Somatic mosaicism in focal epilepsy. Recent findings highlighted the role of somatic parental mosaicism in epileptic encephalopathies. However, somatic mosaicism has also emerged over the last few years as a prominent mechanism in the pathogenesis of lesional focal epilepsies, including focal cortical dysplasia (FCD) type 2 and hemimegalencephaly. Previous studies have identified the role of mosaicism of genes such as MTOR, TSC1/TSC2, and genes encoding components of the PI3K/AKT pathway in patients with epilepsy secondary to brain malformations. A recent study in Annals of Neurology has identified a new unrelated genetic cause of refractory non-lesional focal epilepsy, which leads us to wonder what role mosaicism may be playing in focal epilepsies without obvious findings on MRI.
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