Original Investigation
July 2016

Association of MTOR Mutations With Developmental Brain Disorders, Including Megalencephaly, Focal Cortical Dysplasia, and Pigmentary Mosaicism

Author Affiliations
  • 1Division of Genetic Medicine, Department of Pediatrics, University of Washington, Seattle
  • 2Center for Integrative Brain Research, Seattle Children’s Research Institute, Seattle, Washington
  • 3Novartis Institutes for BioMedical Research Inc, Cambridge, Massachusetts
  • 4Department of Neurology, University of Virginia, Charlottesville
  • 5Center for Developmental Biology and Regenerative Medicine, Seattle Children's Research Institute, Seattle, Washington
  • 6Paediatric Neurology, Neurogenetics, and Neurobiology Unit and Laboratories, A. Meyer Children’s Hospital, Florence, Italy
  • 7Department of Neuroscience, Pharmacology and Child Health, University of Florence, Florence, Italy
  • 8Department of Genetics, Stanford University School of Medicine, Stanford, California
  • 9Department of Radiology, Seattle Children's Hospital, Seattle, Washington
  • 10Department of Medical Genetics, Kasturba Medical College, Manipal University, Manipal, Karnataka, India
  • 11Department of Pediatrics and Adolescent Medicine, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong, China
  • 12PEDEGO Research Group and Medical Research Center Oulu, University of Oulu, Oulu, Finland
  • 13Department of Clinical Genetics, Oulu University Hospital, Oulu, Finland
  • 14Pediatric Neurology and Epilepsy, Kaiser Permanente San Francisco Medical Center, San Francisco, California
  • 15Division of Medical Genetics, Department of Pediatrics, Stanford University School of Medicine, Stanford, California
  • 16Department of Genome Sciences, University of Washington, Seattle
  • 17Department of Pathology, University of Washington, Seattle
  • 18Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas
  • 19Human Genome Sequencing Center, Baylor College of Medicine, Houston, Texas
  • 20Human Genetics Center, University of Texas Health Science Center at Houston, Houston
  • 21Department of Pediatrics, Baylor College of Medicine, Houston, Texas
  • 22Department of Pediatrics, Texas Children’s Hospital, Houston
  • 23Division of Neurology, Pediatrics, and Radiology, University of Washington, Seattle
  • 24Center for Developmental Therapeutics, Seattle Children's Research Institute, Seattle, Washington
  • 25Editas Medicine, Cambridge, Massachusetts
  • 26Department of Neurological Surgery, University of Washington, Seattle
  • 27IRCCS Stella Maris Foundation, Pisa, Italy

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JAMA Neurol. 2016;73(7):836-845. doi:10.1001/jamaneurol.2016.0363

Importance  Focal cortical dysplasia (FCD), hemimegalencephaly, and megalencephaly constitute a spectrum of malformations of cortical development with shared neuropathologic features. These disorders are associated with significant childhood morbidity and mortality.

Objective  To identify the underlying molecular cause of FCD, hemimegalencephaly, and diffuse megalencephaly.

Design, Setting, and Participants  Patients with FCD, hemimegalencephaly, or megalencephaly (mean age, 11.7 years; range, 2-32 years) were recruited from Pediatric Hospital A. Meyer, the University of Hong Kong, and Seattle Children’s Research Institute from June 2012 to June 2014. Whole-exome sequencing (WES) was performed on 8 children with FCD or hemimegalencephaly using standard-depth (50-60X) sequencing in peripheral samples (blood, saliva, or skin) from the affected child and their parents and deep (150-180X) sequencing in affected brain tissue. Targeted sequencing and WES were used to screen 93 children with molecularly unexplained diffuse or focal brain overgrowth. Histopathologic and functional assays of phosphatidylinositol 3-kinase–AKT (serine/threonine kinase)–mammalian target of rapamycin (mTOR) pathway activity in resected brain tissue and cultured neurons were performed to validate mutations.

Main Outcomes and Measures  Whole-exome sequencing and targeted sequencing identified variants associated with this spectrum of developmental brain disorders.

Results  Low-level mosaic mutations of MTOR were identified in brain tissue in 4 children with FCD type 2a with alternative allele fractions ranging from 0.012 to 0.086. Intermediate-level mosaic mutation of MTOR (p.Thr1977Ile) was also identified in 3 unrelated children with diffuse megalencephaly and pigmentary mosaicism in skin. Finally, a constitutional de novo mutation of MTOR (p.Glu1799Lys) was identified in 3 unrelated children with diffuse megalencephaly and intellectual disability. Molecular and functional analysis in 2 children with FCD2a from whom multiple affected brain tissue samples were available revealed a mutation gradient with an epicenter in the most epileptogenic area. When expressed in cultured neurons, all MTOR mutations identified here drive constitutive activation of mTOR complex 1 and enlarged neuronal size.

Conclusions and Relevance  In this study, mutations of MTOR were associated with a spectrum of brain overgrowth phenotypes extending from FCD type 2a to diffuse megalencephaly, distinguished by different mutations and levels of mosaicism. These mutations may be sufficient to cause cellular hypertrophy in cultured neurons and may provide a demonstration of the pattern of mosaicism in brain and substantiate the link between mosaic mutations of MTOR and pigmentary mosaicism in skin.