Paediatric Neurology, Neurogenetics and Neurobiology Unit and Laboratories, Children's Hospital A. Meyer, University of Florence, Florence 50139, Italy.
INMED, Aix-Marseille University, INSERM UMR1249, Marseille 13009, France.
Hum Mol Genet. 2019 Nov 15;28(22):3755-3765. doi: 10.1093/hmg/ddz194.
Single germline or somatic activating mutations of mammalian target of rapamycin (mTOR) pathway genes are emerging as a major cause of type II focal cortical dysplasia (FCD), hemimegalencephaly (HME) and tuberous sclerosis complex (TSC). 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.
哺乳动物雷帕霉素靶蛋白（mTOR）通路基因的单种系或种系突变，正在成为 II 型局灶性皮质发育不良（FCD）、偏侧巨脑畸形（HME）和结节性硬化症（TSC）的主要病因。在少数患者中，基于同一基因的一个等位基因的原发性种系突变和另一个等位基因的二次种系突变，已经证实了 TSC 或 FCD 存在“双打击”机制。在一名患有 HME、严重智力残疾、难治性癫痫和低色素皮肤斑块的患者中，我们鉴定出核糖体蛋白 S6（RPS6）p.R232H 变异，在病变脑组织中以约 15.1%的种系嵌合性存在，在血液中以约 11%的种系嵌合性存在，以及 MTOR p.S2215F 变异，在脑组织中检测到约 8.8%的嵌合性，但在血液中未检测到。在动物模型中独立过表达两种变异体，我们证明 MTOR p.S2215F 导致神经元迁移延迟和巨细胞症，而 RPS6 p.R232H 促使细胞增殖增加。双突变体表现出更严重的表型，在胚胎期表现出增殖和迁移缺陷增加，在出生后阶段，巨细胞表现出偏心核和双核化，这是气球细胞的典型特征。这些发现表明两种变异体存在协同作用。本研究表明，除了 mTOR 通路基因的单种系激活突变和双打击失活突变外，严重形式的皮质发育不良也可能由影响该通路中不同基因的激活突变引起。RPS6 是一个潜在的新的疾病相关基因。
