Guanidinoacetate methyltransferase (GAMT) deficiency: non-invasive enzymatic diagnosis of a newly recognized inborn error of metabolism

Guanidinoacetate methyltransferase deficiency is a newly recognized inborn error of creatine biosynthesis, Manifestation of neurologic symptoms occurs in infancy and is partly reversible upon oral substitution of creatine. In the first two index patients, enzymatic diagnosis was established in a liv er biopsy, and the underlying molecular defect in the GAMT gene has been id entified. In order to provide non-invasive biochemical diagnosis, we have d eveloped an enzyme assay based on the formation of radiolabeled creatine fr om C-14 guanidinoacetate and S-adenosylmethionine in concentrated and dialy zed extracts from cultivated skin fibroblasts, Epstein-Barr virus transform ed lymphoblasts, and cultivated amniotic cells. Cells were investigated fro m controls, from 1 index patient with proven GAMT deficiency and from 3 add itional patients with clinical and biochemical signs of GAMT deficiency. Se paration of C-14 guanidinoacetate from C-14 creatine in the reaction mixtur e was accomplished by HPLC on Hypersil ODS column and radioactivity was det ermined in fractions according to respective UV signals, GAMT activities in control fibroblasts (n = 7), lymphoblasts (n = 8) and in amniotic cells (n = 2) were 0.38-0.56, 0.61-0.84 and 0.38-0.56 nmol/h/mg protein. Apparent K m values were 9.5-14.8 mu M for guanidinoacetate and 68-78 mu M for S-adeno sylmethionine In the index patient and in the three additional patients at risk, GAMT activity was <0.1 nmol/h/mg protein. The assay described here al lows non-invasive diagnosis of GAMT deficiency in patients at risk, (C) 200 0 Elsevier Science B.V. All rights reserved.