UNSTABLE INSERTION IN THE 5'-FLANKING REGION OF THE CYSTATIN-B GENE IS THE MOST COMMON MUTATION IN PROGRESSIVE MYOCLONUS EPILEPSY TYPE-1, EPM1

Progressive myoclonus epilepsy type 1 (EPM1, also known as Unverricht- Lundborg disease) is an autosomal recessive disorder characterized by progressively worsening myoclonic jerks, frequent generalized tonic-cl onic seizures, and a slowly progressive decline in cognition(1). Recen tly, two mutations in the cystatin B gene (also known as stefin B, STF B) mapping to 21q22.3 have been implicated in the EPM1 phenotype: a G- ->C substitution in the last nucleotide of intron 1 that was predicted to cause a splicing defect in one family, and a C-->T substitution th at would change an Arg codon (CGA) to a stop codon (TCA) at amino acid position 68, resulting in a truncated cystatin B protein in two other families(2). A fourth family showed undetectable amounts of STFB mRNA by northern blot analysis in an affected individual. We present haplo type and mutational analyses of our collection of 20 unrelated EPM1 pa tients and families from different ethnic groups. We identify four dif ferent mutations, the most common of which consists of an unstable sim ilar to 600-900 bp insertion which is resistant to PCR amplification. This insertion maps to a 12-bp polymorphic tandem repeat located in th e 5' flanking region of the STFB gene, in the region of the promoter. The size of the insertion varies between different EPM1 chromosomes sh aring a common haplotype and a common origin, suggesting some level of meiotic instability over the course of many generations. This dynamic mutation, which appears distinct from conventional trinucleotide repe at expansions, may arise via a novel mechanism related to the instabil ity of tandemly repeated sequences.