How would you like a clone of yourself stowed away somewhere in case you need a new heart or liver, like a spare tire in the trunk of a car? That, in a nutshell, was the plot of the 2005 high-dollar, low-attendance sci-fi movie, "The Island." Hollywood heartthrobs Scarlett Johansson and Ewan McGregor play dual roles portraying the rich and famous -- and their genetically identical clones. In an appropriate Orwellian twist, doctors must murder the "spare" clones in order to harvest needed body parts.
Chances are, "The Island" isn't a glimpse into the future. Nevertheless, it brings up a relevant point about the potential uses for human reproductive cloning. Organ transplants are difficult undertakings for two major reasons. First, you have to find a donor, and second, there's no guarantee that your body will accept the new organ. Statistically, organ demand far outweighs current supply. According to the Organ Procurement and Transplantation Network, 28,356 Americans received organ transplants in 2007 -- around 78 percent of those came from deceased people. Yet as of August 2008, more than 99,000 people in the United States were on the national waiting list for organs [source: OPTN].
What if you could eliminate the wait time and risky odds with traditional organ transplants by creating custom, cloned organs from your own cells that your body would recognize? Cloning advocates have touted this type of science as therapeutic cloning. This is different from reproductive cloning since therapeutic cloning deals with embryos only, not human babies carried to term.
Embryos contain pluripotent embryonic stem cells, meaning that they can differentiate into more than 200 types of cells. Scientists extract these stem cells when embryos are in the blastocyst phase, the stage when an embryo contains around 150 cells. The stem cells come from the interior of the blastocyst. In November 2007, scientists successfully cloned monkey embryos for the purpose of removing stem cells -- this is the closest we've ever come to performing the same procedure in humans. But removing the stem cells effectively destroys the embryo. Many people within and beyond the scientific community disagree with this practice of cloning that terminates embryos, sparking a continued debate about the bioethics of embryonic stem cell research.
Controversy aside, how would cloned organ transplants work? If you wanted to keep living, doctors obviously couldn't remove your heart and clone a new one, presto-chango. Cloning yourself in order to use the clone's organs wouldn't fly either. Here's where stem cells come in, along with recent scientific breakthroughs that sidestep cloning altogether.
How Organ Cloning Could Work
In order to understand how organ cloning might work, let's first talk about cloning itself. The most common method of therapeutic and reproductive cloning is somatic cell nuclear transfer (SCNT). SCNT involves removing the nucleus from a donor egg, and replacing it with the DNA from the organism meant to be cloned. Scientists could potentially clone organs with SCNT by cloning embryos, extracting the stem cells from the blastocyst, and stimulating the stem cells to differentiate into the desired organ. Coaxing a human stem cell to become a liver, for instance, will require further research. Scientists can reverse engineer cell differentiation processes to understand what chemical or physical signals stem cells receive to properly differentiate. However, that genetic information isn't known for all of the more than 200 types of body cells [source: The National Academies].
Research into human therapeutic cloning has largely come to a halt in the United States [source: Singer]. Aside from bioethical issues, there's a lack of available human eggs for research. Laws and ethical regulations from the National Academy of Sciences and the International Society for Stem Cell Research prohibit monetary compensation for females who donate their eggs for embryonic stem cell research. Coupled with the newness of the science and the potential risks involved with egg donation, stem cell researchers have been hard pressed to find donors. And given the low rate of success with embryonic cloning in general, researchers need an abundance of eggs if they hope to achieve progress. To compensate for the human egg scarcity, Ian Wilmut, who cloned Dolly the sheep, has suggested injecting human DNA into animal eggs instead [source: Singer].
Nevertheless, advancements in therapeutic cloning have been made in animal studies. In March 2008, researchers removed skin cells from mice with Parkinson's disease to test a way to use stem cells as an effective treatment. They inserted the DNA from those skin cells into enucleated eggs (eggs with the nuclei removed) and created cloned mice embryos, via SCNT [source: ScienceDaily]. After extracting stem cells from the cloned embryos, the researchers developed autologous dopamine neurons from them, which are the nerve cells affected by Parkinson's. After implanting the new neurons into the mice, the test animals exhibited signs of recovery [source: ScienceDaily].
Xenotransplantation, or transplanting animal organs into humans, has also been examined as a potential source for organ transplants. But if our bodies sometimes reject transplanted organs from other humans, how would they react to animal organs? In 2002, University of Missouri scientists cloned pigs that lack one of two genes called GATA1, which are primarily responsible for inducing that rejection response in humans [source: CNN]. Though primates would make more genetically suitable candidates for xenotransplantation, pigs are the best alternative until monkey cloning is a more viable option [source: Human Genome Project].
Future stem cell development for growing replacement organs may not even require cloning. In February 2008, a group of scientists at the University of California, Los Angeles derived stem cells from adult human skin cells. They were able to do so by controlling four regulator genes that influence cell differentiation [source: ScienceDaily]. By reprogramming the cells to act as stem cells, the altered skin cells became pluripotent and were called induced pluripotent stem cells. A few months later, Dutch researchers extracted adult stem cells from cellular material left over from open heart surgeries [source: ScienceDaily]. They used those stem cells to grow heart muscle cells, without the use of embryonic stem cells or cloning [source: ScienceDaily].
Because of the ethical gray areas surrounding embryonic stem cell research, people have reacted more positively to alternative methods like the ones described above. In theory, we should be able to eventually grow new organs from stem cells. But the technological advances discussed above indicate that cloning might not be necessary to harness those valuable cells.
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More Great Links
- "Cloning Fact Sheet." Human Genome Project Information. (Sept. 8, 2008)http://www.ornl.gov/sci/techresources/Human_Genome/elsi/cloning.shtml
- "Current U.S. Waiting List -- Overall by Organ." The Organ Procurement and Transplantation Network. Updated Aug. 29, 2008. (Sept. 8, 2008)http://www.optn.org/latestData/rptData.asp
- "Heart Derived Stem Cells Develop Into Heart Muscle." University Medical Center Utrecht. ScienceDaily. April 23, 2008. (Sept. 8, 2008)http://www.sciencedaily.com/releases/2008/04/080423101822.htm
- "Human Skin Cells Reprogrammed Into Embryonic Stem Cells." University of California - Los Angeles. ScienceDaily. February 12, 2008. (Sept. 8, 2008)http://www.sciencedaily.com /releases/2008/02/080211172631.htm
- "Research progress: Pig cloning for organs." CNN. Jan. 3, 2002. (Sept. 8, 2008)http://archives.cnn.com/2002/HEALTH/01/03/pig.cloning/index.html
- Singer, Emily. "Human-Animal Cybrids." Oct. 5, 2007. (Sept. 8, 2008)http://www.technologyreview.com/Biotech/19485/page1/
- Singer, Emily. "Human Therapeutic Cloning at a Standstill." Oct. 9, 2007. (Sept. 8, 2008)http://www.technologyreview.com/Biotech/19488/?a=f
- "Therapeutic Cloning Treats Parkinson's Disease In Mice." Memorial Sloan-Kettering Cancer Center. ScienceDaily. March 24, 2008. (Sept. 8, 2008)http://www.sciencedaily.com/releases/2008/03/080323210229.htm
- "Transplants by Donor Type." The Organ Procurement and Transplantation Network. Updated Sept. 5, 2008. (Sept. 8, 2008)http://www.optn.org/latestData/rptData.asp
- "Understanding Stem Cells." The National Academies. (Sept. 8, 2008)http://dels.nas.edu/dels/rpt_briefs/Understanding_Stem_Cells.pdf