SOX2-MYC Axis: Transcriptional Repression of the MYC Oncogene by SOX2

摘要 已经鉴定了一系列广泛的策略和小分子抑制剂，其靶向积极增殖的 肿瘤细胞然而，静止的肿瘤细胞在很大程度上对用于消除肿瘤的常规疗法具有抗性。 增殖的肿瘤细胞因此，静止的肿瘤细胞构成了主要的临床挑战和显著的未满足的需求。 因为静止细胞代表了肿瘤细胞的残余群体， 导致肿瘤复发。为了解决这一未满足的需求，我们一直在研究转录因子SOX 2， 其升高的表达已显示也限制许多不同肿瘤细胞类型的增殖 和正常胎儿干细胞的增殖一样。我们的工作已经确定，将SOX 2从诱导型 在多种肿瘤细胞类型中，启动子总是导致生长抑制。此外，我们确定， SOX 2在体内导致肿瘤生长停滞的可逆状态，其中当SOX 2返回时肿瘤生长恢复 到内源性水平。为了确定SOX 2是如何限制肿瘤细胞生长的，我们研究了升高SOX 2 改变不同肿瘤细胞类型的转录组。这些研究表明，SOX 2的升高 下调MYC表达和MYC靶基因的表达。为了扩展这些发现，我们 进行了一项MYC拯救研究，并确定当SOX 2升高时， 对细胞有害。因此，这个新发现的SOX 2：MYC轴似乎在生长中起着关键作用。 当SOX 2升高时抑制。同样重要的是，我们确定升高SOX 2下调MYC， 在转录水平上。本文提出的研究旨在确定升高SOX 2如何降低MYC 转录。在增殖细胞中，MYC转录依赖于多个远端增强子，这些增强子激活MYC基因。 通过染色质环的MYC启动子，其形成依赖于β-catenin与Wnt的结合 元件（WRE）存在于每个MYC增强子中。重要的是，SOX 2已被证明干扰Wnt 通过SOX 2的R1反式激活结构域与β-catenin相互作用， 确定抑制肿瘤细胞生长和下调MYC所需的。基于这些发现，我们 假设SOX 2升高通过干扰转录功能下调MYC表达 促进形成驱动MYC所需的关键的长距离增强子-启动子相互作用的因子 转录。这一假设将通过两个具体目标进行检验。目标1将决定如何提高SOX 2改变 MYC基因座的三维染色质结构。目标2将确定升高SOX 2是否会改变结合 转录因子与MYC增强子的结合，改变在活性MYC增强子处发现的组蛋白修饰，和/或 阻断MYC启动子处转录前起始复合物的形成。了解如何提升 SOX 2下调MYC将是非常重要的，因为对这些之间的联系知之甚少。 两个主调节器。最终，我们相信我们的研究将导致发现新的可药物靶点， 根除与癌症复发有关的静止肿瘤细胞。

ABSTRACT A wide array of strategies and small molecule inhibitors have been identified that target actively proliferating tumor cells. However, quiescent tumor cells are largely resistant to conventional therapies used to eliminate proliferating tumor cells. Hence, quiescent tumor cells pose a major clinical challenge and a significant unmet need in the treatment of cancer, because quiescent cells represent a residual population of tumor cells that can drive tumor recurrence. To address this unmet need, we have been studying the transcription factor SOX2, whose elevated expression has been shown to restrict the proliferation of many different tumor cell types as well as the proliferation of normal fetal stem cells. Our work has established that elevating SOX2 from an inducible promoter in multiple tumor cell types invariably leads to growth inhibition. Moreover, we determined that elevating SOX2 in vivo leads to a reversible state of tumor growth arrest, where tumor growth resumes when SOX2 returns to endogenous levels. To determine how SOX2 restricts tumor cell growth, we examined how elevating SOX2 alters the transcriptome in different tumor cell types. These studies demonstrated that elevation of SOX2 downregulates MYC expression and the expression of MYC target genes. To extend these findings, we performed a MYC rescue study and determined that failure to downregulate MYC when SOX2 is elevated is deleterious to the cells. Thus, this newly discovered SOX2:MYC axis appears to play a critical role in growth inhibition when SOX2 is elevated. Equally important, we determined that elevating SOX2 downregulates MYC at the transcriptional level. The studies proposed here seek to determine how elevating SOX2 decreases MYC transcription. In proliferating cells, MYC transcription is dependent on multiple distal enhancers that activate the MYC promoter via chromatin loops whose formation is dependent on the binding of β-catenin to Wnt Response Elements (WREs) present in each MYC enhancer. Importantly, SOX2 has been shown to interfere with Wnt signaling by its interaction with β-catenin via the R1 transactivation domain of SOX2, which we recently determined is required to inhibit tumor cell growth and downregulate MYC. Based on these findings, we hypothesize that elevating SOX2 downregulates MYC expression by interfering with the function of transcription factors that promote the formation of critical long-range enhancer-promoter interactions required to drive MYC transcription. This hypothesis will be tested by two Specific Aims. Aim 1 will determine how elevating SOX2 alters the 3-D chromatin structure of the MYC locus. Aim 2 will determine whether elevating SOX2 alters the binding of transcription factors to MYC enhancers, changes histone modifications found at active MYC enhancers, and/or blocks the formation of the transcription preinitiation complex at the MYC promoter. Understanding how elevating SOX2 downregulates MYC would be highly significant, because little is known about the links between these two master regulators. Ultimately, we believe our studies will lead to the discovery of novel druggable targets to eradicate quiescent tumor cells implicated in cancer recurrence.