Transcriptional Deregulation by MLL Fusion Proteins

描述(由申请人提供)：将混合谱系白血病蛋白(MLL)融合到50多个不同易位伙伴中的一个，将其转化为有效的白血病致癌蛋白。由此产生的融合蛋白通过上调A簇HOX基因，包括HOXA9和HOX辅因子Meis1进行转化。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，通过直接与Hox和Meis1启动子和编码区结合来调节转录。我们发现MLL通过一个小的氨基末端结构域与肿瘤抑制因子Menin相互作用，这种相互作用对正常MLL的功能以及MLL融合蛋白都是重要的。我们的首要目标是促进通过MLL重排开发更有效的白血病治疗方法。为此，拟议的实验将分析MLL融合蛋白转化的几个关键机制。一些MLL融合蛋白将MLL与转录激活子融合，而另一些融合蛋白则使截短的MLL分子二聚化。这两个类别共享MLL氨基末端，这是转化所必需的。在SA#1中，我们将分析MLL氨基末端与薄荷素的相互作用在这两类MLL融合蛋白转化中的作用。SA#2专注于最常见的MLL融合蛋白，涉及易位伙伴AF4、ENL、AF9和AF5q31，它们占MLL相关白血病的70%以上。这四种蛋白质都存在于一个称为MLL伙伴激活复合体(MPAC)的复合体中，该复合体包括组蛋白H3赖氨酸79甲基转移酶Dot1和磷酸化RNA聚合酶II C末端结构域的CDK9/CylinT1/T2。我们将研究MPAC在正常造血和白血病MLL融合蛋白募集到靶点时转录激活的机制。令人惊讶的是，MPAC还含有PC3和Ring1b，这是抑制性多梳蛋白家族的两个成员。我们将确定这些蛋白质的这种酶活性在MPAC的背景下是否完整，以及MLL融合蛋白是否阻止多聚梳子介导的沉默。SA#3评估MPAC激酶和组蛋白甲基转移酶活性在融合蛋白介导的转化中的作用。对这些潜在的“跟腱”的研究将为MLL融合蛋白如何扰乱正常转录机制以及如何将其作为治疗靶点提供有价值的见解。

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.