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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\u2010deficiency. 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\u2009&gt;\u2009C and 506 G\u2009&gt;\u2009A missense mutations. Urinary GAA was increased, but highly variable 347\u20131,624 mmol\/mol creat (Controls &lt;150 mmol\/mol creat). In plasma and CSF the GAA levels were fairly constant at 17.3\u201327.0 \u03bcmol\/L (Controls 1.33\u20133.33) and 11.0\u201312.4 \u03bcmol\/L, respectively (Controls 0.068\u20130.114). 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. \u00a9 2005 Wiley\u2010Liss, Inc.<\/jats:p>","DOI":"10.1002\/ajmg.a.30226","type":"journal-article","created":{"date-parts":[[2005,1,14]],"date-time":"2005-01-14T08:25:00Z","timestamp":1105691100000},"page":"122-127","source":"Crossref","is-referenced-by-count":45,"title":["Guanidinoacetate methyltransferase deficiency identified in adults and a child with mental retardation"],"prefix":"10.1002","volume":"133A","author":[{"given":"H. 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