The carbohydrate-binding domain of Lafora disease protein targets Lafora polyglucosan bodies

Lafora's disease (LD) is an autosomal recessive and fatal form of epilepsy with onset in late childhood or adolescence. One of the characteristic features of LD pathology is the presence of periodic acid-Schiff (PAS) positive Lafora inclusion bodies. Lafora bodies are present primarily in neurons, but they have also been found in other organs. Histochemical and biochemical studies have indicated that Lafora bodies are composed mainly of polysaccharides. The LD gene, EPM2A, encodes a 331 amino acid long protein named laforin that contains an N-terminal carbohydrate-binding domain (CBD) and a C-terminal dual-specificity phosphatase domain (DSPD). Here we demonstrate that the CBD of laforin targets the protein to Lafora inclusion bodies and this property could be evolutionarily conserved. We also tested in vitro the effects of five LD missense mutations on laforin's affinity to Lafora body. While the missense mutant W32G failed to bind to purified Lafora body, four other mutants (S25P, E28L, F88L, and R108C) did not show any effect on the binding affinity. Based on these observations we propose the existence of a laforin-mediated glycogen metabolic pathway regulating the disposal of pathogenic polyglucosan inclusions. This is the first report demonstrating a direct association between the LD gene product and the disease-defining storage product, the Lafora bodies.