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.
ISIS Report 29/03/05
Gene Therapy Woes
Research continues to turn up new obstacles and dangers, and tough
questions are raised over the ethics involved.
Dr. Mae-Wan Ho
The sources for this article is
posted on ISIS members’ website.
Details here
Toxic shock and leukemia
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.
Viral vectors tend to integrate into genes
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.
Immune responses
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.
Risks and questionable ethics
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.
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