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We collected data on 48 patients from 38 families with guanidinoacetate methyltransferase (GAMT) deficiency. Global developmental delay/intellectual disability (DD/ID) with speech/language delay and behavioral problems as the most affected domains was present in 44 participants, with additional epilepsy present in 35 and movement disorder in 13. Treatment regimens included various combinations/dosages of creatine-monohydrate, l-ornithine, sodium benzoate and protein/arginine restricted diets. The median age at treatment initiation was 25.5 and 39 months in patients with mild and moderate DD/ID, respectively, and 11 years in patients with severe DD/ID. Increase of cerebral creatine and decrease of plasma/CSF guanidinoacetate levels were achieved by supplementation with creatine-monohydrate combined with high dosages of l-ornithine and/or an arginine-restricted diet (250 mg/kg/d l-arginine). Therapy was associated with improvement or stabilization of symptoms in all of the symptomatic cases. The 4 patients treated younger than 9 months had normal or almost normal developmental outcomes. One with inconsistent compliance had a borderline IQ at age 8.6 years. An observational GAMT database will be essential to identify the best treatment to reduce plasma guanidinoacetate levels and improve long-term outcomes.

Opitz-Kaveggia syndrome is a rare X-linked multiple congenital anomalies and intellectual disability disorder caused by the recurrent p.R961W mutation in the MED12 gene. Twenty-three affected males from 10 families with this mutation in the MED12 gene have been described so far. Here we report on a new family with three affected cousins, in which we identified a novel MED12 mutation (p.G958E). This is the first demonstration that other mutations in this gene can also lead to Opitz-Kaveggia syndrome. The clinical phenotype of these three new cases is reviewed in detail and compared with the previous reported cases.