Genetic abnormalities in rodent AE-prone strains and in human epilepsy may coincide, but in some cases, they demonstrate differences as well (see below). On the other hand, as mentioned earlier, even in epilepsy clinics, a huge variety of both symptomatic manifestations and their genetic causes could be found in different patients.
Analysis of gene expression profiles and localization of respective mutations as well as the comparisons with each other and with the original AE-non-prone strains could allow revealing both common genetic patterns characteristic of AE and individual strain characteristics, leading to the same result: the development of AE. Using AE-prone strains, it is also possible to compare genetic changes with already established genetic causes of human epilepsy in order to find common mutations and molecular mechanisms leading to seizure proneness. Thus, rodent AE-prone strains increase the scientists’ potential for searching for mutations leading to epilepsy development.
Among AE-prone rodents already described, there are strains with a monogenic inheritance of this trait, and several strains with a presumably polygenic inheritance of the predisposition to AE are now maintained [26][9]. The new rodent AE strains demonstrate the AE monogenic autosomal dominant or recessive inheritance. They were derived either by targeted genetic knockout or N-ethyl-N-nitrosourea mutagenesis followed by screening for mutations of the target gene [171][51]. This approach allows the unambiguous association of certain mutations with epilepsy in parallel with data on the sequencing of large epilepsy patient cohorts in search for orthologous gene mutations. There are also rodent strains with monogenic epilepsy derived by subsequent selection after the discovery of respective spontaneous mutations. Strains with a polygenic type of epilepsy inheritance were also obtained independently in different laboratories. These are considered to be the more preferable models of human seizure states since a significant part of epilepsy clinical cases is caused by a combination of multiple “unfortunate” alleles of different genes. At the same time, the localization of specific mutations leading to the development of AE in these strains is a much more difficult task.
Predisposition to AE and spontaneous death after seizures was shown in serotonin 5-HT2c receptor mutant mice
[188][64]. Audiogenic seizures in this strain started to develop at 60–75 days of age, and by day 120, 100% of animals tested were AE-prone. Data on the role of the serotoninergic system in epilepsy development are inconsistent and depend on the epilepsy form (in humans) or animal model
[189,190,191,192,193][65][66][67][68][69].
One more gene of interest in terms of inherited causes of epilepsy is
GABRB2, which encodes the β2-subunit of the gamma-aminobutyric acid receptor—GABA
A. In humans, various mutant alleles of this gene determine the development of a wide range of epilepsy forms
[194,195,196][70][71][72]. A
Gabrb2-/- knockout mouse strain with a characteristic schizophrenia-like phenotype and AE was obtained
[197][73]. Interestingly, this strain showed signs of neuroinflammation indicated by increased brain levels of the oxidative stress markers, malondialdehyde and proinflammatory cytokines.
The dysfunction of the Lgi1 (leucine-rich repeat glioma-inactivated) protein, originally described as a suppressor of tumor growth and metastasis
[198][74] is associated with epilepsy in humans. Unlike most proteins whose gene mutations are associated with the development of epilepsy, Lgi1 is a secreted protein, the ligand of ADAM22 and ADAM23 (disintegrin and metalloproteinase domain-containing proteins). Lgi1 binds to ADAM22 at the postsynaptic membrane and ADAM23 at the presynaptic membrane, mediating AMPA receptor and potassium channels activity, and thus participating in the regulation of synaptic activity
[199,200][75][76]. The increased expression of Lgi1 can also inhibit ERK1/2-cascade activity
[201][77]. Finally, Lgi1 regulates the activity of inwardly rectifying K
+ (Kir) channels, which contain astrocyte Kir4.1 subunits (Kir4.1 channels) involved in the K
+ homeostasis in the CNS
[200][76]. In humans, at least 43 mutations in the
LGI1 gene are known to be associated with
autosomal dominant lateral temporal lobe epilepsy (ADLTE)
[202][78]. The autoimmune reaction with the development of autoantibodies to Lgi1 causes a form of autoimmune epilepsy
[42,43][79][80]. Finally, loss-of-function mutations in the
KCNJ10 gene encoding Kir4.1 cause the epileptic disorders known as “EAST” (
epilepsy, ataxia, sensorineural deafness, and tubulopathy)
[200][76]. Mutations in the
Lgi1 gene leading to epilepsy cause impaired secretion of the
Lgi1 protein
[200,203][76][81]. Rats carrying a missense mutation (L385R) in the
Lgi1 gene were obtained. Homozygous
Lgi1-mutant rats were predisposed to early-onset spontaneous epileptic seizures and died prematurely. Heterozygous
Lgi1-mutant rats were more susceptible to audiogenic generalized tonic–clonic seizures than wild-type rats.
Lgi1 knockout mice with a similar phenotype were also obtained
[204][82]. The seizure threshold in
Lgi1 heterozygotes decreased with age (13% sensitive animals at 21 days of age, and 52% at 28 days of age)
[204][82].
3.3. Models of Pyridoxine-Dependent Epilepsy and Angelman Syndrome
Mouse knockout of three genes from the family, encoding the proline and acidic amino acid-rich basic leucine zipper (PAR bZip) transcription factor, develops spontaneous and audiogenic seizures. The expression of these genes in most tissues obeys circadian rhythmicity, but in brain tissues they are expressed at almost invariable levels. The scholars attribute the knockout effect of these transcription factors to the fact that one of their targets is the
Pdxk gene encoding pyridoxal kinase, which converts vitamin B6 derivatives into pyridoxal phosphate, involved in amino acid and neurotransmitter metabolism.
Human Angelman syndrome (AS) manifests as defects in intellectual and speech development, emotional retardation, general motor deficits, and epilepsy in 80% of cases. It is caused by a partial deletion of the 15q11.2-q13.3 region in the maternal chromosome, which leads to either loss or mutation of the
UBE3A gene encoding the ubiquitin–protein ligase E3A (UBE3A) also known as E6AP ubiquitin–protein ligase (E6AP). This enzyme is involved in targeting proteins for degradation within the proteasome. Inheriting paternal allele mutation in the same locus leads to the development of Prader–Willi syndrome with more mild neurological manifestations. The molecular mechanisms of the pathophysiology and inheritance of this syndrome were previously described
[217][83]. Mutations specifically affecting the
UBE3A gene occur in 5 to 10% of AS. In more than 75% of cases, large-size deletions within the maternally inherited chromosome 15 appear, herewith the deletion includes not only the
UBE3A gene, but surrounding sequences containing additional genes (namely
GABRB3,
GABRA5 and
GABRG3, encoding different GABA
A receptor subunits)
[218,219][84][85].
3.4. Models with Polygenic or Unknown Inheritance
The first example of a putatively polygenic type of inheritance of AE in rodents is DBA/2J mice. Three loci presumably related to AE in this strain were identified: audiogenic seizure prone 1 (
Asp1),
Asp2, and
Asp3 on chromosomes 12, 4, and 7, respectively
[26,222][9][86]. Subsequently, evidence was obtained that the
Asp1 and
Asp2 loci were mainly responsible for the manifestations of AE
[222][86]. Variations in the coding sequence of the
Kcnj10 gene (localized on chromosome 1) leading to amino acid substitutions in the Kir4.1 potassium channel in DBA/2J relative to the C57BL/6B seizure-insensitive mice were also shown
[223][87]. As cited above, mutations in the
KCNJ10 gene cause epileptic disorders in humans
[200][76]. It is assumed that the functions of the genes localized in the
Asp1 and
Asp2 regions are related to the regulation of Ca
2+-ATPase activity, which is important for synaptic function, in particular affecting the amount of calcium within the synapse, which, in turn, influences the neurotransmitter release from synaptic vesicles
[26,224][9][88].
Two substrains were selected for AE based on the Sprague Dowely population: GEPR-3, displaying moderate seizure intensity, and GEPR-9 with severe AE seizures
[26][9]. Sound-induced seizures appear in GEPR-9 at the age of 25–35 days, with a further increase in intensity and decrease in fit latency
[227][89]. Genetic studies indicate a polygenic inheritance of AE-proneness in GEPRs, while genes responsible for AE are still unknown
[143,228][21][90]. At the same time, GEPRs’ GABAergic system anomalies were described in detail
[26,229][9][91].
The genetics of AE development in three rodent strains obtained by classical selection with a presumably polygenic inheritance of this trait, WAR, KM and GASH/Sal, were described in detail. In KM rats, the audiogenic phenotype appears at 1 month of age, and by the age of 3 months, 100% of animals are AE-prone
[25][20]. In the GASH/Sal strain, the age of AE expression start was not indicated. In WAR, the stable AE phenotype is present at the age of 70–78 days
[25][20]. The cDNA microarray and RNA-seq methods identified the genetic abnormalities in connection with AE-proneness in these strains, followed by mutations in exon sequences, determining the expression of a wide range of genes and differences in the activity of key signaling pathways, demonstrated by the comparison of AE-prone and control strains
[121,230,231,232,233][12][92][93][94][95]. In WAR and GASH/Sal, the transcriptome of the IC, the structure, determining the start of AE seizure, was analyzed for both groups—animals not exposed to sound (naïve) and for animals after a seizure
[230,234][92][96]. In KM rats, the transcriptome of the IC and SC of naïve animals was analyzed
[121][12]. It is the profile of the transcriptome in naïve animals that is of maximal interest, as it allows to evaluate the contribution of various genes and signaling pathways to the AE-proneness. On the other hand, changes in the genes expression levels after seizures may include those involved in the compensatory mechanisms of seizure consequences
[121][12]. However, the identification of specific genes with mutations and/or changes in expression levels that lead to seizure development in epilepsy models with polygenic or unknown nature of inheritance using whole-genome analysis methods faces a fundamental issue which complicates the whole problem. The thing is that most “classical” AE-prone strains (KM, GEPRs, WAR, and GASH/Sal) were bred by selection several decades ago, and after this, they were maintained in isolation from the original populations for a long time. This isolated breeding makes the direct comparison between AE-prone and AE-non-prone strains not very informative, as many genetic events could occur (both in initial and selected strains). The majority of such events should be neutral, i.e., not associated with the trait investigated—AE-proneness. Thus, 71 differential-expressing genes (DEGs) for the WAR model and 64 DEGs for GASH/Sal (in AE vs. non-AE comparison) were detected
[130][4].
The search for genetic and biochemical peculiarities common to various AE-prone strains resulting from spontaneous mutations with further selection could be informative for revealing mechanisms of AE. Since, as mentioned above, some predisposition to AE is characteristic of rodents in general, it can assume the existence of some genetic pattern leading to this phenotype common to different strains and species. In this regard, it is of interest to compare different models for which RNA-seq data are available, i.e., WAR, KM, and GASH/Sal. Analysis of the IC transcriptome of the GASH/Sal hamster and WAR strains revealed several DEGs common to these strains in comparison to the original AE-non-prone strains—this list of genes includes
Egr3,
Rgs2,
Ttr and
Npy in both AE models
[230,234][92][96].
The
Egr3 gene overexpressed in WAR and GASH/Sal encodes a transcriptional regulator that belongs to the EGR (early growth response proteins) family of C2H2-type zinc-finger proteins and regulates NMDA receptor 1 and GABA
A receptor α4 subunit via the BDNF-PKC/MAPK signaling pathway
[83,211,235][97][98][99]. Increased expression of GABA
A receptor α4 subunit and, conversely, decreased expression of α1 subunit of this receptor plays an important role in human
temporal lobe epilepsy and in a mouse model of pilocarpine-induced seizures
[83,235,236][97][99][100]. Thus, increased expression levels of
Egr3 may play a causative role in the development of AE in WAR and GASH/Sal as well. The AE-prone
tremor mouse strain is also characterized by increased expression of the
Egr3 gene
[210][101]. On the other hand, it was unable to find data on the association of the
Egr3 gene with human epilepsy. Mutations in the
Ttr gene in humans cause family amyloid polyneuropathy but not epilepsy
[237,238][102][103]. It is the only gene for which overexpression was found in WAR, KM and GASH/Sal, compared with the original AE-non-prone strains
[121][12]. Transthyretin encoded by the
Ttr gene regulates the activity of GABA
A receptors, which play an important role in the control of predisposition to seizures
[239][104]. Deficiencies of GABA
A receptor-mediated neurotransmission were previously shown in GASH/Sal hamster and WAR rat models. In KM rats, the imbalance of glutamate-GABA content was noted at a neurochemical level as well
[66,142,230,232][19][29][92][94].
In addition to DEGs, sequencing of the transcriptome of GASH/Sal animals in comparison with the control AE-non-prone strain revealed several mutations in genes’ coding regions, and several of these genes were described as being associated with human epilepsy. In this list, mutations in the
Cacna1a and
Cacna2d3 genes encoding subunits of the calcium voltage-gated channel are of the most interest
[232][94]. In humans, mutations in the
CACNA1A induced
developmental epileptic encephalopathies (DEEs)
[249][105].
Grik1 (glutamate ionotropic receptor kainate type subunit 1) gene polymorphism was found in hamsters, while the human ortholog is associated with epilepsy development, including
juvenile absence epilepsy (JAE)
[232,250][94][106].
Cacna1a and
Grick1, together with
Grin2c, in which a substitution was also found in GASH/Sal, are parts of the glutamatergic synapse pathway, which is enhanced in GASH/Sal
[232][94]. The
Grin2c gene encodes the glutamate (NMDA) receptor subunit ε-3
[251][107]. Mutations in genes encoding glutamate receptor subunits ε-1 (
GRIN2A)
[252][108] and ε-2, i.e., genes related to previously mentioned (
GRIN2B)
[253][109], are also associated with human epilepsy. One more GASH/Sal gene
Zeb2 (encoding Zinc finger E-box-binding homeobox 2) carries the substitution (in comparison to the control strain) and is possibly associated with seizure development. The encoded protein is the transcription factor, which participates in the transforming growth factor β (TGFβ) signaling pathway and is essential for early fetal development. Mutations in the
ZEB2 gene in humans are associated with Mowat–Wilson syndrome, a complex disease with epilepsy and severe CNS developmental abnormalities
[232,254,255][94][110][111]. Mutations in several other genes found in GASH/Sal are probably neutral.
WAR transcriptome analysis revealed mutations in
Chrna4,
Grin2a,
Grin2b,
Kcnq3,
Vlgr1 and several other genes
[181][112].
Chrna4 encodes a subunit of the nicotinic acetylcholine receptor, and its mutations are associated with
nocturnal frontal lobe epilepsy.
Kcnq3 encodes a subunit of the voltage-gated potassium channel with mutations associated with
benign familial neonatal seizures [256][113]. Mutations in the
Vlgr1 gene are associated with human epilepsy and with AE in the Frings mice (see above)
[181][112].
One should note that no mutations in exon regions of any genes, associated with the development of epilepsy in humans, were found in the KM rat transcriptome analysis
[121][12].
The comparison of signaling pathways, involved in synaptic regulation mechanisms, could reveal the common pattern of deviations that are specific to the ”epileptic brain” both in AE models and in humans. This pattern could be found despite the differences described in the mutation “list” for AE strains and for that associated with human epilepsy. From this point of view, the KM rat strain
[121][12] was investigated in more detail. In the KM rats’ brain, increased activity of the MAPK signaling cascade was found which has direct pro-epileptogenic effects in the corpora quadrigemina, as well as proapoptotic and proinflammatory signaling pathways differences from Wistar
[121][12]. MAPK/ERK1/2 activity presumably contributes to seizure threshold decrease via the upregulation of glutamatergic synaptic transmission and suppression of GABA
A receptors functions
[53,54,55,56,64,260,261][114][115][116][117][118][119][120].
4. The Use of Rodent Strains with AE in AEDs Screening
All AEDs currently used in clinics are characterized by a wide range of side effects, including rather severe ones, and they are not effective in all cases of epilepsy. For this reason, the search for new AEDs continues in order to find drugs with high efficacy and a smaller spectrum of side effects than those currently used. The development of new AEDs mainly includes studies of modified analogs of conventional drugs, possessing fewer side effects
[264][121].
The “gold standard” models in AEDs testing are the maximal electroshock technique and the PTZ-induced seizure model
[266,267,268][122][123][124]. At the same time, the use of PTZ models proved to be sometimes ineffective, e.g., the absence of the anticonvulsant effect of levetiracetam using this type of seizure
[269][125], whereas in both AE and other types of seizure induction, this drug was effective
[270,271][126][127]. Numerous investigations proved that clinically approved AEDs reduce effectively the seizure intensity in AE-prone rodents. Numerous data, obtained in this field, confirm the applicability of AE-prone rodents for new AEDs generation testing
[135,272,273][10][128][129].
In general, there are probably too many AE-prone rodent strains available today for screening new anticonvulsants and for analyzing the mechanisms of their efficiency. Actually, several strains have rather explicit neurophysiological and biochemical characteristics to be used for this purpose. They are DBA/2 mice, WAR, GEPRs, KM rats and GASH/Sal. Appropriate genetic models described earlier are now used to develop specific treatments for some definite human pathologies, such as fragile X syndrome, Angelman syndrome, etc.
[177,274,275][130][131][132].
The DBA/2 mouse strain was used to reveal the anticonvulsant effects of clobazam
[276][133]; benzodiazepine drugs
[277][134]; adenosine (Ado) type 1 receptors (A1Rs) and their ligands
[278,279][135][136]; effects of serotoninergic system enhancing substances
[280][137]; and structurally diverse GABA
B positive allosteric modulators
[281][138]. The DBA/2 strain was also used to study the synergism of carbenoxolone (the succinyl ester of glycyrrhetinic acid, an inhibitor of 11beta-hydroxy steroid dehydrogenase) and conventional antiepileptic drugs (carbamazepine, diazepam, felbamate, gabapentin, lamotrigine, phenytoin, phenobarbital and valproate)
[282][139] as well as the pharmacodynamic potentiation of antiepileptic drugs’ effects by HMG-CoA reductase inhibitors
[283][140]. In Frings mice, the effects of a new analog of topiramate
[264][121], the anticonvulsive properties of soticlestat, a novel cholesterol 24-hydroxylase inhibitor
[265][141], and the effects of synaptic and extrasynaptic GABA transporters inhibitors were investigated
[284][142].
Fmr-/- mice were used to analyze the possibility of mGluRs and GABA receptors’ activity modulation in fragile X syndrome therapy
[177,275][130][132]. The anticonvulsant properties of lovastatin were investigated in fragile X and Angelman syndrome models
[274][131]. In GEPR-3 and GEPR-9 strains, the anticonvulsant effect of several ion-exchange transporter inhibitors
[285][143] and the ability of liraglutide to reduce tolerance to diazepam
[286][144] were studied. The KM strain was used in studies of the ability of vigabatrin (a GABA decay inhibitor) to reduce the clonic component of audiogenic seizures
[287][145].
In general, one may conclude that along with standard models of seizures induced by maximal electroshock and PTZ, the AE-prone rodents could be widely used in the testing of old and new AEDs.
5. Conclusions
There is currently a rapidly expanding set of new epilepsy models with monogenic inheritance obtained by directed mutagenesis, due to the amazing success of genetic engineering. These achievements have promoted an understanding of the molecular mechanisms of epileptogenesis. In monogenic models, it is possible to establish an unambiguous causal relationship between a known mutation and the development of epileptic events. One may assume that large collections of knockout and transgenic mice (such as Jackson Laboratory), may possess several mutant strains that are predisposed to AE, but this trait has not yet been revealed. This assumption seems legitimate, given that in rodents the AE trait (in some cases) could be determined by mutations in genes for which no association with human epilepsy has been discovered
[215,293][146][147]. An example of such incidental finding of gene knockout association with a predisposition to AE was mentioned previously
[215][146].
The available set of rodent AE-prone strains with a monogenic type of inheritance demonstrates a significant diversity of genes involved in various regulatory processes. These are mutations of genes that control the expression of AE: transcription factors (
Egr3,
Tef), enzymes (
Wwox), receptors (
5-HT2c,
Vlgr1,
Gabrb2), anchor proteins (
Gipc3), regulators of mRNA translation and transport (
Fmr1), etc.
Thus, the models of various forms of epilepsy are available for epilepsy syndromes with different etiologies and in which definite CNS mechanisms are involved, including action potential generation and early developmental events. In several AE models, the altered function of signaling cascades was also described (e.g., MAPK in KM rats)
[121][12] as well as the development of neuroinflammation
[197][73]. Thus, the AE models available provide an opportunity to test a wide range of potential AEDs targeting different proteins involved in various cellular processes, the disruption of which could induce epileptogenesis. Moreover, knockout mouse strains represent an obligatory model system for the development of epilepsy gene therapy methods based on CRISPR/Cas or adenoviral systems targeting, i.e., the development of the individual (associated with epilepsy) mutations “corrections”
[293][147].
Rodent AE models with polygenic inheritance (KM, GEPRs, WAR and GASH/Sal) were developed as the result of numerous spontaneous mutations’ accumulation. Only part of these mutations is responsible for AE-proneness, while others are neutral or even exert compensatory effects for the AE phenotype (having been involuntarily selected for during selection generations). On the one hand, the identification of the genes responsible for audiogenic seizures in these models faces rather serious problems. It is known that several mouse knockouts of genes, whose human orthologs are associated with epilepsy, do not display neither AE nor spontaneous seizures in mice
[293][147]. As an example,
Tsc1 and
Tsc2 (Tuberous sclerosis complex) knockouts do not lead to the development of seizures in mice, unlike tuberous sclerosis cases in humans
[293,294][147][148]. On the other hand, the opposite is also true, i.e., several mutations were described that induce seizures in rodents, and definite disorders in humans
[83,210,211][97][98][101]. Pathological deviations that lead to epileptogenesis in AE-prone strains obtained by selection in independent experiments obviously do not coincide with one another because they could be determined by different mutations. Nevertheless, they could lead to similar results, demonstrating violations of glutamate and GABAergic systems, which provoke seizures. Polygenic AE animal models could be indispensable for studying human epilepsy with a complex type of inheritance.
Most monogenic mouse models of epilepsy were obtained by introducing mutations into the coding regions of target genes (reading frame shifts or substitutions). Many epilepsy cases in humans also describe mutations in the coding regions of certain genes, leading to the loss of functions of the protein encoded. At the same time, a lot of human epilepsy cases were described with mutations affecting not the coding, but the regulatory region of genes
[297,298][149][150]. In such cases, functional disturbances occur due to changes in expression levels relative to the normal ones. In this aspect, the KM rat strain is indicated (in comparison to other rodent models), in which no mutations were found in candidate genes for association with epilepsy, but a wide range of candidate genes could be indicated in which the expression is significantly increased or decreased relative to the conditional norm of Wistar rats
[121][12].
Gene expression could be also affected by the methylation status of gene promoters, as was demonstrated for epilepsy-associated genes
FMR1 and
BRD2 (bromodomain-containing transcriptional activator 2) in humans
[172,299][52][151]. The expression level of many genes could also be regulated by the RNA interference mechanism involving miRNAs (small non-coding RNAs that regulate post-transcriptional gene expression)
[300][152]. It was found that in some cases, miRNAs as well as long non-coding RNAs can act as biomarkers and therapeutic targets in human epilepsy
[301,302,303,304,305,306][153][154][155][156][157][158]. Thus, in the case of some rodent models with polygenic inheritance obtained by the selection, the search for mutations in the regulatory regions of some genes, for epigenetic changes, and changes in the expression of interfering RNAs represent a mandatory field for future research.