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Hopes for ethical, safe therapeutic stem cell production
A new report details the induction of pluripotency in cells derived from human skin cells without the use of viral vectors. Induced pluripotent stem (iPS) cells were first produced in 2007 (see previous news) and were hailed as a potential source of therapeutic stem cells that would not provoke the ethical concerns raised by the use of human embryonic stem (HES) cells; however, the technique used to achieve this cellular reprogramming required the use of viral vectors to introduce genes. This was cause for concern because the use of viruses for therapeutic purposes is known to carry a risk of viral-induced tumour formation, as observed in rare cases for gene therapy (see previous news) and more recently for stem-cell transplantation (see previous news).
The new technique was developed by teams at the University of Edinburgh, UK and the University of Toronto, Canada and published in Nature. It delivers the key genes required to achieve pluripotency in differentiated cells (c-Myc, Klf4, Oct4 and Sox2) via the introduction of a vector comprising short peptide (amino acid) sequences linked to the genetic coding sequences, which directs expression of the attached genes [Kaji K et al. (2009) Nature Mar 1, Epub ahead of print]. The copies of these genes in the genoms of differentiated (specialised) cells are inactive or silenced. Analysis showed that a single copy of the vector was inserted to a specific site within the genome of target human or mouse to permit stable gene expression with minimal modification of the target cell genome. The process was found to be efficient with good expression of the inserted genes, and functional pluripotency demonstrated using in vitro differentiation.
Perhaps more impressively, the researchers were able to subsequently remove the non-viral vector and the genes it carried from target mouse cells using a transposase enzyme without affecting the newly induced pluripotent state of the cells. Active expression of the previously silent, endogenous pluripotency genes was sustained in the absence of the vector over multiple cell passages (ongoing cycles of in vitro growth), suggesting that the system allowed complete removal of the vector and external genes without disturbing the induced pluripotent state of the target cells.
The authors suggest that, while further work is required to optimize the system, it is a method that could provide an efficient means of generating iPS cells for therapeutic stem cell treatments, drug screening and the creation of in vitro models of disease for research purposes. Study leader Dr Keisuke Kaji commented: "It is a step towards the practical use of reprogrammed cells in medicine, perhaps even eliminating the need for human embryos as a source of stem cells" (see BBC news report).
Comment: By both eliminating the need for virus-derived vector sequences to induce pluripotency of human cells, and further demonstrating the potential to remove the vector and genes without disrupting the induced pluripotent state of the target cells, this work represents a major step forward in therapeutic medicine.The next key step would be to demonstrate effective removal of the vector and maintenance of pluripotency in human cells. A number of crucial stages would still remain, notably maintaining stable pluripotency and then reliably inducing the cells to re-differentiate as required for therapeutic purposes. However, this method could permit the creation of a greatly superior source of therapeutic stem cells, greatly reducing the risk of pathogenic processes linked to the inappropriate expression of viral or other externally-derived genes.