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.

Guanidinoacetate methyltransferase deficiency identified in adults and a child with mental retardation

Our study describes the adult clinical and biochemical spectrum of guanidinoacetate methyltransferase (GAMT) deficiency, a recently discovered inborn error of metabolism. The majority of the previous reports dealt with pediatric patients, in contrast to the present study. A total of 180 institutionalized patients with a severe mental handicap were investigated for urine and plasma uric acid and creatinine. Patients with an increased urinary uric acid/creatinine ratio and/or decreased creatinine were subjected to the analysis of guanidinoacetate (GAA). Four patients (three related and one from an unrelated family) were identified with GAMT-deficiency. A fifth patient had died before a biochemical diagnosis could be made. They all had shown a normal psychomotor development for the first year of life, after which they developed a profound mental retardation. Three out of four had convulsions and all four totally lacked the development of speech. Their GAMT activity in lymphoblasts was impaired and two novel mutations were identified: the 59 G > C and 506 G > A missense mutations. Urinary GAA was increased, but highly variable 347-1,624 mmol/mol creat (Controls <150 mmol/mol creat). In plasma and CSF the GAA levels were fairly constant at 17.3-27.0 mumol/L (Controls 1.33-3.33) and 11.0-12.4 mumol/L, respectively (Controls 0.068-0.114). GAMT deficiency in adults is associated with severe mental retardation and absence or limited speech development. Convulsions may be prominent. The nonspecific nature of the clinical findings as well as the limited availability of GAA assays and/or in vivo magnetic resonance spectroscopy of the brain may mean that many more patients remain undiagnosed in institutions for persons with mental handicaps.