Newborn screening strategies for disorders of creatine metabolism

Creatine is necessary to transfer energy between cellular compartments. Creatine is converted to phosphocreatine by the creatine kinase reaction within mitochondria and phosphocreatine generates adenosine triphosphate (ATP) in the cytoplasm. ATP powers most of the energy consuming reactions in cells. Defects in creatine synthesis or transport disrupt this process and result in brain creatine deficiency syndromes. Affected patients have developmental delay, hypotonia, autism, seizures, and impaired motor skill development. Defects of creatine synthesis are caused by impaired activity of the enzymes guanidinoacetate methyltransferase (GAMT) and arginine:glycine amidinotransferase (AGAT), both transmitted as autosomal recessive traits, whereas defect in creatine transport (SLC6A8 Gene) are transmitted in an X-linked recessive manner. Patients with defects in creatine synthesis respond to creatine supplementation and dietary manipulations. This therapy is more effective if initiated before mental retardation is evident. For this reason, diagnosis should be accomplished as soon as possible with newborn screening. Here we report a reliable three-tier testing method for screening for GAMT and AGAT deficiency in newborns' v blood spots. Creatine and guanidinoacetate are detected in newborn screening blood spots by tandem mass spectrometry (MS/MS). Secondtier testing using LC-MS/MS confirms more quantitatively low creatine and increased or decreased guanidinoacetate levels, while third-tier testing consists of DNA sequencing to identify mutations in the GAMT and AGAT Genes. This test can potentially identify newborns with GAMT and AGAT deficiencies with low false positive rate and could be applied to newborn screening nationwide.