Structural Biochemistry/Lafora Disease
Lafora Disease, is a fatal autosomal recessive genetic disorder characterized by the presence of inclusion bodies, known as Lafora bodies, within neurons and the cells of the heart, liver, muscle, and skin. Lafora Disease is caused by mutations in the EPM2A (Epilepsy, progressive myoclonus 2A) gene encoding laforin, a dual-specificity phosphatase, or in the EPM2B (Epilepsy, progressive myoclonus 2B) gene encoding malin, an ubiquitin ligase.
Lafora disease is distinguished by the presence of inclusions called "Lafora bodies" within the cytoplasm, the viscous fluidic matrix inside of cells. Lafora bodies are present primarily in neurons, but they have also been found in other organs. Lafora bodies are composed of abnormal glycogen called polyglucosans. These starch-like polyglucosans are insoluble and hence precipitate inside cells (Lafora Bodies shown in Fig. 1). Polyglucosan bodies appear with age; in Lafora disease, their numbers have increased enormously. Lafora bodies have been observed in virtually all organs of patients with the disease. In the brain, their presence appears to be restricted to neurons; they do not seem to present in astrocytes. Their morphology varies from tissue to tissue, but they generally contain a central core and have a peripheral fluffy appearance. Lafora bodies are composed of glucose polymer (polyglucosan) that is chemically but not structurally related to glycogen (Fig.2).
The protein Laforin has two essential roles: First it dephosphorylates glycogen to inhibit excess glycogen phosphorylation and Lafora Body formation, Secondly it brings the protein Malin to the site of glycogen synthesis.The protein Malin can then ubiquinate (give the kiss of death to) PTG (protein targeting to glycogen), GS (glycogen synthase), GDE (glycogen-debranching enzyme), and other proteins to inhibit the formation of Lafora Bodies. Malin, also interacts with laforin and promote the polyubiquitination and degradation of laforin in vitro and in cultured cells.
Since Laforin acts as a phosphatase it removes phosphomonoesters so that glycogen production proceeds normally, without laforin the phosphomonoesters build up and affect glycogen branching and lead to the formation of Lafora Bodies. The Lafora bodies contain more phosphate and its branching is discontinuous compared to glycogen, which make Lafora Bodies insoluble in water. Due to the mutations/defects with the laforin protein, Lafora bodies begin to build up causing Lafora Disease. Also without the laforin protein malin would not be able to locate PTG, GS, and GDE.
Malin functions to maintain laforin associated with soluble glycogen and that its absence causes sequestration of laforin to an insoluble polysaccharide fraction where it is functionally inert.
The laforin-malin complex acts as a controlled 'garbage disposal' to ubiquitinate and degrade proteins involved in glycogen metabolism. Minor defects with these important proteins will lead to Lafora Disease.
Lafora Disease patients with the malin protein defect live 25% long lives than patients with the laforin defect.
Histology of Lafora Bodies
Lafora bodies vary in size from 1 to 30 micron in diameter and one or more Lafora bodies may be present in the cytoplasm. They may be found in nerve cell processes and apparently free in the neuropil(A region between neuronal cell bodies in the gray matter of the brain and spinal cord). They have a concentric target like lamination, PAS positive, diastases resistant, and Alcian blue positive. The core is more strongly stained than the rim. Lafora bodies are also basophilic, and variably metachromatic (with methyl violet or toluidine blue) inclusion bodies. They are also found in liver, striated muscles, sweat glands.