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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 GTP?S-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.

SUBMITTER: Zhao S

PROVIDER: S-EPMC6802736 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature

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

Zhao Shanshan S Li Zhenghui Z Zhang Muxian M Zhang Lingliang L Zheng Honghua H Ning Jinhuan J Wang Yanyan Y Wang Fengpeng F Zhang Xiaobin X Gan Hexia H Wang Yuanqing Y Zhang Xian X Luo Hong H Bu Guojun G Xu Huaxi H Yao Yi Y Zhang Yun-Wu YW

Experimental & molecular medicine 20190723 7

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 sequ ...[more]

PMID: 31337748

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Project description:ObjectiveThe advent of next-generation sequencing (NGS) enabled the detection of low-level brain somatic variants in postsurgical tissue of focal cortical dysplasia (FCD). The genetic background of FCD Type I remains elusive, while the mammalian target of rapamycin (mTOR) pathway seems to have a relevant role in the pathogenesis of FCD Type II. Our goal was to uncover information on the molecular basis of FCD, performing whole genome sequencing (WGS) in postsurgical tissue to detect candidate brain-specific somatic variants, and evaluate their clinical significance.DesignWGS was performed using paired peripheral venous blood and postsurgical pathological brain deoxyribonucleic acid (DNA) samples. Libraries were prepared using the Roche KAPA HyperPrep polymerase chain reaction (PCR) free library preparation kit. Paired-end 150bp reads were generated on the Illumina NovaSeq platform. The FASTQ files were processed using the nf-core sarek pipeline (version 3.0) to call somatic variants, which were then annotated with ANNOVAR. A screening strategy was applied to obtain relevant variants.ResultsTwo female patients with drug-resistant epilepsy due to FCD who underwent surgical treatment were included. Regarding neuropathological diagnosis, one patient had FCD Type Ia and the other had FCD Type IIa. Five somatic nonsynonymous single nucleotide variants (SNVs) were detected using WGS, three in FCD Ia tissue (WDR24 p.Trp259Gly; MICAL1 p.Lys1036Arg; and KATNB1 p.Leu566Ile) and two in FCD IIa tissue (MATN4 p.Phe91Val and ANKRD6 p.His386Gln). All variants were predicted to be potentially pathogenic by at least two different tools. However, they were classified as variants of uncertain significance (VUS) according to the American College of Medical Genetics and Genomics (ACMG) criteria.ConclusionBrain-specific somatic missense variants were identified by NGS in new candidate genes (WDR24, MICAL1, KATNB1, MATN4, and ANKRD6) using postsurgical FCD tissue, which may contribute to further understanding of the genetic background of FCD. All the reported genes were previously related to epilepsy and/or malformations of central nervous system (CNS) and cortical development. However, the pathogenicity assessment of these variants and, consequently, their impact on clinical practice still poses an important challenge.

Project description:ObjectiveFavorable seizure outcome is reported following resection of bottom-of-sulcus dysplasia (BOSD). We assessed the distribution of epileptogenicity and dysplasia in and around BOSD to better understand this clinical outcome and the optimal surgical approach.MethodsWe studied 27 children and adolescents with magnetic resonance imaging (MRI)-positive BOSD who underwent epilepsy surgery; 85% became seizure-free postresection (median = 5.0 years follow-up). All patients had resection of the dysplastic sulcus, and 11 had additional resection of the gyral crown (GC) or adjacent gyri (AG). Markers of epileptogenicity were relative cortical hypometabolism on preoperative 18 F-fluorodeoxyglucose (FDG) positron emission tomography (PET), and spiking, ripples, fast ripples, spike-high-frequency oscillation cross-rate, and phase amplitude coupling (PAC) on preresection and postresection electrocorticography (ECoG), all analyzed at the bottom-of-sulcus (BOS), top-of-sulcus (TOS), GC, and AG. Markers of dysplasia were increased cortical thickness on preoperative MRI, and dysmorphic neuron density and variant allele frequency of somatic MTOR mutations in resected tissue, analyzed at similar locations.ResultsRelative cortical metabolism was significantly reduced and ECoG markers were significantly increased at the BOS compared to other regions. Apart from spiking and PAC, which were greater at the TOS compared to the GC, there were no significant differences in PET and other ECoG markers between the TOS, GC, and AG, suggesting a cutoff of epileptogenicity at the TOS rather than a tapering gradient on the cortical surface. MRI and tissue markers of dysplasia were all maximal in the BOS, reduced in the TOS, and mostly absent in the GC. Spiking and PAC reduced significantly over the GC after resection of the dysplastic sulcus.SignificanceThese findings support the concept that dysplasia and intrinsic epileptogenicity are mostly limited to the dysplastic sulcus in BOSD and support resection or ablation confined to the MRI-visible lesion as a first-line surgical approach. 18 F-FDG PET and ECoG abnormalities in surrounding cortex seem to be secondary phenomena.

Project description:Focal cortical dysplasia (FCD) type II is a highly epileptogenic developmental malformation and a common cause of surgically treated drug-resistant epilepsy. While clinical observations suggest frequent occurrence in the frontal lobe, mechanisms for such propensity remain unexplored. Here, we hypothesized that cortex-wide spatial associations of FCD distribution with cortical cytoarchitecture, gene expression and organizational axes may offer complementary insights into processes that predispose given cortical regions to harbour FCD. We mapped the cortex-wide MRI distribution of FCDs in 337 patients collected from 13 sites worldwide. We then determined its associations with (i) cytoarchitectural features using histological atlases by Von Economo and Koskinas and BigBrain; (ii) whole-brain gene expression and spatiotemporal dynamics from prenatal to adulthood stages using the Allen Human Brain Atlas and PsychENCODE BrainSpan; and (iii) macroscale developmental axes of cortical organization. FCD lesions were preferentially located in the prefrontal and fronto-limbic cortices typified by low neuron density, large soma and thick grey matter. Transcriptomic associations with FCD distribution uncovered a prenatal component related to neuroglial proliferation and differentiation, likely accounting for the dysplastic makeup, and a postnatal component related to synaptogenesis and circuit organization, possibly contributing to circuit-level hyperexcitability. FCD distribution showed a strong association with the anterior region of the antero-posterior axis derived from heritability analysis of interregional structural covariance of cortical thickness, but not with structural and functional hierarchical axes. Reliability of all results was confirmed through resampling techniques. Multimodal associations with cytoarchitecture, gene expression and axes of cortical organization indicate that prenatal neurogenesis and postnatal synaptogenesis may be key points of developmental vulnerability of the frontal lobe to FCD. Concordant with a causal role of atypical neuroglial proliferation and growth, our results indicate that FCD-vulnerable cortices display properties indicative of earlier termination of neurogenesis and initiation of cell growth. They also suggest a potential contribution of aberrant postnatal synaptogenesis and circuit development to FCD epileptogenicity.

Dataset Information

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

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

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