Molecular and Cellular Basis of Oncogene Addiction

癌症是由癌基因的激活和肿瘤抑制基因的失活引起的。这些基因产物的靶向失活和修复可能是一种特异有效的癌症治疗方法。以前，我们已经证明MYC癌基因的失活足以诱导造血肿瘤的持续消退[2]。随后，我们报道了即使是MYC的短暂失活也足以诱导骨肉瘤的持续肿瘤消退[3，4]。我们的研究结果支持肿瘤表现出癌基因成瘾现象的一般假设[5，6]。为了解释我们的发现，我们推断MYC失活诱导细胞诱导癌症相关基因表达程序的能力发生永久性变化。MYC失活后，基因表达发生特异性和持续变化[Wu et al，PLoS Genetics [1];参见附录，Shachaf et al，Cancer Research，2008]。这些基因表达的变化经常伴随着MYC与启动子位点结合能力的永久性变化，如ChIP所示。我们对MYC进行了初步的ChIP芯片分析，并在全基因组范围内询问了其他转录因子结合的变化。此外，我们还发现基因表达的变化与染色质修饰的特定改变有关。重要的是，MYC似乎不仅通过与典型DNA结合序列（E-Box）的相互作用来调节基因表达，而且其结合特异性可能受到DNA甲基化的调节。我们提供了新的结果，说明TGF-β信号通路可能在MYC失活诱导基因表达和细胞衰老变化的机制中发挥重要作用。此外，我们提供了MYC调节DNA甲基化的证据。因此，我们假设MYC失活通过对DNA甲基化的影响恢复自动调节程序，包括诱导CDKI，导致诱导细胞衰老程序。因此，基于细胞外在受体的机制（包括TGF-β 1信号传导）以及细胞内在机制（包括DNA甲基化的调节）可能对癌基因成瘾至关重要。我们现在提出实验，并要求工资和补助金支持一个由1名首席研究员，2名博士后研究员和1名研究生和一名研究助理组成的团队，以描述TGF-13信号传导和DNA甲基化在MYC失活后肿瘤消退机制中的作用。我们的结果与MYC可能直接调节全局染色质结构的观点一致;并提出了令人惊讶的想法，即MYC诱导的肿瘤细胞仍然不知道或“健忘”-正如我们最近所描述的[10] -它们的癌状态，但仍然准备经历衰老。我们提出的实验结果将对MYC癌基因维持肿瘤发生的机制和癌症治疗新疗法的开发具有重要意义。

Cancer is caused by the activation of oncogenes and the inactivation of tumor suppressor genes. The targeted inactivation and repair of these gene products may be a specific and effective therapy for cancer. Previously, we have shown that the inactivation of the MYC oncogene is sufficient to induce sustained regression of hematopoietic tumors [2]. Subsequently, we reported that even brief inactivation of MYC is sufficient to induce sustained tumor regression of osteosarcoma [3, 4]. Our results support the general hypothesis that tumors exhibit the phenomena of oncogene addiction [5, 6]. To explain our findings, we reasoned that MYC inactivation induces a permanent change in the ability of cells to induce a cancer-associated gene expressionprogram. Upon MYC inactivation, there are specific and sustained changes in gene expression [Wu et al, PLoS Genetics [1]; and see Appendix, Shachaf et al, Cancer Research, 2008]. These changes in gene expression are frequently accompanied by permanent changes in the ability of MYC binding to promoter loci, as shown by ChIP. We performed a preliminary ChIP-on-chip analysis for MYC and interrogated changes in binding of other transcription factors in a genome-wide scale. Moreover, we also found that changes in gene expression are associated with specific alterations in chromatin modifications. Importantly, MYC appears to regulate gene expression not just through interactions with the canonical DNA binding sequence (E-Box), but additionally, its binding specificity may be regulated by DNA methylation. We provide new results that illustrate that the TGF-n signaling pathway may play an important role in the mechanism by which MYC inactivation induces changes in gene expression and cellular senescence. Moreover, we provide evidence that MYC regulates DNA methylation. Hence, we hypothesize that MYC inactivation restores auto-regulatory programs, including the induction of CDKIs, through effects on DNA methylation, resulting in the induction of a cellular senescence program. Hence, both cell extrinsic receptor based mechanisms, including TGF-U signaling, as well as cell intrinsic mechanisms, including regulation of DNA methylation, may be critical to oncogene addiction. We now propose experiments and request salary and grant support for a team of 1 principal investigator, 2 post-doctoral fellows and 1 graduate student and a research associates to delineate the role of TGF-13 signaling and DNA methylation on the mechanism of tumor regression upon MYC inactivation. Our results are consistent with the notion that MYC may directly regulate the global chromatin structure; and suggest the surprising idea that MYC-induced tumor cells remain unaware or "amnesic" - as we have recently described [10] - of their cancerous state, yet remain poised to undergo senescence. The results of our proposed experiments will have important implications for the mechanisms by which the MYC oncogene maintains tumorigenesis and the development of new therapies for the treatment of cancer.