The Role of MLL-AF9 in Acute Myeloid Leukemia

造血干细胞和白血病干细胞（HSC/LSC）的生存依赖于与细胞外基质、可溶性因子和骨髓（BM）微环境的细胞成分的相互作用。假设这些相互作用促进了干细胞的静止及其在生态位中的保留，并影响了对称与非对称自我更新分裂的决定。促成这些决定的信号现在刚刚被确定，可能涉及在干细胞中重要的经典分子途径，包括WNT/catenin， Notch， KitL/Kit和SDF/CXCR4信号。这些信号中的每一个都直接或间接地影响GTPase小蛋白的Rho家族，包括Rac家族。这些蛋白在维持造血和维持干细胞室完整性的生存、增殖和分化决策中发挥着至关重要的作用。我们最近证明在一种新的人类白血病模型中活化的Rac增加。在该模型中，MLL-AF9 （MA9）白血病癌基因被用来转化原代人CD34+细胞，并促进免疫缺陷小鼠的快速急性髓性白血病（AML）。使用Rac抑制剂NSC23766抑制Rac- gtp的形成，或shRNA敲低Rac的表达，在对正常人CD34+脐带血细胞几乎没有抑制作用的情况下，可以有效促进细胞凋亡，抑制细胞增殖。Rac的抑制和凋亡的诱导与Bcl蛋白表达的减少有关，尤其是Bcl- xl，它是Rac2信号传导的下游靶点。我们建议使用我们新开发的白血病模型来研究Rac信号在MLL白血病细胞中的作用，通过体外实验测量与Rac活性相关的特征表型，包括迁移、存活、粘附和增殖（Aim1）。我们将在体内使用异种移植系统和药理学以及RNAi方法来扩展这些结果，以抑制Rac活性和Bcl-xL功能（Aim 2）。这种组合方法将使我们能够确定Rac信号通路在白血病中的重要性，并开始分析MA9 AML中Rac成瘾的具体机制。相关性（见说明）：我们最近已经证明，人类血液干细胞在引入后可以诱导产生白血病

Hematopoietic as well as leukemic stem cells (HSC/LSC) are dependent on interactions with extracellular matrix, soluble factors and cellular components of the bone marrow (BM) microenvironment for their survival. These interactions are hypothesized to promote the quiescence of the stem cell and their retention in the niche and influence the decisions regarding symmetric versus asymmetric self-renewal divisions. The signals that contribute to these decisions are just now being identified and likely involve the classical molecular pathways shown to be important in stem cells, including WNT/catenin, Notch, KitL/Kit and SDF/CXCR4 signaling. Each of these signals impacts directly or indirectly on the Rho family of small GTPase proteins, including the Rac family. These proteins play an essential role in the retention of HSC in the niche and in the survival, proliferation and differentiation decisions that are critical for sustaining hematopoiesis and maintaining the integrity of the stem cell compartment. We have recently demonstrated that activated Rac is increased in a novel model of human leukemia. In this model, the MLL-AF9 (MA9) leukemia oncogene is used to transform primary human CD34+ cells and promotes a rapid acute myeloid leukemia (AML) in immunodeficient mice. Suppression of Rac-GTP formation using the Rac inhibitor NSC23766, or shRNA knockdown of Rac expression, could effectively promote apoptosis and suppress proliferation under conditions where little or no inhibitory effect could be detected on normal human CD34+ cord blood cells. The inhibition of Rac and the induction of apoptosis correlate with the loss of Bcl protein expression, especially of Bcl-xL, which is an identified downstream target of Rac2 signaling. We propose to use our newly developed leukemia model to study the role of Rac signaling in MLL leukemia cells using in vitro assays measuring well-characterized phenotypes associated with Rac activity, including migration, survival, adhesion and proliferation (Aim1). We will extend these results in vivo using a xenograft system and a pharmacologic as well as RNAi approach to inhibit Rac activity as well as Bcl-xL function (Aim 2). This combinatorial approach should enable us to determine the importance of the Rac signaling pathway in leukemia and begin to dissect the specific mechanisms involved in the Rac addiction in MA9 AML. RELEVANCE (See instructions): We have recently shown that human blood stem cells can be induced to make leukemia upon introduction