Multimerization of MYC proteins as oncogenic principle

三种MYC家族癌蛋白是人类肿瘤发生的中心驱动因素。由许多癌基因驱动的肿瘤在其整个生命周期中依赖于升高的MYC表达，这表明靶向MYC功能具有很高的治疗潜力。这一建议解决了什么生化和生物物理机制的基础上这种依赖性的核心问题。MYC蛋白表现出激活转录因子的许多特征，并且传统上被认为是致癌的，因为它们保持肿瘤细胞的特征性基因表达模式。然而，尽管数十年的密集工作，关键MYC靶基因的身份仍然不清楚。事实上，最近的许多发现揭示了MYC蛋白的生物化学特征，这些特征与这种模型不一致，并认为它们具有独立于基因表达的致癌功能。我的实验室已经确定了两个这样的过程：MYC蛋白促进启动子处的转录依赖性双链断裂修复，并解决转录-复制冲突。我们还表明，靶向这些过程具有很高的治疗潜力。当复制叉接近时，MYC蛋白通过终止启动子附近的转录来解析TRCs。这是令人惊讶的，因为如上所述，它们可以作为经典的活化蛋白发挥作用。许多先前的观察已经表明，MYC蛋白可以以两种状态存在，激活状态和抑制状态。此外，MYC蛋白酶体的持续周转是其激活功能所必需的，这表明当周转受阻时，MYC蛋白以抑制状态积累。我们现在已经发现，这两种状态之间的转换对应于MYC蛋白的多聚化和相变。当MYC周转被阻断或细胞受到转录应激时，MYC蛋白以多聚体球形结构积累并定位于染色质上的新位点。引人注目的是，这些站点位于停滞的复制叉附近，并且球形结构围绕着停滞的复制叉。球体形成的阻断导致由复制应力诱导的双链断裂。这表明MYC球体物理地屏蔽了RNA聚合酶的复制叉。我们认为，这些领域发挥了重要的保护功能，复制叉压力条件下，多聚化的能力是中央的普遍致癌功能的MYC。本申请旨在了解MYC多聚化的机制，并开发工具，使我们能够决定多聚化和相变是否确实是MYC致癌功能的核心。

The three MYC family oncoproteins are central drivers of human tumorigenesis. Tumors driven by many oncogenes are dependent on elevated MYC expression throughout their lifespan, suggesting that targeting MYC function has high therapeutic potential. This proposal addresses the central question of what biochemical and biophysical mechanisms underlie this dependence. MYC proteins exhibit many features of activating transcription factors and are traditionally considered oncogenic because they maintain the characteristic gene expression patterns of tumor cells. However, despite decades of intensive work, the identity of critical MYC target genes remains unclear. Indeed, many recent discoveries have revealed biochemical features of MYC proteins that are at odds with this model and argue that they have oncogenic functions independent of gene expression. My laboratory has identified two such processes: MYC proteins promote transcription-dependent double-strand break repair at promoters and they resolve transcription-replication conflicts. We have also shown that targeting these processes has high therapeutic potential. MYC proteins resolve TRCs by terminating transcription near promoters when a replication fork approaches. This is surprising because, as mentioned above, they can function as classical activating proteins. A number of previous observations had already indicated that MYC proteins can exist in two states, an activating and a repressive state. Moreover, continuous proteasomal turnover of MYC is required for its activating functions, suggesting that MYC proteins accumulate in a repressive state when turnover is blocked. We have now found that the transition between the two states corresponds to a multimerization and phase transition of MYC proteins. When MYC turnover is blocked or cells are subjected to transcriptional stress, MYC proteins accumulate in multimeric spherical structures and localize to new sites on chromatin. Strikingly, these sites are located near stalled replication forks, and the spherical structures surround the stalled forks. The blockage of sphere formation resulted in double-strand breaks induced by replication stress. This suggests that MYC spheres physically shield blocked replication forks from RNA polymerase. We propose that such spheres exert an important protective function for replication forks under stress conditions and that the ability to multimerize is central to the pervasive oncogenic functions of MYC. This application aims to understand the mechanisms underlying MYC multimerization and to develop tools that allow us to decide whether multimerization and phase transition are indeed central for the oncogenic functions of MYC.