The effect of oncogenic Ras signaling strength on cancer

描述（由申请人提供）：三分之一的人类癌症中发生了KRAS突变，较小程度上是NRAS突变，很少是HRAS突变，这已被证实会促进肿瘤发生。使用两种独立的方法来操纵致癌KRas信号在内源性水平上，我们表明，这种信号的大小会导致相反的影响，这取决于肿瘤发生的阶段。具体来说，我们在第一次提交的资助报告中报道，野生型HRas和NRas蛋白在致癌KRas下游被激活，当被敲低时，抑制癌细胞系的肿瘤生长。相反，我们现在发现，小鼠中野生型Hras 1的基因消融促进了早期致癌Kras驱动的肿瘤发生。独立于这一调查路线，我们最近发现，KRas的表达非常差相比，HRas由于罕见的密码子的偏见。将罕见密码子改变为常见密码子产生更多的致癌KRas蛋白，增强了已建立的细胞系的肿瘤生长，但抑制了小鼠早期病变的发展。由于致癌Ras可以诱导正常细胞的衰老生长停滞，我们假设KRas在正常细胞中的表达范围非常窄-高到足以促进增殖，但低到足以避免过度衰老-突变时导致增生。我们建议在AIM 1中通过确定操纵野生型Ras蛋白的表达和KRAS的密码子偏好对早期肿瘤发生期间衰老的影响来测试这一假设。然而，低致癌性KRas信号传导仍然抑制了已建立的细胞系的肿瘤生长。因此，我们假设在肿瘤发展的后期阶段存在克服由稀有密码子施加的不良KRas表达的选择性压力。在这方面，在许多人类癌症中突变KRAS等位基因的拷贝数增加，这与更差的预后相关。因此，我们建议在AIM 2中测试在耐受高癌基因表达的环境中将罕见密码子改变为常见密码子是否会抑制肿瘤发生期间Kras拷贝数的增加。我们的初步研究结果表明，拷贝数的增加并不是增加KRas蛋白表达的唯一机制。因此，我们还建议确定KRas蛋白表达在肿瘤发生过程中是如何升高的，以及密码子偏好是否也调节其他癌症相关基因的表达。这项研究的完成将阐明内源性致癌KRas信号传导水平如何差异影响早期和晚期肿瘤发生的分子机制。

DESCRIPTION (provided by applicant): KRAS, to a lesser extent NRAS, and rarely HRAS, are mutated in a third of human cancers, which is well established to promote tumorigenesis. Using two independent approaches to manipulate oncogenic KRas signaling at the endogenous level, we show that the magnitude of this signaling causes opposite effects depending on the stage of tumorigenesis. Specifically, we reported in the first grant submission that wild-type HRas and NRas proteins are activated downstream of oncogenic KRas, and when knocked down, inhibited the tumor growth of cancer cell lines. Conversely, we now find that genetic ablation of wild-type Hras1 in mice promotes early oncogenic KRas-driven tumorigenesis. Independent of this line of investigation, we recently discovered that KRas is expressed very poorly compared to HRas owing to a bias of rare codons. Changing the rare to common codons produced more oncogenic KRas protein, enhanced tumor growth of established cell lines, but inhibited development of early lesions in mice. Since oncogenic Ras can induce a senescent growth arrest in normal cells, we hypothesize that KRas is expressed in a very narrow range in normal cells - high enough to promote proliferation but low enough to avoid excessive senescence - leading to hyperplasia when mutated. We propose to test this hypothesis in AIM 1 by determining the impact of manipulating the expression of wild-type Ras proteins and the codon bias of KRAS on senescence during early tumorigenesis. However, low oncogenic KRas signaling nevertheless inhibited tumor growth of established cell lines. As such, we hypothesize that there is selective pressure to overcome poor KRas expression imposed by rare codons during later stages of tumor development. In this regard, there is a gain in the copy number of the mutant KRAS allele in many human cancers, which is associated with a worse prognosis. We thus propose in AIM 2 to test whether changing the rare to common codons in settings that tolerate high oncogene expression suppresses a gain in Kras copy number during tumorigenesis. Our preliminary findings indicate that a gain in copy number is not the only mechanism to increase KRas protein expression. Thus, we also propose to identify how else KRas protein expression is elevated during tumorigenesis, and whether codon bias also regulates the expression of other cancer-related genes. Completion of this study will elucidate the molecular mechanisms underlying how the level of endogenous oncogenic KRas signaling differentially impacts early and late tumorigenesis.