Schindewolf, C.; Wei, A. D.; Kalume, F.; Torbett, B. E.

The SCN1A gene encodes NaV1.1, a voltage-gated sodium channel protein that is necessary for neuronal excitability and whose loss-of-function mutations cause Dravet syndrome, a treatment-resistant childhood onset epilepsy. Gene replacement strategies for this syndrome are challenged by the large size of SCN1A and difficulty achieving stable cellular expression. Lentiviral vectors (LVVs) offer sufficient packaging capacity and genomic integration for defective SCN1A gene replacement. Here, we evaluated LVV-mediated delivery of different engineered SCN1A transgene sequences in human cells. LVV-transduced cells expressed full-length NaV1.1 protein that trafficked to the membrane and produced functional sodium currents. However, SCN1A transgene expression declined over time despite stable vector copy number, indicating post-integration regulatory limitations. Expression efficiency varied by SCN1A transgene sequence, with a codon-optimized variant showing higher expression despite lower LVV copy number. Treatment with sodium butyrate, a histone deacetylase inhibitor, significantly enhanced SCN1A transgene expression and partially rescued expression decay in a sequence-dependent manner. Incorporation of a ubiquitous chromatin opening element (UCOE) upstream of the promoter to maintain expression resulted in a trend of increased expression and increased responsiveness to butyrate. These findings demonstrate that sequence-specific and epigenetic factors may influence expression of large transgenes following lentiviral delivery, highlighting key challenges and design considerations for therapeutic SCN1A transgene expression.