Stem cell research offers great promise for understanding basic mechanisms of human development and differentiation, as well as the hope for new treatments for diseases such as diabetes, myocardial infarction and Parkinson’s disease. However, human embryonic stem (ES) cell research also raises ethical and political controversies. The groundbreaking work of Shinya Yamanaka has shown that adult mouse and human fibroblasts can be transformed into induced pluripotent stem (iPS) cells by retroviral transduction with only four genes encoding the transcription factors c-Myc, Oct4, Klf4, and Sox2. This work was confirmed by others in murine and human systems. The reprogramming of somatic cells to iPS cells avoids the ethical problems specific to ES cell research. Reprogramming fibroblasts to stem cells makes it possible to create unique pluripotent stem cells to treat individual patients and study human disease. Moreover, concerns regarding the safety of human pluripotent stem cells (hPSCs), in particular their tumorigenic potential must be fully addressed before the hPSC technologies can be widely applied in clinical regenerative medicine. Transplanted cells containing undifferentiated hPSCs may give rise to benign teratomas or malignant tumors. The approaches of viral vector-based techniques can also directly eliminate the tumorigenic potential of hPSCs (Figure 1).
Figure 1. The new approaches of viral vector-based techniques for directly eliminating the tumorigenic potential of hPSCs.
Retroviral vectors are the most widely used vehicles for gene therapy and transgene delivery in cultured cells and animal models. They also played a seminal role in the historical development of iPSC technology because of their ability to efficiently transduce murine cells for a lasting expression of transgenes. LV is ubiquitously used for reprogramming of human cells, rat cells, murine cells, and pig cells. One advantage of LV is their abilities to transduce non-dividing cells. Owing to this trait, LV was used to successfully reprogram murine terminal b cells, which divide only one time in culture. Adenoviral vectors are another widely used vehicle for the delivery of transgenes in reprogramming. Replication-defective adenoviral vectors have proven useful for gene insertion into cells without integration into chromosomal DNA. Gene expression can persist for days, which should provide sufficient time to reprogram fibroblasts to pluripotent cells. It has proved that human iPS cells generated with adenovirus are free of any viral or transgene integration.
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