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Introduction to Gene Therapy

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					                            Introduction to Gene Therapy

   Gene therapy, the therapeutic replacement or repair of abnormal genes in human
cells, is one of the most promising and controversial innovations in biotechnology.
Though gene therapy is possible in the near future for only monogenic diseases (those
caused by one gene), the technique holds enormous possibilities in the distant future
for correcting more complex diseases.

                              Gene Therapy Techniques

   There are three main categories of gene therapy techniques. The first, gene
insertion, is the only one that is possible in the near future. It involves simply adding
the normal version of a defective gene to the genome of the affected cells. When this
gene is expressed, it could produce sufficient quantities of a missing or defective
protein or enzyme, thus "curing" the disease. This method could potentially be
dangerous, however, because a randomly added genetic sequence could disrupt the
function of another vital sequence. For this reason, the other two methods are being
looked into as ideal future methods of gene therapy.

   The second approach, gene modification, entails the direct chemical modification
of of the abnormal DNA sequence in an effort to duplicate the normal sequence in an
abnormal cell. This strategy is much less likely to disrupt the function of other genes
because no new sequences are introduced.

   The third approach, called gene surgery, is considered to be the ultimate goal of
gene therapy. It involves the removal of the precise genetic sequence that is defective,
and replacing it with a cloned copy of the normally functioning sequence. This
strategy is the least likely to cause unwanted side effects, but it is also the most
advanced and therefore is not possible in the near future.

                          Using the Gene-Insertion Strategy

   There are several different ways of inserting a normal copy of a gene into the
defective genome of a cell. All these approaches use vectors, or a "vehicle" used to
get foreign DNA sequences into living cells. The most popular vectors are
retroviruses, or viruses whose genetic information is encoded in RNA.

                                  Retrovirus Vectors

   A retrovirus works by infecting a host cell, then using the enzyme reverse
transcriptase to use its RNA as a template for making DNA which then becomes
integrated into the host cell's chromosomal DNA. The virus can then direct the
formation of more viruses. The fact that retroviruses integrate their genetic material
into that of the host cell makes them ideal for gene therapy, because they efficiently
convey gees into the target cell.

  When retroviruses are used as vectors, a scientist removes the viral genes and
replaces them with therapeutic genes. The virus then transfers these genes, instead of
the virus genes, into the target cell; however, the virus is no longer capable of
replicating itself. This technique is simple and possible in the near future; however, it
has many limitations. The viruses can only accept a certain amount of base pairs,
limiting the size of therapeutic genes to be transferred.In addition, the genetic
information is randomly inserted into the host cell's genome, creating the possibility
of disrupting the function of vital genes. There also exists a concern that the virus
might not be completely "deactivated", and a patient might be inadvertently infected
with a viral disease.

                             Other Insertion Techniques

   Gene therapy that targets blood-cell disorders, such as sickle-cell anemia or
inherited immune deficiencies, could be focused on bone-marrow cells known as stem
cells. Current techniques for gene therapy of these disorders require new treatments
every few months, because the genetically altered blood cells die off. By altering the
stem cells, which carry a full genetic complement and give rise to new blood cells to
replace those that die, one treatment in a lifetime would be enough. Despite the efforts
of scientists, these stem cells remain elusive and difficult to genetically alter.

   Some scientists, concerned about the dangers of using viral vectors, have proposed
creating an artificial chromosome and inserting that into target cells. These scientists
have suggested coating a manmade chromosome in natural proteins, inserting it into a
target cell, and allowing it to express its lifesaving genes.

                               Gene Therapy Successes

   In 1988, researchers began the first test that would definitively show whether
foreign genes, implanted into a human, were as dangerous as some people predicted.
Genes for antibiotic resistance were implanted in blood cells called TILs (tumor-
infiltrating lymphocytes). These modified cells were then given to a 52-year-old man
with malignant melanoma (a fatal skin cancer) which had spread to his liver. He was
told he had two months to live.

   The man received the TILs and lived for nearly a year after the treatment in May
1989 - much longer than previously expected. The genetically altered cells were
detectable in the man's body up to three months after the injection. Most importantly,
this landmark case proved that foreign DNA could be introduced into humans with no
adverse effects, and paved the way for more advanced trials in the future.

   The first true gene therapy took place in 1990, with the treatment of two young
girls who suffered from ADA (adenosine deaminase) deficiency, a rare type of
immune disorder. Victims of the disease have high risk of cancer and are highly
vulnerable to infection. In this trial, viral vectors carried ADA genes into white blood
cells known as T-cells, which were then released into the girls' bodies. The cells
produced the enzyme at about 25% of normal, which is enough to reverse the effects
of ADA deficiency. The technique is not perfect - it requires regular infusions of T-
cells - but it vastly improved the quality of life for the girls who were the first test
subjects.

                          Future Gene Therapy Candidates

   There are many possible candidate diseases for gene therapy, most of which ar
rare, but some of which are fairly common. Diseases targeted for future gene therapy
include cystic fibrosis, a disorder of the secretory cells common in Caucasians; sickle-
cell anemia, a hemoglobin disorder common in people of African descent; Lesch-
Nyhan syndrome, a disease that causes retardation and a self-mutilating tendency; and
familial hypercholesterolemia, a disease in which the ability to absorb a type of
cholesterol is reduced in carriers and absent in full-blown victims. All these diseases
have been researched by biotechnology companies as potential gene therapy
candidates. In addition, the possibility of infecting tumor cells with viruses that render
such cells vulnerable to destruction by normally harmless chemicals is also being
studied.

                         Somatic Cell vs. Germ Cell Therapy

   All the gene therapy techniques discussed so far have been aimed at somatic, or
bodily, cells. These techniques can help individuals with genetic diseases, but do
nothing for their offspring. It has been suggested that the same gene therapy
techniques applied to somatic cells could be applied to germ cells, or reproductive
cells (eggs and sperm),and eliminate certain genetic diseases from the population
(with the exception of spontaneous mutation, which is very rare). This possibility has
been vehemently opposed by many religious and scientific groups, but is also one of
the most attractive applications of gene therapy.

   The idea of removing, say, the gene for Huntington's chorea from the human
population doesn't seem like it could possibly be a bad idea. After all, Huntington's
chorea is one of the most tragic genetic diseases known to mankind. However, many
scientists express a concern that random insertion of replacement genes into germ
cells could disrupt vital genes and create a predisposition to cancer in the unborn
child. (In this case, a predisposition to cancer seems the lesser of two evils, since
Huntington's is fatal to all those who inherit the gene.) Many religious groups also
object on moral grounds, saying that life is sacred and should not be tampered with.

  Needless to say, germ-cell therapy needs to be scrupulously tested to guard against
causing other diseases or cancer risks in children born of altered cells. Of course, this
procedure should be entirely elective, and those who choose not to alter their cells
should not be stigmatized in society. Currently, gene-therapy technology is not
advanced enough to attempt germ-cell therapy.

                                    A Slippery Slope

   Many people see germ-line gene therapy as the beginning of humanity's descent
down a slippery slope. They worry that people with genetic diseases will be
stigmatized as "flawed" in a Nazi-style quest for genetic purity. In addition, they
wonder if, after genetic diseases are dubbed "flaws", other traits such as
nearsightedness, shortness, or athletic and academic inability will also be engineered
out of the population. In this scenario, it is possible for parents to choose their child's
gender, physical appearance, athletic ability, and academic proficiency.

  This scenario, though unlikely, is extremely dangerous - it could disrupt the gender
balance of the population, lead to an extreme loss of genetic diversity, and ultimately
even lead to an increase in genetic diseases because so many engineered children
possess the same "desirable" genes that inbreeding becomes a problem. Also, by
eliminating certain "undesirable" traits (such as sickle-cell trait), we could eliminate
important adaptations to certain environments. (Elimination of sickle cell trait would
probably lead to an increase in malaria, since it confers natural resistance to the
disease.)

                          Gene Therapy: Ethical Principles


      The benefits and the risks of gene therapy for each individual must be
       weighed. With current technology, there exists a risk of infection by viral
       vectors, as well as the risk of disrupting a vital gene and causing another
       disease or a predisposition to cancer.

      Somatic-cell gene therapy is ethically acceptable and should be an option for
       those suffering form a genetic disease for which there is a gene-therapy
       treatment.

      Germ-cell therapy should be scrupulously studied for all potential adverse
       effects before it is tested on humans. It should always be an elective
       procedure, but it should be available to everyone. Dissemination of
       information on the procedure is very important, so people can understand the
       risks. Those who do not choose the procedure should never be stigmatized as
       "flawed".

      Germ-line gene therapy should be limited to elimination of genetic disease.
       Other human attributes should never be altered, because this could lead to
       gender imbalances, inbreeding problems, reduction in genetic diversity, and
       elimination of adaptations associated with carrier status of a disease.



                         Ethical aspects of gene therapy

                                  Alex Mauron
                          Associate professor of bioethics

Gene therapy consists of a wilful modification of the genetic material in cells of
a patient in order to bring about a therapeutic effect. This modification usually
occurs by introducing exogenous DNA using viral vectors or other means.
Although gene therapy is still in its infancy as a clinically useful therapeutic
modality, a discussion of the ethical issues is useful in several respects
because it involves ethical principles of broad applicability in clinical medicine.
Furthermore, many current applications of genetic engineering in medicine
(DNA vaccines, therapeutic use of encapsulated genetically modified cells)
are conceptually close to gene therapy, so that the border between gene
therapy in the narrow sense and other gene-based therapies is getting fuzzier
as time goes by.
Two conceptual distinctions are central to an understanding of the ethical
issues of gene therapy:

1 - Therapy vs. enhancement. There is a consensus that gene therapy
should be therapy, i.e. the correction of bona fide disease conditions, rather
than enhancement, which would mean "improving the human species"
(whatever that means...) and therefore would entail the introduction in human
subjects of novel characteristics going beyond the usual, medical,
understanding of health (i.e. health as absence of serious disease).

2 - Somatic vs. germ line gene therapy. All current research on humans
deals with somatic gene therapy. In these projects somatic cells such as
bone-marrow, liver, lung or vascular epithelium etc. are genetically modified.
Since the germ line is not affected, all effects of therapy end with the life of the
patient, at the very latest. In fact, most somatic therapies will probably require
repeated applications, much like ordinary pharmacological treatments.

Initially, gene therapy was conceptualised mainly as a procedure to correct
recessive monogenic defects by bringing a healthy copy of the deficient gene
in the relevant cells. In fact, somatic gene therapy has a much broader
potential if one thinks of it as a sophisticated means of bringing a therapeutic
gene product to the right place in the body. The field has moved increasingly
from a "gene correction" model to a "DNA as drug" model (ADN médicament,
A. Kahn). This evolution towards an understanding of gene therapy as "DNA-
based chemotherapy" underscores why the ethical considerations for somatic
gene therapy are not basically different from the well-known ethical principles
that apply in trials of any new experimental therapy

      Favourable risk-benefit balance (principle of beneficence/non-
       maleficence);
      Informed consent (principle of respect for persons);
      Fairness in selecting research subjects (principle of justice).

Clearly, the mere fact that gene therapy has to do with genes and the genome
does not, in itself, make it "special" or "suspicious".

A further distinction ought to be made between in vivo and ex vivo somatic
gene therapy. Ex vivo procedures entail the extraction of cells from the
patient's body (for instance bone-marrow cells), genetic modification of the
cells using appropriate vectors or other DNA-transfer methods and
reimplantation of the cells in the patient. In vivo therapy uses a vector or DNA-
transfer technique that can be applied directly to the patient. This is the case
of current experiments aimed at correcting the gene defect of cystic fibrosis by
exposing lung epithelium to adenovirus-derived vectors containing the CFTR
gene. In the in vivo case, the potential for unintended dissemination of the
vector is more of an issue. Therefore, biological safety considerations must
also be subjected to ethical scrutiny in addition to the patient-regarding
concerns already mentioned.
In germ line therapy, the DNA of germ cells would be affected, the objective
being to correct a genetic defect once and for all, in all descendants of the
therapy recipient who will inherit the modified allele. Although germ line
therapy is far more speculative than somatic gene therapy at this time, it is
widely discussed because it raises important and difficult ethical questions
that have relevance for other medical practices as well. The consensus
against germ line therapy is broad, but not unanimous. The ethical debate on
germ line therapy has usually revolved around two kinds of issues:

1 - Germ line therapy is "open-ended" therapy. Its effects extend
indefinitely into the future. This basically fits the objective of germ line therapy
(assuming that it becomes possible one day), namely to correct a genetic
defect once and for all. But precisely there lies also an ethical problem: an
experiment in germ line therapy would be tantamount to a clinical experiment
on unconsenting subjects, which are the affected members of future
generations. This raises a number of very complex questions and is, in my
view, an important but not necessarily overriding argument. A recent
symposium on germ line engineering has concluded with a cautious "yes-
maybe" for germ line gene therapy (see references).

2 - Germ line therapy may involve invasive experimentation on human
embryos. Although there are other potential targets for germ-line
interventions, much of the discussion revolves around the genetic modification
of early embryos, where the germ line has not yet segregated from the
precursors of the various somatic cell types. As a result, the ethical
assessment of germ line gene therapy will hinge in part on the ethical
standing accorded to the early human embryo and the moral (dis)approval of
early embryo experimentation. Those who believe the early embryo to be the
bearer of considerable intrinsic moral worth or even that it is "like" a human
person in a morally-relevant sense will conclude that embryo experimentation
is to be rejected and germ-line therapy as well. Others think that it is only later
in development that humans acquire those features that make them ethically
and legally protected human subjects to the fullest degree. For them, the use
of early embryos is not objectionable and germ line therapy cannot be ruled
out on these grounds alone. As might be expected in view of the moral
pluralism of modern societies, the policies of European countries differ in this
respect: some permit some invasive research on human embryos (UK, Spain,
Denmark), others ban it (Germany, Norway), others are still undecided. More
generally, embryo-centred controversies are expected to increase as the field
of embryonic stem-cell research becomes ever more promising. It is expected
that this field will catch much of the public attention that was devoted to gene
therapy in the nineties.

Clearly, the question of the ethical standing of the human embryo is also of
major importance for other medical procedures in reproductive medicine such
as in-vitro fertilisation, pre-implantation diagnosis, experimentation on human
embryos in general and abortion.

To go back to gene therapy, or rather to the therapeutic innovations due to
genetic engineering such as DNA vaccines: some of these could potentially
benefit a great number of people world-wide, contrary to early developments
of genetic engineering in medicine, which where largely geared towards the
health problems of rich countries. Although the course of biomedical progress
is often unpredictable, the setting of research priorities does raise troubling
issues of social ethics.

				
DOCUMENT INFO
Description: Gene therapy, the therapeutic replacement or repair of abnormal genes in human cells, is one of the most promising and controversial innovations in biotechnology. Though gene therapy is possible in the near future for only monogenic diseases (those caused by one gene), the technique holds enormous possibilities in the distant future for correcting more complex diseases.