by introducing sets of genes into, first, murine somatic cells (Takahashi and Yamanaka, 2006) and, later, human somatic cells (Takahashi et al., 2007; Yu et al., 2007; Park et al., 2008), it seems prudent to consider more explicitly which provisions of the Guidelines should apply also to stem cells of types other than hES cells. This is not to suggest that the need for research with hES cells is supplanted by the availability of other pluripotent stem cells. It is far from clear at this point which cell types will prove to be the most useful for regenerative medicine, and it is likely that each will have some utility. Such iPS cells are currently derived by introduction of retroviruses that carry the inducing genes. This derivation procedure raises serious issues about their potential for use in therapy, inasmuch as it is known that inserted retroviruses can cause cancer, and research will be necessary to develop alternative means to derive iPS cells or to circumvent the potential tumorigenicity. Furthermore, the demonstration that iPS cells are indeed pluripotent relies on careful comparisons with hES cells; for either cell type to be used therapeutically in regenerative medicine, methods need to be developed to promote their differentiation into specialized cell types and to evaluate the safety of introducing cell populations that may contain some pluripotent cells into patients. Much further research will be required on both hES and iPS cells to develop the required procedures, including drawing appropriate comparisons between them. Understanding of the potential for differentiation of hES cells, iPS cells, or, indeed, adult multipotent (capable of differentiation into a limited spectrum of differentiated cell types)3 stem cells will require testing in animals and screening for potential tumorigenicity. Therefore, issues arising from such human-animal chimera experiments pertain to all these cell types.

For those reasons and in response to inquiries from the scientific community, the Human Embryonic Stem Cell Research Advisory Committee has consulted with experts and carefully considered potential modifications of the Guidelines to cover other pluripotent and multipotent stem cells, which the Committee presents herein. The intention is not to extend unnecessarily the oversight of stem cell research where it is already adequately monitored under existing regulations and guidelines. For example, derivation of human pluripotent stem cell lines from sources other than embryos does not involve ethical or policy issues beyond those normally encountered in sampling any tissue from human subjects, although use of such cells may raise issues similar to those for embryonically derived cells. Derivation of iPS cells and

3

A multipotent stem cell can give rise to other types of cells but it is limited in its ability to differentiate. An example is found in the multipotent stem cells in bone marrow that give rise to all blood cells but not other cell types.



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