Gene gold turning to dust?
Governments are sinking further billions into genomics and related research but a new study finds no sign of revolution in healthcare.
Research continues to turn up new obstacles and dangers, and tough questions are raised over the ethics involved. Dr. Mae-Wan Ho
The public first became aware of the dangers of gene therapy when healthy teenage Jesse Gelsinger died in September 1999 as the result of volunteering for a clinical trial for the inherited condition Ornithine Transcarbamylase Deficiency. He died of toxic shock after receiving the adenovirus vector carrying the transgene. The ensuing enquiry turned up more than 600 serious adverse events (including deaths) in other gene therapy clinical trials that were unreported, because they were deemed related to the trial procedure ("Failures of gene therapy", SiS 16). Gelsinger’s father sued the research team and subsequently settled out of court for an undisclosed amount.
In October and December 2002, the Necker Hospital in Paris announced that the two youngest boys enrolled into a gene therapy study for the treatment of X-SCID had developed a form of leukaemia ("Gene therapy’s first cancer victim", SiS 17). The retroviral vector had inserted near the gene LMO2, which encodes a transcription factor, whose over-expression has been implicated in child-hood T-cell acute lymphoblastic leukaemia.disorders. One child has died. A third infant has developed leukaemia by January 2005, which prompted the US Food and Drug Administration to suspend three gene-therapy trials on SCID in the US. The US Panel has announced that gene therapy for X-linked SCID could proceed only if patients have failed to respond to other treatments. This restriction does not apply to other SCID cases.
These two incidents highlight the two major obstacles to gene therapy and the dangers posed: immune reactions against the vectors and transgenes, and inappropriate insertion of vectors and transgenes that can cause mutations leading to cancer.
It is not easy to get foreign genes into the genome, and certainly they cannot be targeted to specific sites. But the insertion sites are not random; they are worse than that. Viral vectors of all kinds tend to insert preferably into genes, and especially those most actively expressed, thus causing the disruption ("Gene therapy risks exposed", SiS 19). They also tend to insert into regions rich in mobile genetic elements that move gene sequences around the genome, thereby compromising the stability of the inserts. In addition, deletions of host DNA tend to occur at the site of insertion.
Gene therapy vectors usually contain parts of bacteria, viruses, or other microorganisms. Immune responses can occur to the viral vector, the transgene product as well as the bacterial plasmid DNA.
Viruses are naturally able to incorporate foreign genetic material in the host cell genome, and therefore are good vectors for gene therapy. However, fighting infection by bacteria and viruses is among the key functions of the immune system. So, bacteria (plasmids) and viruses or parts of them promptly trigger an ‘innate’ immune response as soon as they enter the body, causing cytokine production and an influx of nonspecific inflammatory cells (macrophages, dendritic cells, NK (natural killer) cells, and others).
‘Adaptive’ immunity is stimulated later, when antigen-presenting cells (APCs) carrying antigens from the microorganisms migrate to the lymph nodes. It includes the production of neutralizing antibodies circulating in the blood that are specific of the vector or transgene antigen, and a cell-mediated response involving T cells and NK cells. Adaptive immunity not only contributes to eliminating the vectors and infected cells from the body but also results in a memory response that undermines further attempts to use the same vector or transgene.
Viral vectors are the most likely to induce an immune response, especially those derived from adenovirus and adeno-associated virus (AAV), which express immunogenic proteins within the organism. The innate inflammatory response is high with adenoviral vectors, and almost nil with AAV vectors. Plasmid DNA vectors, because of the presence of CpG dinucleotides, also tend to stimulate the innate inflammatory response.
Specific ‘adaptive’ immune responses are due to capsid antigens in adenoviruses and AAV. Viral gene-encoded proteins in adenoviruses can also be immunogenic.
In the case of retroviral vectors, the immune response is mainly directed at the transgenes located within the vector rather than the antigens in the vector itself. Nonetheless, when used in vivo, they are inactivated in the serum by complement activation and can also trigger a cytotoxic response, in which cells containing the vector are killed.
Bioethicist Jonathan Kimmelman at McGill University, Montreal, Canada, writing in the British Medical Journal in January 2005, highlighted the special risks involved in gene therapy and call for a "central ethical review" of all trial protocols as well as "high scientific standards" for clinical trials.
First, active agents rather than chemicals are used in gene therapy. The vectors are potentially capable of propagating and recombining with other viruses. Second, genetic information is transferred which directly participates in gene expression. Third, it involves both vector and transgene, each of which carries its own risks, but the two may act synergistically to worsen the risks, as in the leukemia that occurred in the X-linked SCID trial; which may be due to the combined toxicity of the vector integrating near an oncogene and the transgene that may have helped to transform T cells. Fourth, gene transfer agents that stably modify cells can involve risks with long latencies, and increase the probability of subtle toxic effects over the long term. This is particularly relevant to treating children, who may be more sensitive to the long-term hazards because their tissues are still developing. Age may indeed have been a factor in the leukemia cases in the X-linked SCID trial. Risks to third parties are possible, such as descendents of the patient getting insertion mutagenesis through the inadvertent modification of germ cells, or the transmission of infectious agents arising in the patient to the general population. Finally, much of the toxicity related to gene therapy is mediated through the immune system (see above).
There are also problems over safety testing. Animal models are often inadequate, as viruses that are pathogenic in humans often behave differently in animals. People previously exposed to viruses similar to the vector can influence their response to the gene transfer, and the dose-toxicity response could be nonlinear.
Although none of the individual risks is unique, the frequency with which they occur in gene therapy trials and at the same time make the risk of gene therapy distinctive.
"The complexity of risk from gene transfer militates against the practice of using only local ethics committees to review trials" wrote Kimmelman.
Furthermore, all major ethic codes require that clinical research be capable of generating valuable medical knowledge. But gene transfer trials have often failed to do so.
Kimmelman pointed out that no gene therapy has been commercialized after 15 years. And because of the uncertainties surrounding gene transfer, "most trials should be conceptualized less as testing an agent’s prospect of commercialization and more as producing information that can be applied to the development of gene transfer."
The same uncertainties make it unethical to recruit healthy volunteers in clinical trials. But if only participants with advanced illness are recruited, such participants are likely to misinterpret the purpose of the trial as providing therapy rather than providing general knowledge; in which case, enrolment in such studies is susceptible to being based on "misinformed" consent. And it is also easy for the experimenter to misinterpret adverse events and deaths as unrelated to the treatment.
Article first published 29/03/05
Got something to say about this page? Comment