Aberrant hematopoiesis: E proteins and AML1-ETO in leukemogenesis

描述（由申请人提供）：急性髓性白血病 (AML) 最常见的原因是 8;21 易位 [t(8;21)]，其导致转录失调。这种易位通过将部分 AML1/RUNX1 转录因子连接到几乎完整的 ETO 蛋白（转录辅阻遏物家族的典型成员）来生成 AML1-ETO 融合蛋白。该提案的长期目标是了解如何选择性干扰 AML1-ETO 活性，从而逆转白血病状态。该应用的直接目标是了解 AML1-ETO 破坏正常转录程序的机制。 AML1-ETO的异常表达是t(8;21) AML的病理原因。 AML1 基因敲除和 AML1-ETO 基因敲入小鼠模型之间的表型差异表明，AML1-ETO 除了解除 AML1 功能的调控外，还具有其他活性。尽管现在已经清楚 AML1-ETO 会干扰造血细胞自我更新、分化和凋亡中涉及的多种细胞事件，但仍不清楚 AML1-ETO 如何解除这些途径的调节。最近，张博士发现了AML1-ETO与I类螺旋-环-螺旋转录因子（E蛋白）之间的分子相互作用。通过 ETO 结构域，AML1-ETO 异常抑制 E 蛋白介导的转录。 E蛋白具有肿瘤抑制活性，但在癌症中经常失活。也就是说，它们促进细胞凋亡并控制造血细胞分化。 AML1-ETO 的致白血病潜力与其对与肿瘤抑制和细胞分化调节相关的 E 蛋白功能的抑制一致。张博士的初步研究表明（i）参与抑制E蛋白依赖性转录的ETO结构域与参与AML1-ETO的致白血病活性的结构域相关； (ii) AML1-ETO 对 E 蛋白依赖性转录的抑制不仅涉及染色质依赖性抑制，还涉及 RNA 聚合酶 II 转录复合物的直接抑制。这些发现得出了一个中心假设：AML1-ETO 必须抑制 E 蛋白介导的染色质依赖性和染色质非依赖性转录，以允许白血病发生。该假设将通过以下两个目标进行检验：（目标1）定义AML1-ETO在染色质水平以及基础转录机制水平上抑制E蛋白依赖性转录的机制； （目标 2）确定 E 蛋白失活对 AML1-ETO 致白血病功能的影响程度，并阐明 t(8;21) 白血病细胞中与 E 蛋白相关的分子途径。更好地了解 t(8;21) AML 的分子机制以及参与白血病发生的蛋白质的异常功能应有助于确定新的治疗靶点和治疗 AML 的策略。

DESCRIPTION (provided by applicant): The most frequent cause of acute myeloid leukemia (AML) is the 8;21 translocation [t(8;21)], which results in transcriptional dysregulation. This translocation generates an AML1-ETO fusion protein by joining part of the AML1/RUNX1 transcription factor to a nearly complete ETO protein, the prototypical member of a family of transcriptional corepressors. The long-term goal of this proposal is to learn how to selectively interfere with AML1-ETO activity, and thereby reverse the leukemogenic state. The immediate goal of this application is to understand the mechanisms by which AML1-ETO disrupts the normal transcriptional program. Aberrant expression of AML1-ETO is the pathological cause of t(8;21) AML. Phenotypic differences between the AML1 knockout and the AML1-ETO knock-in mouse models indicate that AML1-ETO has other activities besides deregulation of AML1 functions. Although it is now clear that AML1-ETO interferes with multiple cellular events involved in hematopoietic cell self-renewal, differentiation, and apoptosis, it remains unclear how AML1-ETO deregulates these pathways. Recently, Dr. Zhang discovered a molecular interaction between AML1-ETO and the class I helix-loop-helix transcriptional factors known as E proteins. Through the ETO domain, AML1-ETO aberrantly represses E protein-mediated transcription. E proteins have tumor- suppressor activities that are frequently inactivated in cancers. That is, they promote apoptosis and control hematopoietic cell differentiation. The leukemogenic potential of AML1-ETO is consistent with its inhibition of E protein functions related to both tumor suppression and regulation of cell differentiation. Dr. Zhang's preliminary studies show (i) that the ETO domains involved in repressing E protein- dependent transcription correlate with those involved in the leukemogenic activities of AML1-ETO; and (ii) that repression of E protein-dependent transcription by AML1-ETO involves not only chromatin-dependent inhibition, but also direct inhibition of the RNA polymerase II transcription complex. These findings led to the central hypothesis that AML1-ETO must repress both the chromatin-dependent and chromatin- independent transcription mediated by E proteins to allow for leukemogenesis. The hypothesis will be tested through the following two aims: (Aim 1) To define the mechanisms by which AML1-ETO represses E protein-dependent transcription at the level of chromatin as well as at the level of basal transcription machinery; and (Aim 2) To determine the extent to which inactivation of E proteins contributes to AML1-ETO leukemogenic function, and to elucidate the molecular pathways associated with E proteins in t(8;21) leukemic cells. A better understanding of the molecular mechanisms underlying t(8;21) AML, and the aberrant functions of proteins involved in leukemogenesis should lead to the identification of new therapeutic targets and strategies for treatment of AML.