Spinal muscular atrophy associated with progressive myoclonus epilepsy.

A rare syndrome characterized by lower motor neuron disease associated with progressive myoclonic epilepsy, referred to as "spinal muscular atrophy associated with progressive myoclonic epilepsy" (SMA-PME), has been described in childhood and is inherited as an autosomal recessive trait. SMA-PME is caused by mutation in the ASAH1 gene encoding acid ceramidase. Ceramide and the metabolites participate in various cellular events as lipid mediators. The catabolism of ceramide in mammals occurs in lysosomes through the activity of ceramidase. Three different ceramidases (acid, neutral and alkaline) have been identified and appear to play distinct roles in sphingolipid metabolism. The enzymatic activity of acid ceramidase is deficient in two rare inherited disorders; Farber disease and SMA-PME. Farber disease is a very rare and severe autosomal recessive condition with a distinct clinical phenotype. The marked difference in disease manifestations may explain why Farber and SMA-PME diseases were not previously suspected to be allelic conditions. The precise molecular mechanism underlying the phenotypic differences remains to be clarified. Recently, a condition with mutation in CERS1, the gene encoding ceramide synthase 1, has been identified as a novel form of PME. This finding underlies the essential role of enzymes regulating either the synthesis (CERS1) or degradation (ASAH1) of ceramide, and the link between defects in ceramide metabolism and PME.