The Role of the APC Tumor Suppressor Gene in Hematopoiesis and Leukemogenesis

描述（由申请人提供）：新出现的证据表明，癌症起源于起源于正常干细胞或祖细胞的癌起始细胞。然而，正常干细胞或祖细胞转化为致癌细胞的分子基础仍然知之甚少。骨髓增生异常综合征（MDS）是造血干细胞（hsc）的克隆性疾病，其特征是造血功能低下，白血病转化率高。APC抑癌基因参与结直肠癌的发生发展。5号染色体长臂缺失或整个5号染色体-5/del（5q）缺失在原发性MDS（10-15%）或治疗相关性MDS （t-MDS）和治疗相关性急性髓性白血病（t-AML）（40%）患者中发生的频率很高。APC基因位于染色体5q23带，95%的-5/del（5q）患者缺失APC基因，这就提出了APC在造血系统中是否起到肿瘤抑制作用的问题。为了研究Apc在造血干细胞功能和正常造血中的作用，我们对有条件Apc等位基因的小鼠进行了实验，结果表明，体内造血细胞中Apc失活会导致骨髓衰竭导致的快速死亡。此外，Apc的缺失导致HSC和祖细胞（HPCs）进入细胞周期增强，凋亡增加，导致HSC和HPC池迅速耗尽。这些研究表明，Apc是造血干细胞和造血干细胞功能以及正常成人造血所必需的。此外，在一项初步研究中，我们发现造血细胞中Apc单个等位基因失活的一组小鼠屈服于伴有巨细胞增生的严重贫血，再现了MDS和-5/del（5q）的t-MDS的几个特征。这些特征表明Apc单倍不足小鼠代表了一种新的有趣的MDS和t-MDS/t-AML的-5/del（5q）疾病模型。我们假设APC的单倍不足通过对造血干细胞和造血干细胞的维持和功能的失调，促进了血液恶性肿瘤的发生和发展。这一假设将在三个具体目标中得到验证：(1)确定Apc单倍性不足是否通过调节造血干细胞和髓系祖细胞的功能而导致血液病的发病机制；(2)确定造血干细胞向白血病起始细胞（lic）转化过程中Apc单倍体不足的基因改变；(3)评估抑制Wnt//2-catenin通路是否能在体内预防Apc一个或两个等位基因失活导致的血液学疾病，并挽救体外-5/del(5q) t-MDS/t-AML患者的髓系白血病细胞生长和分化缺陷。Apc单倍不足模型为我们提供了一个独特的机会来研究正常干细胞或祖细胞转化为LIC时发生的分子事件，从而开发出针对MDS和t-MDS/t- AML的-5/del（5q）靶向白血病起始细胞的有效治疗策略。

DESCRIPTION (provided by applicant): Emerging evidence suggests that cancers are derived from cancer-initiating cells that originate from normal stem cells or progenitors. However, the molecular basis by which a normal stem or progenitor cell is transformed into a cancer-initiating cell is still poorly understood. The myelodysplastic syndromes (MDS) are clonal disorders of hematopoietic stem cells (HSCs), characterized by ineffective hematopoiesis with a high rate of leukemic transformation. The APC tumor suppressor gene is involved in the development of colorectal carcinoma. A deletion of the long arm of chromosome 5 or loss of a whole chromosome 5, -5/del(5q), occurs at a high frequency in patients with primary MDS (10-15%) or therapy-related MDS (t-MDS) and therapy-related acute myeloid leukemia (t-AML) (40%). The APC gene is located on chromosome band 5q23, and is deleted >95% of patients with -5/del(5q), raising the question of whether APC acts as a tumor suppressor in the hematopoietic system. To examine the role of Apc in the function of HSCs and in normal hematopoiesis, we examined mice with a conditional Apc allele, and showed that inactivation of Apc in hematopoietic cells in vivo leads to rapid lethality due to bone marrow failure. In addition, loss of Apc results in enhanced cell cycle entry and increased apoptosis of HSCs and progenitor cells (HPCs), leading to rapid exhaustion of the HSC and HPC pool. These studies suggest that Apc is required for the function of HSCs and HPCs, and normal adult hematopoiesis. Additionally, in a pilot study, we showed that a cohort of mice with inactivation of a single allele of Apc in hematopoietic cells succumb to a severe anemia with macrocytosis, recapitulating several characteristic features of MDS and t-MDS with -5/del(5q). These features suggest that Apc haploinsufficient mice represent a novel and interesting disease model for MDS and t-MDS/t-AML with a -5/del(5q). We hypothesize that haploinsufficiency of APC contributes to the initiation and development of hematological malignancies through deregulation of the maintenance and the function of HSCs and HPCs. This hypothesis will be tested in three specific aims: (1) To determine whether haploinsufficiency of Apc contributes to the pathogenesis of hematological disorders via deregulation of the function of HSCs and myeloid progenitors; (2) To identify the genetic alterations that cooperate with haploinsufficiency of Apc in the transformation of HSCs into leukemia-initiating cells (LICs); and (3) To evaluate whether inhibition of the Wnt//2-catenin pathway prevents hematological disorders induced by inactivation of one or both alleles of Apc in vivo, and rescues the growth and differentiation defects of myeloid leukemia cells from t-MDS/t-AML patients with a -5/del(5q) ex vivo. The Apc haploinsufficient model provides us with a unique opportunity to study the molecular events that occur during the initiation of transformation of a normal stem or progenitor cell into a LIC, leading to the development of new effective therapeutic strategies targeting leukemia-initiating cells for MDS and t-MDS/t- AML with a -5/del(5q).