Mosaicism and Hemimegalencephaly

“somatic mutations in several genes (PIK3CA, AKT3, and mTOR) cause enlargement of just one hemisphere of the brain, a malformation called hemimegalencephaly that is highly associated with epilepsy. These mutations may or may not be found in the blood, the convenient source tissue for DNA analysis, thus presenting a challenge to disease gene identification. Remarkably, patients can show dysfunction of essentially an entire half of their cerebral cortex when only 8 to 35% of the brain cells carry the mutation, suggesting that a minority of cells with a somatic mutation can disrupt the function of widespread cortical circuits. ” http://science.sciencemag.org/content/341/6141/1237758