Is variation in the GABA(B) receptor 1 gene associated with temporal lobe epilepsy?

Genetic Landscape of Common Epilepsies: Advancing towards Precision in Treatment

Epilepsy, a neurological disease characterized by recurrent seizures, is highly heterogeneous in nature. Based on the prevalence, epilepsy is classified into two types: common and rare epilepsies. Common epilepsies affecting nearly 95% people with epilepsy, comprise generalized epilepsy which encompass idiopathic generalized epilepsy like childhood absence epilepsy, juvenile myoclonic epilepsy, juvenile absence epilepsy and epilepsy with generalized tonic-clonic seizure on awakening and focal epilepsy like temporal lobe epilepsy and cryptogenic focal epilepsy. In 70% of the epilepsy cases, genetic factors are responsible either as single genetic variant in rare epilepsies or multiple genetic variants acting along with different environmental factors as in common epilepsies. Genetic testing and precision treatment have been developed for a few rare epilepsies and is lacking for common epilepsies due to their complex nature of inheritance. Precision medicine for common epilepsies require a panoramic approach that incorporates polygenic background and other non-genetic factors like microbiome, diet, age at disease onset, optimal time for treatment and other lifestyle factors which influence seizure threshold. This review aims to comprehensively present a state-of-art review of all the genes and their genetic variants that are associated with all common epilepsy subtypes. It also encompasses the basis of these genes in the epileptogenesis. Here, we discussed the current status of the common epilepsy genetics and address the clinical application so far on evidence-based markers in prognosis, diagnosis, and treatment management. In addition, we assessed the diagnostic predictability of a few genetic markers used for disease risk prediction in individuals. A combination of deeper endo-phenotyping including pharmaco-response data, electro-clinical imaging, and other clinical measurements along with genetics may be used to diagnose common epilepsies and this marks a step ahead in precision medicine in common epilepsies management.

Genetics of temporal lobe epilepsy: a review

Temporal lobe epilepsy (TLE) is usually regarded as a polygenic and complex disorder. To understand its genetic component, numerous linkage analyses of familial forms and association studies of cases versus controls have been conducted since the middle of the nineties. The present paper lists genetic findings for TLE from the initial segregation analysis to the most recent results published in May 2011. To date, no genes have been clearly related to TLE despite many efforts to do so. However, it is vital to continue replication studies and collaborative attempts to find significant results and thus determine which gene variant combination plays a definitive role in the aetiology of TLE.

GABBR1 gene polymorphism(G1465A)isassociated with temporal lobe epilepsy

PURPOSE

γ-Aminobutyric acid B receptor 1(GABBR1) gene G1465A polymorphism has been considered as a potential risk factor for the development of temporal lobe epilepsy (TLE). However, the results were inconsistent. In this study, we performed a meta-analysis to assess the association between GABBR1 G1465A polymorphism and the risk of TLE.

METHODS

Biomedical literature databases including PubMed, ISI web of science and Embase were searched. The studies evaluating the association between GABBR1 G1465A polymorphism and TLE were included. Pooled odds ratio (OR) and 95%CI confidence interval (CI) were calculated using fixed- or random-effects model.

KEY FINDINGS

Seven studies (1011 cases and 2184 controls) met the inclusion criteria and were included in the meta-analysis. The overall result showed that the association between GABBR1 G1465A polymorphism was statistically significant (OR=5.381, 95%CI: 1.726, 16.776, P=0.004). Subgroup analysis showed that the effect estimate was higher in the studies with high quality score (OR=14.220, 95%CI: 6.933, 29.169, P=0.000) than that in the studies with low quality score (OR=1.158, 95%CI: 0.325, 4.123, P=0.821).

SIGNIFICANCE

The present meta-analysis suggests that GABBR1 G1465A polymorphism is associated with the risk of TLE. The role of GABBR1 G1465A polymorphism in the development of TLE merits further investigation.

Expression of GABA(B) receptors is altered in brains of subjects with autism

Autism is a neurodevelopmental disorder that is often comorbid with seizures. Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in brain. GABA(B) receptors play an important role in maintaining excitatory-inhibitory balance in brain and alterations may lead to seizures. We compared levels of GABA(B) receptor subunits GABA(B) receptor 1 (GABBR1) and GABA(B) receptor 2 (GABBR2) in cerebellum, Brodmann's area 9 (BA9), and BA40 of subjects with autism and matched controls. Levels of GABBR1 were significantly decreased in BA9, BA40, and cerebellum, while GABBR2 was significantly reduced in the cerebellum. The presence of seizure disorder did not have a significant impact on the observed reductions in GABA(B) receptor subunit expression. Decreases in GABA(B) receptor subunits may help explain the presence of seizures that are often comorbid with autism, as well as cognitive difficulties prevalent in autism.

Association between the gamma-aminobutyric acid type B receptor 1 and 2 gene polymorphisms and mesial temporal lobe epilepsy in a Han Chinese population

An abnormal gamma-aminobutyric acid B receptor composed of gamma-aminobutyric acid type B receptor 1 (GABBR1) and gamma-aminobutyric acid type B receptor 2 (GABBR2) can provoke seizures. This study was designed to assess the contribution of GABBR1 and GABBR2 in mesial temporal lobe epilepsy (MTLE). Two tag single-nucleotide polymorphisms (SNPs) of GABBR1 and four tag SNPs of GABBR2 were selected and genotyped in 318 MTLE patients and 315 non-epileptic individuals. The rs967932 A-allele of GABBR2 increased the risk of MTLE in an additive and a dominant genetic model, respectively (P=0.018, OR=1.305, 95% CI 1.048-1.624 and P=0.003, OR=1.667, 95% CI 1.186-2.343, respectively). rs1999501 and rs944688 of GABBR2, and rs29259 of GABBR1 were thought to be associated with MTLE; however, after a Bonferroni correction, these associations were not observed and only the rs967932 A-allele was found to increase the risk of MTLE in the dominant model (P=0.036). The frequency at which the haplotype G-C-A-C (rs3780428-rs1999501-rs967932-rs944688) occurred in MTLE patients was significantly higher compared to the controls (12.26% vs. 6.51%, P=0.0004) and patients carrying this haplotype exhibited an earlier onset of MTLE (P=0.028). No evidence of significant allelic, genotypic, or haplotypic associations were identified in the tag SNPs of the GABBR1 gene in patients with MTLE, and the polymorphism at G1465A was not observed in our samples. Our results provide the first evidence that common genetic variations in the GABBR2 gene contribute to the risk of MTLE. Moreover, the present results do not support the hypothesis that common variants of GABBR1 contribute substantially to the epileptogenic effect during MTLE in the Han Chinese.

Analysis of LGI1 promoter sequence, PDYN and GABBR1 polymorphisms in sporadic and familial lateral temporal lobe epilepsy

Autosomal dominant lateral temporal epilepsy (ADTLE) is a genetically transmitted epileptic syndrome characterized by focal seizures with predominant auditory symptoms likely originating from the lateral region of the temporal lobe. Mutations in coding region or exon splice sites of the leucine-rich, glioma-inactivated 1 (LGI1) gene account for about 50% of ADLTE families. De novo LGI1 mutations of the same kind have also been found in about 2.5% of non-familial cases with idiopathic partial epilepsy with auditory features (IPEAF). In both conditions, mutations in the LGI1 promoter region have not been reported. We sequenced the minimal promoter region of LGI1 in the probands of 16 ADLTE families and in 104 sporadic IPEAF patients and no mutations clearly linked to the disease were found. However, two polymorphisms, -500G>A and -507G>A, with potential functional implications were identified and analysed in the cohort of sporadic IPEAF patients but their frequencies did not differ from those found in a control population of similar age, gender and geographic origin. We also analysed in our study population the GABA(B) receptor 1 c.1465G>A and the prodynorphin promoter 68-bp repeat polymorphisms, previously associated with temporal lobe epilepsy. None of these polymorphisms showed a significant association with IPEAF, whereas a tendency towards association with the prodynorphin low expression (L) alleles was found in the small group of ADLTE index cases, in agreement with previous studies suggesting that this polymorphism is a susceptibility factor in familial forms of temporal lobe epilepsy.

Genetic variants in the IMPA2 gene do not confer increased risk of febrile seizures in Caucasian patients

Pathogenesis of febrile seizures (FS), causing the most common of types of seizures in children, remains unknown. Genetic factors appear to play a pivotal role and FS can be inherited as a monogenic or genetically complex disorder. Several risks factors have been proposed but many of the previously reported genetic associations were not replicated. Non-coding polymorphisms in the myo-inositol monophosphatase 2 gene (IMPA2) have been suggested as a susceptibility factor for FS in Japanese patients. It is unknown whether genetic variants in the same gene constitute a risk factor for FS in other ethnic groups because the frequency of FS is significantly higher in Japanese children than in Caucasian patients. We investigated the role of the IMPA2 gene in a cohort of 96 unrelated Caucasian subjects with a history of FS. We did not identify any significant differences in genotypes of cases and matched controls; no mutations or non-synonymous polymorphisms were detected in these individuals. Our data suggest that the genetic variants in the IMPA2 gene are not associated with a risk of FS in Caucasian patients and patients from various genetic groups are likely to have different genetic causes of FS.

Genetic dissection of the common epilepsies

PURPOSE OF REVIEW

Only two functionally validated susceptibility genes, CACNA1H and GABRD, have so far been identified in the common epilepsies using a candidate gene approach. The difficulty with the alternative statistical approach, where none of the suggested candidates has been functionally validated, may partly be due to the posited genetic architecture of the common epilepsies, such as the idiopathic generalized epilepsies. A subset of both rare and common variants from a much larger pool of susceptibility genes may contribute to disease risk. We review methods and designs for the genetic dissection of common epilepsies.

RECENT FINDINGS

Genetic association studies, though theoretically more powerful than linkage analysis, have not yet delivered validated susceptibility genes. Methodological flaws can undermine such studies but are correctable. Concerns remain, however, about the extent of underlying genetic heterogeneity in common epilepsies. Genome-wide association studies are increasingly feasible, but issues remain about their conduct and analysis. Meta-analysis may resolve conflicting association studies, facilitated by the establishment of databases of genetic association studies. Newer multi-locus and admixture mapping approaches are attractive alternatives to traditional association studies and may offer new insights into identifying epilepsy genes.

SUMMARY

We conclude by emphasizing the importance of deeper endophenotyping using electroclinical, imaging, and molecular approaches to dissect the common epilepsies.

Lack of association between a GABA receptor 1 gene polymorphism and temporal lobe epilepsy

PURPOSE

Recently a coding nonsynonymous single-nucleotide polymorphism (SNP; G1465A) in the GABBR1 gene was reported to be associated with the incidence and severity of temporal lobe epilepsy (TLE). To clarify the role of this polymorphism in TLE, we attempted to replicate this study.

METHODS

We genotyped 188 unrelated patients with TLE (110 women, 78 men) and 259 controls of middle European descent by a restriction-length polymerase chain reaction (PCR) assay.

RESULTS

Only two (0.5%) patients and none of the controls exhibited the heterozygous A/G genotype, which was previously reported to be overrepresented among patients as compared with controls.

CONCLUSIONS

Although our study was sufficiently powered, we could not replicate the original association. Potential reasons for this failure could lie in subtle genetic differences between the studied populations or differences in the TLE phenotypes.