Creatine deficiency syndrome. A treatable myopathy due to arginine-glycine amidinotransferase (AGAT) deficiency

We report two sisters, aged 11 and 6years, with AGAT deficiency syndrome (OMIM 612718) which is the least common creatine deficiency syndrome. They were born full-term to consanguineous parents and had moderate developmental delay. Examination showed an important language delay, a progressive proximal muscular weakness in the lower limbs with Gowers sign and myopathic electromyography. Investigations revealed undetectable guanidinoacetate and low level of creatine in plasma and urine, characteristic findings of AGAT deficiency syndrome. Brain magnetic resonance spectroscopy showed a markedly reduced level of creatine. Guanidinoacetate methyltransferase (GATM) gene sequencing revealed a homozygous missense mutation in exon 4:c.608A>C, (p.Tyr203Ser). Thirteen months after beginning the treatment with oral creatine monohydrate 200mg/kg/day, then 400mg/kg/day, there was a dramatic improvement in muscle strength with Gowers sign disappearance in both patients, and a mild improvement in language and cognitive functions. AGAT deficiency syndrome should be considered in all patients with language retardation and cognitive impairment associated to a myopathy of unknown etiology such that early diagnosis must lead to creatine supplementation to cure the myopathy and improve language and cognitive function.