The cloning process also appears to reset the "aging clock" in cloned cells, so that the cells appear younger in some ways than the cells from which they were cloned. In 2000 we reported that telomeres¿the caps at the ends of chromosomes¿from cloned calves are just as long as those from control calves. Telomeres normally shorten or are damaged as an organism ages. Therapeutic cloning may provide "young" cells for an aging population.
A report last July by Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass., and his colleagues gained much attention because it found so-called imprinting defects in cloned mice. Imprinting is a type of stamp placed on many genes in mammals that changes how the genes are turned on or off depending on whether the genes are inherited from the mother or the father. The imprinting program is generally "reset" during embryonic development.
Although imprinting appears to play an important role in mice, no one yet knows how significant the phenomenon is for humans. In addition, Jaenisch and his co-workers did not study mice cloned from cells taken from the bodies of adults, such as fibroblasts or cumulus cells. Instead they examined mice cloned from embryonic cells, which might be expected to be more variable. Studies showing that imprinting is normal in mice cloned from adult cells are currently in press and should be published in the scientific literature within several months.
Meanwhile we are continuing our therapeutic cloning experiments to generate cloned or parthenogenetically produced human embryos that will yield stem cells. Scientists have only begun to tap this important resource.
JOSE B. CIBELLI, ROBERT P. LANZA and MICHAEL D. WEST are vice president of research, vice president of medical and scientific development, and president and CEO, respectively, of Advanced Cell Technology, a privately held biotechnology company in Worcester, Mass. Cibelli received his D.V.M. from the University of La Plata in Argentina and his Ph.D. from the University of Massachusetts at Amherst. His research led to the creation of the first cloned genetically modified calves in 1998. Lanza has an M.D. from the University of Pennsylvania. He is a former Fulbright scholar and is the author or editor of numerous popular and scientific books, including the text Principles of Tissue Engineering. West holds a Ph.D. from Baylor College of Medicine and is particularly interested in aging and stem cells. From 1990 until 1998 he was founder, director and vice president of Geron Corporation in Menlo Park, Calif., where he initiated and managed research programs in the biology of telomeres (the ends of chromosomes, which shrink during aging) and the effort to derive human embryonic stem cells. Carol Ezzell is a staff writer and editor.
Human Therapeutic Cloning. Robert P. Lanza, Jose B. Cibelli and Michael D. West in Nature Medicine, Vol. 5, No. 9, pages 975¿977; September 1999.
Prospects for the Use of Nuclear Transfer in Human Transplantation. Robert P. Lanza, Jose B. Cibelli and Michael D. West in Nature Biotechnology, Vol. 17, No. 12, pages 1171¿1174; December 1999.
The Ethical Validity of Using Nuclear Transfer in Human Transplantation. Robert P. Lanza et al. in Journal of the American Medical Association, Vol. 284, No. 24; December 27, 2000.
The Human Embryo Research Debates: Bioethics in the Vortex of Controversy. Ronald M. Green. Oxford University Press, 2001.
The full text of our article in e-biomed: The Journal of Regenerative Medicine can be viewed at www.liebertpub.com/ebi