Evidence and Recommendation for Guanidinoacetate Methyltransferase Deficiency Newborn Screening

Newborn screening for neuro-metabolic disorders: Strategies, clinical benefits, and prerequisites for program expansion

Newborn screening (NBS) is a successful program of secondary prevention for rare diseases, such as neuro-metabolic diseases, enabling early identification of affected individuals and pre-symptomatic treatment. Driven by innovations in high-throughput sequencing technologies, NBS panels have continued to grow and will probably be extended further in the future. However, implementing NBS for a disease is subject to various preconditions to maximize the benefit for the affected children, while avoiding harm to the screened healthy cohort, their families and the society. Ideally, data on clinical long-term benefit of NBS and early treatment is collected prior to NBS implementation through long-term observational studies and registries. In addition, NBS should be implemented as an iteratively evaluated public health program and the data collection should be accompanied by intra-operable long-term observational studies, ideally extended in international cooperations. In this review, the current expertise in NBS, the screening strategies and possible long-term clinical benefits are presented and discussed for several neuro-metabolic diseases, including propionic acidemia and isolated methylmalonic acidemias, homocystinurias, remethylation defects, acquired cobalamin (vitamin B12) deficiency, urea cycle disorders, tetrahydrobiopterin (BH4) and primary neurotransmitter disorders, as well as lysosomal storage disorders. Given these prerequisites, several of the neuro-metabolic diseases discussed here might be part of future NBS programs worldwide.

Diagnostic delay in cerebral creatine deficiency disorders: lessons learned from a cross-sectional single center study, and guanidinoacetate and creatine measurements in Switzerland between 2015 and 2023

BACKGROUND

Cerebral creatine deficiency disorders (CCDD) are rare diseases caused by defects in the enzymes L-arginine: glycine amidinotransferase (AGAT) or guanidinoacetate-N-methyltransferase (GAMT), which are involved in synthesis of creatine; or by a defect in the creatine transporter (CRTR), which is essential for uptake of creatine as important energy source into the target cells. Patients with CCDD can present with a variety of unspecific symptoms: global developmental delay, speech-language disorder, behavioral abnormalities and seizures. Early treatment initiation is essential in AGAT and GAMT deficiencies to achieve a favorable outcome. This study describes the CCDD patient cohort in a single center, and an analysis of the referrals to two Swiss laboratories in Lausanne and Zurich between 2015 and 2023 for the two marker metabolites guanidinoacetate and creatine.

RESULTS

The patient cohort comprised 6 patients (defects: 2 GAMT, 4 CRTR), who were initially seen by different subspecialties depending on first symptoms. There was a diagnostic and therapeutic delay between 3 and 32 months (mean 13.8). Numbers of biomarker requests showed a constant increase during the study period, with a majority of tests performed in urine, the preferred sample for CCDD detection. Almost all samples (93.3%) were sent in by large hospitals (mainly from neurology, developmental pediatrics and metabolism) and only few (5.2%) by pediatricians in private practice, although those usually see the patients first.

CONCLUSIONS

The data from this study demonstrate a relevant delay in identifying patients with these rare conditions, and a predominance of biomarker analysis requested from pediatric subspecialties that are involved in patient management often long after occurrence of symptoms. To reduce the diagnostic delay and the outcome of patients, the current practice of sample referral should be reflected and first-contact healthcare providers should be encouraged to initiate selective screening.

Current Status of Newborn Bloodspot Screening Worldwide 2024: A Comprehensive Review of Recent Activities (2020-2023)

Newborn bloodspot screening (NBS) began in the early 1960s based on the work of Dr. Robert "Bob" Guthrie in Buffalo, NY, USA. His development of a screening test for phenylketonuria on blood absorbed onto a special filter paper and transported to a remote testing laboratory began it all. Expansion of NBS to large numbers of asymptomatic congenital conditions flourishes in many settings while it has not yet been realized in others. The need for NBS as an efficient and effective public health prevention strategy that contributes to lowered morbidity and mortality wherever it is sustained is well known in the medical field but not necessarily by political policy makers. Acknowledging the value of national NBS reports published in 2007, the authors collaborated to create a worldwide NBS update in 2015. In a continuing attempt to review the progress of NBS globally, and to move towards a more harmonized and equitable screening system, we have updated our 2015 report with information available at the beginning of 2024. Reports on sub-Saharan Africa and the Caribbean, missing in 2015, have been included. Tables popular in the previous report have been updated with an eye towards harmonized comparisons. To emphasize areas needing attention globally, we have used regional tables containing similar listings of conditions screened, numbers of screening laboratories, and time at which specimen collection is recommended. Discussions are limited to bloodspot screening.

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.

Long term follow-up in GAMT deficiency - Correlation of therapy regimen, biochemical and in vivo brain proton MR spectroscopy data

GAMT deficiency is a rare autosomal recessive disease within the group of cerebral creatine deficiency syndromes. Cerebral creatine depletion and accumulation of guanidinoacetate (GAA) lead to clinical presentation with intellectual disability, seizures, speech disturbances and movement disorders. Treatment consists of daily creatine supplementation to increase cerebral creatine, reduction of arginine intake and supplementation of ornithine for reduction of toxic GAA levels. This study represents the first long-term follow-up over a period of 14 years, with detailed clinical data, biochemical and multimodal neuroimaging findings. Developmental milestones, brain MRI, quantitative single voxel 1H magnetic resonance spectroscopy (MRS) and biochemical analyses were assessed. The results reveal insights into the dose dependent effects of creatine/ornithine supplementation and expand the phenotypic spectrum of GAMT deficiency. Of note, the creatine concentrations, which were regularly monitored over a long follow-up period, increased significantly over time, but did not reach age matched control ranges. Our patient is the second reported to show normal neurocognitive outcome after an initial delay, stressing the importance of early diagnosis and treatment initiation.

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.