AUTHOR=Yang Ping , Broadbent Robert , Prasad Chitra , Levin Simon , Goobie Sharan , Knoll Joan H. , Prasad Asuri N. 

TITLE=De novo STXBP1 Mutations in Two Patients With Developmental Delay With or Without Epileptic Seizures

JOURNAL=Frontiers in Neurology

VOLUME=Volume 12 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/neurology/articles/10.3389/fneur.2021.804078

DOI=10.3389/fneur.2021.804078

ISSN=1664-2295

ABSTRACT=Objectives: Mutations in the STXBP1 gene have been associated with epileptic encephalopathy. Previous studies from in vitro neuroblastoma 2A cells showed that haploinsufficiency of STXBP1 is the mechanism for epileptic encephalopathy. In this ex vivo study, STXPB1 DNA mutations and RNA expressions were assessed from two patients for helping understanding the impact of STXBP1 mutations on the disease etiology and mechanism.
Methods: Microarray and DNA sequencing were performed on two children with development delay with or without infantile spasms, respectively. Pathogenic mutations of STXBP1 were identified. Then STXBP1 expression assays by RT-Q-PCR on the RNA extracted from the patients’ blood samples were performed. 
Results: Pathogenic deletion [of exons 13 to 20 and 3’ downstream of STXBP1] and nonsense mutation [c.1663G>T (p.Glu555X) in exon 18 of STXBP1] were detected from the two patients, respectively. RT-Q-PCR analysis of the RNAs showed that 1) the deletion mediated RNA decay, and 2) no RNA decay was identified in the nonsense mutation at codon 555 which predicted production of truncated STXBP1 protein. 
Significance: Our RNA assays from the patient blood samples are the first ex vivo assay to support that both haploinsufficiency and truncation of STXBP1 protein (presumed dominant negative) are causative mechanisms for epileptic encephalopathies, intellectual disability and developmental delay. The RNA assay also indicates that escape from nonsense-mediated RNA decay is possible where the nonsense mutation resides <50 nucleotides upstream of the last coding exon-exon junction even presence of additional non-coding exons at the 3’ down stream of the coding exon.