PT - JOURNAL ARTICLE AU - Guerrini, Renzo AU - Cavallin, Mara AU - Pippucci, Tommaso AU - Rosati, Anna AU - Bisulli, Francesca AU - Dimartino, Paola AU - Barba, Carmen AU - Garbelli, Rita AU - Buccoliero, Anna Maria AU - Tassi, Laura AU - Conti, Valerio TI - Is Focal Cortical Dysplasia/Epilepsy Caused by Somatic <em>MTOR</em> Mutations Always a Unilateral Disorder? AID - 10.1212/NXG.0000000000000540 DP - 2021 Feb 01 TA - Neurology Genetics PG - e540 VI - 7 IP - 1 4099 - http://ng.neurology.org/content/7/1/e540.short 4100 - http://ng.neurology.org/content/7/1/e540.full SO - Neurol Genet2021 Feb 01; 7 AB - Objective To alert about the wide margin of unpredictability that distribution of somatic MTOR mosaicism may have in the brain and the risk for independent epileptogenesis arising from the seemingly healthy contralateral hemisphere after complete removal of epileptogenic focal cortical dysplasia (FCD).Methods Clinical, EEG, MRI, histopathology, and molecular genetics in 2 patients (1 and 2) treated with focal resections and subsequent complete hemispherectomy for epileptogenic FCD due to somatic MTOR mutations. Autoptic brain study of bilateral asymmetric hemispheric dysplasia and identification of alternative allele fraction (AAF) rates for AKT1 (patient 3).Results The strongly hyperactivating p.Ser2215Phe (patient 1) and p.Leu1460Pro (patient 2) MTOR mutations were at low-level AAF in the dysplastic tissue. After repeated resections and eventual complete hemispherectomy, both patients manifested intractable seizures arising from the contralateral, seemingly healthy hemisphere. In patient 3, the p.Glu17Lys AKT1 mutation exhibited random distribution and AAF rates in different tissues with double levels in the more severely dysplastic cerebral hemisphere.Conclusions Our understanding of the distribution of somatic mutations in the brain in relation to the type of malformation and its hypothesized time of origin may be faulty. Large studies may reveal that the risk of a first surgery being disappointing might be related more to the specific somatic mammalian target of rapamycin mutation identified than to completeness of resection and that the advantages of repeated resections after a first unsuccessful operation should be weighed against the risk of the contralateral hemisphere becoming in turn epileptogenic.AAF=alternative allele fraction; AED=antiepileptic drug; DMEG=dysplastic megalencephaly; FCDII=focal cortical dysplasia type II; HME=hemimegalencephaly; MCD=malformations of cortical development; MEG=megalencephaly; mTOR=mammalian target of rapamycin; OFC=occipital frontal circumference (head circumference); smMIP=single-molecule molecular inversion probe; WES=whole-exome sequencing