I know Kharn, this is grave digging, but hey, what the hell? It's a good topic.
Cloning and stem cells
Baby steps
Dec 30th 2003
From The Economist print edition
New work shows the promise, and pitfalls, of embryonic-stem-cell research
FOR believers, Christmas is a celebration of the virgin birth. This season, however, researchers are pondering another sort of immaculate conception. Somatic nuclear transfer, or “cloning” as it is more popularly known, is a way of creating embryos without the conventional meeting of egg and sperm that is needed to provide a full complement of the genetic material to make a new individual. Embryos can also be created by parthenogenesis—a nifty biological trick in which an unfertilised egg cell is coaxed into providing all the genetic material required for embryonic development. In the January issue of Wired, a popular technology magazine, researchers at Advanced Cell Technology, an American biotechnology company, are reported to have used both techniques to create human embryos healthy enough to make it through at least the earliest stages of development.
Parthenogenesis occurs in the animal world. Many species of fish, birds and snakes reproduce naturally through parthenogenesis. And cloning has been performed in a wide variety of species, Dolly the sheep being its most famous offspring. Many researchers would like to apply these techniques to people.
Their interest is not procreation. Human reproductive cloning is so fraught with technical risks and ethical dilemmas that few respectable scientists will touch it, and parthenogenesis, while occasionally also observed in mammals, never produces viable offspring. But if such embryos could be coaxed far enough along the track of development to form multicellular structures known as blastocysts, they could prove a useful source of stem cells. Unlike most of the body's cells, which are locked into a single job for life, embryonic stem cells are able to reproduce themselves repeatedly, and when given the right biological signals, can mature into a wide range of different cell types. This makes them a potential source of new cells to repair damaged tissue in, say, diabetes or Parkinson's disease.
Michael West, Advanced Cell Technology's chief executive, is reluctant to confirm Wired's story, which claims that the firm has managed to coax one of its cloned embryos into a 16-cell entity called a morula—the predecessor of a blastocyst. But Dr West does say that his firm has managed to generate stem cells through parthenogenesis, and is working towards building a bank of stem cells with particular immunological features, so that patients might one day be treated using those that pose the lowest risk of graft rejection.
As Advanced Cell Technology has yet to publish the current research in a peer-reviewed journal, and the firm is known to be searching for funds, many experts are sceptical of its claims. Among the critics is Gerald Schatten, a researcher at the University of Pittsburgh who is working on generating embryonic stem cells from cloned monkey embryos. Primates (humans included) are tougher to clone than other animals, he says, because the molecular machinery needed for cell division is somehow impaired during the process of cloning. Though Advanced Cell Technology may have managed to create early-stage embryos, Dr Schatten awaits experimental evidence that these can develop far enough for stem cells to be derived from them and that those stem cells will, in turn, develop normally in a test tube.
Parthenogenesis, on the other hand, is proving a reliable source of embryonic stem cells, at least in monkeys. Jose Cibelli, who once worked at Advanced Cell Technology and is now a researcher at Michigan State University, has managed to coax parthenogenetically derived embryonic stem cells into forming various cell types, including nerve cells that produce dopamine, a brain chemical whose deficiency leads to Parkinson's disease. The next step is to inject these dopamine-producing cells back into monkeys to see if they can do the same job inside a warm body.
An embryonic science
While research is yielding novel sources of embryonic stem cells, and ways to give ordinary cells stem-like powers, it is also revealing new challenges in translating this research from the laboratory bench to the patient's bedside. One problem has to do with the way human embryonic stem cells are cultivated in a test tube. Stem cells are hard to grow in sufficient quantity and purity for research, let alone for therapeutic applications. They dislike solitude and refuse to thrive unless they are in contact with so-called “feeder” cells. These provide biochemical signals that allow stem cells to maintain their unique characteristics and prevent them from differentiating into mature cell types.
Mouse cells are normally used as feeders for human embryonic stem cells, but many researchers are worried about employing differentiated cells derived from such lines as actual human therapies, for fear they may have picked up nasty viruses from their mouse feeders. Indeed, a recent report from a team of stem-cell experts co-ordinated by Johns Hopkins University, in Baltimore, has concluded that “it would be unethical to expose human subjects to stem-cell lines that have been derived with mouse feeder layers.”
A number of groups are working on alternatives. One possibility is to use human, rather than mouse, feeder cells. Ariff Bongso, a stem-cell researcher at the National University of Singapore, has tested a number of human cell types and found that adult skin cells work just as well as mouse feeders at keeping human embryonic cells healthy and happy.
Another option is to dispense with feeder cells altogether. In the January issue of Nature Medicine, Ali Brivanlou and his team of researchers at Rockefeller University in New York, the University of Athens in Greece and the Centre National de la Recherche Scientifique in Roscoff, France, have found a way of cultivating human embryonic stem cells for several generations in the absence of feeder cells. Instead, they grow their cells with a molecule called 6-bromoindirubin-3'-oxime (more catchily known as BIO).
BIO is known to inhibit the activity of a molecule that acts as a brake on a system of molecular signals called the Wnt pathway. Growing human embryonic stem cells in the presence of BIO allows the Wnt pathway to function fully. That, in turn, keeps the cells dividing without differentiating into other cell types. Yet when exposed to the right environment, BIO-treated stem cells still mature into special types, such as nerve cells, as efficiently as those grown in more conventional conditions. Dr Brivanlou and his colleagues are now trying to develop better molecules than BIO, in the hope of using them to gain even more effective control over the process of differentiation.
Lines in the sand
Despite these promising new approaches, many hurdles lie in the immediate path of human embryonic stem cells. One is regulatory. On August 9th 2001, President George Bush announced that federal funding for human-embryonic-stem-cell research would be restricted to work done on those cell lines established by that date. At the time, more than 60 such lines from laboratories around the world were provisionally eligible. Today, however, only a dozen or so lines are actually available for federally funded researchers in America, since these are the only ones that laboratory tests have so far proved have all the properties of bona fide human embryonic stem cells.
This is a far cry from the rich pickings which such researchers were first promised, and even this slender selection of eligible cells may not be so desirable after all. It turns out that all the approved lines were grown using mouse feeder cells. Yet scientists risk their federal grants if they try to derive embryonic stem cells using alternative methods.
Another challenge has to do with the long-term behaviour of human embryonic stem cells—no matter their provenance—both in the test tube and in the human body. In the January issue of Nature Biotechnology, Peter Andrews of the University of Sheffield, in England, James Thomson of the University of Wisconsin, and their colleagues, report that human-embryonic-stem-cell lines grown over several months in the laboratory can develop genetic abnormalities. In particular, the embryonic cells they studied gained extra bits of chromosomes 12 or 17, which the group speculates may help these cells retain their ability to renew themselves. But the researchers also note that similar chromosomal changes are found in some types of cancer cell, yet another concern when it comes to moving into the clinic.
How specialised cells derived from embryonic stem cells will behave in the human body is a further uncertainty. As the Johns Hopkins report points out, whether grafts of these fruits of embryonic stem cells will settle down in the right place in the body, and keep doing their intended job rather than becoming other cell types or, even worse, cancer cells, is unknown.
But results from animal experiments demonstrate their tantalising potential. In the December issue of Laboratory Investigation, Joseph Itskovitz-Eldor and his team at the Technion-Israel Institute of Technology in Haifa show how human embryonic stem cells can be coaxed (in a test tube) into becoming the various cell types that make up blood vessels. Indeed, subsequent experiments have demonstrated that, when injected into special experimental mice, these differentiated cells are able to form fully functional blood vessels which join up with the animal's own circulatory system. This is a potential treatment for such things as ischemic heart disease.
But that path is littered with more than just technical obstacles. Embryonic stem cells raise profound questions about the morality of using existing embryos, or creating new ones, for the purposes of research on a medical treatment, no matter how great the benefit. As the scientific barriers to embryonic-stem-cell therapy fall, those ethical debates are set to deepen.
