Juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy
Classification and external resources
ICD-10	G40.3
ICD-9	345.1
OMIM	606904 254770 604827 608816
DiseasesDB	32202
eMedicine	neuro/416
MeSH	D020190

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 are more common in the arms than the legs. Other seizure types such as generalized tonic-clonic and absence seizures can also occur. Myoclonic seizures can also occur in the later part of the day, and are not limited to the arms and hands. They can also occur in the lower legs, causing sufferers to trip over their own feet. Clusters of myoclonics can lead to absence seizures, and clusters of absence seizures can lead to generalized tonic-clonic seizures. Expression can occur as young as 18 months of age. Males in some families do not survive the crisis point at 24 months of age.

Pathophysiology

CACNB4

CACNB4 encodes a calcium channel β subunit. β subunits are important regulators of calcium channel current amplitude, voltage dependence, and also regulate channel trafficking. The β₄ 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 β₄ subunit associated with JME, C104F and R482X. When wild-type α_1A and β₄ subunits are expressed in oocytes they produce large Ba²⁺ currents that inactivate slowly. Incorporation of either of the mutant β₄ 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.^[1]

GABRA1

GABRA1 encodes an α 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 α₁β₂γ₂ combination of subunits in HEK 293 cells produces 6-fold greater current than similar subunits compositions containing mutant α₁ subunits. The mutation also results in greatly decreased sensitivity in the receptor for activation by GABA.^[2] 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 α₁ 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.^[3][4]

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 (E_Cl). As chloride is conducted through the normally inhibitory GABA receptors, this alteration in E_Cl 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.^[5]

GABRD

GABRD encodes the δ 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.^[6] 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.^[7] It should be noted, however, that these mutations are very rare as causes of JME.^[8]

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. 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.^[9][10] 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.^[11]

Other loci

There is also evidence linking a gene or genes on chromosome 15 (15q14) as well as the BRD2 gene on chromosome 6 (6p21)^[12] to JME. Causative genes in this region, however, have not been shown.

Relation to other rare disorders

Until recently, the medical literature did not indicate a connection among many genetic disorders, both genetic syndromes and genetic diseases, that are now being found to be related. As a result of new genetic research, some of these are, in fact, highly related in their root cause despite the widely-varying set of medical symptoms that are clinically visible in the disorders. This emerging class of diseases are called ciliopathies. The underlying cause may be a dysfunctional molecular mechanism in the primary cilia structures of the cell, organelles which are present in many cellular types throughout the human body. The cilia defects adversely affect "numerous critical developmental signaling pathways" essential to cellular development and thus offer a plausible hypothesis for the often multi-symptom nature of a large set of syndromes and diseases. Known ciliopathies include primary ciliary dyskinesia, Bardet-Biedl syndrome, polycystic kidney and liver disease, nephronophthisis, Alstrom syndrome, Meckel-Gruber syndrome and some forms of retinal degeneration.^[13] It has been suggested that juvenile myoclonic epilepsy may be a ciliopathy.^[14]

Diagnosis

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

Treatment/Management

Main article: epilepsy

Before giving Carbamazepine do ask for history of early morning seizures and or jerks/twitches especially in a young person. Carbamazepine aggravates these.

References

1. ^ Escayg A, De Waard M, Lee D, Bichet D, Wolf P, Mayer T, Johnston J, Baloh R, Sander T, Meisler M (2000). "Coding and noncoding variation of the human calcium-channel beta4-subunit gene CACNB4 in patients with idiopathic generalized epilepsy and episodic ataxia". Am J Hum Genet 66 (5): 1531–1539. doi:10.1086/302909. PMC 1378014. PMID 10762541. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1378014. 
2. ^ Cossette P, Liu L, Brisebois K, Dong H, Lortie A, Vanasse M, Saint-Hilaire J, Carmant L, Verner A, Lu W, Wang Y, Rouleau G (2002). "Mutation of GABRA1 in an autosomal dominant form of juvenile myoclonic epilepsy". Nat Genet 31 (2): 184–189. doi:10.1038/ng885. PMID 11992121. 
3. ^ Gallagher M, Song L, Arain F, Macdonald R (2004). "The juvenile myoclonic epilepsy GABA(A) receptor alpha1 subunit mutation A322D produces asymmetrical, subunit position-dependent reduction of heterozygous receptor currents and alpha1 subunit protein expression". J Neurosci 24 (24): 5570–5578. doi:10.1523/JNEUROSCI.1301-04.2004. PMID 15201329. 
4. ^ Krampfl K, Maljevic S, Cossette P, Ziegler E, Rouleau G, Lerche H, Bufler J (2005). "Molecular analysis of the A322D mutation in the GABA receptor alpha-subunit causing juvenile myoclonic epilepsy". Eur J Neurosci 22 (1): 10–20. doi:10.1111/j.1460-9568.2005.04168.x. PMID 16029191. 
5. ^ Haug K, Warnstedt M, Alekov A, Sander T, Ramírez A, Poser B, Maljevic S, Hebeisen S, Kubisch C, Rebstock J, Horvath S, Hallmann K, Dullinger J, Rau B, Haverkamp F, Beyenburg S, Schulz H, Janz D, Giese B, Müller-Newen G, Propping P, Elger C, Fahlke C, Lerche H, Heils A (2003). "Mutations in CLCN2 encoding a voltage-gated chloride channel are associated with idiopathic generalized epilepsies". Nat Genet 33 (4): 527–532. doi:10.1038/ng1121. PMID 12612585. 
6. ^ Dibbens L, Feng H, Richards M, Harkin L, Hodgson B, Scott D, Jenkins M, Petrou S, Sutherland G, Scheffer I, Berkovic S, Macdonald R, Mulley J (2004). "GABRD encoding a protein for extra- or peri-synaptic GABAA receptors is a susceptibility locus for generalized epilepsies". Hum Mol Genet 13 (13): 1315–1319. doi:10.1093/hmg/ddh146. PMID 15115768. 
7. ^ Feng H, Kang J, Song L, Dibbens L, Mulley J, Macdonald R (2006). "Delta subunit susceptibility variants E177A and R220H associated with complex epilepsy alter channel gating and surface expression of alpha4beta2delta GABAA receptors". J Neurosci 26 (5): 1499–1506. doi:10.1523/JNEUROSCI.2913-05.2006. PMID 16452673. 
8. ^ Lenzen K, Heils A, Lorenz S, Hempelmann A, Sander T (2005). "Association analysis of the Arg220His variation of the human gene encoding the GABA delta subunit with idiopathic generalized epilepsy". Epilepsy Res 65 (1–2): 53–57. doi:10.1016/j.eplepsyres.2005.04.005. PMID 16023832. 
9. ^ Suzuki T, Delgado-Escueta A, Aguan K, Alonso M, Shi J, Hara Y, Nishida M, Numata T, Medina M, Takeuchi T, Morita R, Bai D, Ganesh S, Sugimoto Y, Inazawa J, Bailey J, Ochoa A, Jara-Prado A, Rasmussen A, Ramos-Peek J, Cordova S, Rubio-Donnadieu F, Inoue Y, Osawa M, Kaneko S, Oguni H, Mori Y, Yamakawa K (2004). "Mutations in EFHC1 cause juvenile myoclonic epilepsy". Nat Genet 36 (8): 842–849. doi:10.1038/ng1393. PMID 15258581. 
10. ^ Suzuki T, Delgado-Escueta A, Alonso M, Morita R, Okamura N, Sugimoto Y, Bai D, Medina M, Bailey J, Rasmussen A, Ramos-Peek J, Cordova S, Rubio-Donnadieu F, Ochoa A, Jara-Prado A, Inazawa J, Yamakawa K (2006). "Mutation analyses of genes on 6p12-p11 in patients with juvenile myoclonic epilepsy". Neurosci Lett 405 (1–2): 126–131. doi:10.1016/j.neulet.2006.06.038. PMID 16876319. 
11. ^ Ma, S; Blair, MA; Abou-Khalil, B; Lagrange, AH; Gurnett, CA; Hedera, P (2006). "Mutations in the GABRA1 and EFHC1 genes are rare in familial juvenile myoclonic epilepsy". Epilepsy Res 71 (2–3): 129–134. doi:10.1016/j.eplepsyres.2006.06.001. PMID 16839746. 
12. ^ Pal, D.; Evgrafov, O.; Tabares, P.; Zhang, F.; Durner, M.; Greenberg, DA (2003). "BRD2 (RING3) Is a Probable Major Susceptibility Gene for Common Juvenile Myoclonic Epilepsy" (PDF). Am. J. Hum. Genet 73 (2): 261–270. doi:10.1086/377006. PMC 1180366. PMID 12830434. http://geneticsofepilepsy.org/wp-content/uploads/2009/01/identifiction-of-brd2-as-epilepsy-gene.pdf. 
13. ^ Badano, Jose L.; Norimasa Mitsuma, Phil L. Beales, Nicholas Katsanis (September 2006). "The Ciliopathies : An Emerging Class of Human Genetic Disorders". Annual Review of Genomics and Human Genetics 7: 125–148. doi:10.1146/annurev.genom.7.080505.115610. PMID 16722803. http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.genom.7.080505.115610. Retrieved 2008-06-15. 
14. ^ Delgado-Escueta, Antonio V. (May 2007). "Advances in Genetics of Juvenile Myoclonic Epilepsies". Epilepsy Currents : Reviews and Critical Analysis 7 (3): 61–67. doi:10.1111/j.1535-7511.2007.00171.x. PMC 1874323. PMID 17520076. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1874323.