Contribution of rare genetic variants to drug response in absence epilepsy.

Objective: We investigated the possible significance of rare genetic variants to response to valproic acid (VPA) and ethosuximide (ETX) in patients with absence epilepsy. Our primary hypothesis was that rare CACNA1H variants are more frequent in ETX-non-responsive patients compared to ETX-responsive. Our secondary hypothesis was that rare variants in GABA-receptor genes are more frequent in VPA-non-responsive patients compared to VPA-responsive.

Methods: We recruited patients with absence epilepsy treated with both VPA and ETX, and performed whole exome sequencing in order to investigate the potential role of rare variants in CACNA1H, other voltage-gated calcium channel (VGCC) genes, or GABA-receptor genes in predicting response to ETX or VPA.

Results: Sixty-two patients were included; 12 were ETX-responsive, 14 VPA-responsive, and 36 did not have a clear positive response to either medication. We did not find significant enrichment inCACNA1H rare variants in ETX-responsive patients (odds ratio 3.43; 0.43-27.65; p = 0.20), nor was there enrichment for other VGCC genes. No significant enrichment of GABA-receptor gene rare variants was seen for VPA-non-responsive patients versus VPA-responsive. We found enrichment of rare GABA-receptor variants in our absence cohort compared to controls (odds ratio 3.82; 1.68-8.69). There was no difference in frequency of CACNA1H rs61734410 and CACNA1I rs3747178 polymorphisms between ETX-responsive and ETX-non-responsive groups; these polymorphisms have previously been reported to predict lack of response to ETX in absence epilepsy.

Significance: We conclude that if CACNA1H rare variants predict lack of response to ETX, a larger sample is necessary to test this with sufficient power. Increased GABA-receptor gene rare variant frequency in absence epilepsy patients who fail initial anti-seizure therapy suggests subtle GABA receptor dysfunction may contribute to the underlying pathophysiology.