Targeting PML for leukemia therapy.

描述(由申请人提供)：白血病的维持已被证明依赖于大量白血病人群中具有自我更新特性的一小部分细胞，并被称为“白血病启动细胞”(LIC)。LIC与常规干细胞具有相同的机械特性，包括更多的静止性质，这被认为是它们对标准的基于化疗的治疗产生抵抗力的中介。未能有效地针对LIC可能会导致疾病复发。慢性粒细胞白血病(CML)是一种被广泛研究的干细胞疾病，在这种疾病中，LIC池并不总是通过当前的靶向治疗而被根除，导致疾病在停药后复发。我们已经证明，早幼粒细胞白血病(PML)肿瘤抑制因子在规则的造血干细胞(HSCs)和CML原始细胞中的表达都令人惊讶地高，并且PML表达的缺失预示着CML的预后更有利。我们随后证明了PML在维持HSCs/LICs的静止和自我更新特性方面发挥了关键作用，尽管涉及的确切分子机制尚不清楚。众所周知，PML还可以通过用砷处理细胞而受到药理上的抑制，这会特别降低蛋白质的稳定性。利用这一点，我们已经在CML的小鼠模型中使用了砷介导的PML消融，成功地靶向了LIC。然而，PML靶向在临床上的有效性还需要评估。因此，为了了解PML下游维持LIC所需的关键途径，并将PML去除LIC靶向转移到临床，我们提出了以下具体目标：(1)研究PML在静止期LIC中的分子功能及其在LIC和骨髓生态位之间的串扰中的作用；(2)在更多临床相关的CML小鼠模型中评估砷的治疗效果；(3)建立一项联合应用砷和达沙替尼治疗慢性期CML的临床试验。这些目标的实现可以极大地改善慢性粒细胞白血病患者的治疗，有可能在LIC根除后停止治疗。此外，了解PML的功能可以为LIC和肿瘤干细胞消融提供其他治疗靶点。 公共卫生相关性：我们定义了PML在维持CML启动细胞中的重要作用，并提出了一种通过使用砷抑制PML功能来靶向静止期白血病启动细胞(LICs)的新治疗方法。我们的发现支持这样的观点，即用砷消融PML可能是使CML启动细胞对抗肿瘤治疗更敏感的有效工具。这项工作有可能对慢性粒细胞白血病的治疗产生重大影响。酪氨酸激酶抑制剂在大多数CML患者中成功地抑制了恶性克隆。然而，一旦停止治疗，这种疾病就会复发，部分患者会出现对TKI治疗具有抵抗力的疾病。在这项建议中，我们将评估砷与TKI联合治疗的效果，目标是针对LICS，从而导致对这种疾病的根治性治疗而不是抑制性治疗。阐明PML调节LIC池维持的机制可能反过来导致治疗白血病的新的治疗方法。

DESCRIPTION (provided by applicant): Maintenance of leukemia has been demonstrated to be dependent upon a small sub-population of cells within the bulk leukemic population that have self-renewal properties and are termed "leukemia-initiating cells" (LICs). LICs share mechanistic properties with regular stem cells including a more quiescent nature, which is thought to mediate their resistance to standard chemotherapy-based treatment. Failure to effectively target LICs can result in disease relapse. Chronic Myelogenous Leukemia (CML) is an extensively studied stem cell disorder in which the LIC pool is not always eradicated by current targeted therapy, leading to disease relapse upon drug discontinuation. We have demonstrated that expression of the Promyelocytic Leukemia (PML) tumor suppressor is surprisingly high in both regular hematopoietic stem cells (HSCs) and in CML blasts and that loss of PML expression predicts a more favorable outcome in CML. We subsequently demonstrated that PML plays a key role in maintaining the quiescence and self-renewal properties of HSCs/LICs although the exact molecular mechanisms involved are poorly understood. PML is also known to be pharmacologically inhibited by treatment of cells with arsenic, which specifically decreases the stability of the protein. Taking advantage of this, we have used arsenic-mediated ablation of Pml in a mouse model of CML to successfully target LICs. However, the effectiveness of PML targeting in the clinic needs to be assessed. Therefore, in order to understand the key pathways downstream PML required for LIC maintenance and to translate PML-ablative LIC targeting to the clinic we propose the following Specific Aims: (1) to study the molecular function of PML in quiescent LICs and its role in the cross-talk between LICs and the bone marrow niche; (2) to assess effectiveness of arsenic treatment in more clinically relevant mouse models of CML; (3) to develop a clinical trial of combination arsenic+dasatinib treatment for chronic phase CML. Accomplishment of these aims could greatly improve treatment of patients with CML, possibly allowing discontinuation of therapy after LIC eradication. Further, understanding PML function could provide other therapeutic targets for LIC and cancer stem cell ablation. PUBLIC HEALTH RELEVANCE: We have defined the essential role of PML in the maintenance of CML-initiating-cells, and present a new therapeutic approach for targeting quiescent leukemia initiating cells (LICs) by using arsenic to inhibit the function of PML. Our findings support the notion that PML-ablation by arsenic may be an effective tool to render CML-initiating cells more sensitive to anti-tumor therapy. This work has the potential to have a significant impact on the treatment of CML. Tyrosine kinase inhibitors are successful at suppressing the malignant clone in the majority of CML patients. However, the disease recurs upon discontinuation of therapy, and a subset of patients develops disease that is resistant to TKI therapy. In this proposal, we will evaluate the effect of combining arsenic with TKI therapy, with goal of targeting the LICs in order to result in curative rather than suppressive therapy for this disease. The elucidation of the mechanisms by which PML regulates the maintenance of the LIC pool could in turn lead to novel therapeutic approaches to treat leukemia.