Mental retardation: a common clinical hallmark of creatine deficiency disorders

Introduction: In Western countries, mental retardation (MR) affects about 3% of the general population. For the majority of the cases of inherited MR, the genetic causes are not yet elucidated. Patients with creatine deficiency disorders (CDD) may present with MR/developmental delay as well as expressive speech and language delay, autism and epilepsy. They represent a group of treatable inborn errors of creatine biosynthesis and transport (SLC6A8) across the blood brain barrier. Patients and Methods: A group of children and young adults with MR were studied for defects in creatine metabolism. We started with the determination of guanidinoacetate and creatine in 6,600 urine samples by GC-MS-SIM. DNA mutation analysis was performed in all suspected cases. Results: Urine biochemical analysis revealed seven cases compatible with GAMT deficiency and 15 patients suggestive of a defect in SLC6A8. All GAMT deficient patients show the same mutation which suggests a founder effect in our population. SLC6A8 deficiency patients revealed a large spectrum of mutations. Discussion: So far, 22 patients with CDD were identified in our laboratory (1:300). We believe these defects are still under diagnosed, so the possibility should be considered in all children affected by unexplained MR, seizures, and speech delay.