Abstracts

Rationale: Focal cortical dysplasia (FCD) Type IIb is a cortical malformation characterized by cortical architectural abnormalities, dysmorphic neurons, and balloon cells. It has been suggested that FCDs are caused by somatic mutations in cells in the developing brain. Here, we explore the possible involvement of somatic mutations in FCD Type IIb.Methods: We collected a total of 24 blood-brain paired samples with FCD, including 13 individuals with FCD Type IIb, five with Type IIa, and six with Type I. We performed whole exome sequencing using paired samples from nine of the FCD Type IIb subjects. Somatic MTOR mutations were identified and further investigated using all 24 paired samples by deep sequencing of the entire gene’s coding region. Somatic MTOR mutations were confirmed by droplet digital PCR. The effect of MTOR mutations on mTOR kinase signaling was evaluated by immunohistochemistry and western blot analyses of brain samples and by in vitro transfection experiments.Results: We identified four lesion-specific somatic MTOR mutations in six of 13 (46%) individuals with FCD Type IIb showing mutant allele rates of 1.11–9.31%. Functional analyses showed that phosphorylation of ribosomal protein S6 in FCD Type IIb brain tissues with MTOR mutations was clearly elevated compared with control samples. Transfection of any of the four MTOR mutants into HEK293T cells led to elevated phosphorylation of 4EBP, the direct target of mTOR kinase.Conclusions: We found low-prevalence somatic mutations in MTOR in FCD Type IIb, indicating that activating somatic mutations in MTOR cause FCD Type IIb.