Hidden in inhibitory neurons: cell-type genetics reveals KCNIP2 as a candidate epilepsy drug target

For roughly one in three people with epilepsy, existing antiseizure medications simply do not stop the seizures. The drugs on the market act on a narrow set of neuronal targets — ion channels and inhibitory signalling — leaving a pressing need for mechanistically distinct therapeutic options. Now, a study published in Frontiers in Genetics offers a striking illustration of why previous genetic searches for new drug targets may have been looking in the wrong place, or rather, at the wrong resolution. The research, led by Aytac Gul of Hatay Mustafa Kemal University, demonstrates that whether a druggable epilepsy gene is detectable can hinge entirely on the cell type in which its expression is measured.

The study tackled a persistent problem in epilepsy drug discovery. Human genetics is one of the most effective routes to identifying new drug targets — targets with genetic support show roughly a two-fold higher probability of progressing to approval — but previous epilepsy target-discovery efforts have relied on expression measured in blood plasma or in bulk brain tissue, where signals from millions of different cells are averaged together. These approaches have yielded inconsistent candidate lists, with different studies nominating entirely different genes. The new work set out to test whether this instability has a structural explanation: that a gene detectable in one cell type is simply missed when expression is measured in another.

To do this, Gul took the genes prioritised at the genome-wide significant loci of the largest epilepsy meta-analysis, conducted by the ILAE Consortium on Complex Epilepsies, and screened 23 of them using a two-step genetic design. First, drug-target Mendelian randomisation (MR) was used to ask whether genetically predicted expression of each gene was associated with epilepsy risk. MR exploits the random allocation of alleles at conception to estimate causal effects largely free from the confounding and reverse causation that plague observational studies. When the gene encodes a druggable protein, the MR estimate further predicts the effect of modulating that target with a drug. But MR alone can be misled: two nearby variants inherited together can produce a spurious association even when one controls the gene and a different one affects disease. So the second step applied Bayesian colocalisation, testing whether the expression signal and the disease signal at each locus were driven by a single shared causal variant. A gene was retained only when both steps agreed.

The crucial innovation was the resolution at which expression was measured. Rather than relying on bulk tissue, the study drew on SingleBrain, a single-nucleus RNA-sequencing brain eQTL meta-analysis of four European-ancestry cohorts, providing expression quantitative trait loci (eQTL) for inhibitory neurons, excitatory neurons, and four glial cell types. Bulk cortex eQTL came from an independent resource, MetaBrain. Each gene was assessed across all seven panels, allowing a direct comparison of colocalisation posteriors across resolutions while holding the epilepsy outcome and colocalisation window fixed.

Of the 23 genes screened, MR nominated ten as having a significant effect on epilepsy risk. Colocalisation retained only two. The first, KCNIP2, produced the study’s most striking result. Its expression in inhibitory neurons colocalised with genetic generalised epilepsy at a posterior probability of 0.95 — well above the 0.8 threshold — and higher expression was associated with lower risk, at an odds ratio of 0.64 (p = 2.5 × 10−11). In bulk cortex, however, the same gene carried a strong expression signal that mapped to a completely different causal variant, with the posterior for a shared variant falling to zero. The bulk-cortex eQTL lead variant was in negligible linkage disequilibrium with the inhibitory-neuron lead (r² = 0.016), and the inhibitory-neuron lead carried no detectable effect on KCNIP2 expression in bulk tissue. The two resolutions were measuring different regulatory events, not one diluting the other.

This pattern offers a mechanism for the weak and inconsistent results of earlier analyses. Bulk tissue does not merely average away a cell-type-restricted signal — at this locus, it replaces the neuronal signal with a separate variant that does not colocalise with epilepsy at all. A plasma or bulk-tissue screen would discard KCNIP2 entirely. The finding was formally tested rather than inferred: the full glial summary statistics showed that KCNIP2 carries a genuine cis-eQTL in oligodendrocytes, with a directionally concordant but sub-threshold colocalisation posterior of 0.53, while three other glial panels carried no eQTL reaching their discovery thresholds. Cochran’s Q across the seven panels confirmed significant heterogeneity carried by inhibitory neurons against the rest.

The second retained gene, AP3D1, behaved differently. It colocalised with genetic generalised epilepsy across inhibitory neurons, excitatory neurons, and bulk cortex alike (posteriors 0.94, 0.94, and 0.92), with higher expression associated with increased risk (OR 1.46). Its pattern was set by the phenotype rather than by resolution — for all epilepsy, it did not colocalise in any panel. Both genes were then tested in FinnGen, an independent Finnish biobank. The directions of effect reproduced at the phenotype-matched generalised-epilepsy endpoint, with KCNIP2 protective (OR 0.51) and AP3D1 risk-increasing (OR 1.67). Colocalisation itself did not reproduce in FinnGen, which the study attributes to the smaller Finnish case count and different registry ascertainment rather than a contradiction of the discovery result.

The biology of KCNIP2 lends the genetic finding particular weight. The gene encodes KChIP2, an EF-hand calcium-sensing accessory subunit of the Kv4 A-type potassium channel. KChIP2 raises the A-type potassium current, and its deletion reduces that current, increases neuronal excitability, and raises kindling-induced seizure susceptibility in mice. The protective direction estimated by MR — higher expression, lower risk — is exactly what this channel biology predicts. Human genetics implicates the same channel from the pore-forming side as well: a de novo variant in KCND2, which encodes the Kv4.2 subunit that KChIP2 modulates, alters A-type-current inactivation and causes intractable seizures. The Kv4-KChIP2 complex is a structurally resolved target, making positive modulation of the A-type current the candidate on-target therapeutic direction. Yet the study is careful to note that KChIP2 also regulates the equivalent potassium current in cardiac muscle, so a potentiating strategy carries an on-target cardiac liability, and the closely related subunit family complicates selective targeting.

The study is equally candid about its limits. KCNIP2 cannot be separated from its neighbour CUEDC2 on genetic evidence alone: the two share a single inhibitory-neuron regulatory variant, with their strongest eQTL variants in near-complete linkage disequilibrium (r² = 0.975), and conditioning on the shared variant removed the epilepsy association for both. KCNIP2 is nominated on the strength of its genome-wide significant eQTL, its larger expression effect, and the established channel biology — not because CUEDC2 has been excluded. Every statement about KCNIP2, the study notes, should be read as a statement about the KCNIP2/CUEDC2 locus. AP3D1, meanwhile, encodes a subunit of the intracellular AP-3 vesicle-trafficking adaptor, a protein class with no catalytic ligand pocket and therefore low small-molecule tractability, leaving it a supported epilepsy gene but a weak drug-target candidate.

The demonstration that cell-type resolution determines detectability rests mainly on this single gene, and colocalisation has power only at genome-wide significant loci, so the discovery space was necessarily small. No functional work was performed, and the causal role of the two genes in seizure liability remains to be established. Still, the proof of principle is clear: at this locus, single-cell resolution is what made the signal detectable. Cell-type-restricted candidates have now been reported in ischaemic stroke and migraine as well, suggesting the approach may generalise. For a field where a third of patients lack effective treatment, the message is that the search for new epilepsy drug targets may need to look not just at the right genes, but at the right cells.

Subject of Research: Genetics

Article Title: Cell-type-resolved colocalisation prioritises KCNIP2 as a candidate epilepsy drug-target detectable at inhibitory-neuron but not bulk-tissue resolution: a Mendelian randomisation and colocalisation study

Article References: Gul, A. (2026). Cell-type-resolved colocalisation prioritises KCNIP2 as a candidate epilepsy drug-target detectable at inhibitory-neuron but not bulk-tissue resolution: a Mendelian randomisation and colocalisation study. Frontiers in Genetics, 17, Article 1920945. https://doi.org/10.3389/fgene.2026.1920945

Image Credits: AI Generated

DOI: 10.3389/fgene.2026.1920945

Keywords: epilepsy, KCNIP2, Mendelian randomisation, colocalisation, inhibitory neurons, drug targets, genetics, eQTL, Kv4 potassium channel, CUEDC2, AP3D1, FinnGen