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Progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1; MIM 254800) is an autosomal recessive disorder that occurs with a low frequency in many populations but is more common in Finland and the Mediterranean region^1,2. It is characterized by stimulus-sensitive myoclonus and tonic-clonic seizures with onset at age 6–15 years, typical electroencephalographic abnormalities and a variable rate of progression between and within families^3–5. Following the initial mapping of the EPM1 gene to chromosome 21 (ref. 6) and the refinement of the critical region to a small interval^7–9, positional cloning identified the gene encoding cystatin B (C5T6), a cysteine protease inhibitor, as the gene underlying EPM1 (ref. 10). Levels of messenger RNA encoded by CST6 were dramatically decreased in patients. A 3′ splice site and a stop codon mutation were identified in three families, leaving most mutations …