PRC2 Heterogeneity Drives Tumor Growth in Medulloblastoma [mouse]

Intratumor epigenetic heterogeneity is emerging as a key mechanism underlying tumor evolution and drug resistance. Epigenetic abnormalities frequently occur in medulloblastoma, the most common childhood malignant brain tumor. Medulloblastoma is classified into four subtypes including SHH medulloblastoma, which is characterized by elevated SHH signaling and a cerebellum granule neuron precursor (CGNP) cell-of-origin. Here we report that the histone H3K27 methyltransferase polycomb repressor complex 2 (PRC2) is often heterogeneous within individual SHH medulloblastoma tumors. In mouse models, complete deletion of the PRC2 core subunit EED inhibited medulloblastoma growth, while a mosaic deletion of EED significantly enhanced tumor growth. EED is intrinsically required for CGNP maintenance by inhibiting both neural differentiation and cell death. Complete deletion of EED led to CGNP depletion and reduced occurrence of medulloblastoma. Surprisingly, medulloblastomas with mosaic EED levels grew faster than control wildtype tumors and expressed increased levels of oncogenes such as Igf2, which is directly repressed by PRC2 and has been demonstrated to be both necessary and sufficient for SHH medulloblastoma progression. IGF2 mediated the oncogenic effects of PRC2 heterogeneity in tumor growth. Assessing clones of a human medulloblastoma cell line with different EED levels confirmed that EEDlow cells can stimulate the growth of EEDhigh cells through paracrine IGF2 signaling. Thus, PRC2 heterogeneity plays an oncogenic role in medulloblastoma through both intrinsic growth competence and non-cell autonomous mechanisms in distinct tumor subclones. Total RNAs were extracted from mouse medulloblastoma samples with different Eed genotypes . Gene expression profiles were examined uisng RNA-seq.

肿瘤内表观遗传异质性正逐渐成为肿瘤演化与耐药性产生的核心机制。表观遗传异常在髓母细胞瘤中十分常见，而髓母细胞瘤是儿童群体中最高发的恶性脑肿瘤。髓母细胞瘤可分为四种亚型，其中音猬因子 (Sonic Hedgehog, SHH) 型髓母细胞瘤以SHH信号通路激活及小脑颗粒神经元前体细胞 (cerebellum granule neuron precursor, CGNP) 为细胞起源特征。本研究发现，组蛋白H3K27甲基转移酶多梳抑制复合体2 (polycomb repressor complex 2, PRC2) 在单个SHH型髓母细胞瘤中常存在异质性。在小鼠模型中，完全敲除PRC2的核心亚基EED可抑制髓母细胞瘤的生长，而嵌合敲除EED则显著促进肿瘤生长。EED对于CGNP的维持具有内在必要性：其可同时抑制神经分化与细胞死亡。完全敲除EED会导致CGNP耗竭，并降低髓母细胞瘤的发生率。令人意外的是，EED表达水平呈嵌合状态的髓母细胞瘤，其生长速度快于野生型对照肿瘤，且癌基因（如胰岛素样生长因子2 (insulin-like growth factor 2, Igf2)）的表达水平上调；Igf2可被PRC2直接抑制，且已被证实对于SHH型髓母细胞瘤的进展既是必需的，也是充分的。IGF2介导了PRC2异质性在肿瘤生长中的致癌作用。对不同EED表达水平的人髓母细胞瘤细胞系克隆进行检测后证实，EED低表达（EEDlow）细胞可通过旁分泌IGF2信号通路，促进EED高表达（EEDhigh）细胞的生长。综上，PRC2异质性通过两种机制在髓母细胞瘤中发挥致癌作用：一是不同肿瘤亚克隆的内在生长能力，二是非细胞自主性机制。研究人员从携带不同Eed基因型的小鼠髓母细胞瘤样本中提取总RNA，并通过RNA测序 (RNA-seq) 检测基因表达谱。