<p>Epilepsies are the results of abnormal brain hyperactivities caused by brain injury, drug intoxication, and genetic perturbations. In the group of genetic-related epilepsies, the ion channel mutations contribute 25% of total epilepsy cases. Many studies suggest some forms of severe epilepsies can start early in patients’ lives, with epilepsy starting during infancy and childhood. With the wide adoption of genomic sequencing in children having seizures, an increasing number of <em>SCN2A</em> genetic variants have been revealed as genetic causes of epilepsy. Voltage-gated sodium channel Nav1.2, encoded by gene SCN2A, is predominantly expressed in the pyramidal excitatory neurons and supports action potential (AP) firing. One recurrent SCN2A genetic variant is L1342P, which was identified in multiple patients with epileptic encephalopathy and intractable seizures. However, the mechanism underlying L1342P-mediated seizures and the pharmacogenetics of this variant in human neurons remain unknown. To probe the potential hypothesized biophysical property changes, we used a heterologous expression system expressing the Nav1.2-L1342P. We observed prominent but quite complex gating kinetics without significant changes in window current. To understand the core phenotypes of the L1342P variant in human neurons, we took advantage of a reference human-induced pluripotent stem cell (hiPSC) line from a male donor, in which L1342P was introduced by CRISPR/Cas9-mediated genome editing. Using patch-clamping and microelectrode array (MEA) recordings, we revealed that cortical neurons derived from hiPSCs carrying heterozygous L1342P variant have significantly increased intrinsic excitability, higher sodium current density, and enhanced bursting and synchronous network firing, suggesting hyperexcitability phenotypes. Interestingly, L1342P neuronal culture displayed a degree of resistance to the anticonvulsant medication phenytoin, which recapitulated aspects of clinical observation of patients carrying the L1342P variant. In contrast, phrixotoxin-3 (PTx3), a compound showing greater specificity on Nav1.2 over other sodium channel subtypes, can potently alleviate spontaneous and chemically induced hyperexcitability of neurons carrying the L1342P variant. Our results reveal a possible pathogenic underpinning of Nav1.2-L1342P mediated epileptic seizures and demonstrate the utility of genome-edited hiPSCs as an in vitro platform to advance personalized phenotyping and drug discovery.</p>