Single germline or somatic activating mutations of mammalian target of rapamycin (mTOR) pathway genes are emerging as a major cause of type II focal cortical dysplasia (FCD), hemimegalencephaly (HME) and tuberous sclerosis complex (TSC). A double-hit mechanism, based on a primary germline mutation in one allele and a secondary somatic hit affecting the other allele of the same gene in a small number of cells, has been documented in some patients with TSC or FCD. In a patient with HME, severe intellectual disability, intractable seizures and hypochromic skin patches, we identified the ribosomal protein S6 (RPS6) p.R232H variant, present as somatic mosaicism at ~15.1% in dysplastic brain tissue and ~11% in blood, and the MTOR p.S2215F variant, detected as ~8.8% mosaicism in brain tissue, but not in blood. Overexpressing the two variants independently in animal models, we demonstrated that MTOR p.S2215F caused neuronal migration delay and cytomegaly, while RPS6 p.R232H prompted increased cell proliferation. Double mutants exhibited a more severe phenotype, with increased proliferation and migration defects at embryonic stage and, at postnatal stage, cytomegalic cells exhibiting eccentric nuclei and binucleation, which are typical features of balloon cells. These findings suggest a synergistic effect of the two variants. This study indicates that, in addition to single activating mutations and double-hit inactivating mutations in mTOR pathway genes, severe forms of cortical dysplasia can also result from activating mutations affecting different genes in this pathway. RPS6 is a potential novel disease-related gene.
By converting physical forces into electrical signals or triggering intracellular cascades, stretch-activated ion channels (SACs) allow the cell to respond to osmotic and mechanical stress. Knowledge of the pathophysiological mechanisms underlying associations of SACs with human disease is limited. Here we describe 16 unrelated patients, with severe early onset developmental and epileptic encephalopathy (DEE), intellectual disability, and severe motor and cortical visual impairment, associated with progressive neurodegenerative brain changes carrying ten distinct de novo variants of TMEM63B. Variants were missense, including the recurrent V44M in 7/16 patients, or in-frame, and affected highly conserved residues located in transmembrane regions of the protein. In 12 patients, haematological abnormalities co-occurred, such as macrocytosis and haemolysis, requiring blood transfusions in some. We modelled V44M, R443H, and T481N in transfected Neuro2a cells and demonstrated leak inward cation currents across the mutated channel even in isotonic conditions, while the response to hypo-osmotic challenge was impaired, as were the Ca2+ transients generated under hypo-osmotic stimulation. Ectopic expression of the V44M and G580C variants in Drosophila resulted in early death. TMEM63B-associated DEE represents a novel clinicopathological entity in which altered cation conductivity results in a severe neurological phenotype with progressive brain damage and early onset epilepsy, associated with haematological abnormalities in most patients.
Epilepsy is a heterogenous group of disorders defined by recurrent seizure activity due to abnormal synchronized activity of neurons. A growing number of epilepsy cases are believed to be caused by genetic factors and copy number variants (CNV) contribute to up to 5% of epilepsy cases. However, CNVs in epilepsy are usually large deletions or duplications involving multiple neurodevelopmental genes. In patients who underwent seizure focus resection for treatment-resistant epilepsy, whole genome DNA methylation profiling identified 3 main clusters of which one showed strong association with receptor tyrosine kinase (RTK) genes. We identified focal copy number gains involving epidermal growth factor receptor (EGFR) and PDGFRA loci. The dysplastic neurons of cases with amplifications showed marked overexpression of EGFR and PDGFRA, while glial and endothelial cells were negative. Targeted sequencing of regulatory regions and DNA methylation analysis revealed that only enhancer regions of EGFR and gene promoter of PDGFRA were amplified, while coding regions did not show copy number abnormalities or somatic mutations. Somatic focal copy number gains of noncoding regulatory represent a previously unrecognized genetic driver in epilepsy and a mechanism of abnormal activation of RTK genes. Upregulated RTKs provide a potential avenue for therapy in seizure disorders.
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