Controversies in Treatment Approaches: Gene Therapy, IVF, Stem Cells, and Pharmacogenomics

The concept of changing a person's DNA, even to cure a fatal genetic disease, differs from more traditional remedies like surgery, pharmaceuticals, and physical therapy, and it is frightening to some people. Successful treatment approaches are available for a handful of single-gene disorders, most of which are enzyme deficiencies, including Gaucher's disease (a lysosomal storage disorder) and phenylketonuria (phenylalanine hydroxylase deficiency). Other disorders, including Duchenne muscular dystrophy (DMD), are the result of the complete loss of a functional protein. DMD is an X-linked recessive disorder that is caused by a mutation in the dystrophin gene. Mutation of this gene results in failure of muscle regeneration, leading to progressive muscle weakness and eventually causing fibrosis in essential organs (i.e., the heart and diaphragm). At this time, no medical, surgical, or other option exists to correct the underlying genetic cause of DMD and preserve muscle function in affected males. For individuals with DMD, the promise of gene therapy through one of the methods listed above means the possibility of leading a normal life.

Despite such potential benefits, many people oppose gene therapy on religious grounds, believing that altering genetic material is against God's will. This argument appears to hold the most sway because it raises the specter of "playing God". Table 1 lists some other frequently debated topics related to the inclusion of gene therapy in mainstream medical treatment. Many of these questions can be applied to any type of human research.

Table 1: Topics of Discussion in Gene Therapy Ethics

Adapted from Genetics Home Reference at http://ghr.nlm.nih.gov/handbook/therapy/ethics and from Kolehmainen, 2000

The scientific obstacles to gene therapy are related to the vehicles used to deliver normal genes. Most attempts at gene therapy involve inserting a normal or slightly modified version of a gene sequence into a viral vector. This virus then carries the gene into the patient's body and to the targeted tissue. Indeed, some success has been achieved in treating DMD using this method. However, this and other uses of viral vectors are often hindered by issues related to the patient's immune response, the specificity of delivery, and insertional mutagenesis.

As previously mentioned, one complicating factor in gene therapy is that many of the body's immunological defenses that are used to tackle infections are activated when a virus is present, including those viruses used as vectors to carry transgenes designed to cure genetic disease. Adenoviral vectors are the most immunogenic of all the viral vector groups, and they can have devastating effects. For example, an immune response to this vector was the cause of death in the case of Jesse Gelsinger, a young man who died during an experimental gene therapy trial in 1999.

Moreover, although natural infections with wild-type viruses are restricted to certain tissues based on the route of transmission, recombinant vectors are not subject to the same physical limitations. In other words, viral vectors are "promiscuous," and systemic delivery generally leads to unwanted vector uptake by many different cell types in multiple organs. Indeed, there are concerns that random insertion of these integrating viral vectors, many of which are derived from retroviruses, can inadvertently lead to mutagenesis and cancer (Hacein-Bey-Abina, 2003; Thomas, 2003).

Despite such problems, the realm of gene therapy has evolved since the 1999 study that took Gelsinger's life, and a number of successes have been documented. The first gene therapy success was in 2000, when three children were cured of a fatal immunodeficiency disorder. Later, in 2005, Qiao et al. reversed muscular dystrophy in mice using a single injection of the normal version of the dystrophin gene. More recently, two independent studies reported partial sight restoration in four young adults who were born with severe blindness, again with a single injection of a curative gene (Bainbridge et al., 2008). The years since the Gelsinger tragedy have thus been filled with a mixture of successes and failures on the gene therapy front. And, although advances continue to be made, much is left to be done before this type of therapy is ready for full clinical integration.

Following IVF, chromatin from donor cells joins to form pronuclei in the oocyte cytoplasm. The membranes of these pronuclei eventually disintegrate, creating a single-celled zygote with a diploid number of chromosomes. Then, over the next few days in cell culture, a series of cleavage divisions take the zygote through the two-, four-, eight-, and sixteen-cell stages. Eventually, a hollow ball of cells (the blastocyst) is formed, and embryonic stem cells (ES cells) can be extracted from the inner cell mass within the blastocyst. After several more months, millions of stem cells exist, all derived from the dozens of cells that were part of the inner cell mass. If the resulting cells have not differentiated and are genetically normal, they constitute an embryonic stem cell line.

At this point, each ES cell is pluripotent, meaning that it retains the ability to differentiate into any of the three germ layers: endoderm (the layer that forms the organs and tissues associated with digestion and respiration), mesoderm (the layer that forms the muscles, bones, and reproductive structures), and ectoderm (the layer that forms the skin, brain and nervous system). Therein lies the value of ES cells.

ES cell lines can remain undifferentiated for years. However, because their fate is undetermined, they have the potential to become any type of cell in the human body under the right conditions. If these conditions are met, ES cells can be placed in any area of the body affected by disease to produce healthy cells and tissues (in a process known as cell-based therapy). To date, no human ES-cell-based therapies have made it to clinical trial.

All tissues and organs contain undifferentiated cells. Unlike ES cells, however, adult stem cells differentiate into the cell type from which they were derived. Thus far, the most successful adult stem cell therapies have been transplantations of bone marrow and peripheral blood stem cells to treat cancers and blood diseases such as Fanconi's anemia and severe combined immunodeficiency disease (SCID or "bubble boy disease").

In recent years, induced pluripotent stem (iPS) cells that rival ES cells in their ability to differentiate have been generated using adult human somatic cells (Takahashi et al., 2007; Yu et al., 2007). At this time, the process requires the use of retroviral vectors. Retroviruses insert randomly, which creates the risk of disrupting normal cell function and cell cycle regulation. Virus-free iPS cells will be required before they can be safely used in regenerative medicine.

Treatments using adult stem cells rarely generate controversy. As a matter of fact, bone marrow transplants have become so familiar as a cancer treatment that many people may not fully grasp the concept that stem cells are being used.

In contrast, ES cell technology has produced a firestorm of controversy, as outlined in Table 2. Arguments against ES-cell-based therapies arise from the belief that life/consciousness starts at the moment of conception or within the first six days after conception, as well as from concern regarding the source of the embryonic cells.

Table 2: Arguments For and Against Federal Funding of Embryonic Stem Cell Research

Adapted from http://www.sourcewatch.org/index.php?title=U.S._federal_stem_cell_legislation#_note-39

Some religious beliefs define life as beginning at the moment of conception. Therefore, individuals with these beliefs feel that all research using ES cells is immoral. This point of view is usually countered with the argument that the embryos that are used are completely outside a woman's body, which negates the usual view of conception. Some opponents also present the argument of consciousness in embryos, which has been countered with evidence that embryos lack a nervous system until after implantation.

Some scientists are trying to perfect techniques that use only one cell or just a few cells from an embryo, rather than all of them, similar to the techniques routinely performed in preimplantation genetic testing. Efforts are also underway to engineer ES cells that stop developing before they reach a certain point in development. The ability to induce pluripotency in adult somatic cells, however, might soon make the entire discussion of the uses of ES cells irrelevant.

The United States has attempted to legislate a middle ground in this debate by making a distinction between publicly and privately funded research entities. Publicly funded research is restricted to pre-existing cell lines, although ES cells can be generated anew if private funding is used. Although publicly funded research is generally made publicly available, private companies are under no obligation to provide open access to their data. As a result, two questions have arisen:

• What oversight is there for ES cell research that is privately funded?
• Does having two disparate policies that depend solely on the source of funding hinder the potential for research advances?