Modeling hematologic malignancy and self-renewal with gain-of-function MLL1

描述(由申请人提供)：表观遗传调控因子作为人类疾病致病过程的参与者最近受到越来越多的关注，以及由于其酶活性而成为令人兴奋的新药靶点。这项提议的一个目标是产生 一种动物模型，在该动物模型中，一个聚合的表观遗传调节因子MLL1可以被调节，以研究各种人类疾病，这种广泛表达的蛋白质在其中发挥重要作用。该动物模型利用FLP重组酶定向整合，将表位标记的全长MLL1基因导入胚胎干细胞，其中多西环素可诱导MLL1基因的表达。MLL1在T细胞记忆、神经干细胞规范、造血、自闭症谱系/认知障碍、Weideman-Steiner综合征、皮肤和肌肉干细胞功能、神经上皮癌和血液病、血管和颅面发育中的作用已通过各种不同的途径得到证实，但缺乏研究其发病机制的动物模型。在这项提案中，我们将建立一个动物模型，以加速在所有这些领域的发现。这一模型将被共同采购经理人用来检验两个主要假设。原癌基因MLL1在几种类型的白血病中被染色体易位或扩增破坏，预后特别差。相反，在造血系统中MLL1的丢失会导致造血干细胞(HSCs)自我更新和增殖缺陷的丧失。基于我们的初步数据，我们将检验这一假设，即瞬时诱导MLL1蛋白可以以一种可能在临床上有用的方式增强胎儿和/或成人HSCs的自我更新。因此，MLL1水平升高的功能后果将使用该模型来确定，特别是对HSC自我更新和分化的影响。对有助于自我更新的MLL1目标基因的表观遗传修饰的影响也将被确定。促进造血干细胞自我更新途径的发现可能与治疗脐带血或用于移植的多能造血细胞来源的努力直接相关。第二种假设是，长期持续表达野生型MLL1蛋白会导致骨髓增生异常综合征或白血病。我们的初步数据表明，这种使用MLL1融合癌蛋白的FLP介导的策略是可行的，并可以产生传统逆转录病毒介导的MLL1融合白血病模型所不可能的新见解。目前，还没有动物模型能够以反映基因扩增的方式维持MLL1的表达，因此没有临床前模型来测试可能对携带MLL1扩增的急性白血病有效的新的治疗策略。确定MLL1在骨髓增生异常综合征/白血病中持续表达的充分性，将为测试这种表观遗传调节因子与急性白血病中发生的其他常见突变的作用奠定基础。

DESCRIPTION (provided by applicant): Epigenetic regulators have recently received increasing attention as participants in pathogenic processes in human disease, as well as exciting new drug targets due to their enzymatic activities. A goal of this proposal is to generate an animal model in which a paradigmatic epigenetic regulator, MLL1, can be modulated to study a wide variety of human diseases in which this broadly-expressed protein plays an important role. This animal model utilizes Flp recombinase-directed integration to introduce an epitope-tagged, full-length MLL1 cDNA into embryonic stem cells in which doxycycline addition induces MLL1 cDNA expression. Roles for MLL1 in T-cell memory, neural stem cell specification, hematopoiesis, autism spectrum/cognitive disorders, Weideman- Steiner syndrome, skin and muscle stem cell function, neuroepithelial and hematologic cancers, vascular and craniofacial development have been established through a variety of individual approaches, however, the animal models to study the disease mechanisms have been lacking. In this proposal, we will generate an animal model that can accelerate discoveries in all of these fields. This model will be utilized by the co-PIs to test two major hypotheses. The proto-oncogene MLL1 is disrupted by chromosomal translocations or amplification in several types of leukemia with particularly poor prognosis. In contrast, loss of MLL1 in the hematopoietic system results in loss of self-renewal and proliferation defects in hematopoietic stem cells (HSCs). Based on our preliminary data, we will test the hypothesis that transient induction of MLL1 protein can enhance self-renewal in fetal and/or adult HSCs in a manner that could be clinically useful. Thus, the functional consequences of increased MLL1 levels will be determined using this model, particularly the effect on self- renewal and differentiation of HSCs. The impact on epigenetic modification of MLL1 target genes instrumental in self-renewal will be also be determined. The discovery of pathways that enhance self-renewal in HSCs may be directly relevant to efforts to treat umbilical cord blood or pluripotent sources of hematopoietic cells for transplantation. The second hypothesis is that chronic, sustained expression of wild-type MLL1 protein will lead to a myelodysplastic syndrome or leukemia. Our preliminary data illustrate that this Flp-mediated strategy using MLL1-fusion oncoproteins is feasible and can yield novel insights not possible with traditional retroviral- mediated MLL1-fusion leukemia models. Currently, there are no animal models in which MLL1 expression can be sustained in a manner that reflects gene amplification, thus no pre-clinical model for testing novel therapeutic strategies that may be effective in acute leukemia harboring MLL1 amplifications. Establishing the sufficiency of sustained MLL1 expression in myelodysplastic syndrome/leukemia will set the stage for testing the role of this epigenetic regulator in conjunction with other common mutations that occur in acute leukemia.