Expression of iPS Factors from Transfected mRNA

Differentiated cells can be reprogrammed to pluripotency by enforced expression of certain combinations of stem cell-specific protein factors in them. The power of this method was first demonstrated by Yamanaka’s group using retroviruses carrying Oct3/4, Sox2, c-Myc, and Klf4. Alternative factors such as Lin28 and Nanog, and additional factors such as the human telomerase gene hTert and shRNA against p53 were also shown to contribute to reprogramming. From the very beginning it was realized that viral integration would pose a major problem in using the induced pluripotent stem cells (iPSCs) for clinical purposes. There have been multiple attempts to circumvent this problem by using non-integrating vectors such as plasmid, minicircle DNA, adenovirus, baculovirus, removable transposons, episomal DNA, or by introducing recombinant proteins with a transmembrane domain into target cells. From reports in the field and customer feedbacks it seems that retroviral or lentiviral systems are still the most efficient in reprogramming. mRNA is about the only option left unreported, until an article by Warren et al was published in Cell Stem Cell online recently.

From that report, it is clear that the reason that it took so long for RNA-induced iPSCs (RiPSCs) to appear in the literature was because synthetic mRNAs activate interferon responses in mammalian cells, reminding us of the early days of RNAi. The authors took a number of steps to reduce interferon responses, including adding a 5’-cap (actually a fairly standard step in in vitro transcription), using a phosphatase to remove 5’ triphosphates on uncapped mRNAs, and using modified C and U bases (5-methucytidine or 5mC and pseudouridine or psi) during T7 promoter-driven in vitro transcription. The prepared mRNA was then administered everyday for 17 days at an amount not clearly defined in the paper. The main benefit of this method is of course that there is no gene integration to alter the chromosome. The efficiency of the new method was also compared to using viral vectors and it was shown that 1.4% conversion efficiency was achieved vs retroviral systems’ 0.01% (although we have experienced better results using lentivirus, at least the 4-in-1 version).

The DNA templates used for in vitro transcription of the iPS factors were created by multiple PCR reactions and bridged ligation; it could also be done by other cloning strategies. For those excited about trying this new way of making iPSCs, the major hassle would be preparing modified mRNAs good and abundant enough for 17 consecutive transfections. Allele Biotech would like to provide custom services, before offering shelf products, for creating such mRNAs as the method sounds potentially very helpful to many researchers in the iPSC field.

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Wednesday, October 6th, 2010 iPSCs and other stem cells, Viruses and cells

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