Stadtfeld M, Nagaya M, Utikal J, Weir G, Hochedlinger K.
Induced pluripotent stem cells generated without viral integration.
Science. 2008 Nov 7;322(5903):945-9.
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This work builds on the work by Yamanaka and others showing that relatively short periods of exposure and/or sequential exposure to reprogramming signals is sufficient to transform cells into pluripotent cells. This raised the possibility that episomal/transient vectors, protein transduction strategies and small molecules may work.
In this manuscript the authors have shown that inducible adenovirus persists for sufficiently long periods to reprogram cells and as such minimizes risks associated with nonrandom integration and disruption of potentially important genes. These induced cells appeared similar to cells derived by other integrating methods and were capable of robust chimera formation.
While clearly an important step forward, several issues remain. The authors note that adenovirus infection efficiency is variable in different cell types. Persistence and levels of expression are variable as well, and both of these likely reduce the efficiency of reprogramming. Indeed, the authors used liver cells for their experiments as these are much more efficiently infected with adenovirus as compared to fibroblasts. Experiments were performed with rodent cells, and it is unclear if the longer cycle time and longer period of induction to pluripotency required will represent a benefit or a hindrance to this methodology. Some of the adeno-associated viruses integrate into the genome, albeit at a very low frequency, and it will be important to test for such integration.
Nevertheless, these experiments represent an important first step in the transition to the clinic. Other episomal viruses exist (some of which persist for longer periods), several methods can be used to improve viral uptake, and some viruses appear to infect far more efficiently. We have no doubt that this group and others are already trying these alternatives.