Lafora bodies in skeletal muscle are fiber type specific

Abstract

Glycogen is the largest soluble cytosolic macromolecule, containing up to 55,000 glucoses per molecule. It is formed by glycogen synthase (GS) and branching enzyme (BE), which acting coordinately lead to branching after every sixth glucose and ultimately to a spherical shape that allows solubility. Polyglucosans are malformed glycogen molecules containing much less branching. They precipitate and accumulate into polyglucosan bodies (PB). PB characterize adult polyglucosan body disease (APBD) and Lafora disease (LD). In APBD, PB form in and often obstruct axons. APBD is an axonopathy with upper and lower motor neuron signs and no epilepsy. In LD, PB, called Lafora bodies (LB), occupy neuronal perikarya and dendrites. LD is a progressive myoclonus epilepsy, with no long tract or peripheral nerve deficits. APBD is caused by BE deficiency. LD is caused by mutations in the EPM2A and EPM2B genes encoding respectively the laforin phosphatase and the malin ubiquitin E3 ligase, which regulates laforin. In LD, glycogen becomes progressively phosphorylated, a pathologic process normally prevented by laforin. The charged phosphates unfold glycogen, expose its hydrophobic regions, and lead it to precipitate. GS precipitates with glycogen, but BE does not. Subsequent extension by GS unchecked by branching would convert the glycogen to polyglucosan.1,2 The next step in LD research is to understand the basis of the progressive phosphorylation. What is known is …