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 liver biopsy, and the underlying molecular defect in the GAMT gene has been identified. In order to provide non-invasive biochemical diagnosis, we have developed an enzyme assay based on the formation of radiolabeled creatine from ¹⁴C guanidinoacetate and S-adenosylmethionine in concentrated and dialyzed extracts from cultivated skin fibroblasts, Epstein–Barr virus transformed lymphoblasts, and cultivated amniotic cells. Cells were investigated from controls, from 1 index patient with proven GAMT deficiency and from 3 additional patients with clinical and biochemical signs of GAMT deficiency. Separation of ¹⁴C guanidinoacetate from ¹⁴C creatine in the reaction mixture was accomplished by HPLC on Hypersil ODS column and radioactivity was determined 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 Km values were 9.5–14.8 μM for guanidinoacetate and 68–78 μM for S-adenosylmethionine. 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 allows non-invasive diagnosis of GAMT deficiency in patients at risk.