Nonsense Mediated mRNA Decay Pathway

信使RNA(Messenger RNA，mRNA)降解是一个在基因表达调控中起重要作用的过程。信使核糖核酸的衰减率变化很大，可以根据环境信号进行调节。许多研究表明，信使核糖核酸的周转与翻译有关。一条已经被广泛研究并清楚地例证了翻译和信使核糖核酸转换之间的联系的途径是无意义介导的信使核糖核酸衰变通路(NMD)。基因的无义突变会加速该基因转录的信使核糖核酸的衰变。真核生物中的NMD途径是一种保守的机制，可以使细胞摆脱干扰细胞功能的缺陷基因产物。大约15%到30%的遗传疾病是由无义突变引起的。因此，在研究了调节NMD途径的顺式和反式作用因素后，了解这一途径的功能应该能够导致治疗所有患有目前尚无治愈方法的遗传性疾病的患者。UPF1/MOF4/IFS2、UPF2/NMD/SUA/IFS1、UPF3/SUA6、MOF2/SU11、MOF5和MOF8基因突变导致含有无义的mRNAs积累增加，而对大多数野生型转录本的丰度没有影响。这些研究也证明了翻译的多重过程与衰败之间的联系。特别是，我们已经证明在翻译终止、过早终止翻译的转录本的降解和核糖体移码之间存在串扰。基于这些观察，我们假设存在一个“监视复合体”来监视这些过程并提高它们的保真度。我们还表明，监视复合体通过与终止密码子的RNA结合蛋白3‘相互作用来识别RNA为异常。在这些研究的基础上，我们提出了一个模型，该模型认为，在翻译的最初几轮中，监督复合体通过监测RNP从核形式转换到细胞质形式的转变来评估翻译的终止。这项拨款计划中实验的目的是研究NMD途径中涉及的序列和因素，以剖析其机制和它是如何被调控的。

Messenger RNA (mRNA) degradation is a process that plays an important role in the regulation of gene expression. mRNA decay rates vary greatly and can be modulated in response to environmental signals. Many studies have demonstrated that mRNA turnover an be linked to translation. One pathway that has been extensively investigated and clearly exemplified the link between translation and mRNA turnover is the nonsense-mediated mRNA decay pathway (NMD). Nonsense mutations in a gene can accelerate the decay of the mRNA transcribed from that gene. The NMD pathway in eukaryotes is a conserved mechanism that rids cells of faulty gene products that can interfere with cell function. Approximately fifteen- to thirty percent of all genetic disorders arise as a consequence of nonsense mutations. Therefore, understanding how this pathway functions should e able to lead to treatments for a subset of all patients suffering from genetic disorders in which there is presently no cure, having been investigating both the cis- and trans-acting factors involved in regulating the NMD pathway. Mutations in the UPF1/MOF4/IFS2, UPF2/NMD/SUA/IFS1, UPF3/SUA6, MOF2/SU11, MOF5, and MOF8 genes result in an increased accumulation of nonsense-containing mRNAs while having no effect on the abundance of most wild-type transcripts. These studies also demonstrated a link between multiple process of translation and decay. In particular, we have shown that there is a cross-talk between the processes of translation termination, degradation of transcripts harboring premature translation termination, and ribosomal frameshifting. Based on these observations, we have hypothesized the presence of a "surveillance complex" that monitors these processes and increases their fidelity. We have also shown that the surveillance complex recognizes an RNA as aberrant by interacting with RNA binding proteins 3' of the termination codon. Based on these studies, we have proposed a model that suggests that the surveillance complex assesses translation termination by monitoring the transition of an RNP as it is converted from a nuclear to a cytoplasmic form during the initial rounds of translation. The goals of the experiments in this grant proposal are to investigate the sequences and factors involved in the NMD pathway in order to dissect its mechanism and how it is regulated.