Generalized epilepsy with febrile seizures plus (GEFS+) is a
syndromic autosomal dominant
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
disorder where affected individuals can exhibit numerous
epilepsy
Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...
phenotypes.
GEFS+ can persist beyond early childhood (i.e., 6 years of age). GEFS+ is also now believed to encompass three other epilepsy disorders: severe myoclonic epilepsy of infancy (SMEI), which is also known as
Dravet's syndrome, borderline SMEI (SMEB), and
intractable epilepsy of childhood (IEC).
There are at least six types of GEFS+, delineated by their causative gene. Known causative gene mutations are in the
sodium channel
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the channel ...
α subunit genes
SCN1A
Sodium channel protein type 1 subunit alpha (SCN1A), is a protein which in humans is encoded by the ''SCN1A'' gene.
Gene location
The SCN1A gene is located on chromosome 2 of humans, and is made up of 26 exons spanning a total length of 6030 nu ...
, an associated β subunit
SCN1B
Sodium channel subunit beta-1 is a protein that in humans is encoded by the ''SCN1B'' gene.
Voltage-gated sodium channels are essential for the generation and propagation of action potentials in striated muscle and neuronal tissues. Biochemically ...
, and in a
GABAA receptor γ subunit gene, in GABRG2 and there is another gene related with
calcium channel
A calcium channel is an ion channel which shows selective permeability to calcium ions. It is sometimes synonymous with voltage-gated calcium channel, although there are also ligand-gated calcium channels.
Comparison tables
The following tables e ...
the PCDH19 which is also known as
Epilepsy Female with Mental Retardation.
Penetrance
Penetrance in genetics is the proportion of individuals carrying a particular variant (or allele) of a gene (the genotype) that also express an associated trait (the phenotype). In medical genetics, the penetrance of a disease-causing mutation is t ...
for this disorder is estimated at 60%.
Signs and symptoms
Individuals with GEFS+ present with a range of
epilepsy phenotypes. These include febrile seizures that end by age 6 (FS), such seizures extending beyond age 6 that may include afebrile
tonic-clonic,
myoclonic
Myoclonus is a brief, involuntary, irregular (lacking rhythm) twitching of a muscle or a group of muscles, different from clonus, which is rhythmic or regular. Myoclonus (myo "muscle", clonic "jerk") describes a medical sign and, generally, i ...
,
absence,
atonic seizure An atonic seizure (also called drop seizure, akinetic seizure, astatic seizure, or drop attack) is a type of seizure that consists of partial or complete loss of muscle tone that is caused by temporary alterations in brain function. These seizures ...
s and
myoclonic-astatic epilepsy
Myoclonic astatic epilepsy (MAE), also known as myoclonic atonic epilepsy or Doose syndrome, is a generalized idiopathic epilepsy. MAE was first coined in 1970 by Dr. Hermann Doose. It is characterized by the development of myoclonic seizures and/ ...
. Individuals may also present with SMEI, characterized by generally tonic-clonic seizures, impaired
psychomotor development,
myoclonic seizures
Myoclonus is a brief, involuntary, irregular (lacking rhythm) twitching of a muscle or a group of muscles, different from clonus, which is rhythmic or regular. Myoclonus (myo "muscle", clonic "jerk") describes a medical sign and, generally, is ...
,
ataxia
Ataxia is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements. Ataxia is a clinical manifestation indicating dysfunction of ...
, and poor response to many anticonvulsants.
Pathophysiology
Type 1
GEFS+ type 1 is a subtype of GEFS+ in which there are mutations in SCN1B, a gene encoding a
sodium channel
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the channel ...
β subunit. The β subunit is required for proper channel inactivation. There are two known mutations in SCN1B that lead to GEFS+ (Figure 1). The first and best characterized of these mutations is C121W. This mutation alters a
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
involved in a
disulfide bond
In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
in the extracellular
N-terminus
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
of the protein. This extracellular region is similar to the cell adhesion molecule contactin and other
cell adhesion molecule
Cell adhesion molecules (CAMs) are a subset of cell surface proteins that are involved in the binding of cells with other cells or with the extracellular matrix (ECM), in a process called cell adhesion. In essence, CAMs help cells stick to each ...
s. It is believed that the disulfide bond disrupted by the C121W mutation is required for the proper folding of this N-terminus motif. Coexpression of SCN1B with sodium channel α subunits in
oocyte
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...
s and other cells results in channels that inactivate more slowly. Expression of C121W mutant along with wild-type α subunits produces current indistinguishable from that through α subunits alone.
Further investigation of this mutation has indicated that it results in decreased frequency dependent rundown and, thus, likely hyperexcitability when compared to cells expressing the wild-type subunit. This mutation also disrupts the subunit's ability to induce cellular aggregation. The importance of this last fact is unclear, though it is presumed that proper channel aggregation within cells and cell-cell contact are required for normal neuronal function.
A second mutation has been found in one kindred with GEFS+ type 1. This mutation is in a splice acceptor site of
exon
An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequen ...
3. The loss of this acceptor site reveals a downstream cryptic acceptor site and a protein missing 5 amino acids in the N-terminus (I70_E74del). This mutation has not been further characterized.
Type 2
A second subtype of GEFS+, type 2, is the result of mutations in SCN1A, a gene encoding a sodium channel α subunit. There are currently almost 90 known mutations in the SCN1A gene throughout the entirety of the channel (see table 1). These mutations result in almost any imaginable mutation type in the gene, short of duplications. The results of these mutations are highly variable, some producing functional channels while others result in non-functional channels. Some functional channels result in membrane hyperexcitability while others result in hypoexcitability. Most of the functional mutant channels result in hyperexcitability due to decreased frequency dependent rundown. An example of this is the D188V mutation. A 10 Hz stimulation of wild-type channels causes current to decrease to approximately 70% of maximum whereas the same stimulation of mutant channels results in rundown to 90% of maximum. This is caused by an expedited recovery from inactivation for mutant channels versus wild-type. The D188V mutant, for example, recovers to 90% maximal current in 200ms while wild-type channels are unable to recover to this degree in >1000ms.
Some other functional mutations that lead to hyperexcitability do so by other means, such as decreasing the rate of entrance into the slow inactivated state.
Some of the other functional mutations are believed to result in hypoexcitability. The R859C mutation, for example, has a more depolarized voltage dependence of activation, meaning that the membrane must be more depolarized for the channel to open. This mutant also recovers more slowly from inactivation.
The nonfunctional channels are believed to produce similar changes in cell excitability. Likewise, many of the
nonsense mutation
In genetics, a nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a ''nonsense codon'' in the transcribed mRNA, and in leading to a truncated, incomplete, and usually nonfunctional protein produc ...
s likely result in nonfunctional channels and hypoexcitability, though this has yet to be tested. It is also unclear how this membrane hypoexcitability leads to the GEFS+ phenotype.
Type 3
Patients with GEFS+ type 3 have mutations in the GABRG2 gene, which encodes the
GABAA γ2 subunit (figure 2). The first mutation discovered in GABRG2 was K289M, in the extracellular region linking membrane-spanning domains M2 and M3. Oocytes injected with α1, β2, and γ2 subunits produce large GABA inducible currents whereas those injected with K289M mutant instead of wild-type subunits produce currents much smaller (about 10% of wild-type). This abnormal current is not the result of non-incorporation of mutant subunits since mutant containing receptors are still sensitive to
benzodiazepine
Benzodiazepines (BZD, BDZ, BZs), sometimes called "benzos", are a class of depressant drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. They are prescribed to treat conditions such as anxiety disorders, ...
s, a property for which functional γ subunits are required. Because of these results, it is believed that the GEFS+ phenotype in these individuals is a result of hyperexcitability.
Concurrent with the previous mutation, a second group found a second mutation in GABRG2 associated with GEFS+. This mutation, R43Q, is located in the one of two benzodiazepine binding-sites located in the extracellular N-terminus. Benzodiazepines, such as
Diazepam
Diazepam, first marketed as Valium, is a medicine of the benzodiazepine family that acts as an anxiolytic. It is commonly used to treat a range of conditions, including anxiety, seizures, alcohol withdrawal syndrome, muscle spasms, insomnia, a ...
, potentiate
GABA induced current. This potentiation is abolished in cells expressing the R43Q mutant subunit instead of the wild-type γ subunit. This mutation does not affect the subunit's ability to coassemble into function receptors as it still confers resistance to GABA current blockade by
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
. As with the previous mutation, this mutation is expected to result in neuronal hyperexcitability.
The final known GEFS+ type 3 mutation is a
nonsense mutation
In genetics, a nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a ''nonsense codon'' in the transcribed mRNA, and in leading to a truncated, incomplete, and usually nonfunctional protein produc ...
, Q351X, located in the intracellular region linking the third and fourth membrane spanning segments. When this mutant subunit is expressed in cells with wild-type α and β subunits it produces non-functional receptors. Since wild-type α and β subunits expressed alone are able to produce GABA inducible current this indicates that the mutation either prevents both coassembly of the mutant and wild-type subunits but also coassembly of the wild-type α and β subunits or prevents proper trafficking of the formed receptor to the membrane. Fusion of GFP onto this mutated subunit has indicated that it is localized to the
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
instead of the
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
. As with other known GEFS+ type 3 mutation, Q351X likely results in neuronal hyperexcitability.
SCN2A mutations
The final type of GEFS+ is caused by mutations in the SCN2A gene, which encodes a
sodium channel
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the channel ...
α subunit. The first associated mutation in this gene is R187W, located on the intracellular region linking membrane spanning units two and three in the first domain (D1S2-S3, figure 3). Patients with this mutation have both febrile and afebrile seizures.
Electrophysiological
Electrophysiology (from Greek , ''ēlektron'', "amber" etymology of "electron"">Electron#Etymology">etymology of "electron" , ''physis'', "nature, origin"; and , ''-logia'') is the branch of physiology that studies the electrical properties of bi ...
examination of this mutant revealed that it increases the time constant for inactivation, presumably increasing sodium current and leading to hyperexcitability. However, this mutation also yields channels that inactivate at more hyperpolarized potentials relative to wild-type channels, indicative of hypoexcitability. Whether the result on membrane excitability of this mutation is hyperexcitability or hypoexcitability is, as yet, unclear.
The second known mutation in SCN2A associated with GEFS+ is R102X. This mutation is located in the intracellular N-terminus (figure 3) and results in SMEI in patients. The result of this mutation is completely non-functional channels and membrane hypoexcitability. The truncated mutant protein also seems to cause wild-type channels to inactivate at more hyperpolarized potentials, indicating that it also acts in a
dominant negative
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
manner.
Management
Long term management is by use of anticonvulsant medication, principally
valproate
Valproate (VPA) and its valproic acid, sodium valproate, and valproate semisodium forms are medications primarily used to treat epilepsy and bipolar disorder and prevent migraine headaches. They are useful for the prevention of seizures in those ...
,
stiripentol
Stiripentol, sold under the brand name Diacomit, is an anticonvulsant medication used for the treatment of Dravet syndrome - a serious genetic brain disorder.
The most common side effects include loss of appetite, weight loss, insomnia (difficu ...
,
topiramate
Topiramate, sold under the brand name Topamax among others, is a carbonic anhydrase inhibitor medication used to treat epilepsy and prevent migraines. It has also been used in alcohol dependence. For epilepsy this includes treatment for gener ...
or
clobazam
Clobazam, sold under the brand names Frisium, Onfi and others, is a benzodiazepine class medication that was patented in 1968. Clobazam was first synthesized in 1966 and first published in 1969. Clobazam was originally marketed as an anxioselec ...
. Ketogenic diet has also been found useful in certain cases
Management of breakthrough seizures is by benzodiazepine such as midazolam.
See also
*
Febrile seizure
A febrile seizure, also known as a fever fit or febrile convulsion, is a seizure associated with an increased body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even whe ...
s
*
Idiopathic generalized epilepsy
Idiopathic generalized epilepsy (IGE) is a group of epileptic disorders that are believed to have a strong underlying genetic basis. Patients with an IGE subtype are typically otherwise normal and have no structural brain abnormalities. People als ...
*
Dravet Syndrome Foundation
*
International Dravet Epilepsy Action League
References
External links
{{DEFAULTSORT:Generalized Epilepsy With Febrile Seizures Plus
Channelopathies
Epilepsy types