Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India.
Oncogene. 2013 Aug 29;32(35):4148-59. doi: 10.1038/onc.2012.422. Epub 2012 Oct 1.
Tumor-suppressor protein p53, the 'guardian of the genome', is critical in maintaining cellular homeostasis and genomic stability. Earlier, we have reported the discovery of internal ribosome entry sites (IRESs) within the p53 mRNA that regulate the translation of the full length and its N-terminal-truncated isoform, ΔN-p53. Polypyrimidine tract-binding protein (PTB) is an IRES trans-acting factor that positively regulates the IRES activities of both p53 isoforms by relocating from nucleus to the cytoplasm during stress conditions. Here we have demonstrated the putative contact points of PTB on the p53 IRES RNA. Studies on mutations that occur naturally in the 5' untranslated region (5' UTR) in p53 mRNA were lacking. We have investigated a naturally occurring C-to-T single-nucleotide polymorphism (SNP) first reported in human melanoma tumors. This SNP is at position 119 in the 5' UTR of p53 mRNA and we demonstrate that it has consequences on the translational control of p53. Introduction of this SNP has led to decrease in cap-independent translation from p53 5' UTR in bicistronic reporter assay. Further, the effects of this SNP on cap-independent translation have been studied in the context of p53 cDNA as well. Interestingly, the 5' UTR with this SNP has shown reduced binding to PTB that can be corroborated to its weaker IRES activity. Previously, it has been shown that G2-M checkpoint, DNA-damaging stress and oncogenic insult favor IRES-mediated translation. Under similar conditions, we demonstrate that this SNP interferes with the enhancement of the IRES activity of the 5' UTR. Taken together, the results demonstrate for the first time that SNP in the 5' UTR of the p53 mRNA might have a role in translational control of this critical tumor-suppressor gene.
抑癌蛋白 p53 是基因组的“守护者”，对于维持细胞内稳态和基因组稳定性至关重要。我们之前曾报道过 p53 mRNA 内存在内部核糖体进入位点（IRES），可以调节全长 p53 及其 N 端截断型异构体 ΔN-p53 的翻译。多嘧啶 tract 结合蛋白（PTB）是一种 IRES 反式作用因子，在应激条件下从核内转移到细胞质中，可正向调节这两种 p53 异构体的 IRES 活性。本文我们已证实了 PTB 在 p53 IRES RNA 上的假定结合点。目前还缺乏关于 p53 mRNA 5'非翻译区（5'UTR）中自然发生突变的研究。我们研究了一种在人类黑色素瘤肿瘤中首次报道的自然发生的 C 到 T 单核苷酸多态性（SNP）。该 SNP 位于 p53 mRNA 5'UTR 的第 119 位，我们证明其对 p53 的翻译调控有影响。在双顺反子报告实验中，引入该 SNP 会导致 p53 5'UTR 无帽依赖翻译减少。此外，我们还在 p53 cDNA 背景下研究了该 SNP 对无帽依赖翻译的影响。有趣的是，具有该 SNP 的 5'UTR 与 PTB 的结合减少，这与其较弱的 IRES 活性相符。之前的研究表明，G2-M 检查点、DNA 损伤应激和致癌损伤有利于 IRES 介导的翻译。在类似条件下，我们证明该 SNP 会干扰 5'UTR 的 IRES 活性增强。综上所述，这些结果首次表明，p53 mRNA 5'UTR 中的 SNP 可能在该关键肿瘤抑制基因的翻译调控中发挥作用。
