Hartmann Wolfgang, Digon-Söntgerath Boris, Koch Arend, Waha Anke, Endl Elmar, Dani Indra, Denkhaus Dorota, Goodyer Cynthia G, Sörensen Niels, Wiestler Otmar D, Pietsch Torsten
Department of Neuropathology, University of Bonn Medical Center, Bonn, Germany.
Clin Cancer Res. 2006 May 15;12(10):3019-27. doi: 10.1158/1078-0432.CCR-05-2187.
Medulloblastomas represent the most frequent malignant brain tumors of childhood. They are supposed to originate from cerebellar neural precursor cells. Recently, it has been shown that Sonic Hedgehog-induced formation of medulloblastoma in an animal model is significantly enhanced by activation of the phosphatidylinositol 3'-kinase (PI3K) signaling pathway.
To examine a role for PI3K/AKT signaling in the molecular pathogenesis of human medulloblastoma, we did an immunohistochemical study of the expression of Ser473-phosphorylated (p)-AKT protein in 22 medulloblastoma samples: All samples displayed p-AKT expression. To investigate if an activated PI3K/AKT pathway is required for medulloblastoma cell growth, we treated five human medulloblastoma cell lines with increasing concentrations of the PI3K inhibitor LY294002 and analyzed cellular proliferation and apoptosis. The antiproliferative effect could be antagonized by overexpressing constitutively active AKT. As the activation of PI3K/AKT signaling may be associated with alterations of the PTEN gene located at 10q23.3, a chromosomal region subject to frequent allelic losses in medulloblastoma, we screened PTEN for mutations and mRNA expression.
Proliferation of all of the medulloblastoma cell lines was dependent on PI3K/AKT signaling, whereas apoptosis was not prominently affected. Allelic loss was detected in 16% of the cases. One medulloblastoma cell line was found to carry a truncating mutation in the PTEN coding sequence. Even more important, PTEN mRNA and protein levels were found to be significantly lower in medulloblastomas compared with normal cerebellar tissue of different developmental stages. Reduction of PTEN expression was found to be associated with PTEN promoter hypermethylation in 50% of the tumor samples.
We conclude that activation of the PI3K/AKT pathway constitutes an important step in the molecular pathogenesis of medulloblastoma and that dysregulation of PTEN may play a significant role in this context.
髓母细胞瘤是儿童期最常见的恶性脑肿瘤。它们被认为起源于小脑神经前体细胞。最近研究表明，在动物模型中，音猬因子诱导的髓母细胞瘤形成通过磷脂酰肌醇3'-激酶（PI3K）信号通路的激活而显著增强。
为了研究PI3K/AKT信号在人类髓母细胞瘤分子发病机制中的作用，我们对22例髓母细胞瘤样本中Ser473磷酸化（p）-AKT蛋白的表达进行了免疫组织化学研究：所有样本均显示p-AKT表达。为了研究激活的PI3K/AKT通路是否是髓母细胞瘤细胞生长所必需的，我们用浓度递增的PI3K抑制剂LY294002处理了5种人类髓母细胞瘤细胞系，并分析了细胞增殖和凋亡情况。组成型活性AKT的过表达可拮抗抗增殖作用。由于PI3K/AKT信号的激活可能与位于10q23.3的PTEN基因改变有关，10q23.3是髓母细胞瘤中经常发生等位基因缺失的染色体区域，我们筛查了PTEN的突变和mRNA表达。
所有髓母细胞瘤细胞系的增殖均依赖于PI3K/AKT信号，而凋亡未受到显著影响。16%的病例检测到等位基因缺失。发现一个髓母细胞瘤细胞系在PTEN编码序列中存在截短突变。更重要的是，与不同发育阶段的正常小脑组织相比，髓母细胞瘤中PTEN mRNA和蛋白水平显著降低。在50%的肿瘤样本中发现PTEN表达降低与PTEN启动子高甲基化有关。
我们得出结论，PI3K/AKT通路的激活是髓母细胞瘤分子发病机制中的一个重要步骤，并且PTEN的失调可能在这种情况下起重要作用。
