Does NMD prevent the disruption of genes from genomis integration of Alu elements?

尽管人类基因组中最丰富的散置元件--内含子Alu元件已经被预测具有成为替代外显子的能力，但只有少数基因通过未知的基因组基础以剪接变体的形式真正整合了Alu元件。在这里，我们测试了我们的假设，无义介导的mRNA衰变（NMD），mRNA的监视系统，其中含有提前终止密码子（PTC）的mRNA被选择性地检测和破坏，可以作为转录后消除剂的含剪接变异体（ASV）。对21个人类基因中ASV表达水平的检测显示，ASV通常作为次要等位基因存在于多种人类组织以及HeLa和SH-SY 5 Y细胞中。在这些细胞中去除NMD导致ASV在一些基因中的上调，这表明NMD在抑制ASV表达中的部分作用。此外，在ADARB 1基因中连续引入PTC（ASV作为主要等位基因表达）未能触发NMD。为了阐明这种对NMD的不敏感性是否受ASV低表达的调节，ASV的表达受剪接效率的改变的影响，剪接效率的改变是由核苷酸取代引起的，而不是由NMD引起的。ASV中供体剪接位点的强度弱于组成型和选择性外显子，ESES的数量也少于组成型和选择性外显子。这些结果表明，大多数含有PTC的ASV在天然状态下以低水平产生，并且仅对NMD部分敏感。

Although thousands of intronic Alu elements, the most abundant interspersed elements in human genome, have predicted capability to become alternative exons, only a small number of genes actually incorporate Alu elements as expressed splicing variants by unknown genomic basis. Here we tested our hypothesis that nonsense-mediated mRNA decay (NMD), an mRNA surveillance system by which mRNAs containing premature termination codons (PTCs) are selectively detected and disrupted, can serve as post-transcriptional eliminator of Alu-containing splicing variants (ASVs). Examination of the expression level of ASVs in 21 human genes revealed that ASVs are generally present as minor alleles in multiple human tissues as well as in HeLa and SH-SY5Y cells. Removal of NMD in these cells resulted in an upregulation of ASVs in some genes, suggesting a partial role of NMD in suppressing the expression of ASVs Furthermore, a serial introduction of PTCs in the ADARB1 gene, of which ASV is expressed as a major allele, failed to trigger NMD. To delineate whether this insensitivity to NMD is modulated by low expression of ASVs, The expression of ASVs is affected by the change of splicing efficiency, which caused by substitutions of nucleotide, rather than by NMD. The strength of donor splice sites in ASVs was weaker than in constitutive and alternative exons and the number of ESEs in ASVs were smaller than in constitutive and alternative exons. These results suggested that the majority of ASVs containing PTCs are produced at low level in native status and are only partially sensitive to NMD.