Mislocalization of syntaxin‐1 and impaired neurite growth observed in a human iPSC model for STXBP 1 ‐related epileptic encephalopathy

Syntaxin‐binding protein 1 ( STXBP 1) is essential for synaptic vesicle exocytosis. Mutations of its encoding gene, STXBP 1, are among the most frequent genetic causes of epileptic encephalopathies. However, the precise pathophysiology of STXBP 1 haploinsufficiency has not been elucidated. Using patient‐derived induced pluripotent stem cells ( iPSC s), we aimed to establish a neuronal model for STXBP 1 haploinsufficiency and determine the pathophysiologic basis for STXBP 1 encephalopathy. We generated iPSC lines from a patient with Ohtahara syndrome ( OS ) harboring a heterozygous nonsense mutation of STXBP 1 (c.1099C>T; p.R367X) and performed neuronal differentiation. Both STXBP 1 messenger RNA ( mRNA ) and STXBP 1 protein expression levels of OS ‐derived neurons were approximately 50% lower than that of control‐derived neurons, suggesting that OS ‐derived neurons are a suitable model for elucidating the pathophysiology of STXBP 1 haploinsufficiency. Through Western blot and immunocytochemistry assays, we found that OS ‐derived neurons show reduced levels and mislocalization of syntaxin‐1, a component of soluble N ‐ethylmaleimide‐sensitive factor attachment receptor ( SNARE ) proteins. In addition, OS ‐derived neurons have impaired neurite outgrowth. In conclusion, this model enables us to investigate the neurobiology of STXBP 1 encephalopathy throughout the stages of neurodevelopment. Reduced expression of STXBP 1 leads to changes in the expression and localization of syntaxin‐1 that may contribute to the devastating phenotype of STXBP 1 encephalopathy.