Transcriptional Deregulation by MLL Fusion Proteins

描述（由申请人提供）：混合谱系白血病蛋白（MLL）与超过50种不同易位伴侣中的一种融合，将其转化为有效的白血病致癌蛋白。所产生的融合蛋白通过上调包括Hoxa9和Hox辅助因子Meis1在内的A簇Hox基因转化。我们在了解MLL和MLL融合蛋白的转录调控机制方面取得了相当大的进展。我们发现MLL是一种组蛋白H3赖氨酸4甲基转移酶，通过直接结合Hox和Meis1启动子和编码区来调节转录。我们发现MLL通过一个小的氨基末端结构域与肿瘤抑制蛋白menin相互作用，这种相互作用对正常MLL和MLL融合蛋白的功能都很重要。我们的首要目标是促进对MLL重排白血病更有效治疗的发展。为此，提出的实验将分析MLL融合蛋白转化的几个关键机制。一些MLL融合蛋白将MLL融合到转录激活因子上，而另一些则将截断的MLL分子二聚化。这两个类共享转换所需的MLL氨基末端。在SA#1中，我们将分析两类MLL融合蛋白转化过程中MLL氨基端与menin相互作用的作用。SA#2关注的是最常见的MLL融合蛋白，包括易位伴侣AF4、ENL、AF9和AF5q31，它们占MLL相关白血病的70%以上。这四种蛋白在MPAC （MLL Partner Activation complex）复合体中有物理关联，该复合体包括Dot1（一种组蛋白H3赖氨酸79甲基转移酶）和CDK9/CyclinT1/T2（磷酸化RNA聚合酶II的C端结构域）。我们将研究MPAC在正常造血和被白血病MLL融合蛋白募集到靶位点时的转录激活机制。令人惊讶的是，MPAC还含有抑制Polycomb蛋白家族的两个成员Pc3和Ring1B。我们将确定这些蛋白的酶活性在MPAC环境下是否完整，以及MLL融合蛋白是否阻止polycomb介导的沉默。SA#3评估了MPAC激酶和组蛋白甲基转移酶活性在融合蛋白介导转化中的作用。对这些潜在的“阿喀琉斯之踵”的研究将为MLL融合蛋白如何破坏正常的转录机制以及如何靶向治疗提供有价值的见解。

DESCRIPTION (provided by applicant): Fusion of the mixed lineage leukemia protein (MLL) to one of over 50 different translocation partners converts it into a potent leukemogenic oncoprotein. The resulting fusion proteins transform by upregulation of A cluster Hox genes including Hoxa9 and the Hox cofactor Meis1. We have made considerable progress toward understanding the mechanism of transcriptional regulation by MLL and MLL fusion proteins. We discovered that MLL is a histone H3 lysine 4 methyltransferase that regulates transcription by direct binding to both Hox and Meis1 promoters and coding regions. We found that MLL interacts with the tumor suppressor menin via a small amino terminal domain and that this interaction is important for both the function of normal MLL as well as for MLL fusion proteins. Our overriding goal is to facilitate the development of more effective therapies for leukemias with MLL rearrangements. To this end, the proposed experiments will analyze several mechanisms pivotal for transformation by MLL fusion proteins. Some MLL fusion proteins fuse MLL to transcriptional activators while others dimerize the truncated MLL molecule. Both classes share the MLL amino terminus, which is required for transformation. In SA#1 we will analyze the role of the interaction of the MLL amino terminus with menin in transformation by both classes of MLL fusion proteins. SA#2 focuses on the most common MLL fusion proteins, involving translocation partners AF4, ENL, AF9 and AF5q31, which account for more than 70% of MLL-associated leukemias. All four proteins are physically associated in a complex called MPAC (MLL Partner Activation Complex) that includes Dot1, a histone H3 lysine 79 methyltransferase, and CDK9/CyclinT1/T2, which phosphorylates the C terminal domain of RNA polymerase II. We will study the mechanism of transcriptional activation by MPAC in both normal hematopoiesis and when recruited to target loci by leukemogenic MLL fusion proteins. Surprisingly, MPAC also contains Pc3 and Ring1B, two members of the repressive Polycomb group protein family. We will determine if this enzymatic activity of these proteins is intact in the context of MPAC and whether MLL fusion proteins prevent Polycomb-mediated silencing. SA#3 assesses the role of MPAC kinase and histone methyltransferase activity on fusion protein-mediated transformation. Studies of these potential "Achilles heels" will provide valuable insights into how normal transcriptional mechanisms are disrupted by MLL fusion proteins and how these may be targeted therapeutically.