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AML 1/RUNX 1转录因子连接到几乎完整的ETO蛋白（转录辅阻遏物家族的原型成员）而产生AML 1-ETO融合蛋白。该提案的长期目标是了解如何选择性地干扰AML 1-ETO活性，从而逆转白血病状态。本申请的直接目标是了解AML 1-ETO破坏正常转录程序的机制。AML 1-ETO的异常表达是t（8;21）AML的病理学原因。AML 1敲除和AML 1-ETO敲入小鼠模型之间的表型差异表明，AML 1-ETO除了使AML 1功能失调外还具有其他活性。虽然现在已经清楚AML 1-ETO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AML 1-ETO如何去调节这些通路。最近，Zhang博士发现了AML 1-ETO与I类螺旋-环-螺旋转录因子（称为E蛋白）之间的分子相互作用。通过ETO结构域，AML 1-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。AML 1-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。Zhang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AML 1-ETO致白血病活性的ETO结构域相关;（ii）AML 1-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，还涉及对RNA聚合酶II转录复合物的直接抑制。这些发现导致了中心假设，即AML 1-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。本研究的主要目的是：（1）明确AML 1-ETO在染色质水平和基础转录机制水平上抑制E蛋白依赖性转录的机制;和（目的2）为了确定E蛋白的失活对AML 1-ETO致白血病功能的贡献程度，并阐明t（8;21）白血病细胞中E蛋白相关的分子通路。更好地了解t（8;21）AML的分子机制，以及白血病发生中蛋白质的异常功能，将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.