Use of Stem Cells to Cure Human Disease

Remember that normally, these embryonic stem cells (ES cells) form mixed-tissue tumors if they are not suppressed by being cultured in mitomycin-arrested, or irradiated, mouse-fibroblast feeder layers. Even human embryonic stem cells must be grown on these feeder layers of mouse cells, or else they simply differentiate (in an undirected, chaotic fashion) into mixed-tissue tumors. It is not as though you can simply take these embryonic stem cells and grow them into whatever replacement tissue you want, such as substantia nigra brain cells to cure Muhammad Ali’s Parkinson’s disease, or spinal-cord cells to cure quadriplegia, or pancreatic islet cells to cure insulin-requiring diabetes. We have no idea at the present moment how to direct these undifferentiated stem cells into truly organized tissue for curing human disease. Furthermore, there is every reason to suspect that if simply replaced back in humans, or allowed to grow in routine culture, they would do nothing but produce tumors.

When taken off of their feeder layers, ES cells will differentiate spontaneously into a multitude of cell types in a random, uncontrolled fashion. If you were to inject these ES cells into a mouse whose immune system is compromised, they would develop into mixed-tissue tumors called teratomas, representing all the different embryonic tissue types, but still not in a predictable fashion. There is only one way that ES cells can be used, even in the mouse, for gene therapy or tissue replacement, and that was alluded to when I briefly explained the use of the knockout mouse.

Remember, these ES cells thus far cannot be magically formed into a kidney, a heart, brain cells, or pancreatic cells, or fix a severed spinal cord. The only way that the stem cells can achieve their totipotential development is by injecting them into an existing blastocyst, where they mingle with the ICM cells within that blastocyst and become an integral part of that developing embryo (see figure below). The ES cells injected into the blastocyst cavity will find their way into every cell lineage in the adult that develops from that embryo — into every cell, into every tissue, into every organ of the body, and even into its sperm and eggs.

So now you’re probably scratching your head, wondering what good this does and how this can help us cure human disease. Stem cells can be used to correct disease-causing defective genes in much the same way as the mouse experiment in which they are used to create so-called knockout mice. First, let me warn you that the only way stem cells can be of any benefit is by deriving them from an embryo cloned from the individual who has the disease you wish to cure. The ultimate goal would be to develop tissue that will not be rejected by the immune system of the person whose sickness you are trying to cure. Thus, stem cell therapy, if ever to work, is always going to be intricately associated with the concept of therapeutic, though not reproductive, cloning. With all the excitement and talk about the need for funding for stem cell research, scientists should be much clearer on this point because of the huge moral implications of using embryonic stem cell research for curing disease.

In fact, adult stem cells have already been used successfully for decades in humans to cure cancer. You can wipe out many widespread cancers by giving otherwise lethal doses of chemotherapy and radiation. These doses would be lethal because they destroy the blood-producing cells in your bone marrow that make your blood cells, and that control your immune system. For decades, oncologists have been able to solve this by replacing your destroyed bone marrow with bone marrow stem cells from a donor. These infused stem cells repopulate the cancer victim’s body and accomplish two ends: (1) They replace the bone marrow stem cells of the patient that were destroyed by the massive doses of radiation and chemotherapy, and (2) they provide immune cells from a different person, the donor, that will attack any residual surviving cancer cells.

This long-established and successful form of adult stem cell therapy is often confused with the hype about embryonic stem cells. Embryonic stem cell therapy is just an illusion that will probably require years of basic animal research before it might (or might not) ever become a reality. The reason is that all cells in your body have some adult stem cells that are responsible for tissue renewal. Aside from the bone marrow, which is composed of just disorganized cells, most of the tissue of your body is extremely organized in an intricate fashion. We have no idea how either adult stem cells, or even embryonic cells, accomplish this organization. That is why if you try to infuse embryonic stem cells into animals, you just get mixed-tissue tumors. To try such infusions in humans would be absurd.

Stem cell therapy will work in only two possible ways: (1) an embryo from a diseased individual would have to be cloned to make stem cells. These stem cells would then have to be injected into blastocysts derived from standard IVF, which would then grow into healthy babies that could donate immune-compatible tissue to the diseased individual; or (2) adult stem cells (in contrast to embryonic stem cells) from the sick patient would have to be injected into blastocysts that would then develop into a baby whose tissue is compatible with that of the patient. The latter approach has fewer ethically explosive implications because no cloning is involved and no embryos are destroyed. These are the only two ways, at present, in which stem cells can be used for tissue replacement. Those who are morally against cloning embryos which will then be destroyed would prefer using adult stern cells as opposed to embryonic stem cells. But in either case, human IVF would be required, and in either case, a baby is being created just for the purpose of providing replacement tissue. The idea of using stem cells just to form into tissue-replacement cells is otherwise just far-fetched hype. In fact, we need much more animal research before even dreaming of doing intelligent research with human embryonic stem cells.

Korean stem cell researchers were thought to have taken us a step closer to this goal. They had led us to believe that cloned stem cell lines from adults can be made relatively efficiently. It was even possible that new stem cell lines could be made for each new patient by injecting his nuclei into existing stem cells, which would completely avoid destroying embryos. The only question remaining was how these cloned stem cell lines might be used to cure the disease of the adults from whom they were derived.

Because of the massive press hype and competition for huge research grants and great public recognition, there has been a disconcerting degree of scientific fraud surrounding the issue of human cloning and stem cell research. The Korean stem cell research team achieved great honor and fame, with huge grants from the Korean government, and were hailed as national heroes for claiming, in meticulously written scientific papers, that they could efficiently make cloned embryos from patients with a specific disease and then grow stem cells for tissue replacement (that won’t be rejected) from that patient who was cloned. It appeared that the Koreans had solved the number one problem in all human cloning and stem cell research.

The problem is that it was all a lie. The scientific world was rocked by the discovery and admission that the Korean scientists had faked it all. Even some naive American scientists who put their name on the paper with the Koreans were duped. So, to boil down the hype and the fraud: no cloned animal is normal. No human embryo has yet been cloned. We have no way yet of making stem cells from cloned humans. Stem cells from noncloned human embryos would have limited usefulness because of rejection problems. Even if we had cloned stem cells, we have no basic scientific understanding of how to use pluripotent stem cells to differentiate into organized replacement tissue.

Combining therapeutic cloning with gene therapy is the new frontier in treating genetic disorders, but overzealous and misapplied use in humans could be tragic. The dangerous nature of such an idea was borne out in 2001 when brain cells donated from aborted fetuses were transplanted into Parkinson’s patients. The researchers hoped that these fetal cells (which would have otherwise gone to no use) could substitute for the deficient number of dopamine-secreting cells in the Parkinson’s patient. Although these cells were not specifically derived from cloned cells of the sick patients, and immune problems would have to be treated with antirejection medicines, the immune problems did not turn out to be the reason these experiments in humans failed utterly. In about 15 percent of the patients, the transplanted dopamine-secreting cells grew very well, and they were not rejected by the patient’s immune system. In those cases, however, there ensued an uncontrolled oversecretion of dopamine, so that the Parkinson’s patients were completely unable to control their movements. They writhed with constant, jerky tremors that were much worse than those caused by the original Parkinson’s disease. These same side affects can be caused by an overdose of the Parkinson’s drug L-dopa, but at least the dose can be reduced. But in this situation there was no way to remove or deactivate the transplanted cells.

This was a severe blow to what had been considered a highly promising approach for treating Parkinson’s disease, Alzheimer’s, and other neurodegenerative diseases. The same problem will come up with any effort to use embryonic stem cells to create tissue in a dish and then use that replacement tissue to cure disease. It is the organization of these differentiated cells in the brain, or in the heart, or wherever the tissue replacement has to be done, that is so complicated and completely beyond our current understanding. Simply injecting these differentiated replacement cells into an organ is not going to ensure any sort of proper organized development. The uncontrollable movements in the Parkinson’s patients mentioned above was reported to be “absolutely devastating.” They chewed constantly, their fingers went up and down, their wrists flexed and distended, and they jerked their heads and flung their arms out in a completely uncontrollable way. Despite prior ad hoc reports of spectacular results and miraculous cures, this careful NIH-funded study demonstrated just how far we are from any cure for these patients. Furthermore, in those who did not have this disastrous side affect of oversecretion of dopamine, there was no improvement whatsoever compared to control Parkinson’s patients. So the problem is not only with the generation of tissue that can be used for replacement in disease, but rather with the organization of that tissue in a properly functioning fashion.

However, stem cell and cloning technology could possibly be the most likely avenue to curing genetically lethal hematologic diseases caused by gene mutations in children or adults who would otherwise die. In chapter 13 we talked about a little girl who was dying of a condition called Fanconi’s anemia, in which she could not make blood cells. Her parents underwent IVF (with PGD) to have another baby not only to make sure that the next baby did not have the mutated Fanconi genes, but also to ensure that the new baby would be a good HLA tissue match for their dying daughter. They gave birth to a healthy baby without Fanconi’s disease who was a perfect tissue match for their dying daughter just in time for the doctors to perform a bone-marrow transplant to save their daughter’s life. Therapeutic cloning and stem cell technology have now been shown in mice to be able to accomplish a similar result for a limitless variety of genetic diseases.

Let me close this section by summarizing an amazing study from Rudolf Jaenisch’s lab at the Whitehead Institute at MIT (which is right next door to David Page’s lab, where our male-infertility genomics are performed). First published in 2002 in Cell , this landmark mouse study will be the model for all successful human gene therapy in the future. The Boston researchers took immune-deficient mice called RAG 2 and cloned them by injecting tail-tip donor cells into enucleated donor eggs using the Honolulu technique. These cloned embryos were then cultured to blastocyst stage, and the ICM cells moved into cultures of mouse embryonic fibroblasts to make embryonic stem cells that were genetically identical to the immune-deficient adult mice. These embryonic stem cells derived from nuclear transfer behaved like all embryonic stem cells in that when they were injected into another embryo they penetrated into every cell tissue type of that developing embryo. Remember, injecting stem cells into an adult is problematic, but when they are injected into a blastocyst, they develop normally. Gene defects can be repaired or created in these embryonic stem cells by culturing them in the presence of appropriate genes using homologous recombination, a process that automatically occurs within the stern cell culture. That is the basis of all knockout mouse studies.

The genetic defect of the RAG 2 mice embryonic stem cells was “repaired” by culture with a normal gene. These genetically repaired embryonic stem cells were then transferred into otherwise normal blastocysts that did not have an inner cell mass. (Actually, this is a rather complex process developed at the Whitehead Institute called tetraploid embryo complementation, for which I am only giving a vastly simplified description.) All of the cells of this embryo were entirely derived from the repaired embryonic stem cells. Thus, viable, healthy, fertile mice, genetically identical to the immune-deficient mouse but having the proper gene to correct that immune deficiency, were born. These mice that were derived from the repaired embryonic stem cells were then used as otherwise genetically identical tissue donors for the original diseased mice, resulting in a complete cure of their immune deficiency. Thus, unlike PGD, in which an embryo can be selected and will result in a baby that can be a reasonable tissue match to cure a dying child, an absolutely identical tissue match in these mice could be created, in which the specific gene defect has been repaired.

Once these researchers proved that the genetically repaired stem cells could result in a genetically normal mouse that is otherwise a clone of the diseased mouse, they went a step further. They tried to get these stern cells to differentiate in vitro into blood-producing cells, and to skip the step of making a new mouse that could be used as a donor. However, these cells were able to colonize the bone marrow of the diseased mouse only slightly. Thus, therapy using this approach is still a long way off.

No legitimate scientist or physician has any interest in cloning human beings or in destroying life. But the knockout mouse technology and stem cells developed in the 1980s, along with modern IVF, carry great hope for the cure of human genetic disease. The infertile couple going through IVF may be confused by all the hype in the press over unused frozen embryos and the need for stem cell research. In fact, there are very few unused frozen human embryos, and we lack the basic understanding of cell differentiation and dedifferentiation that would be required for stem cell therapy or therapeutic cloning. Thus, regardless of any political or religious debate, what is most needed to bring relief to human disease via stern cells is basic research in animal models such as the mouse. For the infertile couple, the misleading hype that their embryos may be used to help paraplegics walk again is regrettable. But IVF, which has completely weathered more than twenty-five years of moral debate, is clearly an established therapy and will continue to bring joy and family into countless modern lives.