Somatic mutations involving TSC 1 and TSC2 genes in two children with focal cortical dysplasia

Experimental models of human cortical malformations: from mammals to 'acortical' zebrafish

Human neocortex controls and integrates cognition, emotions, perception and complex behaviors. Aberrant cortical development can be triggered by multiple genetic and environmental factors, causing cortical malformations. Animal models, especially rodents, are a valuable tool to probe molecular and physiological mechanisms of cortical malformations. Complementing rodent studies, the zebrafish (Danio rerio) is an important model organism in biomedicine. Although the zebrafish (like other fishes) lacks neocortex, here we argue that this species can still be used to model various aspects and brain phenomena related to human cortical malformations. We also discuss novel perspectives in this field, covering both advantages and limitations of using mammalian and zebrafish models in cortical malformation research. Summarizing mounting evidence, we also highlight the importance of translationally-relevant insights into the pathogenesis of cortical malformations from animal models, and discuss future strategies of research in the field.

mTOR pathway: Insights into an established pathway for brain mosaicism in epilepsy

The mechanistic target of rapamycin (mTOR) signaling pathway is an essential regulator of numerous cellular activities such as metabolism, growth, proliferation, and survival. The mTOR cascade recently emerged as a critical player in the pathogenesis of focal epilepsies and cortical malformations. The 'mTORopathies' comprise a spectrum of cortical malformations that range from whole brain (megalencephaly) and hemispheric (hemimegalencephaly) abnormalities to focal abnormalities, such as focal cortical dysplasia type II (FCDII), which manifest with drug-resistant epilepsies. The spectrum of cortical dysplasia results from somatic brain mutations in the mTOR pathway activators AKT3, MTOR, PIK3CA, and RHEB and from germline and somatic mutations in mTOR pathway repressors, DEPDC5, NPRL2, NPRL3, TSC1 and TSC2. The mTORopathies are characterized by excessive mTOR pathway activation, leading to a broad range of structural and functional impairments. Here, we provide a comprehensive literature review of somatic mTOR-activating mutations linked to epilepsy and cortical malformations in 292 patients and discuss the perspectives of targeted therapeutics for personalized medicine.

Identification of genetic characteristics in pediatric epilepsy with focal cortical dysplasia type 2 using deep whole-exome sequencing

BACKGROUND

Focal cortical dysplasia type 2 (FCD2) is a malformation of cortical development that constitutes a common cause of pediatric focal epilepsy. Germline or somatic variants in the mammalian target of rapamycin (mTOR) signaling pathway genes are the pathogenesis of FCD2.

OBJECTIVE

In this study, whole-exome deep sequencing was performed on dysplastic cortex from focal epilepsy in children to explore genetic characteristics in FCD2.

METHODS

Resected core lesions of FCD2 were confirmed by pathology, and peripheral blood was collected from 11 patients. Deep whole-exome sequencing (>500X) was performed on derived genomic DNA, germline, or somatic variants in brain-specific genes were analyzed and identified.

RESULTS

In 11 patients, a heterozygous likely pathogenic germline variant of DEPDC5 was identified in one case, while somatic variants were found in four brain samples. The frequencies of the somatic variant allele were 2.52%-5.12%. Somatic variants in AKT3, TSC2, and MTOR (mTOR signaling pathway genes) were found in three samples. Besides, one somatic variant was detected in MED12 which has not been reported to associate with FCD2.

CONCLUSION

Our study expanded the variant spectrum in the mTOR-GATOR pathway, and also detected a somatic variant in MED12 which was potentially associated with FCD 2.

CfDNA Measurement as a Diagnostic Tool for the Detection of Brain Somatic Mutations in Refractory Epilepsy

Epilepsy is a neurological disorder that affects more than 50 million people. Its etiology is unknown in approximately 60% of cases, although the existence of a genetic factor is estimated in about 75% of these individuals. Hundreds of genes involved in epilepsy are known, and their number is increasing progressively, especially with next-generation sequencing techniques. However, there are still many cases in which the results of these molecular studies do not fully explain the phenotype of the patients. Somatic mutations specific to brain tissue could contribute to the phenotypic spectrum of epilepsy. Undetectable in the genomic DNA of blood cells, these alterations can be identified in cell-free DNA (cfDNA). We aim to review the current literature regarding the detection of somatic variants in cfDNA to diagnose refractory epilepsy, highlighting novel research directions and suggesting further studies.