Polyglucosan storage myopathies

Polyglucosan is an amylopectin-like polysaccharide associated with defective glycogen metabolism and, unlike normal glycogen, it is to some extent resistant to α-amylase digestion. It also has a characteristic fibrillar appearance under the electron microscope. Polyglucosan may aggregate into dense inclusions known as polyglucosan bodies. Its accumulation can be found in various tissues to some degree in normal ageing, but it is also the hallmark of some diseases associated with defects in glycogen metabolism. These diseases frequently involve both skeletal and cardiac muscle tissue, causing myopathy with muscle weakness and wasting, and cardiomyopathy with arrhythmia, conduction block, and cardiac failure. Mutations in eight human genes are known to be associated with polyglucosan storage involving muscle, namely GYG1, GBE1, RBCK1 (HOIL-1), PFKM, EPM2A, EPM2B (NHLRC1), PRDM8, and PRKAG2. There is also a common equine polysaccharide storage myopathy belonging to this group of diseases involving the GYS1 gene. The pathogenic mechanisms that cause the abnormal glycogen accumulation appearing as polyglucosan have been studied in some of these diseases. In most cases the pathogenesis is largely unknown. In this review, we summarize the polyglucosan storage diseases from a clinical, morphological, and genetic standpoint. We also identify some important similarities and differences regarding the morphological appearance of polyglucosan accumulation and discuss pathogenic pathways.