Lafora disease is a fatal, recessive neurodegenerative epilepsy caused by mutations in the EPM2A gene that encodes for the glycogen phosphatase Laforin. In the absence of laforin activity, glycogen transforms into hyper-phosphorylated, water-insoluble starch-like particles called Lafora bodies (LBs). LBs are the suspected cause of neuronal apoptosis, neurodegeneration, and eventual death of LD patients. Laforin is the founding member of the glucan phosphatase family and also the only known human phosphatase with a carbohydrate-binding module (CBM), followed by a dual specificity phosphatase domain (DSP). The CBM is responsible for glycogen binding while the DSP possesses phosphatase activity. At present, there are over 30 single-point mutations identified in LD patients. However, the molecular mechanism of glycogen dephosphorylation by laforin and the formation of LBs are not understood. The X-ray crystal structure reveals that the patient point mutations are scattered between specific regions of the CBM and DSP domains of laforin suggesting multiple mechanisms that causes LD. To understand the mechanism of phosphatase activity, stability, and glycogen binding of the mutants, we employed several biophysical methods (DSF, SAXS, AUC) and biochemical assays (glycogen phosphatase, ConA, etc.). Our data shows that patient mutations are described to have loss of functional interactions either in the CBM binding site, DSP active site, CBM/DSP interface, or at the dimerization interface. Moreover, the analyses of LD patient mutations define the mechanism by which subsets of mutations disrupt laforin function. Our data provides fundamental insights connecting glycogen metabolism to this neurodegenerative disease.

Function of laforin in glycogen metabolism and lafora disease

249th National Meeting (2015)

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