Elucidating the Functional and Mechanistic Roles of LINE-1 Retrotransposons in Myeloid Leukemia

项目摘要 逆转录转座子在人类癌症中的作用历来被低估，这是因为它们 被认为是不重要的“垃圾”DNA，而且由于它们的高重复和变异， 技术挑战。LINE-1（或L1）逆转录转座子约有500，000个拷贝，占总转座子的17% 人类基因组，但只有约100个全长人类特异性L1元件（L1 H）能够激活 反转录转位L1编码的蛋白质也动员非自主反转录转座子，非编码RNA和 mRNA，导致人类DNA的三分之一的产生。因此，L1主要被认为是 有害的，因为它们的活性可导致基因突变和染色体改变， 包括癌症在内的人类疾病。通过对人类染色质调节因子进行基于结构域的CRISPR筛选， MPP 8是HUSH复合体的一种成分，负责L1逆转录转座子的沉默， 急性髓性白血病（AML）细胞的选择性表观遗传依赖性。而对于正常的 造血，MPP 8损失通过重新激活L1损害AML的启动和维持。异位L1激活 表型复制MPP 8损失，而阻断L1逆转录转座废除的表型。L1抑制是 与人类AML患者的治疗抗性和不良预后相关。因此，虽然反转录转座子 在历史上被认为是遗传不稳定性和体细胞遗传的来源， 突变，我们的研究结果支持了反转录转座子作为选择性肿瘤抑制因子的意想不到的“肿瘤抑制”功能。 对骨髓性白血病的依赖这个项目的目标是阐明的功能和机制的作用， L1反转录转座子作为AML病理生理学的新调控途径中心假设是L1 逆转录转座子在AML启动中调节DNA损伤反应和/或炎症信号传导 细胞，L1活性控制治疗反应的DNA损伤诱导剂在髓性白血病。 我们的假设已经制定了广泛的初步研究的基础上，使用正交 在生理学相关AML模型中调节L1功能的方法和一种新发现的分子 逆转录转座子的抑制和癌症起始细胞的增殖之间的联系。时遵循这些 根据研究结果，这些假设将通过三个具体目标进行检验：1）确定母语的功能角色 逆转录转座子在髓系白血病克隆进化中的作用2)阐明L1的机械作用 逆转录转座子在AML发病机制中的作用3)确定调节反转录转座子活性对 髓系白血病对DNA损伤诱导剂的治疗反应这些研究将 不仅阐明了反转录转座子意想不到的“肿瘤抑制”功能的机制基础， 活性，而且还建立了一个新的范例，移动的DNA元件作为以前的 白血病生物学中未知的调节途径，其干预可能导致改善抗白血病 治疗

PROJECT SUMMARY The role of retrotransposons in human cancer has been historically underappreciated both because they have been dismissed as unimportant ‘junk’ DNA, and because their high repetition and variation pose significant technical challenges. There are ~500,000 copies of LINE-1 (or L1) retrotransposons occupying 17% of the human genome, but only ~100 full-length human-specific L1 elements (L1Hs) are capable of active retrotransposition. L1-encoded proteins also mobilize non-autonomous retrotransposons, noncoding RNAs and mRNAs, leading to the generation of a third of human DNA. As such, L1s have been considered mostly deleterious because their activity can lead to genetic mutations and chromosomal alterations that contribute to human disorders including cancer. By domain-based CRISPR screens of human chromatin regulators, we identified MPP8, a component of the HUSH complex responsible for the silencing of L1 retrotransposons, as a selective epigenetic dependency of acute myeloid leukemia (AML) cells. While dispensable for normal hematopoiesis, MPP8 loss impaired AML initiation and maintenance by reactivating L1s. Ectopic L1 activation phenocopied MPP8 loss, whereas blocking L1 retrotransposition abrogated the phenotypes. L1 suppression is associated with therapy resistance and poor prognosis in human AML patients. Hence, while retrotransposons are historically recognized for their cancer-promoting roles as sources of genetic instability and somatic mutations, our findings support an unexpected ‘tumor-suppressive’ function for retrotransposons as a selective dependency for myeloid leukemia. The goal of this project is to elucidate the functional and mechanistic roles of L1 retrotransposons as a new regulatory pathway in AML pathophysiology. The central hypotheses are that L1 retrotransposons function to modulate DNA damage response and/or inflammatory signaling in AML-initiating cells, and that L1 activity controls therapeutic response to DNA damage-inducing agents in myeloid leukemia. Our hypotheses have been formulated on the basis of extensive preliminary studies using orthogonal approaches to modulate L1 function in physiologically relevant AML models and a newly discovered molecular link between suppression of retrotransposons and propagation of cancer-initiating cells. Guided by these findings, these hypotheses will be tested by three specific aims: 1) Establish the functional roles of L1 retrotransposons in the clonal evolution of myeloid leukemia. 2) Elucidate the mechanistic roles of L1 retrotransposons in AML pathogenesis. 3) Determine the effects of modulating retrotransposon activity on therapeutic response to DNA damage-inducing agents in myeloid leukemia. Taken together, these studies will not only elucidate the mechanistic basis for the unexpected ‘tumor-suppressive’ function for retrotransposon activity in myeloid leukemia, but also establish a new paradigm for mobile DNA elements as a previously uncharted regulatory pathway in leukemia biology, whose intervention may lead to improved anti-leukemia therapies.