GATM and atrial conduction disease: Cerebral creatine deficiency disorders (CCDD) are rare diseases caused by defects in the enzymes L-arginine: glycine amidinotransferase (AGAT) or guanidinoacetate-N-methyltransferase (GAMT), which are involved in synthesis of creatine; or by a defect in the creatine transporter (CRTR), which is essential for uptake of creatine as important energy source into the target cells.