Loss of lysosomal association of cystatin B proteins representing progressive myoclonus epilepsy, EPM1, mutations.
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.
Folkhälsan Institute of Genetics, Department of Medical Genetics and Neuroscience Center, Biomedicum Helsinki, 00014 University of Helsinki, Finland.
Kirsi
K
Alakurtti
Ekkehard
E
Weber
Riitta
R
Rinne
Gerit
G
Theil
Gerrit-Jan
GJ
de Haan
Dick
D
Lindhout
Paula
P
Salmikangas
Pekka
P
Saukko
Ulla
U
Lahtinen
Anna-Elina
AE
Lehesjoki
D000818:Animals; D019556:COS Cells; D002454:Cell Differentiation / Q000235:genetics; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism; D006224:Cricetinae; D055313:Cystatin B; D015891:Cystatins / Q000235:genetics* / Q000378:metabolism; D003593:Cytoplasm / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008247:Lysosomes / Q000235:genetics / Q000378:metabolism*; D018598:Minisatellite Repeats / Q000235:genetics*; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D020125:Mutation, Missense / Q000235:genetics*; D020191:Myoclonic Epilepsies, Progressive / Q000235:genetics* / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics*; D021381:Protein Transport / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism
C524994:CSTB protein, human; D015891:Cystatins; D012333:RNA, Messenger; D055313:Cystatin B