Plasmid transfection influences the readout of nonsense-mediated mRNA decay reporter assays in human cells

Messenger RNA (mRNA) turnover is a crucial and highly regulated step of gene expression in mammalian cells. This includes mRNA surveillance pathways such as nonsense-mediated mRNA decay (NMD), which assesses the fidelity of transcripts and eliminates mRNAs containing a premature translation termination codon (PTC). When studying mRNA degradation pathways, reporter mRNAs are commonly expressed in cultivated cells. Traditionally, the molecular mechanism of NMD has been characterized using pairs of reporter constructs that express the same mRNA with (PTC-containing mRNA) or without (wild-type mRNA) a PTC. Cell lines stably expressing an NMD reporter have been reported to yield very robust and highly reproducible results, but establishing the cell lines can be very time-consuming. Therefore, transient transfection of such reporter constructs is frequently used and allows analysis of many samples within a short period of time. However, the behavior of transiently and stably transfected NMD constructs has not been systematically compared so far. Here, we report that not all commonly used human cell lines degrade NMD targets following transient transfection. Furthermore, the degradation efficiency of NMD substrates can depend on the manner of transfection within the same cell line. This has substantial implications for the interpretation of NMD assays based on transient transfections.