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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.<\/jats:p>","DOI":"10.1093\/hmg\/ddz194","type":"journal-article","created":{"date-parts":[[2019,8,13]],"date-time":"2019-08-13T15:10:03Z","timestamp":1565709003000},"page":"3755-3765","source":"Crossref","is-referenced-by-count":68,"title":["Somatic double-hit in MTOR and RPS6 in hemimegalencephaly with intractable epilepsy"],"prefix":"10.1093","volume":"28","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3610-5819","authenticated-orcid":false,"given":"Cristiana","family":"Pelorosso","sequence":"first","affiliation":[{"name":"Paediatric Neurology, Neurogenetics and Neurobiology Unit and Laboratories, Children\u2019s Hospital A. Meyer, University of Florence, Florence 50139, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fran\u00e7oise","family":"Watrin","sequence":"additional","affiliation":[{"name":"INMED, Aix-Marseille University, INSERM UMR1249, Marseille 13009, France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Valerio","family":"Conti","sequence":"additional","affiliation":[{"name":"Paediatric Neurology, Neurogenetics and Neurobiology Unit and Laboratories, Children\u2019s Hospital A. Meyer, University of Florence, Florence 50139, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Emmanuelle","family":"Buhler","sequence":"additional","affiliation":[{"name":"INMED, Aix-Marseille University, INSERM UMR1249, Marseille 13009, France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Antoinette","family":"Gelot","sequence":"additional","affiliation":[{"name":"Service d'Anatomie Pathologique, H\u00f4pital Trousseau, H\u00f4pitaux Universitaires de l'Est Parisien, Universit\u00e9 Pierre et Marie Curie, Paris 75012, France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0219-0023","authenticated-orcid":false,"given":"Xiaoxu","family":"Yang","sequence":"additional","affiliation":[{"name":"Department of Neuroscience, Howard Hughes Medical Institute, Rady Children\u2019s Institute of Genomic Medicine, University of California, San Diego, La Jolla, CA 92037, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Davide","family":"Mei","sequence":"additional","affiliation":[{"name":"Paediatric Neurology, Neurogenetics and Neurobiology Unit and Laboratories, Children\u2019s Hospital A. Meyer, University of Florence, Florence 50139, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jennifer","family":"McEvoy-Venneri","sequence":"additional","affiliation":[{"name":"Department of Neuroscience, Howard Hughes Medical Institute, Rady Children\u2019s Institute of Genomic Medicine, University of California, San Diego, La Jolla, CA 92037, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jean-Bernard","family":"Manent","sequence":"additional","affiliation":[{"name":"INMED, Aix-Marseille University, INSERM UMR1249, Marseille 13009, France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Valentina","family":"Cetica","sequence":"additional","affiliation":[{"name":"Paediatric Neurology, Neurogenetics and Neurobiology Unit and Laboratories, Children\u2019s Hospital A. Meyer, University of Florence, Florence 50139, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Laurel L","family":"Ball","sequence":"additional","affiliation":[{"name":"Department of Neuroscience, Howard Hughes Medical Institute, Rady Children\u2019s Institute of Genomic Medicine, University of California, San Diego, La Jolla, CA 92037, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Anna Maria","family":"Buccoliero","sequence":"additional","affiliation":[{"name":"Pathology Unit, Children\u2019s Hospital A. Meyer, University of Florence, Florence 50139, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Antonin","family":"Vinck","sequence":"additional","affiliation":[{"name":"INMED, Aix-Marseille University, INSERM UMR1249, Marseille 13009, France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Carmen","family":"Barba","sequence":"additional","affiliation":[{"name":"Paediatric Neurology, Neurogenetics and Neurobiology Unit and Laboratories, Children\u2019s Hospital A. Meyer, University of Florence, Florence 50139, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Joseph G","family":"Gleeson","sequence":"additional","affiliation":[{"name":"Department of Neuroscience, Howard Hughes Medical Institute, Rady Children\u2019s Institute of Genomic Medicine, University of California, San Diego, La Jolla, CA 92037, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Renzo","family":"Guerrini","sequence":"additional","affiliation":[{"name":"Paediatric Neurology, Neurogenetics and Neurobiology Unit and Laboratories, Children\u2019s Hospital A. 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