Improving treatment of guanidinoacetate methyltransferase deficiency: reduction of guanidinoacetic acid in body fluids by arginine restriction and ornithine supplementation
Guanidinoacetate methyltransferase (GAMT) deficiency (McKusick 601240), an inborn error of creatine biosynthesis, is characterized by creatine depletion and accumulation of guanidinoacetate (GAA) in the brain. Treatment by oral creatine supplementation had no effect on the intractable seizures. Based on the possible role of GAA as an epileptogenic agent, we evaluated a dietary treatment with arginine restriction and ornithine supplementation in order to achieve reduction of GAA. In an 8-year-old Kurdish girl with GAMT deficiency arginine intake was restricted to 15 mg/kg/day (0.4 g natural protein/kg/day) and ornithine was supplemented with 100 mg/kg/day over a period of 14 months. The diet was enriched with 0.4 g/kg/day of arginine-free essential amino acid mixture and creatine treatment remained unchanged (1.1 g/kg/day). Guanidino compounds in blood, urine, and CSF were measured by means of cation-exchange chromatography. The combination of arginine restriction and ornithine supplementation led to a substantial and permanent decrease of arginine without disturbance of nitrogen detoxification. Formation of GAA was effectively reduced after 4 weeks of treatment and sustained thereafter. Biochemical effects were accompanied by a marked clinical improvement. Distinctly reduced epileptogenic activities in electroencephalography accompanied by almost complete disappearance of seizures demonstrates the positive effect of GAA reduction. This indicates for the first time that GAA may exert an important epileptogenic potential in man. Arginine restriction in combination with ornithine supplementation represents a new and rationale therapeutic approach in GAMT deficiency.