Aberrant hematopoiesis: E proteins and AML1-ETO in leukemogenesis

描述(申请人提供)：急性髓系白血病(AML)最常见的原因是8；21易位[t(8；21)]，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蛋白上产生AML1-ETO融合蛋白，ETO蛋白是转录辅阻遏子家族的典型成员。这项提议的长期目标是学习如何选择性地干扰AML1-ETO活性，从而逆转白血病状态。这项应用的直接目标是了解AML1-ETO扰乱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基因敲除小鼠模型和AML1-ETO敲入小鼠模型之间的表型差异表明，AML1-ETO除了解除对AML1功能的调控外，还具有其他活动。尽管目前已清楚AML1-ETO可干扰涉及造血细胞自我更新、分化和凋亡的多种细胞事件，但AML1-ETO如何解除对这些途径的调控仍不清楚。最近，张博士发现了AML1-ETO和被称为E蛋白的I类螺旋-环-螺旋转录因子之间的分子相互作用。AML1-ETO通过ETO结构域异常抑制E蛋白介导的转录。E蛋白具有肿瘤抑制活性，这种活性在癌症中通常是失活的。也就是说，它们促进细胞凋亡并控制造血细胞分化。AML1-ETO的致白血病潜能与其抑制E蛋白功能一致，E蛋白功能与肿瘤抑制和细胞分化调节有关。张博士的初步研究表明：(I)参与抑制E蛋白依赖转录的ETO结构域与参与AML1-ETO致白血病活性的结构域相关；(Ii)AML1-ETO对E蛋白依赖转录的抑制不仅涉及染色质依赖的抑制，还包括对RNA聚合酶II转录复合体的直接抑制。这些发现导致了一个中心假设，即AML1-ETO必须同时抑制E蛋白介导的染色质依赖和染色质非依赖的转录，以允许白血病的发生。这一假说将通过以下两个目的进行验证：(1)确定AML1-ETO在染色质水平和基础转录机制水平上抑制E蛋白依赖转录的机制；(2)确定E蛋白失活在多大程度上促进AML1-ETO的白血病发生功能，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，将有助于识别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.