Host RNA Processing Factors: Friend or Foe of Oncogenic Retroelements

描述(申请人提供)：逆转录病毒是一种普遍存在的导致人类和动物癌症的病原体。对于逆转录病毒小鼠乳腺肿瘤病毒(MMTV)来说，当病毒遗传物质整合到宿主染色体中并激活细胞原癌基因的表达时，就会发生癌症。虽然在20世纪30年代被发现，但像MMTV这样的逆转录病毒用来复制自己基因组的机制和细胞用来防御逆转录病毒感染的策略还不是很清楚。除了外源性逆转录病毒的威胁外，内源性逆转录元件，如LINE-1反转录转座子，通过失调控制细胞生长的细胞基因的表达，导致DNA双链断裂，并通过诱导基因组不稳定，在细胞内活跃复制并导致癌症。我们的初步数据表明，MMTV可能与细胞质中与LINE-1反转录转座子报道的同一组宿主因子相互作用，这提出了一种有趣的可能性，即共同的细胞途径已经进化成对多种类型的逆转录元件的防御。在MMTV中，病毒衣壳在细胞质中与宿主mRNA处理因子Yb1、Mov10和Ago2相关的离散位置形成。我们设想，MMTV RNA可能会被走私到这些网站，以避免翻译机器，而是被包装成组装病毒衣壳。然而，对于LINE-1，这些相同的宿主因子似乎在宿主防御努力中发挥核心作用，以降解LINE-1RNA并限制逆转录转座。综上所述，这些观察结果导致了一种假设，即mRNA处理的细胞质部位代表了试图促进自身复制的MMTV和LINE-1编码因子与试图限制复制的宿主因子之间的战场。我们将通过两个具体目标来检验这一假设。首先，我们将以致癌逆转录病毒MMTV为研究对象，研究细胞内加工因子mRNA表达水平的改变对病毒衣壳组装、病毒产量和病毒感染性的影响。我们的目标是确定信使核糖核酸加工因子是促进病毒复制的步骤，还是干扰MMTV的组装和传染性。无论结果如何，这些实验都将是信息量大的，并导致对MMTV生物学的更深入理解。其次，我们将通过检查MMTV衣壳和LINE-1编码的核糖核蛋白复合体在同一细胞中的定位来确定是否有共同的细胞途径作用于MMTV和LINE-1逆转录元件。最后，将使用域交换功能增益方法来确定控制其亚蜂窝贩运的MMTV和LINE-1元件的特征。这些实验结果将为有助于癌症发展的逆转录元件-宿主相互作用提供新的见解。在未来，我们可能会发现，如果这些宿主相互作用被其他逆转录元件共享，这些结果可能会广泛适用。最终，这项研究可能会为逆转录病毒和逆转录转座子引起的癌症的预防或治疗提供新的靶点。

DESCRIPTION (provided by applicant): Retroviruses are ubiquitous agents that cause cancer in humans and animals. For the retrovirus mouse mammary tumor virus (MMTV), cancer arises when the viral genetic material becomes integrated into the host chromosome and activates the expression of cellular proto-oncogenes. Although discovered in the 1930s, the mechanisms used by retroviruses like MMTV to replicate their genomes and the strategies used by cells to defend against retrovirus infection are not well understood. In addition to the threat of exogenous retroviruses, endogenous retro-transcribing elements, like the LINE-1 retrotransposon, actively replicate in cells and contribute to cancer by dysregulating the expression of cellular genes that control cell growth, by causing DNA double-stranded breaks, and by inducing genomic instability. Our preliminary data suggest that MMTV may interact with the same group of host factors in the cytoplasm as reported for the LINE-1 retrotransposon, raising the intriguing possibility that a common cellular pathway has evolved to defend against multiple types of retro-transcribing elements. In the case of MMTV, viral capsids are formed at discrete sites in the cytoplasm that associate with host mRNA processing factors Yb1, Mov10, and Ago2. We envision that the MMTV RNA might be trafficking to these sites to avoid translation machinery, instead being packaged into assembling virus capsids. However, for LINE-1, these same host factors appear to play a central role in host defensive efforts to degrade LINE-1 RNA and limit retrotransposition. Together, these observations led to the hypothesis that cytoplasmic sites of mRNA processing represent the battleground between MMTV and LINE-1- encoded factors trying to promote their own replication and host factors trying to restrict replication. We will test this hypothesis through two specific aims. First, focusing on the oncogenic retrovirus MMTV, we will examine the effect of altered expression levels of cellular mRNA processing factors on viral capsid assembly, virus production, and virus infectivity. Our goal is to determine whether mRNA processing factors promote steps in virus replication or interfere with MMTV assembly and infectivity. Regardless of the outcome, these experiments will be informative and lead to a deeper understanding of MMTV biology. Second, we will determine whether a common cellular pathway acts on MMTV and LINE-1 retroelements by examining the localization of MMTV capsids and LINE-1 encoded ribonucleoprotein complexes in the same cells. Finally, features of MMTV and LINE-1 elements that control their subcellular trafficking will be identified using a domain-swapping gain-of-function approach. These experimental results will provide novel insights into retroelement-host interactions that contribute to the development of cancer. In the future, we may find that these results are broadly applicable if these host interactions are shared by other retro-transcribing elements. Ultimately, this research may provide new targets for the prevention or treatment of cancers caused by retroviruses and retrotransposons.