Laforin is a glycogen phosphatase, deficiency of which leads to elevated phosphorylation of glycogen in vivo

*Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202;
^†The Hospital for Sick Children, Toronto, ON, Canada M5G 1X8; and
^§California Comprehensive Epilepsy Program, David Geffen School of Medicine, University of California, Los Angeles, CA 90073

Edited by Edmond H. Fischer, University of Washington, Seattle, WA, and approved October 19, 2007 (received for review August 22, 2007)

Abstract

Lafora disease is a progressive myoclonus epilepsy with onset typically in the second decade of life and death within 10 years. Lafora bodies, deposits of abnormally branched, insoluble glycogen-like polymers, form in neurons, muscle, liver, and other tissues. Approximately half of the cases of Lafora disease result from mutations in the EPM2A gene, which encodes laforin, a member of the dual-specificity protein phosphatase family that additionally contains a glycogen binding domain. The molecular basis for the formation of Lafora bodies is completely unknown. Glycogen, a branched polymer of glucose, contains a small amount of covalently linked phosphate whose origin and function are obscure. We report here that recombinant laforin is able to release this phosphate in vitro, in a time-dependent reaction with an apparent K _m for glycogen of 4.5 mg/ml. Mutations of laforin that disable the glycogen binding domain also eliminate its ability to dephosphorylate glycogen. We have also analyzed glycogen from a mouse model of Lafora disease, Epm2a^−/− mice, which develop Lafora bodies in several tissues. Glycogen isolated from these mice had a 40% increase in the covalent phosphate content in liver and a 4-fold elevation in muscle. We propose that excessive phosphorylation of glycogen leads to aberrant branching and Lafora body formation. This study provides a molecular link between an observed biochemical property of laforin and the phenotype of a mouse model of Lafora disease. The results also have important implications for glycogen metabolism generally.

Footnotes

^¶To whom correspondence should be addressed. E-mail: proach{at}iupui.edu
Author contributions: V.S.T., A.V.S., A.A.D.-R., and P.J.R. designed research; V.S.T., W.W., A.V.S., and A.A.D.-R. performed research; J.T., J.-M.G., X.Z., A.V.D.-E., and B.A.M. contributed new reagents/analytic tools; P.J.R. analyzed data; and V.S.T. and P.J.R. wrote the paper.
↵ ^‡Present address: Department of Medicine, School of Medicine, University of California at San Diego, La Jolla, CA 92093.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0707952104/DC1.