A brain somatic RHEB doublet mutation causes focal cortical dysplasia type II

Abstract

Focal cortical dysplasia type II (FCDII) is a cerebral cortex malformation characterized by local cortical structure disorganization, neuronal dysmorphology, and refractory epilepsy. Brain somatic mutations in several genes involved in the PI3K/AKT/mTOR pathway are associated with FCDII, but they are only found in a proportion of patients with FCDII. The genetic causes underlying the development FCDII in other patients remain unclear. Here, we carried out whole exome sequencing and targeted sequencing in paired brain–blood DNA from patients with FCDII and identified a brain somatic doublet mutation c.(A104T, C105A) in the Ras homolog, mTORC1 binding (RHEB) gene, which led to the RHEB p.Y35L mutation in one patient with FCDII. This RHEB mutation carrier had a dramatic increase of ribosomal protein S6 phosphorylation, indicating mTOR activation in the region of the brain lesion. The RHEB p.Y35L mutant protein had increased GTPλS-binding activity compared with wild-type RHEB. Overexpression of the RHEB p.Y35L variant in cultured cells also resulted in elevated S6 phosphorylation compared to wild-type RHEB. Importantly, in utero electroporation of the RHEB p.Y35L variant in mice induced S6 phosphorylation, cytomegalic neurons, dysregulated neuron migration, abnormal electroencephalogram, and seizures, all of which are found in patients with FCDII. Rapamycin treatment rescued abnormal electroencephalograms and alleviated seizures in these mice. These results demonstrate that brain somatic mutations in RHEB are also responsible for the pathogenesis of FCDII, indicating that aberrant activation of mTOR signaling is a primary driver and potential drug target for FCDII.

Convergent and Divergent Mechanisms of Epileptogenesis in mTORopathies

“…In the context of epilepsy, spontaneous seizures were reported in all studies that evaluated this phenotype. Postnatal rapamycin treatment, starting before or after seizure onset, sufficiently suppresses seizures in multiple models (Lim et al, 2015(Lim et al, , 2017Hsieh et al, 2016;Zhao et al, 2019;Onori et al, 2020). Interestingly, these studies consistently reported that mTORC1induced seizures occur independently of neuronal placement, and preventing this alteration did not prevent seizures.…”

RAB39B Deficiency Impairs Learning and Memory Partially Through Compromising Autophagy

“…Upregulation of autophagy has been found to reverse disease-like phenotypes in animal models of FCD, PD, AD, etc. ( Lee et al, 2013 ; Frake et al, 2015 ; Zhao S. et al, 2019 ). Herein, we also found that rapamycin treatment improved novel object recognition memory and rescued LTP deficits in Rab39b KO mice, suggesting that rapamycin may also alleviate symptoms in XLID patients caused by RAB39B mutations.…”

The Neurodevelopmental Pathogenesis of Tuberous Sclerosis Complex (TSC)

“…Incidentally, the role of signals that regulate hamartin and tuberin activity as well as the mTORC1 substrates responsible for specific developmental events are only now being determined. Perhaps most striking is the fact that somatic mutations that cause mTORC1 pathway activation have been identified as a cause of a plethora of neurological diseases (Lee et al, 2012 ; Poduri et al, 2012 ; Parker et al, 2013 ; Lal et al, 2014 ; Baek et al, 2015 ; Baulac et al, 2015 ; Crino, 2015 ; D’Gama et al, 2015 , 2017 ; Leventer et al, 2015 ; Lim et al, 2015 ; Baulac, 2016 ; Korenke et al, 2016 ; Møller et al, 2016 ; Hanai et al, 2017 ; Park et al, 2018 ; Iffland and Crino, 2019 ; Kim et al, 2019 ; Pelorosso et al, 2019 ; Salinas et al, 2019 ; Zhao et al, 2019 ; Garcia et al, 2020 ). Thus, what has been learned by studying the TSC pathway may now be applied to an expanding number of patients.…”