Juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME), also known as Janz syndrome, is a fairly common form of idiopathic generalized epilepsy, representing 5-10% of all epilepsies. This disorder typically first manifests itself between the ages of 12 and 18 with myoclonus occurring early in the morning. Most patients also have tonic-clonic seizures and many also have absence seizures. Linkage studies have demonstrated at least 6 loci for JME, 4 with known causative genes. Most of these genes are ion channels with the one non-ion channel gene having been shown to affect ion channel currents.

Signs and symptoms
Signs of JME are myoclonus occurring early in the morning. This rarely results in patients falling, but rather dropping objects. Attacks of myoclonia is more common in the arms than the legs. Other seizure types such as generalized tonic-clonic and absence seizures can also occur.

CACNB4
CACNB4 encodes a calcium channel &beta; subunit. &beta; subunits are important regulators of calcium channel current amplitude, voltage dependence, and also regulate channel trafficking. The &beta;4 isoform encoded by CACNB4 is most prevalent in the cerebellum. In mice, a naturally occurring null mutation leads to the "lethargic" phenotype, which is similar to JME. There are at least two mutations in the &beta;4 subunit associated with JME, C104F and R482X. When wild-type &alpha;1A and &beta;4 subunits are expressed in oocytes they produce large Ba2+ currents that inactivate slowly. Interestingly, incorporation of either of the mutant &beta;4 subunit into channels instead of wild-type subunits produces currents that are larger by 30-40%. The R482X mutation also increases the rate of fast inactivation of the channel. Since these effects are subtle, it is believed that they are contributory rather than completely causative for JME.

GABRA1
GABRA1 encodes an &alpha; subunit of the GABA A receptor, which encodes one of the major inhibitory neurotransmitter receptors. There is one known mutation in this gene that is associated with JME, A322D, which is located in the third segment of the protein. Expression of the &alpha;1&beta;2&gamma;2 combination of subunits in HEK 293 cells produces 6-fold greater current than similar subunits compositions containing mutant &alpha;1 subunits. The mutation also results in greatly decreased sensitivity in the receptor for activation by GABA. This combination of mutant containing receptors also activates far more slowly than wild-type containing receptors. Although originally not reported to result in altered protein trafficking, more recent study has indicated that the A322D mutation reduced &alpha;1 subunit trafficking to the membrane by >90%. Heterozygous expression of wild-type and mutant subunits produces current approximately 50% the size of wild-type due to this altered trafficking.

CLCN2
The CLCN2 gene encodes a chloride channel that is heavily expressed in brain regions inhibited by GABA. It is believed to be important in maintaining a proper chloride reversal potential needed in inhibitory neurotransmission by GABA. There are three known mutations in CLCN2 associated with JME, M200fsX231, 74_117del, and G715E. Neither the M200fsX231 nor the 74_117del mutation yield current when expressed in cells. Since these channels are responsible for the removal of intracellular chloride, these mutations are expected to lead to increased chloride concentrations and, thus, altered chloride reversal potential (ECl). As chloride is conducted through the normally inhibitory GABA receptors, this alteration in ECl may lead to either decreased GABAergic currents or GABAergic currents that are actually excitatory. The G715E mutation, on the other hand, produces normal sized currents but has altered voltage dependent activation. For this mutant, activation occurs at more positive potentials compared to wild-type channels. This may cause increased neuronal excitability.

GABRD
GABRD encodes the &delta; subunit of the GABA receptor, which is a subunit yielding receptors which do not desensitize and are localized outside of the synapse. There are three mutations in this gene associated with JME, E177A, R220C, and R220H, all located in the N-terminus of the protein. The last of these mutations is also present in normal controls. Receptors containing the E177A mutation have greatly decreased current compared to wild-type. This is not the case for the R220C mutation but is similar to the R220H mutation, though to a lesser extent than the E177A mutation. More recent study has found that the E177A mutant also has greatly decreased desensitization compared to wild-type receptors. Receptors containing only E177A or R220H mutants, versus heterozygotes, had significantly decreased surface expression compared to wild-type or heterozygotic receptors. These mutants also have decreased single-channel open times compared to wild-type. It should be noted, however, that these mutations are very rare as causes of JME.

EFHC1
The final known associated gene is EFHC1, which is poorly understood. EFHC1 has three DM10 domains (themselves of unknown function) and an EF hand motif, which is known to bind intracellular calcium. EFHC1 is expressed in many tissues, including the brain, where it is localized to the soma and dendrites of neurons, particularly the hippocampal CA1 region, pyramidal neurons in the cerebral cortex, and Purkinje cells in the cerebellum. There are 5 mutations in EFHC1 associated with JME; D210N, F229L, D253Y, P77T and R221H. The last two mutations were originally detected as a pair in the same individual. EFHC1 seems to be involved in programmed cell death as EFHC1 transfected cells have a higher rate of apoptosis. This rate is decreased by the double mutations P77T + R221H. Interestingly, wild-type EFHC1 increased the R-type calcium channel currents in transfected cells. This stimulation is decreased by JME associated mutations. Because of this, programmed cell death is decreased and the pruning of unwanted neurons may be hampered. As with some other loci, mutations in EFHC1 is not a common loci for JME. More recently, R221H has been found without P77T in one JME kindred.

Other loci
There is also evidence linking a gene or genes on chromosome 15 (15q14) and chromosome 6 (6p21) to JME. Causative genes in this region, however, have not been shown.

Diagnosis
Diagnosis is typically made based on patient history. EEG recordings are also sometimes used as confirmation.

Treatment/Management
See the equivalent section in the main epilepsy article.