Epilepsy's genetic architecture is complex, but generalized forms may be unusually tractable
Epilepsy is not one disease. It is a constellation of seizure disorders affecting roughly 50 million people worldwide, and for about one-third of them, existing medications do not work. The genetic story behind these conditions has unfolded along two parallel tracks for decades. One track, focused on severe early-onset epilepsies caused by single-gene mutations, has been remarkably productive, identifying over a thousand implicated genes. The other track, focused on the common epilepsies that make up the vast majority of cases, has moved more slowly. A mini-review published in Genomic Psychiatry explains why, and argues that the common-epilepsy track may be closer to a payoff than most people realize.
Twin studies set the baseline
The genetic contribution to epilepsy has been recognized since the 1930s, when twin studies first showed higher concordance rates among identical twins compared to fraternal twins. The largest study, covering 47,626 twin pairs, found 28 percent concordance in identical twins versus 7 percent in fraternal twins. But that average masks enormous differences between subtypes. For genetic generalized epilepsy (GGE), identical twin concordance reached 77 percent. For focal epilepsy, it was 40 percent, with fraternal concordance dropping to just 3 percent.
These numbers indicate that GGE is substantially more heritable than focal epilepsy, and modern molecular methods have confirmed this. The SNP-heritability of GGE, the proportion of risk attributable to common genetic variants, is estimated at roughly three times that of focal epilepsy. Specific subtypes like juvenile myoclonic epilepsy and childhood absence epilepsy show even higher estimates.
Where rare and common variants converge
The review, led by Dr. Olav B. Smeland at the Centre for Precision Psychiatry in Oslo, traces how both rare and common genetic variants contribute to epilepsy risk. On the rare-variant side, copy number variant studies involving 13,420 epilepsy cases found increased burden across all common epilepsy types, with a deletion at the 15q13.3 locus carrying an odds ratio of 36.04 for GGE. Whole-exome sequencing has identified protein-truncating variants in genes encoding the GATOR1 complex, a regulator of the mTORC1 pathway, as contributors to focal epilepsy risk.
On the common-variant side, the largest genome-wide association study (GWAS) to date, with 29,944 cases and 52,538 controls, identified 26 genome-wide significant loci. The distribution was strikingly lopsided: 22 loci came from just 7,407 GGE cases, while focal epilepsy, with more than twice as many cases, produced none.
The most integrative insight from the review is that genes appearing in rare-variant analyses, such as SCN1A, SCN8A, DEPDC5, and NPRL3, also show up in common-variant studies. The same biological pathways, particularly those involving ion channel function and excitatory-inhibitory balance, carry traffic from both directions.
The power projection that changes the calculus
One of the review's most striking analytical contributions involves GWAS power projections. At current sample sizes for GGE (effective N of about 23,000), only around 1.5 percent of SNP-heritability is captured by genome-wide significant variants. By comparison, a stroke GWAS with an effective sample size ten times larger explains a similar fraction. The math is clear: if GGE GWAS were scaled to comparable sizes, roughly 50 percent of common genetic variance could be captured.
This favorable ratio of heritability to polygenicity means that GGE may be one of the most cost-efficient targets in complex disease genomics. The investment required is smaller than for most other brain disorders, and the yield would be disproportionately large.
Epilepsy genes overlap with psychiatric disorder genes
The review documents extensive genetic overlap between epilepsy and major psychiatric conditions. Using the bivariate MiXeR model, the authors show that most variants associated with GGE are also associated with schizophrenia, major depression, bipolar disorder, and anxiety. This overlap is substantial even when traditional genetic correlations appear modest, because many shared variants push risk in opposite directions for different conditions, masking the overlap in standard analyses.
The genome-wide genetic correlation between focal and generalized epilepsy is 0.61, confirming substantial shared biology. Both epilepsy types show negative correlations with cognitive ability, consistent with the cognitive impairment frequently observed in patients.
What remains out of reach
The review is candid about limitations. Over 92 percent of cases in the largest epilepsy GWAS are of European ancestry, severely limiting the generalizability of findings and polygenic risk scores to other populations. Polygenic risk scores currently show a hazard ratio of 1.73 per standard deviation for GGE, comparable to cardiology applications but insufficient for population screening.
Focal epilepsy remains poorly characterized genetically. Current sample sizes provide insufficient statistical power for reliable analysis, leaving a major portion of the epilepsy spectrum unmapped. The broad phenotypic categories used in GWAS, grouping all focal epilepsies or all generalized epilepsies together, may obscure subtype-specific signals that could be clinically relevant.
Routine genetic testing for common epilepsies remains premature. Unlike severe early-onset epilepsies where identifying a specific mutation can guide treatment, the polygenic architecture of common epilepsies does not yet support clinical decision-making at the individual level.
The opportunity, the authors argue, is to scale up. Larger, more ancestrally diverse GWAS, integrated with clinical data, neuroimaging, and longitudinal health records, could bring the genetics of common epilepsy from academic interest to practical tool. The genetic signal is there. The question is whether the epilepsy research community will mobilize the coordinated effort to capture it.