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STANFORD MEDICINE
Volume 19 Number 1 Winter 2002
Stem Cell Pursuits
by Ruthann Richter
Portrait photo by Leslie WilliamsonIrving Weissman, first to capture the stem cell, explores its potential for science and medicine
Irving Weissman was fresh from the wilds of Montana when he arrived at Stanford in 1960 as a medical student, eager to find a niche in a research lab. Shortly thereafter, two researchers from the Ontario Cancer Institute in Toronto confirmed the existence of the stem cell, that elusive entity now considered the essential seed for many, and possibly all, human tissues.
It was auspicious timing, for while Weissman did not know it then, his life would eventually revolve around catching the stem cell and putting it to work. As he matured as a researcher, Weissman went on to develop a method for isolating the stem cell - laying the groundwork for the dozens of experiments now exploring this cell's power to fight illnesses as diverse as cancer and Parkinson's disease.
Weissman's work focuses exclusively on adult stem cells, not the embryonic stem cells that have inspired political controversy of late. While embryonic cells have the remarkable capacity to produce a wide range of tissues, they disappear four or five days after conception. Adult stem cells, however, are not so all-powerful and tend to be committed to making just one type of cell but they remain in the body throughout an individual's lifetime.
While Weissman was busy at Stanford as a medical student, the Toronto researchers - James Till, PhD, and Ernest McCulloch, MD - were searching for a remedy for the radiation effects observed in post-war Japan. In doing so, the researchers delivered lethal radiation doses to laboratory mice and were able to rescue them with injections of fresh bone marrow, which completely rebuilt the animals' stores of red and white blood cells. When the scientists used genetic markers to track the origins of the new blood cells, they found that all the cells emanated from a single source; this, they knew, had to be a stem cell.
This rare, blood-forming cell had another distinguishing quality, the researchers found: It had the ability to renew itself, thus serving as a constant source of replenishment for the blood. They now had indirect proof that this type of cell existed and they estimated it comprised one of every 2,000 cells in the bone marrow. But they could not lay their hands on the cell itself.
Weissman was well aware of these experiments as a young medical student when he wrangled his way into the lab of distinguished cancer researcher Henry Kaplan, MD. Weissman at that time was interested in lymphocytes, disease-fighting white blood cells of the immune system, and in how they go awry to produce lymphomas such as Hodgkin's. His work came to focus on the two types of lymphocytes - T cells and B cells. Weissman knew it would be important to learn where these cells came from, so he worked backward, looking for a common ancestor.
"We got the earliest precursors of all T cells and the earliest precursors of all B cells - but we were stuck there," recalls Weissman, MD, now a professor of pathology and the Karel H. and Avice N. Beekhuis Professor in Cancer Biology. "We got to the point where we couldn't find the precursors to these precursors without a whole new system to analyze the blood cells."
It would take a few decades and an entirely new approach to the problem before he could home in on his prize. Finally in July 1988, Weissman and his colleagues reported in Science that they had used a process of elimination to isolate the blood-forming stem from a pool of bone marrow cells. To accomplish this they had amassed as many blood-cell-binding antibodies as they could, including some they developed on their own. They used these antibodies to select out non-stem cells; when an antibody attached to a cell, then that cell was eliminated from competition. To the small group that remained, the researchers added two antibodies known from previous research to bind to stem cells.
The researchers took the few cells that were left in the pool and tested whether they were really stem cells by injecting them into mice whose blood systems had been destroyed by lethal radiation. They found that as few as 30 of these rarefied cells saved half of the mice, renewing these survivors' entire blood supplies.
"It was really unbelievable," Weissman says. "We had been working in systems where you'd have to transfer several hundred thousand to a million bone marrow cells to save a lethally irradiated mouse. So now we could do it with 30 to 100 cells. That meant we could hope to do the same in humans. It also meant that for the first time, we could study the whole life span of a single stem cell and of the daughter cells in a living body. That just turned out to be a bonanza scientifically."
It was also a reason to celebrate, which Weissman did by sharing a fine bottle of wine with his co-authors and indulging in an extended fly-fishing trip, one of his favorite pastimes.
Finding the stem cell in humans would be a bit more challenging, as one could not subject humans to the same experiments in mice. To speed the search, Weissman and a postdoc in his lab, Michael McCune, MD, set out to make mice more like humans. They created a special mouse, which they dubbed SCID-hu, that had all the components of the human immune system. Together they formed the company SyStemix to finance the hunt for the human stem cell in these mixed-up mice.
SyStemix, located in Palo Alto, developed high-speed sorting machines to help in the search, investing some $50 million in the undertaking, says Weissman. By April 1992, Weissman and collaborators at the company reported in the Proceedings of the National Academy of Sciences that they had found a candidate for the human blood-forming stem cell.
Putting stem cells to work
Beginning in 1996, SyStemix sponsored several clinical trials using these newly discovered stem cells in patients with various forms of cancer. One trial, conducted at Stanford and at the Karmanos Cancer Institute in Detroit, involved 25 women with late-stage breast cancer that had spread throughout their bodies. The study attempted to overcome a problem observed in many cancer patients who had been treated with bone marrow transplants that relied on the patients' own tissues for the transplant material: Tumor cells remained in the blood, says Robert Negrin, MD, associate professor of medicine and a collaborator on the study. In the trial, the researchers extracted bone-marrow-rich blood from the women, purified the stem cells at the SyStemix lab and put the fresh cells in a tiny vial for reinfusion back into the patients. In the meantime, the women were given very high doses of chemotherapy to kill off all of the cancer cells. The purified stem cells were tested after treatment, and indeed no residual cancer cells were found, Negrin says.
"About half the women remained free of disease, which is remarkable for patients with metastatic breast cancer, a disease that is very difficult to treat," Negrin says. "I think it's a very promising approach and something that has broad application in many clinical settings, such as transplantation, as in this study, as well as for treatment of autoimmune diseases and possibly in promoting tolerance to organ transplantation." Results of the study were published in December 2000 in Biology of Blood and Marrow Transplantation.
In a separate SyStemix-sponsored study, researchers at the University of Nebraska used purified stem cells to treat patients with non-Hodgkin's lymphoma. The results have yet to be published, but Weissman says they were encouraging as well, with more than half the patients remaining free of disease four years later.
Based on results of these initial studies, Weissman and Negrin say they're eager to repeat the trials in a controlled fashion with large numbers of patients suffering from various forms of cancer.
"The principle is always the same," Weissman says. "If you want to give a potentially lethal dose of chemotherapy or radiotherapy to try to kill all of the cancer cells, you need to put back stem cells to save them. It's probably not a good idea to give back stem calls that also contain cancer cells."
Until this point, most of Weissman's work involved one specialized group of adult stem cells - those that spawn the blood system. But in recent years scientists have unearthed more than a dozen different types of stem cells, each with the ability to generate a specific kind of tissue. Weissman joined the search for these other stem cells in the late 1990s, collaborating with scientists from the Salk Institute in La Jolla, Calif., and from Cal Tech in Pasadena, Calif., to form a new company, StemCells Inc. in Palo Alto, to help speed the process along.
In December 2000, the group reported isolating an adult stem cell in humans that is capable of generating various kinds of brain cells. When the researchers transplanted the cells of this type into the brains of immunodeficient newborn mice, the cells took hold and began to reproduce and differentiate into other brain tissues, a finding the researchers reported in the Proceedings of the National Academy of Science.
Weissman and his colleagues at StemCells Inc. now plan to test these findings in various mouse models to see if these cells might have some clinical value. In theory, these neural stem cells could be enormously powerful, with potential to replace cells lost or damaged in diseases such as Alzheimer's, Parkinson's, Huntington's, Tay-Sachs, Gaucher's and even spinal cord injury and stroke, Weissman says.
"At StemCells, we're now looking at each of those mouse models to see if the human cells will work," he says. "It may work in some or none at all. But it is clearly a new way of looking at regenerating nervous system cells."
At the same time, these new cells could offer great insight into the biology of the brain and how it forms and develops, he says.
"I think the ability to get human or mouse neural stem cells, to modify them, transplant them and follow them should be a new way, a complementing way, to do neurobiology that hasn't been there before," he says.
Weissman says his work remains unaffected by the new federal restrictions on embryonic stem cell research, announced by President George W. Bush on Aug. 9, 2001. Nevertheless, says Weissman, because embryonic stem cells have enormous scientific value and possible clinical application, the new rules could hamper important research advances. The regulations hold that federal funds can support embryonic stem cell research only if the research makes use of embryonic stem cell lines that existed when the regulations were announced. At that time, there were perhaps 60 in the country. And while stem cell research can be done on just a few cultures, notes Weissman, the restriction concerns him because it is important that these cultures be viable and be derived from a diverse cross-section of the population.
"It's unlikely in those existing 60 cell lines that you'll find that diversity," Weissman says. "If the research is going to be important, it ought to be broad and you want to be able to obtain samples from people with a known genetic predilection for a disease, so that when you develop a blood vessel cell from an embryonic stem cell, it comes from someone with a family with a high risk of heart disease and stroke. That way you would be working on something really relevant to that disease."
He says the protests against embryonic stem cell research today are reminiscent of the protests that accompanied the early recombinant DNA experiments in the 1970s, which spawned many clinically valuable therapies.
"There were the critics then, the same people saying, 'You're acting like God, you're creating life,'" Weissman says. "But today there are hundreds of thousands of people who are alive because of erythropoietin (a protein that stimulates red blood cell production), GCSF (a protein used to treat some cancer patients) and interferons. So those who would ban this research must take the responsibility for the lives that would be lost because they banned the research."
And so, more than 40 years later, Weissman remains as convinced as ever of the power of the stem cell
