Alcantara Diana, Timms Andrew E, Gripp Karen, Baker Laura, Park Kaylee, Collins Sarah, Cheng Chi, Stewart Fiona, Mehta Sarju G, Saggar Anand, Sztriha László, Zombor Melinda, Caluseriu Oana, Mesterman Ronit, Van Allen Margot I, Jacquinet Adeline, Ygberg Sofia, Bernstein Jonathan A, Wenger Aaron M, Guturu Harendra, Bejerano Gill, Gomez-Ospina Natalia, Lehman Anna, Alfei Enrico, Pantaleoni Chiara, Conti Valerio, Guerrini Renzo, Moog Ute, Graham John M, Hevner Robert, Dobyns William B, O'Driscoll Mark, Mirzaa Ghayda M
Genome Damage and Stability Centre, University of Sussex, Sussex, UK.
Center for Developmental Biology and Regenerative Medicine, Seattle Children's Research Institute, Seattle, WA, USA.
Brain. 2017 Oct 1;140(10):2610-2622. doi: 10.1093/brain/awx203.
Mutations of genes within the phosphatidylinositol-3-kinase (PI3K)-AKT-MTOR pathway are well known causes of brain overgrowth (megalencephaly) as well as segmental cortical dysplasia (such as hemimegalencephaly, focal cortical dysplasia and polymicrogyria). Mutations of the AKT3 gene have been reported in a few individuals with brain malformations, to date. Therefore, our understanding regarding the clinical and molecular spectrum associated with mutations of this critical gene is limited, with no clear genotype-phenotype correlations. We sought to further delineate this spectrum, study levels of mosaicism and identify genotype-phenotype correlations of AKT3-related disorders. We performed targeted sequencing of AKT3 on individuals with these phenotypes by molecular inversion probes and/or Sanger sequencing to determine the type and level of mosaicism of mutations. We analysed all clinical and brain imaging data of mutation-positive individuals including neuropathological analysis in one instance. We performed ex vivo kinase assays on AKT3 engineered with the patient mutations and examined the phospholipid binding profile of pleckstrin homology domain localizing mutations. We identified 14 new individuals with AKT3 mutations with several phenotypes dependent on the type of mutation and level of mosaicism. Our comprehensive clinical characterization, and review of all previously published patients, broadly segregates individuals with AKT3 mutations into two groups: patients with highly asymmetric cortical dysplasia caused by the common p.E17K mutation, and patients with constitutional AKT3 mutations exhibiting more variable phenotypes including bilateral cortical malformations, polymicrogyria, periventricular nodular heterotopia and diffuse megalencephaly without cortical dysplasia. All mutations increased kinase activity, and pleckstrin homology domain mutants exhibited enhanced phospholipid binding. Overall, our study shows that activating mutations of the critical AKT3 gene are associated with a wide spectrum of brain involvement ranging from focal or segmental brain malformations (such as hemimegalencephaly and polymicrogyria) predominantly due to mosaic AKT3 mutations, to diffuse bilateral cortical malformations, megalencephaly and heterotopia due to constitutional AKT3 mutations. We also provide the first detailed neuropathological examination of a child with extreme megalencephaly due to a constitutional AKT3 mutation. This child has one of the largest documented paediatric brain sizes, to our knowledge. Finally, our data show that constitutional AKT3 mutations are associated with megalencephaly, with or without autism, similar to PTEN-related disorders. Recognition of this broad clinical and molecular spectrum of AKT3 mutations is important for providing early diagnosis and appropriate management of affected individuals, and will facilitate targeted design of future human clinical trials using PI3K-AKT pathway inhibitors.
磷脂酰肌醇-3-激酶（PI3K）-AKT-雷帕霉素靶蛋白（mTOR）信号通路中的基因突变是脑过度生长（巨头畸形）以及节段性皮质发育异常（如半侧巨脑症、局灶性皮质发育异常和多小脑回畸形）的常见病因。迄今为止，已报道少数患有脑畸形的个体存在AKT3基因突变。因此，我们对与这一关键基因突变相关的临床和分子谱系的了解有限，尚无明确的基因型-表型相关性。我们试图进一步描绘这一谱系，研究嵌合水平，并确定AKT3相关疾病的基因型-表型相关性。我们通过分子倒置探针和/或桑格测序对具有这些表型的个体进行AKT3靶向测序，以确定突变的类型和嵌合水平。我们分析了突变阳性个体的所有临床和脑成像数据，其中一例进行了神经病理学分析。我们对携带患者突变的AKT3进行了体外激酶分析，并检测了定位在pleckstrin同源结构域的突变的磷脂结合谱。我们鉴定出14名携带AKT3突变的新个体，其表型因突变类型和嵌合水平而异。我们全面的临床特征分析以及对所有先前发表患者的回顾，大致将携带AKT3突变的个体分为两组：由常见的p.E17K突变导致高度不对称皮质发育异常的患者，以及携带AKT3胚系突变且表现出更多可变表型的患者，包括双侧皮质畸形、多小脑回畸形、室管膜下结节性异位和无皮质发育异常的弥漫性巨头畸形。所有突变均增加了激酶活性，pleckstrin同源结构域突变体表现出增强的磷脂结合。总体而言，我们的研究表明，关键的AKT3基因激活突变与广泛的脑受累相关，范围从主要由AKT3嵌合突变引起的局灶性或节段性脑畸形（如半侧巨脑症和多小脑回畸形）到由AKT3胚系突变引起的弥漫性双侧皮质畸形、巨头畸形和异位。我们还首次对一名因AKT3胚系突变导致极度巨头畸形的儿童进行了详细的神经病理学检查。据我们所知，这个孩子是有记录的儿科最大脑尺寸之一。最后，我们的数据表明，AKT3胚系突变与巨头畸形相关，无论是否伴有自闭症，这与PTEN相关疾病类似。认识到AKT3突变的这种广泛临床和分子谱系对于为受影响个体提供早期诊断和适当管理很重要，并且将有助于使用PI3K-AKT通路抑制剂进行未来人类临床试验的靶向设计。
