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

TitleAssociation of MTOR Mutations With Developmental Brain Disorders, Including Megalencephaly, Focal Cortical Dysplasia, and Pigmentary Mosaicism.
Publication TypeJournal Article
Year of Publication2016
AuthorsMirzaa, GM, Campbell, CD, Solovieff, N, Goold, CP, Jansen, LA, Menon, S, Timms, AE, Conti, V, Biag, JD, Olds, C, Boyle, EAugust, Collins, S, Ishak, G, Poliachik, SL, Girisha, KM, San Yeung, K-, Chung, BHon Yin, Rahikkala, E, Gunter, SA, McDaniel, SS, Macmurdo, CForsyth, Bernstein, JA, Martin, B, Leary, RJ, Mahan, S, Liu, S, Weaver, M, Dorschner, MO, Jhangiani, S, Muzny, DM, Boerwinkle, E, Gibbs, RA, Lupski, JR, Shendure, J, Saneto, RP, Novotny, EJ, Wilson, CJ, Sellers, WR, Morrissey, MP, Hevner, RF, Ojemann, JG, Guerrini, R, Murphy, LO, Winckler, W, Dobyns, WB
JournalJAMA Neurol
Volume73
Issue7
Pagination836-45
Date Published2016 Jul 01
ISSN2168-6157
Abstract

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.

DOI10.1001/jamaneurol.2016.0363
Alternate JournalJAMA Neurol
PubMed ID27159400
PubMed Central IDPMC4979321
Grant ListU54 HD083091 / HD / NICHD NIH HHS / United States
R01 NS092772 / NS / NINDS NIH HHS / United States
U54 HG006542 / HG / NHGRI NIH HHS / United States
UM1 HG006542 / HG / NHGRI NIH HHS / United States
K02 NS072162 / NS / NINDS NIH HHS / United States
K08 NS092898 / NS / NINDS NIH HHS / United States
R01 NS058721 / NS / NINDS NIH HHS / United States