Role of the mitochondrial RNA polymerase POLRMT in MYC function

描述(由申请人提供)：癌细胞表现出与正常细胞代谢的巨大差异。目前，人们非常乐观地认为，癌细胞特有的代谢特征可能代表着可行的治疗靶点。包括我们在内的许多研究小组的工作已经明确地表明，MYC癌蛋白是发生在人类癌细胞中的新陈代谢重新编程的中心调节因子。为了深入了解MYC在人类癌症中的作用，我们最近进行了一次无偏见的表达谱筛选，以确定其转录与恶性转化密切相关的关键下游靶点。通过筛选，我们鉴定了POLRMT基因。POLRMT是一个核基因，它编码单链RNA聚合酶分子mtRNAP，负责从小的环状线粒体基因组(MtDNA)转录。(此前，MYC曾被证明调节三种主要的核RNA聚合酶分子。)对MYC诱导POLRMT/mtRNAP的功能研究表明，MYC通过控制POLRMT的表达来调节线粒体内的所有转录。此外，由于mtRNAP转录复制mtDNA所需的RNA引物，MYC也调节细胞线粒体基因组的内容。基因拯救实验表明，这两种影响完全依赖于mtRNAP，而不依赖于最近描述的POLRMT编码的核异构体spRNAP-IV。MYC和POLRMT的表达之间的联系已经在各种人类癌细胞系中得到证实，并得到了公共癌症基因表达数据集的证据支持。MYC还调节原代人类细胞中POLRMT的表达。最重要的是，阻断MYC诱导POLRMT表达的能力会导致强烈的细胞凋亡，这严格依赖于激活的MYC的存在。重新激活MYC潜在的凋亡潜能或“固有的肿瘤抑制活性”的策略被广泛认为是未来抗癌治疗的一个令人兴奋的机会。最后，阻断POLRMT诱导的MYC依赖性细胞凋亡似乎是由于POLRMT/mtRNAP在电子传递链线粒体编码成分的表达中的独特要求以及随后对嘧啶生物合成途径的影响所致。几十年来，靶向嘧啶生物合成一直被认为是一种成功的抗癌治疗方法，我们的数据表明，诱导POLRMT/mtRNAP可能是干预这一途径的一个新的节点，特别是在表达高水平MYC的人类癌症中。这里提出的研究将验证这一假说，并提供我们已经确定的途径的详细机制理解，以及它为肿瘤细胞提供的基本的MYC依赖的生存功能。

DESCRIPTION (provided by applicant): Cancer cells exhibit profound metabolic differences from their normal counterpart. Currently, there is significant optimism that the metabolic traits unique to cancer cells may represent viable therapeutic targets. Work from a number of groups, ours included, has firmly implicated the MYC oncoprotein as a central regulator of the metabolic reprogramming that takes place in human cancer cells. To gain insight into MYC's role in human cancer, we recently performed an unbiased expression profiling screen to identify essential downstream targets whose transcription is tightly correlated with malignant transformation. Via the screen, we identified the POLRMT gene. POLRMT is a nuclear gene that encodes the single chain RNA polymerase molecule, mtRNAP that is responsible for transcription from the small, circular mitochondrial genome (mtDNA). (MYC had previously been shown to regulate the three major nuclear RNA polymerase molecules.) Functional studies of the induction of POLRMT/mtRNAP by MYC show that MYC regulates all transcription within the mitochondria via its control of POLRMT expression. Furthermore, because mtRNAP transcribes the RNA primers required for replication of the mtDNA, MYC regulates cellular mitochondrial genome content as well. Genetic rescue experiments show that both of these efects depends exclusively on mtRNAP and not on a recently described nuclear isoform encoded by POLRMT, spRNAP-IV. The link between MYC and POLRMT expression has been confirmed in a variety of human cancer cell lines and is supported by evidence from public cancer gene expression datasets. MYC also regulates POLRMT expression in primary human cells. Most importantly, blocking the ability of MYC to induce POLRMT expression results in robust apoptosis that strictly depends on the presence of activated MYC. Strategies that reactivate the latent apoptotic potential or "intrinsic tumor suppressor activity" of MYC are widely viewed as an exciting opportunity for future anti-cancer therapy. Finally, the MYC-dependent apoptosis that results from blocking POLRMT induction appears to result from a unique requirement for POLRMT/mtRNAP in the expression of mitochondrial-encoded components of the electron transport chain and subsequent effects on the pyrimidine biosynthesis pathway. Targeting pyrimidine biosynthesis has been recognized as a successful anti-cancer therapy for many decades and our data suggest that induction of POLRMT/mtRNAP may represent a novel node for intervention in this pathway, specifically in the forms of human cancer that express high levels of MYC. The studies proposed here will test this hypothesis and provide a detailed mechanistic understanding of the pathway we have identified, along with the essential, MYC-dependent survival function it provides to tumor cells.