Whole genome sequencing identifies putative associations between genomic polymorphisms and clinical response to the antiepileptic drug levetiracetam

In the context of pharmacogenomics, whole genome sequencing provides a powerful approach for identifying correlations between response variability to specific drugs and genomic polymorphisms in a population, in an unbiased manner. In this study, we employed whole genome sequencing of DNA samples from patients showing extreme response (n=72) and non-response (n=27) to the antiepileptic drug levetiracetam, in order to identify genomic variants that underlie response to the drug. Although no common SNP (MAF>5%) crossed the conventional genome-wide significance threshold of 5x10-8, we found common polymorphisms in genes SPNS3, HDC, MDGA2, NSG1 and RASGEF1C, which collectively predict clinical response to levetiracetam in our cohort with [~]91% predictive accuracy ([~]94% positive predictive value, [~]85% negative predictive value). Among these genes, HDC, NSG1, MDGA2 and RASGEF1C are potentially implicated in synaptic neurotransmission, while SPNS3 is an atypical solute carrier transporter homologous to SV2A, the known molecular target of levetiracetam. Furthermore, we performed gene- and pathway-based statistical analysis on sets of rare and low-frequency variants (MAF<5%) and we identified associations between genes or pathways and response to levetiracetam. Our findings include a) the genes PRKCB and DLG2, which are involved in glutamatergic neurotransmission, a known target of anticonvulsants, including levetiracetam; b) the genes FILIP1 and SEMA6D, which are involved in axon guidance and modelling of neural connections; and c) pathways with a role in synaptic neurotransmission, such as WNT5A-dependent internalization of FZD4 and disinhibition of SNARE formation. Targeted analysis of genes involved in neurotransmitter release and transport further supports the possibility of association between drug response and genes NSG1 and DLG2. In summary, our approach to utilise whole genome sequencing on subjects with extreme response phenotypes is a feasible route to generate plausible hypotheses for investigating the genetic factors underlying drug response variability in cases of pharmaco-resistant epilepsy. AUTHOR SUMMARYLevetiracetam (LEV) is a prominent antiepileptic drug prescribed for the treatment of both focal and generalised epilepsy. The molecular mechanism mediating its action is not well understood, but it involves the modulation of synaptic neurotransmition through binding to the synaptic vesicle glycoprotein SV2A. Identifying genomic polymorphisms that predict response to the drug is important, because it can help clinicians prescribe the most appropriate treatment in a patient-specific manner. In this study, we employed whole genome sequencing (WGS) of DNA samples from extreme responders or non-responders to LEV and we identified a small group of common variants, which successfully predict response to the drug in our cohort. These variants are mostly located in genes implicated in synaptic function. Furthermore, we identified significant associations between clinical response to LEV and low-frequency variants in genes and pathways involved in excitatory neurotransmission or in the moulding of neural networks in the brain. Our approach to utilise WGS on subjects with extreme response phenotypes is a feasible route to generate plausible hypotheses on the genomic basis of pharmaco-resistant epilepsy. We expect that the rapidly decreasing cost of WGS will allow conducting similar studies on a larger scale in the near future.