Bid Function in Myeloid Homeostasis and Leukemogenesis

描述(由申请人提供)：bcl-2家族构成了细胞死亡途径中的一个关键检查点。BID是该家族“BH3-Only”子集的成员之一，需要其保守的BH3结构域才能发挥促凋亡功能。Bid在肿瘤坏死因子RI和Fas(1-7)等死亡受体下游的凋亡途径中起重要作用。半胱氨酸天冬氨酸氨基转移酶8的裂解和肉豆蔻酰化是一个激活分子开关，促进Bid靶向线粒体(8)，在那里它激活多结构域促凋亡成员，如BAK或Bax，介导细胞色素c的释放和下游效应物caspase的激活(9)。Bid缺陷小鼠存活和发育正常，但肝细胞对抗Fas诱导的肝细胞凋亡具有抵抗力(10)。因此，Bid在某些细胞类型，如肝细胞中，在死亡受体下游的线粒体扩增环中起着重要的作用。然而，许多其他细胞，包括激活的淋巴细胞，在体外实验中被Fas杀死，直接激活下游效应半胱氨酸天冬氨酸酶促进细胞凋亡。然而，随着Bid缺陷小鼠年龄的增长或受到慢性病毒感染的挑战，我们发现在体内也特别需要Bid来维持所谓的I型细胞的动态平衡，包括B细胞、T细胞和粒单核细胞系。随着Bid基因缺陷小鼠年龄的增长，它们表现为髓系增生，然后发展为慢性粒-单核细胞白血病，发病率较高。Bcl2基因是在滤泡性B细胞淋巴瘤的分子标志t(14：18)染色体断裂点上克隆的，它建立了一类新的癌基因，其中的异常是细胞死亡而不是增殖(13-15)。在髓系细胞中过度表达bcl2的转基因小鼠会发展成骨髓增生性疾病，当与携带Fas受体突变的小鼠杂交时，就会发展成AML，这意味着Fas途径和Bcl2在抑制髓系细胞的肿瘤方面发挥了协同作用(16)。促凋亡成员在正常髓系内稳态中的作用尚未得到很好的描述。我们认为Bid是介导髓系死亡受体信号的关键促凋亡中间体，它的失活通过延长这些脆弱细胞的寿命促进了髓系白血病的发生，允许积累更多的突变。因此，这种单一的“BH3-Only”蛋白在维持正常的髓系稳态和抑制肿瘤方面具有潜在的作用。在此背景下，我提出了以下具体目标：1.明确Bid在髓系内稳态中的作用。2.明确Bid在髓系白血病发生中的作用。3.确定白血病发生过程中与BID丢失相关的继发性基因改变。

DESCRIPTION (provided by applicant): The BCL-2 family constitutes a crucial checkpoint in the cell death pathway. BID is a member of the "BH3-only" subset of this family and requires its conserved BH3 domain for its pro-apoptotic function. BID plays an important role in the apoptotic pathway downstream of death receptors such as TNF RI and Fas (1-7). Cleavage by caspase 8 and myristoylation serves as an activating molecular switch which facilitates targeting of BID to mitochondria(8) where it activates multidomain proapoptotic members such as BAK or BAX, mediating the release of cytochrome c and activation of downstream effector caspases (9). Bid-deficient mice are viable and develop normally, however hepatocytes are resistant to anti-Fas-induced hepatocellular apoptosis(10). BID thus plays an important role in mediating a mitochondrial amplification loop downstream of death receptors in certain cell types such as hepatocytes. Yet many other cells, including activated lymphocytes, are killed by Fas in in vitro assays, directly activating downstream effector caspases for the promotion of apoptosis(11). However, as Bid-deficient mice age or are challenged by chronic viral infection, we've discovered that BID is also singularly required in vivo to maintain homeostasis in so-called Type I cells including B cells, T cells, and the myelomonocytic lineage(12). As Bid-deficient mice age, they display a myeloid hyperplasia and then progress to Chronic myelomonocytic leukemia at high incidence. The founding family member, BCL-2 was cloned at the t(14:18) chromosomal breakpoint, the molecular hallmark of follicular B cell lymphoma establishing a new class of oncogenes in which the aberration is in cell death rather than proliferation(13-15). Transgenic mice over expressing BCL-2 in myeloid cells develop a myeloproliferative disorder, and when crossed with mice harboring a mutation in the Fas receptor, progress to AML, implicating a synergistic role for the Fas pathway and Bcl-2 in tumor suppression of the myeloid lineage(16). The role of pro-apoptotic members in normal myeloid homeostasis has not been well characterized. We propose that BID is the critical proapoptotic intermediate that mediates death receptor signaling in the myeloid lineage and that its inactivations promotes myeloid leukemogenesis by prolonging the life of these vulnerable cells, allowing accumulation of additional mutations. This single "BH3-only" protein therefore has potential to play a potent role in maintenance of normal myeloid homeostasis as well as tumor suppression. In this context, I propose the following specific aims: 1. Define the role of Bid in myeloid homeostasis. 2. Define the role of Bid in myeloid leukemogenesis. 3. Identify the secondary genetic changes that cooperate with loss of Bid in leukemogenesis.