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Unformatted text preview: lly identical to the recipient of the transplant, who was the donor of the adult somatic cell nucleus. This bypasses the barrier of the immune system in rejecting the transplanted tissue.
This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. 2009 Sinauer Associates, Inc. UNCORRECTED PAGE PROOFS
CELL DEATH AND CELL RENEWAL 27 FIGURE 17.26 Therapeutic cloning In therapeutic cloning, the nucleus of a patient's cell would be transferred to an enucleated egg, which would be cultured to an early embryo. Embryonic stem cells would then be derived, differentiated into the desired cell type and transplanted back into the patient. The transplanted cells would be genetically identical to the recipient (who was the donor of the adult nucleus), so complications of immune rejection would be avoided. Unfertilized egg Remove egg chromosomes Adult somatic cell from patient Transfer nucleus to enucleated egg The possibility of therapeutic cloning provides the most general approach to treatment of the wide variety of devastating disorders for which stem cell transplantation therapy could be applied. However, although some success has been achieved in animal models, there remain major obstacles that would need to be overcome before therapeutic cloning could be applied to humans. Substantial improvements would be needed to overcome the low efficiency with which embryos are generated by somatic cell nuclear transfer. In addition, therapeutic cloning by somatic cell nuclear transfer raises ethical concerns, not only with respect to the possibility of cloning human beings (reproductive cloning), but also with respect to the destruction of embryos that serve as the source of embryonic stem cells. These concerns may be alleviated by recent advances in reprogramming somatic cells to a pluripotent state resembling embryonic stem cells. Culture to early embryo Blastocyst Culture embryonic stem cells Induced Pluripotent Stem Cells
Given both the technical and ethical difficulties in the derivation of embryonic stem cells by somatic cell nuclear transfer, a major advance in the field has come from studies demonstrating that adult somatic cells can be directly converted to pluripotent stem cells in culture. This circumvents the need for derivation of embryos and provides a direct mechanism for converting somatic cells to stem cells which, like embryonic stem cells, have the potential of developing into all tissues of an organism. The conversion (or reprogramming) of somatic cells to pluripotent stem cells was first reported by Kazutoshi Takahashi and Shinya Yamanaka in 2006. They found that mouse fibroblasts could be reprogrammed to cells resembling embryonic stem cells (called induced pluripotent stem cells) by the action of only four transcription factors introduced by retroviral gene transfer ( Figure 17.27 ). Subsequent studies have shown that induced pluripotent stem cells, like embryonic stem cells, are capable of differentiating into all cell types when introduced into early mouse embryos. That the action of only four key transcription factors is sufficient to reprogram adult somatic cells to pluripotent stem cells is a remarkable finding, which raises a number of intriguing questions as to the transcriptional programs that control cell fate. Importantly, further research has also demonstrated that adult human fibroblasts can be reprogrammed to pluripotency by a similar procedure. Thus, it is now possible to convert skin cells from a patient directly to induced pluripotent stem cells in culture, providing a new route to the derivation of pluripotent stem Differentiate to desired cell type (e.g., neurons) Transplant back to patient This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. 2009 Sinauer Associates, Inc. UNCORRECTED PAGE PROOFS
CHAPTER 17 Adult mouse fibroblast FIGURE 17.27 Induced pluripotent stem cells Adult mouse fibroblasts in culture are converted to pluripotent stem cells by infection with retroviral vectors (see Figure 4.34) carrying genes for four transcription factors: Oct3/4, Sox2, Klf4, and c-Myc. Infection with retroviruses carrying genes for Oct3/4, Sox2, Klf4 and c-Myc Induced pluripotent stem cell cells for use in transplantation therapy. However, problems still remain to be addressed. Some of the transcription factors (e.g., c-Myc) used to reprogram fibroblasts to pluripotent stem cells can act as oncogenes to cause cancer, although it may be possible to substitute these potentially harmful transcription factors with others that do not have oncogenic potential. Additionally, the retroviral vectors that have been used to introduce genes into fibroblasts can themselves cause harmful mutations leading to cancer development, so alternative methods of gene transfer will need to be developed for safe therapeutic applications. Nonetheless, overcoming these difficulties ma...
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