1. ¹Folkhälsan Institute of Genetics, Department of Medical Genetics and Neuroscience Center, Biomedicum Helsinki, University of Helsinki, Finland
2. ²Institute of Physiological Chemistry, Martin Luther University Halle-Wittenberg, Halle, Germany
3. ³Department of Forensic Medicine, University of Turku, Finland
4. ⁴Epilepsy Institute Netherlands SEIN, Heemstede, The Netherlands
5. ⁵DBG-Department of Medical Genetics, University Medical Center Utrecht, The Netherlands

Correspondence: Dr U Lahtinen, Folkhalsan Institute of Genetics, Biomedicum Helsinki, PO Box 63, 00014 University of Helsinki, Finland. Tel: +358-9-191 250 54; Fax: +358-9-191 250 73; E-mail: ulla.lahtinen@helsinki.fi

Received 17 June 2004; Revised 24 August 2004; Accepted 26 August 2004; Published online 13 October 2004.

Abstract

Loss-of-function mutations in the cystatin B (CSTB), a cysteine protease inhibitor, gene underlie progressive myoclonus epilepsy of Unverricht–Lundborg type (EPM1), characterized by myoclonic and tonic–clonic seizures, ataxia and a progressive course. A minisatellite repeat expansion in the promoter region of the CSTB gene is the most common mutation in EPM1 patients and leads to reduced mRNA levels. Seven other mutations altering the structure of CSTB, or predicting altered splicing, have been described. Using a novel monoclonal CSTB antibody and organelle-specific markers in human primary myoblasts, we show here that endogenous CSTB localizes not only to the nucleus and cytoplasm but also associates with lysosomes. Upon differentiation to myotubes, CSTB becomes excluded from the nucleus and lysosomes, suggesting that the subcellular distribution of CSTB is dependent on the differentiation status of the cell. Four patient mutations altering the CSTB polypeptide were transiently expressed in BHK-21 cells. The p.Lys73fsX2-truncated mutant protein shows diffuse cytoplasmic and nuclear distribution, whereas p.Arg68X is rapidly degraded. Two missense mutations, the previously described p.Gly4Arg affecting the highly conserved glycine, critical for cathepsin binding, and a novel mutation, p.Gln71Pro, fail to associate with lysosomes. These data imply an important lysosome-associated physiological function for CSTB and suggest that loss of this association contributes to the molecular pathogenesis of EPM1.