[1.1] Overview of Idiopathic Generalized Epilepsies
Idiopathic Generalized Epilepsies have many traits that distinguish it from other forms of epilepsies. For one, they are less common and usually arise in early childhood.[1] Not all cases extend throughout a patient’s life and IGEs are often classified as a pediatric problem. The term idiopathic refers to the nature of the epilepsies as they have a tendency to arise spontaneously. Of the six forms of generalized epilepsies, three have been linked with IGEs; absence, myoclonic and tonic-clonic.[2]

  1. Absence epilepsies arise during childhood and last for about 3-15 seconds. Children have no recollection of the seizure and they normally go undetected. Oftentimes they extend into adulthood when there is a family history of epilepsies or when a child experiences other forms of seizures in combination.[3]
  2. Myoclonic epilepsies also arise in late childhood and extend towards adulthood but unlike other forms of epilepsy the patient does not lose consciousness. Characteristically they are linked with sudden twitching, usually of the arms for 1 s and are sometimes in conjunction to falling or sleep deprivation.[4]
  3. Tonic-clonic epilepsies come in two varieties, primary and secondary. Primary tonic-clonic epilepsies are associated with IGEs and involve the entire brain. Occurring during childhood, key characteristics include tonic extension of the limbs for 20-40 seconds, jerking of extremities for 30-50 seconds, loss of consciousness and failure to breath properly.[5]
A distinguishable factor of IGEs from any other form of epileptic diseases is that many scientists are convinced it is genetic in origin. A wide range of genes have been linked to various forms of epilepsies including GABRA1, CLCN2, SCN1A, KCNQ3, SLC2A1 and 15q13.3. Proving all IGEs have a genetic origin has been difficult and confusing for scientists but major advances have been made to understand the nature of mendelian and monogenic IGEs.[6]


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Sample tracings from patients with idiopathic generalized epilepsy. Ambulatory EEG were recorded at a speed of 15mm/s and an amplitude of 50 mV/7mm. EDA 5 epileptiform discharges on awakening; JME 5 juvenile myoclonic epilepsy; A-GTCS 5 epilepsy with generalized tonic-clonic seizures on awakening; R-GTCS 5 epilepsy with generalized tonic-clonic seizures at random; CAE 5 childhood absence epilepsy. Time is reported in hours:minutes:seconds. (adapted from Fittipaldi, F. (2001))
[7]



[1.1.1] GABA receptors

GABAa receptors are composed of two α subunits, two β subunits and one subunit each of γ and δ.9 The GABA pathway is responsible for releasing inhibitory tones to the brain that balance excitatory neuronal circuits that cause convulsions by hyperpolarizing neurons via the influx of chloride ions through 19 subunits.[8] GABAa is also associated with numerous anti-epileptic drug through enhancement of the pathway.[9] Mutations have commonly been found in α1, β3, γ 2, and δ subunits and can be classified as four different types; missense, nonsense and frameshifts in coding sequences and non-coding sequences.[10]



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Schematic representation of the GABAA receptor subunit topology, showing the location of autosomal dominant epilepsy mutations associated with different idiopathic generalized epilepsies. SMEI, myoclonic epilepsy of infancy; CAE, childhood absence epilepsy; GEFS+, generalized epilepsy with febrile seizures plus; JME, juvenile myoclonic epilepsy; FS, febrile seizures. (adapted from Macdonald, R. (2009))

1.Missense refers to the addition of a different amino acid to a peptide group that alters the triplet genetic code resulting in one of three things; benign polymorphism, increased susceptibility to a disease or a mutation. For GABAa receptors, missense usually result in reduced surface expression either due to impaired folding, assembly or ER retention.[11]

Two specific missense mutations in the γ2 subunit have been linked to certain types of IGE seizures. One refers to the alteration of the proline 83 to a serine found in a family that exhibited febrile seizures and IGE over three generations. Eight of the family members suffered from this mutation that affects benzodiazepine binding to the extracellular domain of the GABAa receptor.
[12] Of the 190 control patients tested, none were found to possess this mutation. Four different programs, PolyPhen, SIFT, SNAP, PANTHER were used to predict what effect this mutation might have on the protein and it was found to have damaging effects not only on structure but more importantly on function.[13]


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Segregation profiles of the novel GABAA receptor mutants in French Canadian families affected with IGE and related phenotypes. (A and B) The (A) K353delins18X and (B) D219N mutations in GABRA1 were found in four individuals of the same family over two generations. (C) The P83S mutation in GABRG2 was found in nine individuals of the same family over two generations. Blackened symbols indicate affected individuals. (adapted from Lachance-Touchette, P. (2011))


The second missense mutation found in the γ2 subunit that changes lysine 289 into methionine is located in an area responsible for the gating of the ion channels and is often associated with autosomal dominant generalized epilepsy. By applying GABA for 2-5 ms durations and recording via outside patch clamps scientists discovered the receptors had more rapid deactivation with open times a quarter of the length of wild types. The entire cell was able to deactivate faster resulting in disinhibition that may lead to epilepsy. K289M is also linked to clonic epilepsy with febrile seizures as a result of the alteration of the extracellular domains.[14]

2. Nonsense mutations refer to changes in the nucleotide sequence resulting in a stop codon or premature translation-termination codon. This leads to either a shortened protein or activate nonsense-mediated decay (NMD) eliminating the production of the mutant protein depending on the location within the sequence of the mutation.[15]

One example of a nonsense mutation found in relation to IGEs is the premature stop codon at Q351X between the third and fourth transmembrane domains. An Australian family with a history of seizures and epilepsies were screened for the Q351X mutation. One mother possessed the mutation as did her two children while her brother and other son did not. Scientists decided to test the effect of the Q351X mutation by creating a premature stop codon in position 351 of a mature GABRG2 protein via PCR-mutagenesis. Oocytes exhibiting wild-type subunits responded well to GABA injections while those without GABRG2 protein contained α and β subunits that failed to assemble into complexes and thus did not respond to GABA injections. Through GFP-GABRG2[Q351X] the receptors were found to become isolated inside the lumen of the endoplasmic reticulum thus explaining why they were not receptive to GABA. The mutation had truncated the protein so that there was no transmembrane anchor therefore trapping the complex within the ER and reducing the density of surface receptors that is crucial for the function of GABRG2 proteins.[16]

3. Frameshift mutations refer to the deletion or insertion of nucleotides causing a change downstream of the sequence. Oftentimes they also produce a premature translation-termination codon.[17]
A deletion in 975delC producing a frameshift and premature stop codon in GABRA1 was found in a single individual during a study of 98 IGE patients. All three other family members (two parents and a brother) carried the wild type allele thus confirming this mutation arose sporadically. No GABA-evoked currents were found in the mutated receptor while normal levels were detected in the wild type patients. Again GFP proved that mutated receptors were located in the cytoplasm as opposed to the surface membrane and lead to the degradation of the protein. GABAergic pathway is considered the most important inhibitory pathway in the brain and impairment could lead to major epileptic seizures.[18]




[1.1.2] Glucose transporters

Glucose transporter type I (GLUT1) has been linked to a multitude of neurobiological diseases ranging from retardation to microcephaly. A mutation in the SLC2A1 gene has been the main focus for scientists and is found in more than 10% of all patients with early-onset absence epilepsies. In a study of 95 families each with at least one individual exhibiting IGE one Italian family was found to show a single point mutation where cysteine 232 was substituted with argenine. Found in all eight living family members as well as four healthy adults the mutation was not found in 846 control individuals including 190 Italians. The family consisted of three generations of IGE patients all showing no other forms of disorders and all exhibiting absence seizures. Uptake of glucose was limited by 30% as well as maximum uptake velocity but no change was found to the affinity for glucose. The protein also regularly appeared on the surface membrane of oocytes. Lower glucose transport across the blood-brain barrier has been linked to generalized epileptic activity and PET scans have showed a permanent hypometabolism in the frontal lobes of patients with generalized seizures.[19]


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Pedigree of the idiopathic generalized epilepsies (IGE) family carrying the SLC2A1 mutation and evolutionary conservation of the mutated residue (A) (B) Evolutionarily conservation of the 232 arginine residue in different species.mmutated allele;normal allele. (adapted from Striano, P. (2012))

Another study of the SLC2A1 mutation featuring 34 patients all with early-onset absence epilepsies revealed that 12% were affected leading to reduced protein function. Three missense mutations and one splice-site mutation was found in Patients 1, 2, 3 and 4 while none were found in the 276 control subjects. In all three mutations glucose transport was decreased compared to the wild type as well as the maximum velocity.[20]

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(adapted from Suls, A. (2009))

In a study that included 564 unrelated IGE patients, 127 were screened for mutations. In total 17 SNPs were tested in both IGE patients and controls and on all counts the controls gave rise to genotype numbers predicted by the Hardy-Weinberg equilibrium. Seven SNPs (1, 6, 7, 10, 11, 16 and 17) were associated with an IGE phenotype with the most significant being SNP10 located within the first intron of the GRM4 gene which encodes for the group III metabotropic glutamate receptor 4 located on chromosomal segment 6p21.3 that has been linked to increased susceptibility for juvenile myoclonic epilepsies. Variations in intron DNA sequences are known to affect gene expression leading to irregular GRM4 regulation. For childhood absence epilepsies two SNPs (7 and 9) showed significant differences between IGE patients and controls and five markers were associated with the onset of juvenile myoclonic epilepsies.[21]

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(adapted from Muhle, H. (2010))




[1.1.3] Nicotinergic acetylcholine receptors

The CHRNA7 gene which codes for the subunit of nicotinergic acetylcholine receptor found normally in the reticular thalamus of the brain has also been noted as a possible genetic factor. Located in the area of the brain where IGE seizures are believed to manifest, a 15q13.3 microdeletion in the area has been linked to increase susceptibility to IGEs. Two studies were performed to determine the correlation between the deletion and the increased risk to epilepsies. One composed of 647 unrelated IGE Western European patients and 1202 German controls. Seven were found to contain the deletion while none were found in the controls. Another test was conducted simultaneously in Switzerland and North America involving 576 patients and 2497 controls. Five were found to contain the deletion while again, none in the controls. A deletion between BP4-BP5 contains at least seven different genes but the main candidate has been isolated as CHRNA7. Cholinergic pathways as well as nicotinergic acetylcholine receptors have been found throughout the central nervous system both pre and postsynaptically controlling inhibitory and excitatory pathways. However because CHRNA7 has been found so predominantly in the reticular thalamus, an area crucial to the modulation of thalamocortical pathways and central to the generation of IGE seizures it has been tagged by scientists as a primary suspect for the increase of susceptibility of individuals to IGE.[22]





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