport and synthesis in the CNS, as well as the clinical and biochemical characteristics of creatine-deficient patients, our aim was to delineate a clearer view of the roles of the blood-brain and blood-cerebrospinal fluid barriers in the transport of creatine and guanidinoacetate between periphery and CNS, and on the intracerebral synthesis and transport of creatine. This review also addresses the question of guanidinoacetate toxicity for brain cells, as probably found under GAMT deficiency.

Expanded newborn screening by mass spectrometry: New tests, future perspectives

Tandem mass spectrometry (MS/MS) has become a leading technology used in clinical chemistry and has shown to be particularly sensitive and specific when used in newborn screening (NBS) tests. The success of tandem mass spectrometry is due to important advances in hardware, software and clinical applications during the last 25 years. MS/MS permits a very rapid measurement of many metabolites in different biological specimens by using filter paper spots or directly on biological fluids. Its use in NBS give us the chance to identify possible treatable metabolic disorders even when asymptomatic and the benefits gained by this type of screening is now recognized worldwide. Today the use of MS/MS for second-tier tests and confirmatory testing is promising especially in the early detection of new disorders such as some lysosomal storage disorders, ADA and PNP SCIDs, X-adrenoleucodistrophy (X-ALD), Wilson disease, guanidinoacetate methyltransferase deficiency (GAMT), and Duchenne muscular dystrophy. The new challenge for the future will be reducing the false positive rate by using second-tier tests, avoiding false negative results by using new specific biomarkers and introducing new treatable disorders in NBS programs.

Review: Human guanidinoacetate n-methyl transferase (GAMT) deficiency: A treatable inborn error of metabolism

The creatine biosynthetic pathway is essential for cellular phosphate associated energy production and storage, particularly in tissues having higher metabolic demands. Guanidinoacetate N-Methyl transferase (GAMT) is an important enzyme in creatine endogenous biosynthetic pathway, with highest expression in liver and kidney. GAMT deficiency is an inherited autosomal recessive trait that was the first among creatine deficiency syndrome to be reported in 1994 having characteristic features of no comprehensible speech development, severe mental retardation, muscular hypotonia, involuntary movements and seizures that partly cannot be treated with anti-epileptic drugs. Due to problematic endogenous creatine biosynthesis, systemic depletion of creatine/phosphocreatine and accumulation of guanidinoacetate takes place that are the diagnostic features of this disease. Dietary creatine supplementation alone or along with arginine restriction has been reported to be beneficial for all treated patients, although to various extent. However, none of the GAMT deficient patient has been reported to return to complete normal developmental level.

Feasibility of newborn screening for guanidinoacetate methyltransferase (GAMT) deficiency

Guanidinoacetate methyltransferase (GAMT) deficiency causes brain creatine deficiency characterized by developmental delays, speech delay, seizures and autism-like behavior. Identification and therapy at birth because of a positive family history has prevented intellectual disability and seizures in all cases reported. The objective of this study was to develop a method to identify patients with GAMT deficiency from newborn screening blood spots. Creatine and guanidinoacetate were extracted from 10,000 deidentified blood spots using the same protocol routinely used for newborn screening and quantified by stable isotope dilution using deuterated creatine and guanidinoacetate as internal standards. Residual dried blood spots from three infants with GAMT deficiency were used to evaluate the sensitivity of the method. A second tier test using UPLC-MS/MS was performed to analyze samples with a concentration of guanidinoacetate >2.44 μmol/L (99.5th centile of the normal population). Fifty four blood spots required second tier testing in addition to seven blood spots from three patients with GAMT deficiency retrospectively analyzed. With second tier testing, only the samples from GAMT deficiency patients had elevated concentration of guanidinoacetate. Our results show that GAMT deficiency can be identified in newborns using routine extraction methods. The cost of this additional screening is minimal, as it does not require additional instrumentation, procedure, or sample collection. The use of a second tier test can reduce the false positive rate to a minimum. Summary Brain creatine deficiency syndromes cause mental retardation that can be prevented if therapy is initiated early in life. This manuscript reports that infants with GAMT deficiency (one of the brain creatine deficiency syndromes) can be identified from elevated guanidinoacetate in newborn blood spots with virtually absent false-positive results using a second tier test.

Guanidinoacetate methyltransferase (GAMT) deficiency: outcomes in 48 individuals and recommendations for diagnosis, treatment and monitoring

We collected data on 48 patients from 38 families with guanidinoacetate methyltransferase (GAMT) deficiency. Global developmental delay/intellectual disability (DD/ID) with speech/language delay and behavioral problems as the most affected domains was present in 44 participants, with additional epilepsy present in 35 and movement disorder in 13. Treatment regimens included various combinations/dosages of creatine-monohydrate, l-ornithine, sodium benzoate and protein/arginine restricted diets. The median age at treatment initiation was 25.5 and 39 months in patients with mild and moderate DD/ID, respectively, and 11 years in patients with severe DD/ID. Increase of cerebral creatine and decrease of plasma/CSF guanidinoacetate levels were achieved by supplementation with creatine-monohydrate combined with high dosages of l-ornithine and/or an arginine-restricted diet (250 mg/kg/d l-arginine). Therapy was associated with improvement or stabilization of symptoms in all of the symptomatic cases. The 4 patients treated younger than 9 months had normal or almost normal developmental outcomes. One with inconsistent compliance had a borderline IQ at age 8.6 years. An observational GAMT database will be essential to identify the best treatment to reduce plasma guanidinoacetate levels and improve long-term outcomes.

Evidence-based treatment of guanidinoacetate methyltransferase (GAMT) deficiency

BACKGROUND

Guanidinoacetate methyltransferase (GAMT) deficiency causes cerebral creatine deficiency. Patients can have autistic behavior, seizures, intellectual disability, and severe speech delay. The goal of therapy is to increase creatine while reducing potentially neurotoxic guanidinoacetate concentrations. Here we evaluate how different therapies affect plasma guanidinoacetate levels in patients with GAMT deficiency.

METHODS

Retrospective analysis of data from five new patients with GAMT deficiency (four with delays and seizures, one diagnosed at birth).

RESULTS

The four symptomatic patients had decreased brain creatine by magnetic resonance spectroscopy and three also had abnormal globi pallidi by MRI. GAMT sequencing identified four previously reported mutations and one novel missense mutation (c.233T>A/p.V78E). Treatment with creatine (250-1000 mg/kg/day), ornithine (100-800 mg/kg/day), and sodium benzoate (50-135 mg/kg/day) supplements along with dietary protein restriction (0.8-1.5 g/kg/day) improved seizures and development with all patients becoming verbal. The patient treated at birth remains developmentally normal. Reduction in glycine and increase in ornithine levels significantly decreased plasma guanidinoacetate, with glycine levels being the best predictor of guanidinoacetate levels. In contrast, arginine levels were not significantly correlated with plasma guanidinoacetate.

CONCLUSIONS

Our results show that supplements of creatine, sodium benzoate (to reduce glycine) and ornithine reduce guanidinoacetate levels in patients with GAMT deficiency (dietary therapy was not evaluated in our study). Normal development with early therapy renders GAMT deficiency an ideal candidate for inclusion in newborn screening panels.

Biochemical, molecular, and clinical diagnoses of patients with cerebral creatine deficiency syndromes

Cerebral creatine deficiency syndromes (CCDS) are a group of inborn errors of creatine metabolism that involve AGAT and GAMT for creatine biosynthesis disorders and SLC6A8 for creatine transporter (CT1) deficiency. Deficiencies in the three enzymes can be distinguished by intermediate metabolite levels, and a definitive diagnosis relies on the presence of deleterious mutations in the causative genes. Mutations and unclassified variants were identified in 41 unrelated patients, and 22 of these mutations were novel. Correlation of sequencing and biochemical data reveals that using plasma guanidinoacetate (GAA) as a biomarker has 100% specificity for both AGAT and GAMT deficiencies, but AGAT deficiency has decreased sensitivity in this assay. Furthermore, the urine creatine:creatinine ratio is an effective screening test with 100% specificity in males suspected of having creatine transporter deficiency. This test has a high false-positive rate due to dietary factors or dilute urine samples and lacks sensitivity in females. We conclude that biochemical screening for plasma GAA and measuring of the urine creatine:creatinine ratio should be performed for suspected CCDS patients prior to sequencing. Also, based on the results of this study, we feel that sequencing should only be considered if a patient has abnormal biochemical results on repeat testing.

Elevation of guanidinoacetate in newborn dried blood spots and impact of early treatment in GAMT deficiency

Guanidinoacetate methyltransferase (GAMT) deficiency is a good candidate disorder for newborn screening because early treatment appears to improve outcomes. We report elevation of guanidinoacetate in archived newborn dried blood spots for 3 cases (2 families) of GAMT deficiency compared with an unaffected carrier and controls. We also report a new case of a patient treated from birth with normal developmental outcome at the age of 42 months.

[Cerebral creatine deficiency syndromes]

INTRODUCTION

Creatine deficiency syndromes are a recently described group of diseases characterized by inborn errors of creatine metabolism. Clinical features include a spectrum of neurodevelopment disorders of diverse severity. They are characterized by low levels of cerebral creatine caused by different pathogenic mutations concerning the genes coding for creatine synthesis enzymes [arginine: glicyne amidinotransferase (AGAT, EC 2.1.4.1) and guanidinoacetate methyltansferase (GAMT, EC 2.1.1.2)], AGAT and GAMT, respectively, or its transporter (CT1 deficiency), SLC6A8. Enzymatic deficiencies are transmitted as autosomal recessive traits, whereas the transporter deficit is X-linked.

OBJECTIVES

To characterize the clinical and laboratorial presentation, diagnosis and treatment of cerebral creatine deficiency patients, followed in Hospital Pediátrico Carmona da Mota. The awareness of these inborn errors of metabolism as neurological disorders, namely of neurodevelopment, among the medical community is a secondary aim of the present work.

METHODS AND MATERIAL

Retrospective analysis of the clinical files of patients followed in our Hospital and diagnosed with cerebral creatine deficiency syndrome.

RESULTS

Twelve patients belonging to seven different families were diagnosed with creatine deficiency syndromes. Five presented GAMT deficiency and seven CT1 deficiency. Present ages are 2 to 38 years old. The most common clinical presentations were: global development delay in seven patients (two with epilepsy), and speech delay in two patients. Only one patient had communication and social interaction dysfunction. In all, global development delay in the range of intellectual delay was identified. The pathognomonic pattern of cerebral creatine deficiency in the brain image was demonstrated in eight patients. Pathogenic mutations in GAMT or SLC6A8 genes were identified in all cases.

CONCLUSIONS

The suspicion of cerebral creatine depletion must be considered in all children presenting unexplained global psychomotor development delay. Pre-symptomatic therapy has shown promising results, especially in GAMT deficiency patients. The high rate of asymptomatic carriers of GAMT mutations in our population makes this disorder eligible to neonatal screening in Portugal.

Guanidinoacetate methyltransferase deficiency: first steps to newborn screening for a treatable neurometabolic disease

BACKGROUND

GAMT deficiency is an autosomal recessive disorder of creatine biosynthesis resulting in severe neurological complications in untreated patients. Currently available treatment is only successful to stop disease progression, but is not sufficient to reverse neurological complications occurring prior to diagnosis. Normal neurodevelopmental outcome in a patient, treated in the newborn period, highlights the importance of early diagnosis.

METHODS

Targeted mutation analysis (c.59G>C and c.327G>A) in the GAMT gene by the QIAxcel system and GAA measurement by a novel two-tier method were performed in 3000 anonymized newborn blood dot spot cards.

RESULTS

None of the targeted mutations were detected in any newborn. Two novel heterozygous variants (c.283_285dupGTC; p.Val95dup and c.278_283delinsCTCGATGCAC; p.Asp93AlafsX35) were identified by coincidence. Carrier frequency for these insertion/deletion types of GAMT mutations was 1/1475 in this small cohort of newborns. GAA levels were at or above the 99th percentile (3.12 μmol/l) in 4 newborns. Second-tier testing showed normal results for 4 newborns revealing 0.1% false positive rate. No GAMT mutations were identified in 4 of the newborns with elevated GAA levels in the first tier testing.

CONCLUSION

This is the first two-tier study to investigate carrier frequency of GAMT deficiency in the small cohort of newborn population to establish evidence base for the first steps toward newborn screening for this treatable neurometabolic disorder.

Creatine metabolism in urea cycle defects

Creatine (Cr) and phosphocreatine play an essential role in energy storage and transmission. Maintenance of creatine pool is provided by the diet and by de novo synthesis, which utilizes arginine, glycine and s-adenosylmethionine as substrates. Three primary Cr deficiencies exists: arginine:glycine amidinotransferase deficiency, guanidinoacetate methyltransferase deficiency and the defect of Cr transporter SLC6A8. Secondary Cr deficiency is characteristic of ornithine-aminotransferase deficiency, whereas non-uniform Cr abnormalities have anecdotally been reported in patients with urea cycle defects (UCDs), a disease category related to arginine metabolism in which Cr must be acquired by de novo synthesis because of low dietary intake. To evaluate the relationships between ureagenesis and Cr synthesis, we systematically measured plasma Cr in a large series of UCD patients (i.e., OTC, ASS, ASL deficiencies, HHH syndrome and lysinuric protein intolerance). Plasma Cr concentrations in UCDs followed two different trends: patients with OTC and ASS deficiencies and HHH syndrome presented a significant Cr decrease, whereas in ASL deficiency and lysinuric protein intolerance Cr levels were significantly increased (23.5 vs. 82.6 μmol/L; p < 0.0001). This trend distribution appears to be regulated upon cellular arginine availability, highlighting its crucial role for both ureagenesis and Cr synthesis. Although decreased Cr contributes to the neurological symptoms in primary Cr deficiencies, still remains to be explored if an altered Cr metabolism may participate to CNS dysfunction also in patients with UCDs. Since arginine in most UCDs becomes a semi-essential aminoacid, measuring plasma Cr concentrations might be of help to optimize the dose of arginine substitution.

Evaluation of two year treatment outcome and limited impact of arginine restriction in a patient with GAMT deficiency

A 4-year-old female with history of developmental regression and autistic features was diagnosed with guanidinoacetate methyltransferase deficiency at age 21 months. Upon treatment, she showed improvements in her developmental milestones, sensorial-neural hearing loss and brain atrophy on cranial-MRI. The creatine/choline ratio increased 82% in basal ganglia and 88% in white matter on cranial MR-spectroscopy. The CSF guanidinoacetate decreased 80% after six months of ornithine and creatine supplementation and an additional 8% after 18 months of additional arginine restricted diet. We report the most favorable clinical and biochemical outcome on treatment in our patient.

Guanidinoacetate methyltransferase (GAMT) deficiency in two Tunisian siblings: clinical and biochemical features

BACKGROUND

Guanidinoacetate methyltransferase (GAMT) deficiency is a recently described disorder and few cases have been reported to date. As it is a treatable pathology, we seek to contribute to its better understanding, particularly to further elucidate its biochemical diagnosis for early treatment.

METHODS

The patients, two brothers aged 13 years (P1) and 11 years (P2), have been explored for signs and symptoms suggestive of inborn errors of metabolism. The quantification of creatine (Cr), guanidinoacetate (GAA), and GAMT activity was performed by liquid chromatography-tandem mass spectrometry (LC-MS/MS).

RESULTS

The two brothers presented a similar clinical picture: developmental delay, epilepsy, axial hypotonia, spastic tetraparesis, severe mental and language delay, and autistic behaviour. GAA concentrations were markedly increased in plasma and in urine [2796 and 3342 micromol/mmol creatinine (control range: 4 - 220 micromol/mmol creatinine)/14 and 29 micromol/L (control range: 0.35 - 1.8 micromol/L), respectively] while plasma and urine creatine concentrations were at the lower normal range limit. Activity of GAMT in lymphoblasts was extremely reduced (< 0.01 nmol/mg protein/hour) compared to healthy subjects. GAMT activity was found to be intermediary in patients' parents.

CONCLUSIONS

It appears that the clinical picture is heterogeneous but should be considered as potential signs of creatine metabolism disorders, however, the biochemical diagnosis is reliable as the enzyme activity is zero in most cases. To date, it is still too early to establish correlations between symptoms and biochemical profile. However, the identification of additional cases of GAMT deficiency should help elucidate such relationships and the progress of patients treated with creatine in conjunction with ornithine supplementation.

Creatine prevents the inhibition of energy metabolism and lipid peroxidation in rats subjected to GAA administration

Guanidinoacetate methyltransferase (GAMT) deficiency is an inherited neurometabolic disorder, biochemically characterized by the tissue accumulation of guanidinoacetate (GAA). Affected patients present epilepsy and mental retardation whose etiopathogeny is unclear. Previous reports have shown that GAA alters brain energy metabolism and that creatine, which is depleted in patients with GAMT deficiency, can act as a neuroprotector; as such, in the present study we investigated the effect of creatine administration on some of the altered parameters of energy metabolism (complex II, Na(+),K(+)-ATPase and creatine kinase) and lipid peroxidation caused by intrastriatal administration of GAA in adult rats. Animals were pretreated for 7 days with daily intraperitonial administrations of creatine. Subsequently, these animals were divided into two groups: Group 1 (sham group), rats that suffered surgery and received saline; and group 2 (GAA-treated). Thirty min after GAA or saline, the animals were sacrificed and the striatum dissected out. Results showed that the administration of creatine was able to reverse the activities of complex II, Na(+),K(+)-ATPase and creatine kinase, as well as, the levels of thiobarbituric acid reactive substances (TBARS), an index of lipid peroxidation. These findings indicate that the energy metabolism deficit caused by GAA may be prevented by creatine, which probably acts as an antioxidant since it was able to prevent lipid peroxidation. These data may contribute, at least in part, to a better understanding of the mechanisms related to the energy deficit and oxidative stress observed in GAMT deficiency.

Guanidinoacetate methyltransferase deficiency (GAMT)

An increasing number of disorders of metabolism are becoming amenable to the treatment, and GAMT deficiency is one of them. The symptoms and signs are reviewed, emphasising that delayed language development is a particular feature. Other symptoms include learning disorders, autistic behaviour, epileptic seizures, and movement disorders. The condition is inherited in an autosomal recessive manner, and mutations in the GAMT gene severely affect the activity of guanidinoacetate. The MRI scan shows an increased signal in the globus pallidus, and the diagnosis is confirmed by finding increased guanidinoacetate in the urine and a low plasma creatine. Other methods of diagnosis are discussed. Treatment is based on giving creatine supplementation orally and a low-protein diet with restricted arginine and increased ornithine. This results in improvement of many of the symptoms, especially of the epileptic seizures and the abnormal movements. It is justifiable to consider this condition in any patient with unexplained learning disorders.

Guanidinoacetate methyltransferase (GAMT) deficiency diagnosed by proton NMR spectroscopy of body fluids

In patients with guanidinoacetate methyltransferase (GAMT) deficiency several parameters may point towards the diagnosis of GAMT deficiency. These include the low levels of creatine and creatinine in urine, the high concentration of guanidinoacetic acid (GAA) in urine and the low levels of creatine and creatinine in the cerebrospinal fluid (CSF). In this study, body fluids from 10 GAMT deficient patients were analysed using (1)H NMR spectroscopy. The urine 1D (1)H NMR spectra of all the patients showed a doublet resonance at 3.98 ppm (pH 2.50) derived from GAA present in high concentration. For this compound, a good recovery and good correlation was found between an LC-MS/MS method and (1)H NMR spectroscopy. In CSF NMR spectra of these patients, the singlet resonances of creatine and creatinine (3.05 and 3.13 ppm, respectively) were absent (normally always present in (1)H NMR spectra of CSF). Due to overlap by other resonances, the doublet of GAA could not be observed. Our data demonstrate that (1)H NMR spectroscopy of urine and CSF can be used to diagnose patients with GAMT deficiency.

Expanded clinical and molecular spectrum of guanidinoacetate methyltransferase (GAMT) deficiency

Guanidinoacetate methyltransferase (GAMT) deficiency is a disorder of creatine biosynthesis, characterized by excessive amounts of guanidinoacetate in body fluids, deficiency of creatine in the brain, and presence of mutations in the GAMT gene. We present here 8 new patients with GAMT deficiency along with their clinical, biochemical and molecular data. The age at diagnosis of our patients ranges from 0 to 14 years. The age of onset of seizures usually ranges from infancy to 3 years. However, one of our patients developed seizures at age 5; progressing to myoclonic epilepsy at age 8 years and another patient has not developed seizures at age 17 years. Five novel mutations were identified: c.37ins26 (p.G13PfsX38), c.403G>T (p.D135Y), c.507_521dup15 (p.C169_S173dup), c.402C>G (p.Y134X) and c.610_611delAGinsGAA (p.R204EfsX63). Six patients had the c.327G>A (last nucleotide of exon 2) splice-site mutation which suggests that this is one of the most common mutations in the GAMT gene, second only to the known Portuguese founder mutation, c.59G>C (p.W20S). Our data suggests that the clinical presentation can be variable and the diagnosis may be overlooked due to unawareness of this disorder. Therefore, GAMT deficiency should be considered in the differential diagnosis of progressive myoclonic epilepsy as well as in unexplained developmental delay or regression with dystonia, even if the patient has no history of seizures. As more patients are reported, the prevalence of GAMT deficiency will become known and guidelines for prenatal diagnosis, newborn screening, presymptomatic testing and treatment, will need to be formulated.

AGAT, GAMT and SLC6A8 distribution in the central nervous system, in relation to creatine deficiency syndromes: a review

Creatine deficiency syndromes, either due to AGAT, GAMT or SLC6A8 deficiencies, lead to a complete absence, or a very strong decrease, of creatine within the brain, as measured by magnetic resonance spectroscopy. While the mammalian central nervous system (CNS) expresses AGAT, GAMT and SLC6A8, the lack of SLC6A8 in astrocytes around the blood-brain barrier limits the brain capacity to import creatine from the periphery, and suggests that the CNS has to rely mainly on endogenous creatine synthesis through AGAT and GAMT expression. This seems contradictory with SLC6A8 deficiency, which, despite AGAT and GAMT expression, also leads to creatine deficiency in the CNS. We present novel data showing that in cortical grey matter, AGAT and GAMT are expressed in a dissociated way: e.g. only a few cells co-express both genes. This suggests that to allow synthesis of creatine within the CNS, at least for a significant part of it, guanidinoacetate must be transported from AGAT- to GAMT-expressing cells, possibly through SLC6A8. This would explain the creatine deficiency observed in SLC6A8-deficient patients. By bringing together creatine deficiency syndromes, AGAT, GAMT and SLC6A8 distribution in CNS, as well as a synthetic view on creatine and guanidinoacetate levels in the brain, this review presents a comprehensive framework, including new hypotheses, on brain creatine metabolism and transport, both in normal conditions and in case of creatine deficiency.

Global developmental delay in guanidionacetate methyltransferase deficiency: differences in formal testing and clinical observation

Guanidinoacetate N-methyltransferase (GAMT) deficiency is a defect in the biosynthesis of creatine (Cr). So far, reports have not focused on the description of developmental abilities in this disorder. Here, we present the result of formal testing of developmental abilities in a GAMT-deficient patient. Our patient, a 3-year-old boy with GAMT deficiency, presented clinically with a severe language production delay and nearly normal nonverbal development. Treatment with oral Cr supplementation led to partial restoration of the cerebral Cr concentration and a clinically remarkable acceleration of language production development. In contrast to clinical observation, formal testing showed a rather harmonic developmental delay before therapy and a general improvement, but no specific acceleration of language development after therapy. From our case, we conclude that in GAMT deficiency language delay is not always more prominent than delays in other developmental areas. The discrepancy between the clinical impression and formal testing underscores the importance of applying standardized tests in children with developmental delays. Screening for Cr deficiency by metabolite analysis of body fluids or proton magnetic resonance spectroscopy of the brain deficiency should be considered in any child with global developmental delay/mental retardation lacking clues for an alternative etiology.

Successful treatment of a guanidinoacetate methyltransferase deficient patient: findings with relevance to treatment strategy and pathophysiology

Biochemical and developmental results of treatment of a guanidinoacetate methyltransferase (GAMT) deficient patient with a mild clinical presentation and remarkable developmental improvement after treatment are presented. Treatment with creatine (Cr) supplementation resulted in partial normalization of cerebral (measured with magnetic resonance proton spectroscopy) and plasma levels of Cr and guanidinoacetate (GAA). Addition of high dose ornithine to the treatment led to further normalization of plasma GAA, while cerebral Cr and GAA did not improve further.

Severe speech delay as the presenting symptom of guanidinoacetate methyltransferase deficiency

Guanidinoacetate methyltransferase deficiency typically presents with muscular hypotonia, global developmental delay, extrapyramidal signs, and seizures during infancy and childhood. The authors report a 5-year-old child with guanidinoacetate methyltransferase deficiency who presented with severe speech delay, emphasizing the importance of an early screening for disorders of creatine synthesis and transport in every infant or child with isolated speech delay of unknown cause.

A prevalent pathogenic GAMT mutation (c.59G>C) in Portugal

Guanidinoacetate methyltransferase (GAMT) deficiency (MIM 601240), an autosomal recessive disorder of creatine biosynthesis, presents with mental retardation, extrapyramidal symptoms, autistic-like behavior and epilepsy. Other hallmarks are cerebral creatine deficiency, increased levels of guanidinoacetate in body fluids and mutations in the GAMT gene. Creatine supplementation partially restores cerebral creatine content. Worldwide, 29 patients have been identified and 15 different mutations have been reported in the GAMT gene. Ten out of these 29 patients are of Portuguese origin. Likely, a founder effect and a high carrier rate in Portugal exist, since in 17 out of the 20 Portuguese alleles the c.59G>C; p.Trp20Ser mutation was found. We investigated the carrier rate of the c.59G>C; p.Trp20Ser mutation in different regions of Portugal and confirmed the pathogenic nature of this missense mutation by transient transfections. Anonymous bloodspots (1002) were screened for the presence of the c.59G>C; p.Trp20Ser mutation by SNaPshot (Single Nucleotide Polymorphism Multiplex Kit). Eight carriers of c.59G>C; p.Trp20Ser were detected of which four are derived from the Archipelagos. This suggests that the carrier rate of the c.59G>C; p.Trp20Ser mutation is relatively high in these islands, as well as in other parts of Portugal. It also implies that newborn screening in these regions is warranted for this treatable disorder.

Creatine uptake in brain and skeletal muscle of mice lacking guanidinoacetate methyltransferase assessed by magnetic resonance spectroscopy

Creatine (Cr) levels in skeletal muscle and brain of a mouse model of Cr deficiency caused by guanidinoacetate methyltransferase absence (GAMT-/-) were studied after Cr supplementation with 2 g.kg body wt-1.day-1 Cr for 35 days. Localized 1H magnetic resonance spectroscopy (MRS) was performed in brain (cerebellum and thalamus/hippocampus) and in hind leg muscle of GAMT-/- mice before and after Cr supplementation and in control (Con) mice. As expected, a signal for Cr was hardly detectable in MR spectra of GAMT-/- mice before Cr supplementation. In the thalamus/hippocampus region of these mice, an increase in N-acetylasparate (NAA) was observed. During Cr administration, Cr levels increased faster in skeletal muscle compared with brain, but this occurred only during the first day of supplementation. Thereafter, Cr levels increased by 0.8 mM/day in all studied locations. After 35 days of Cr supplementation, Cr levels in all locations were higher compared with Con mice on a Cr-free diet and NAA levels normalized. Only because of the repeated MRS measurements performed in this longitudinal Cr supplementation study on GAMT-/- mice were we able to discover the initial faster uptake of Cr in skeletal muscle compared with brain, which may represent muscular Cr uptake independent of Cr transporter expression. Our results can provide the basis for additional experiments to optimize Cr supplementation in GAMT deficiency, as increases in brain Cr are slow in patients after Cr supplementation.

Treatment monitoring of brain creatine deficiency syndromes: a 1H- and 31P-MR spectroscopy study

BACKGROUND AND PURPOSE

Brain creatine (Cr) deficiencies (BCr-d) are rare disorders of creatine biosynthesis and transport. We performed consecutive measures of total Cr (tCr) and of its phosphorylated fraction, phosphocreatine (PCr), in the brains of children affected by Cr synthesis defects during a long period of therapy. The aim was to identify the optimal treatment strategy for these disorders.

MATERIALS AND METHODS

Two patients with guanidinoacetate methyltransferase defect (GAMT-d) were treated with different amounts of Cr and with diet restrictions aimed at reducing endogenous guanidinoacetate (GAA) synthesis. Three patients with arginine:glycine amidinotransferase defect (AGAT-d) were treated with different Cr intakes. The patients' treatments were monitored by means of (1)H- and (31)P-MR spectroscopy.

RESULTS

Cr and PCr replenishment was lower in GAMT-d than in AGAT-d even when GAMT-d therapy was carried out with a very high Cr intake. Cr and especially PCr replenishment became more efficient only when GAA blood values were reduced. Adenosine triphosphate (ATP) was increased in the baseline phosphorous spectrum of GAMT-d, and it returned to a normal value with treatment. Brain pH and brain P(i) showed no significant change in the AGAT-d syndrome and at any Cr intake. However, 1 of the 2 GAMT-d patients manifested a lower brain pH level while consuming the GAA-lowering diet.

CONCLUSIONS

AGAT-d treatment needs lower Cr intake than GAMT-d. Cr supplementation in GAMT-d treatment should include diet restrictions aimed at reducing GAA concentration in body fluids. (1)H- and especially (31)P-MR spectroscopy are the ideal tools for monitoring the therapy response to these disorders.

Guanidinoacetate methyltransferase deficiency masquerading as a mitochondrial encephalopathy

Guanidinoacetate methyltransferase (GAMT) deficiency is a rare disorder of creatine synthesis. We report a patient who presented at 10 months of age with hypotonia and global developmental delay. Subsequently, she developed seizures and choreoathetosis. Magnetic resonance imaging showed high signal bilaterally in the globus pallidus on T2-weighted images. Mitochondrial respiratory chain studies revealed low complex I activity (in muscle 0.052 nmol NADH oxidized per min per unit citrate synthase, controls 0.166 +/- 0.047; in fibroblasts 0.080 nmol NADH oxidized per min per unit citrate synthase, controls 0.197 +/- 0.034). The true diagnosis was suspected at 21 months of age because of persistent low plasma and urine creatinine concentrations. GAMT activity was undetectable in fibroblasts and compound heterozygous mutations were found in the GAMT gene (c.327G>A and c.522G>A). The patient was treated with creatine, dietary arginine restriction and ornithine supplements. Her movement disorder and seizures resolved but she still has severe cognitive impairment and no expressive language. The occurrence of secondary respiratory chain abnormalities in GAMT deficiency may lead to misdiagnosis, particularly as the clinical and radiological features resemble those seen in mitochondrial encephalopathies. It is important to establish the correct diagnosis because specific treatment is available.

Cerebral creatine deficiency syndromes: clinical aspects, treatment and pathophysiology

Cerebral creatine deficiency syndromes (CCDSs) are a group of inborn errors of creatine metabolism comprising two autosomal recessive disorders that affect the biosynthesis of creatine--i.e. arginine:glycine amidinotransferase deficiency (AGAT; MIM 602360) and guanidinoacetate methyltransferase deficiency (GAMT; MIM 601240)--and an X-linked defect that affects the creatine transporter, SLC6A8 deficiency (SLC6A8; MIM 300036). The biochemical hallmarks of these disorders include cerebral creatine deficiency as detected in vivo by 1H magnetic resonance spectroscopy (MRS) of the brain, and specific disturbances in metabolites of creatine metabolism in body fluids. In urine and plasma, abnormal guanidinoacetic acid (GAA) levels are found in AGAT deficiency (reduced GAA) and in GAMT deficiency (increased GAA). In urine of males with SLC6A8 deficiency, an increased creatine/creatinine ratio is detected. The common clinical presentation in CCDS includes mental retardation, expressive speech and language delay, autistic like behaviour and epilepsy. Treatment of the creatine biosynthesis defects has yielded clinical improvement, while for creatine transporter deficiency, successful treatment strategies still need to be discovered. CCDSs may be responsible for a considerable fraction of children and adults affected with mental retardation of unknown etiology. Thus, screening for this group of disorders should be included in the differential diagnosis of this population. In this review, also the importance of CCDSs for the unravelling of the (patho)physiology of cerebral creatine metabolism is discussed.

Methyl balance and transmethylation fluxes in humans

Various questions have been raised about labile methyl balance and total transmethylation fluxes, and further discussion has been encouraged. This report reviews and discusses some of the relevant evidence now available. The fact that, if needed, labile methyl balance is maintained by methylneogenesis appears to be established, but several aspects of transmethylation remain uncertain: definitive measurements of the rate of total transmethylation in humans of both sexes on various diets and at various ages; the extent to which synthesis of phosphatidylcholine has been underestimated; and the relative contributions of the 2 pathways for the formation of sarcosine (ie, N-methylglycine). The available evidence indicates that the quantitatively most important pathways for S-adenosylmethionine-dependent transmethylation in mammals are the syntheses of creatine by guanidinoacetate methyltransferase, of phosphatidylcholine by phosphatidylethanolamine methyltransferase, and of sarcosine by glycine N-methyltransferase. Data presented in this report show that S-adenosylmethionine and methionine accumulate abnormally in the plasma of humans with glycine N-methyltransferase deficiency but not of those with guanidinoacetate N-methyltransferase deficiency or in the plasma or livers of mice devoid of phosphatidylethanolamine N-methyltransferase activity. The absence of such accumulations in the latter 2 conditions may be due to removal of S-adenosylmethionine by synthesis of sarcosine. Steps that may help clarify the remaining issues include the determination of the relative rates of synthesis of sarcosine, creatine, and phosphatidylcholine by rapid measurement of the rates of radiolabel incorporation into these compounds from L-[methyl-3H]methionine administered intraportally to an experimental animal; clarification of the intracellular hepatic isotope enrichment value during stable-isotope infusion studies to enhance the certainty of methyl flux estimates during such studies; and definitive measurement of the dietary betaine intake from various diets.

Pre-symptomatic treatment of creatine biosynthesis defects

Recent observations in two patients, one with AGAT deficiency (AGAT-D) and one with GAMT deficiency (GAMT-D), both diagnosed already at birth, provide first evidence for important therapeutic effects of pre-symptomatic treatment with creatine (Cr) supplementation in AGAT-D and Cr supplementation plus guanidinoacetate lowering strategies in GAMT-D. Although long-term data are lacking, the results suggest that complete prevention of neurological sequelae in early treated patients could be feasible (Battini et al., 2006; Schulze et al., 2006).

Overexpression of GAMT restores GAMT activity in primary GAMT-deficient fibroblasts

Guanidinoacetate methyltransferase deficiency (MIM 601240) is an autosomal recessive disorder of creatine biosynthesis. Patients present with mental retardation, extrapyramidal symptoms, autistic-like behavior, epilepsy, cerebral creatine deficiency and increased levels of guanidinoacetate. So far 15 mutations have been reported, including six missense variants that are highly likely to be pathogenic mutations. To prove that mutations in the GAMT gene are responsible for GAMT deficiency we overexpressed the GAMT open reading frame in GAMT-deficient fibroblasts by stable transfection. In addition, HeLa cells were transiently transfected with the same expression vector. In contrast to mock transfectants transfection of primary GAMT-deficient fibroblasts with wild-type GAMT results in the restoration of GAMT activity as measured by GC-MS using stable isotope labeled substrates. Moreover, the expression of the GAMT-EGFP fusion protein was analyzed by Western blot, confirming the presence of GAMT fusion protein, both in the stable as well as in the transient transfectants. Here, we prove that mutations in the GAMT gene are responsible for GAMT deficiency, since overexpression of the GAMT open reading frame restores GAMT activity in GAMT-deficient fibroblasts. Furthermore, the transient transfection of HeLa cells will be important for functional analysis of variants of unknown consequence (i.e., missense mutations).

Presymptomatic treatment of neonatal guanidinoacetate methyltransferase deficiency

Prospective observation in a neonate with guanidinoacetate methyltransferase deficiency (GAMT-D), a severe neurometabolic disorder, revealed increased guanidinoacetate levels at birth. After 14-month treatment with creatine, high-dose ornithine, benzoate, and an arginine-restricted diet, the patient's development is normal and she does not present any symptoms of GAMT-D. The authors' observation indicates that early detection of GAMT-D is possible in the neonatal period, and presymptomatic treatment may prevent its manifestation.

GAMT deficiency: Features, treatment, and outcome in an inborn error of creatine synthesis

BACKGROUND

Guanidinoactetate methyltransferase (GAMT) deficiency is an autosomal recessive disorder of creatine synthesis. The authors analyzed clinical, biochemical, and molecular findings in 27 patients.

METHODS

The authors collected data from questionnaires and literature reports. A score including degree of intellectual disability, epileptic seizures, and movement disorder was developed and used to classify clinical phenotype as severe, moderate, or mild. Score and biochemical data were assessed before and during treatment with oral creatine substitution alone or with additional dietary arginine restriction and ornithine supplementation.

RESULTS

Intellectual disability, epileptic seizures, guanidinoacetate accumulation in body fluids, and deficiency of brain creatine were common in all 27 patients. Twelve patients had severe, 12 patients had moderate, and three patients had mild clinical phenotype. Twenty-one of 27 (78%) patients had severe intellectual disability (estimated IQ 20 to 34). There was no obvious correlation between severity of the clinical phenotype, guanidinoacetate accumulation in body fluids, and GAMT mutations. Treatment resulted in almost normalized cerebral creatine levels, reduced guanidinoacetate accumulation, and in improvement of epilepsy and movement disorder, whereas the degree of intellectual disability remained unchanged.

CONCLUSION

Guanidinoactetate methyltransferase deficiency should be considered in patients with unexplained intellectual disability, and urinary guanidinoacetate should be determined as an initial diagnostic approach.

Intrastriatal administration of guanidinoacetate inhibits Na+, K+-ATPase and creatine kinase activities in rat striatum

Guanidinoacetate methyltransferase deficiency (GAMT deficiency) is an inherited neurometabolic disorder clinically characterized by epilepsy and mental retardation and biochemically by accumulation of guanidinoacetate (GAA) and depletion of creatine. Although this disease is predominantly characterized by severe neurological findings, the underlying mechanisms of brain injury are not yet established. In the present study, we investigated the effect of intrastriatal administration of GAA on Na+, K+-ATPase activity, total (tCK), cytosolic (Cy-CK), and mitochondrial (Mi-CK) creatine kinase (CK) activities in rat striatum. We verified that Na+, K+-ATPase, tCK, and Mi-CK activities were significantly inhibited by GAA, in contrast to Cy-CK which was not affected by this guanidino compound. Since these enzyme activities can be affected by reactive species, we also investigated the effect of intrastriatal administration of GAA on thiobarbituric acid reactive substances (TBARS), an index of lipid peroxidation in rats. We found that this metabolite significantly increased this oxidative stress parameter. Considering the importance of Na+, K+-ATPase and CK activities for brain metabolism homeostasis, our results suggest that the inhibition of these enzymes by increased intracerebral levels of GAA may contribute to the neuropathology observed in patients with GAMT-deficiency.

Quantitative determination of guanidinoacetate and creatine in dried blood spot by flow injection analysis-electrospray tandem mass spectrometry

BACKGROUND

Guanidinoacetate (GAA) and creatine (Cr) are reliable biochemical markers of primary creatine disorders. The aim of this study was to develop and validate a method for the determination of GAA and Cr in dried blood spot through the use of stable isotope dilution and flow injection analysis (FIA)-ESI-MS/MS.

METHODS

Dried blood spots were extracted using methanol-water solution containing D3-Cr. After evaporation and formation of butyl esters, samples were analyzed using multiple reaction monitoring mode (m/z 174.2-->101.1 for GAA, 188.3-->90.1 for Cr and 191.3-->93.1 for D3-Cr).

RESULTS

The analysis was very fast (1 min). The detection limits were 0.34 micromol/l of blood and 0.30 micromol/l of blood for Cr and GAA, respectively, and the response was linear over the range 0.25-12.5 micromol/l of blood for GAA and 3.57-624.7 micromol/l of blood for Cr. Recovery range was 93-101% for Cr and 94-105% for GAA and between-run CVs were 5.3% for GAA and 4.5% for Cr. Ion suppression effect was also studied. The method was applied to spots obtained from two patients affected by GAMT deficiency, four patients affected by AGAT deficiency (including a newborn) as well as 282 healthy subjects.

CONCLUSIONS

The detection of GAA in dried blood spot by FIA-ESI-MS/MS is a highly reliable and high throughput method for the diagnosis of GAMT and AGAT deficiencies and a possible tool for newborn screening of both these tractable disorders.

[Radiological innovations in the screening and diagnosis of the inborn errors of metabolism]

New metabolic diseases are regularly identified by a genetic or biochemical approach. Indeed, the metabolic diseases result from an enzymatic block with accumulation of a metabolite upstream to the block and deficit of a metabolite downstream. The characterization of these abnormal metabolites by MRI spectroscopy permitted to identify the deficient enzyme in two new groups of diseases, creatine deficiencies and polyol anomalies. Creatine deficiency is implicated in unspecific mental retardation. A low peak of creatine at MRI spectroscopy is evocating of creatine deficiency which is treatable by creatine administration. Deficiency of synthesis of polyols, metabolites on the pentose pathway, represent new described metabolic diseases with variable symptoms including a neurological distress, liver disease, splenomegaly, cutis laxa and renal insufficiency. The deficit of ribose-5-phosphate isomerase, one of the enzymes whose diagnosis is evoked in front of the accumulation of ribitol, arabitol and xylitol leads to a leucodystrophy in adults. This new deficit was highlighted by the identification of an abnormal peak in cerebral MRI-spectroscopy corresponding to the abnormal accumulation of polyols in brain. Congenital hyperinsulinism (HI) is characterized by profound hypoglycaemia related to inappropriate insulin secretion. Focal and diffuse forms of hyperinsulinism share a similar clinical presentation but their treatment is dramatically different. Until recently, preoperative differential diagnosis was based on pancreatic venous sampling, an invasive and technically demanding technique. Positron emission tomography (PET) after injection of [18F]Fluoro-L-DOPA has been evaluated for the preoperative differentiation between focal and diffuse HI, by imaging uptake of radiotracer and the conversion of [18F]Fluoro-L-DOPA into dopamine by DOPA decarboxylase. PET with [18F]Fluoro-L-DOPA has been validated as a reliable test to differentiate diffuse and focal HI and is now a major differential diagnosis tool in infantile hyperinsulinemic hypoglycaemia.

Use of denaturing HPLC to provide efficient detection of mutations causing guanidinoacetate methyltransferase deficiency

Guanidinoacetate methyltransferase (GAMT) deficiency is an autosomal recessive error of creatine synthesis characterized by cerebral creatine deficiency, accumulation of guanidinoacetate, mental retardation, epilepsy, and extrapyramidal symptoms. To date, 14 mutations of the GAMT gene in 27 patients have been reported. Mutation analysis was done using direct sequencing of PCR products and denaturing gradient gel electrophoresis in combination with direct sequencing. In contrast, we evaluated the efficiency of a newly developed DHPLC method to detect mutations in the GAMT gene by analysing DNA from 14 GAMT patients with known mutations. PCR amplification of both patient and control DNA was followed by formation of homoduplices and heteroduplices, and their detection by DHPLC. DHPLC identified all mutations tested and is the preferred choice of analytical method.