Recent advancements in genetic research have shed light on the complex polygenic architecture underlying common epilepsies, a group of neurological disorders affecting approximately 50 million people worldwide. A comprehensive review published in Genomic Psychiatry, led by Dr. Olav B. Smeland from the Centre for Precision Psychiatry at Oslo University Hospital, highlights the intricate genetic landscape that defines these conditions, revealing insights beyond those offered by earlier twin studies.
Epilepsy is not a singular disease but a constellation of seizure disorders that can result in increased mortality and psychiatric comorbidities. For around one-third of patients, existing medications fail to provide relief. The review explores two parallel tracks of genetic research: one focusing on severe monogenic epilepsies and the other on common forms, including genetic generalized epilepsy and focal epilepsy. Although the former has identified over a thousand implicated genes, the latter has progressed more slowly due to the complexities of polygenic inheritance.
Historical Context and Modern Insights
The review traces the history of epilepsy research back to twin studies from the 1930s, which revealed that monozygotic twins had a higher concordance rate for epilepsy than dizygotic twins. A significant study analyzed 47,626 twin pairs, finding a concordance rate of 28% in monozygotic twins compared to just 7% in dizygotic twins. These figures further diverged when examining specific epilepsy subtypes, with genetic generalized epilepsy showing a concordance of 77% among monozygotic twins, compared to 35% for dizygotic twins. In contrast, focal epilepsy had a much lower concordance rate of 40% and 3%, respectively.
Modern molecular methods have quantified heritability more precisely, estimating that SNP-heritability is about three times larger for genetic generalized epilepsy than for focal epilepsy. Specific subtypes, such as juvenile myoclonic epilepsy and childhood absence epilepsy, exhibit even higher heritability estimates, emphasizing the importance of diagnostic precision in genetic research.
Rare Genetic Variants and Their Role
The review also discusses the contribution of rare genetic variants (with a minor allele frequency below 1%) to epilepsy risk. A study involving 13,420 epilepsy cases demonstrated a heightened burden of copy number variants across various epilepsy types, with genetic generalized epilepsy exhibiting the highest burden. Notably, recurrent deletions at the 15q13.3 locus emerged as a significant risk factor, having an odds ratio of 36.04.
whole-exome sequencing has identified ultrarare protein-truncating variants in genes related to the GATOR1 complex, a negative regulator of the mTORC1 pathway, as substantial contributors to non-acquired focal epilepsy risk. These findings underline the convergence of signals from both rare and common variants, suggesting shared biological pathways involving ion channel function and synaptic excitability.
Expanding the Landscape of Common Variants
The largest genome-wide association study (GWAS) to date, conducted by the International League Against Epilepsy, involved 29,944 cases and 52,538 controls and identified 26 genome-wide significant loci. A striking observation was that 22 of these loci were associated with genetic generalized epilepsy, which was derived from only 7,407 cases, while focal epilepsy, which had more than double the cases, showed no significant associations. This discrepancy is attributed to fundamental differences in the genetic architecture of these epilepsy subtypes.
Dr. Smeland noted, “The genetic architecture of generalized epilepsies offers an unusually favorable ratio of heritability to polygenicity. Our power projections suggest that a modestly larger GWAS for genetic generalized epilepsy could capture approximately 50% of its common genetic variance, making it remarkably cost-efficient compared to other complex brain disorders.” Among the 29 potential causal genes identified, ten are already established monogenic epilepsy genes.
Genetic Pleiotropy and Clinical Implications
The review emphasizes the phenomenon of genetic pleiotropy, where certain genetic variants influence multiple phenotypes. The genetic correlation between focal and genetic generalized epilepsy is reported at 0.61, indicating shared risk factors. Both forms of epilepsy exhibit negative correlations with cognitive ability, consistent with the cognitive impairments often observed in patients.
Using the bivariate MiXeR model, the authors identified that most variants linked to genetic generalized epilepsy are also associated with major psychiatric disorders, including schizophrenia and major depression. This genetic overlap provides a molecular explanation for the comorbidities frequently seen in clinical settings. Naz Karadag, the study’s first author, stated, “Understanding these shared genetic foundations may eventually help identify epilepsy patients at elevated risk for psychiatric comorbidities.”
Interestingly, approximately 30% to 40% of common variants influencing epilepsy risk overlap with those affecting cortical thickness and surface area, despite a lack of significant genetic correlations between these traits.
Looking Ahead: Challenges and Opportunities
The review underscores that while genetic testing is well-established for severe early-onset epilepsies, it remains premature for common epilepsies due to the complexity of their inheritance. Polygenic risk scores indicate an increased lifetime risk of epilepsy associated with higher genetic scores, yet their current performance is inadequate for population screening. Over 92% of cases in the largest epilepsy GWAS are of European ancestry, which limits the applicability of these findings across diverse populations.
Dr. Smeland cautioned that while polygenic risk scores hold promise for specific clinical contexts, they should not be used for routine decision-making until the diversity of study populations is improved. One of the most compelling aspects of the review is its call for larger, more diverse studies to better understand the genetic underpinnings of common epilepsies and their implications for clinical practice.
As the field progresses, integrating genetic data with other modalities, such as clinical records and neuroimaging, will be crucial. Large biobanks, like the UK Biobank and All of Us Research program, will provide essential resources for future research. The review concludes with an optimistic view of the potential for genetic discovery and clinical application in epilepsy, suggesting that the next generation of GWAS could significantly enhance understanding and treatment options for this complex group of disorders.
