Mapping the gene for juvenile myoclonic epilepsy.

The practice of epileptology at a molecular level, where gene products are identified by gene mapping, will soon be possible for a growing number of epilepsies. Juvenile myoclonic epilepsy (JME) is the first of such epilepsies to be mapped to a chromosome, namely chromosome 6p21.3. Family studies of 68 JME probands from California revealed 50% of all families reported seizures in first- or second-degree relatives. Twelve percent of all family members other than the proband had epileptic seizures. Eighty percent of symptomatic siblings and 6% of asymptomatic siblings had diffuse 4- to 6-Hz multispike-wave complexes. Twelve percent of asymptomatic parents had diffuse, nonspecific slow waves mixed with spikes or sharp waves. JME is tightly linked to the Bf-HLA loci in chromosome 6. No matter what mode of inheritance is assumed, linkage to the clinical manifestations of JME and its associated EEG traits is indicated by lod scores over 3.0, as long as "EEG affected" but clinically asymptomatic family members are counted as affected during LIPED analysis. Studies are now being done to further localize the JME site. At the same time, further linkage studies should decide if JME is heterogeneous within itself and whether the same JME site in 6p21.3 underlies absence and grand mal epilepsies.