Newborn screening · Other inherited disorder
Guanidinoacetate Methyltransferase Deficiency · GAMT
Cerebral creatine deficiency. Treatable with creatine and ornithine supplementation if caught before symptoms.
Description
Guanidinoacetate methyltransferase deficiency, GAMT deficiency, is an autosomal recessive cerebral creatine deficiency syndrome. The GAMT enzyme converts guanidinoacetate (GAA) to creatine using S-adenosylmethionine. Two damaging copies of the GAMT gene on chromosome 19p13.3 leave the brain unable to make creatine and cause GAA to accumulate in blood, urine, and cerebrospinal fluid. Creatine is required for energy buffering in neurons and muscle, and high GAA appears to be neurotoxic. Inheritance is autosomal recessive.
Untreated children typically present in the first one to three years with global developmental delay, intellectual disability, treatment-resistant epilepsy, and a movement disorder that can include dystonia, chorea, or autistic features. Brain magnetic resonance spectroscopy shows a near-absent creatine peak. Serum and urine GAA are elevated, and urine creatine to creatinine ratio is low. Confirmation uses GAMT gene sequencing and, when needed, fibroblast enzyme activity. Stockler-Ipsiroglu et al. (Molecular Genetics and Metabolism, 2014) summarized the international natural history across 48 cases and documented the seizure and developmental phenotype that defines the untreated course.
GAMT deficiency was added to the federal Recommended Uniform Screening Panel (RUSP) in January 2023, the first metabolic addition to the RUSP since Pompe and MPS I in 2015 and 2016. Newborn screening uses the dried blood spot to measure GAA, with a creatine ratio as a second-tier marker. Reported birth incidence is roughly 1 in 250,000 to 1 in 500,000, with founder clusters described in specific populations including the Portuguese Roma.
Treatments to date
Treatment is high-dose oral creatine monohydrate, oral L-ornithine, and dietary restriction of arginine to reduce GAA production. Sodium benzoate is sometimes added to lower GAA further by conjugating glycine. The approach was established and refined by Stockler and colleagues from the original Austrian case in 1994 forward, and the combined regimen is now standard of care across metabolic clinics. There is no FDA-approved disease-specific drug. Creatine monohydrate is sold as a dietary supplement, and metabolic teams prescribe pharmaceutical-grade preparations through specialty pharmacies.
Outcomes diverge sharply by timing. Children identified before symptom onset and started on treatment as newborns can have normal or near-normal development on follow-up, with controlled GAA, suppressed seizures, and typical school performance reported across multiple cohorts. Children diagnosed after symptoms appear retain the intellectual disability and movement features established before treatment, although seizure control and behavior often improve. This pre-symptomatic versus post-symptomatic gap is the central argument for newborn screening and the basis of the 2023 RUSP addition.
Pilot newborn screening programs in Utah and New York preceded the federal RUSP decision and provided implementation data the ACHDNC reviewed. As of 2026 most state programs are at various stages of adoption, and uptake across all 50 states is incomplete. Active research includes substrate-reduction strategies aimed at lowering GAA more efficiently, AAV-mediated gene therapy in preclinical models, and longitudinal outcome studies of the first newborn-screened cohorts.