Dysregulation of NEUROG2 plays a key role in focal cortical dysplasia

Abstract

ObjectiveFocal cortical dysplasias (FCDs) are an important cause of drug‐resistant epilepsy. In this work, we aimed to investigate whether abnormal gene regulation, mediated by microRNA, could be involved in FCD type II.MethodsWe used total RNA from the brain tissue of 16 patients with FCD type II and 28 controls. MicroRNA expression was initially assessed by microarray. Quantitative polymerase chain reaction, in situ hybridization, luciferase reporter assays, and deep sequencing for genes in the mTOR pathway were performed to validate and further explore our initial study.Resultshsa‐let‐7f (p = 0.039), hsa‐miR‐31 (p = 0.0078), and hsa‐miR34a (p = 0.021) were downregulated in FCD type II, whereas a transcription factor involved in neuronal and glial fate specification, NEUROG2 (p < 0.05), was upregulated. We also found that the RND2 gene, a NEUROG2‐target, is upregulated (p < 0.001). In vitro experiments showed that hsa‐miR‐34a downregulates NEUROG2 by binding to its 5′‐untranslated region. Moreover, we observed strong nuclear expression of NEUROG2 in balloon cells and dysmorphic neurons and found that 28.5% of our patients presented brain somatic mutations in genes of the mTOR pathway.InterpretationOur findings suggest a new molecular mechanism, in which NEUROG2 has a pivotal and central role in the pathogenesis of FCD type II. In this way, we found that the downregulation of hsa‐miR‐34a leads to upregulation of NEUROG2, and consequently to overexpression of the RND2 gene. These findings indicate that a faulty coupling in neuronal differentiation and migration mechanisms may explain the presence of aberrant cells and complete dyslamination in FCD type II. Ann Neurol 2018;83:623–635

Genomic and Epigenetic Advances in Focal Cortical Dysplasia Types I and II: A Scoping Review

“…Cell-type-specific analysis using laser microdissection of DNs and BCs detected a PTEN somatic missense variant, not present in the adjacent normal brain tissue of the same patient, and associated with substantial expression of phospho-Akt (Schick et al, 2006). Variants of unknown significance in the AKT1 gene were detected in FCD type IIa and IIb tissues (Avansini et al, 2018;Kobow et al, 2019), Baldassari et al (2019b) reported two patients with FCD IIa with a somatic missense mutation (p.Glu17Lys) in AKT3, with VAF of 1.1-2.3%.…”

“…Loss-of-function mosaic mutations in negative regulators of mTOR kinase, as TSC1 and TSC2, induce mTOR hyperactivation by disturbing the function of the TSC1-TSC2 complex (Lim et al, 2017;Baldassari et al, 2019b). The missense, nonsense, and frameshift indel variants detected in TSC1 and TSC2 genes were present in both FCD type IIa and IIb specimens, with VAF ranging from 1.0 to 6.7% (D'Gama et al, 2017;Lim et al, 2017;Avansini et al, 2018;Baldassari et al, 2019b;Sim et al, 2019;Zhang et al, 2020). Contrary to the germline variants, somatic TSC mutations were associated with isolated FCD without the typical systemic tuberous sclerosis complex clinical manifestations (D'Gama et al, 2017;Lim et al, 2017;Baldassari et al, 2019b).…”

“…Brain-only somatic MTOR mutations accounted for 3.8-57.1% of patients with FCD II across different studies and represented the most commonly affected gene (Leventer et al, 2015;Lim et al, 2015;Nakashima et al, 2015;Mirzaa et al, 2016;Moller et al, 2016;D'Gama et al, 2017;Avansini et al, 2018;Baldassari et al, 2019b;Sim et al, 2019;Zhang et al, 2020). The variant allelic frequencies (VAF) detected in the brain-derived DNA were low, ranging from 0.25 to 15.67% for FCD IIa and from 0.74 to 10.6% for FCD IIb (Leventer et al, 2015;Lim et al, 2015;Nakashima et al, 2015;Mirzaa et al, 2016;Moller et al, 2016;D'Gama et al, 2017;Avansini et al, 2018;Baldassari et al, 2019b;Sim et al, 2019;Zhang et al, 2020). All MTOR somatic variants were missense, leading to a substitution of an amino acid in the protein.…”

“…Moreover, some FCD subtypes (Ia, IIa, and IIb) can be distinguished based on DNA methylation profiles, highlighting the possibility of disease-specific signatures to be used as biomarkers (Kobow et al, 2019). Dysregulation of microRNAs, a class of small non-coding single-stranded RNA molecules that act as posttranscriptional regulators of gene expression, may contribute to a failure in the neuronal differentiation and migration process (Wang et al, 2016;Che et al, 2017;Avansini et al, 2018).…”

Toward a refined genotype–phenotype classification scheme for the international consensus classification of Focal Cortical Dysplasia

“…Several studies using in vitro electrophysiological recording in surgical specimens from patients with FCD Type 2B have shown that BC are essentially electrically silent, whereas neurons display hyper‐excitable intrinsic membrane properties (37, 38). There is also a substantial amount of data demonstrating the persistence of immature features (39), suggesting, in particular, a deregulation of inhibitory synaptic transmission involving altered γ‐aminobutyric acid (GABA)‐A‐mediated currents (40). Interestingly, enrichment of somatic variants of the mTOR pathway has been shown in both DN and BC using advanced laser microdissection or single‐cell DNA sequencing techniques (4, 41).…”