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The Progress of Somatic Gene Therapy
Aaron M. Pieroni
Fox School of Business, Temple University, Philadelphia, PA 19122
The Bionic Human
November 2015
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Abstract
The current field of somatic gene therapy is constantly changing with the discovery of
new types of delivery methods, diseases and techniques. This paper will explain the differences
between these methods and techniques while also delving into the success of Gendicine and
Glybera. Currently the most promising research being done in this field relates to the cure of
HIV and the adeno-associated virus vector associated with it. Despite the potential ability to cure
a multitude of diseases somatic gene therapy is not without its problems. The societal and
ethical question relating to the changing of the human genome or constantly dissected and
securitized, and for good reason.
Introduction
Gene therapy has been around for some time and is classified into two distinct groups,
somatic gene therapy and germ line gene therapy. Germ line gene therapy involves changing the
genome of sperm and of the egg before conception to prevent diseases from ever effecting the
child of this egg and sperm. Somatic gene therapy, the topic of this paper, is the manipulating of
an adult’s genes for the sake of producing a product that alters the state of the individual’s genes
being expressed. This paper will discuss the significant historical markers of this technology
from the mid-1960s to present day. This paper will also discuss the way in which the procedures
are conducted, the differences between them and the differences between in vivo and ex vivo.
The types of techniques being used currently will be compared and contrasted as well as the
multitude of delivery systems and their limitations. Finally this paper will discuss the success
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and improvements of gene therapy while explaining the ethical and societal implications that
arise when researching gene therapy.
History
The thought of genetic therapy has its roots set in the mid-1960s where many new
findings revealed the depths in which virus actually altered inherited DNA. It was Edward
Tatum who, in 1966, aroused the link between virus and somatic-cell genetics and its possible
effectiveness in genetic therapy. Genetic therapy, as we see it today, was first envisioned by
Stanfield Roger’s in 1970 when he proposed that defective DNA could be replaced with a
sample of non-defective DNA. Theodore Friedmann and Richard Roblin (SOURCE) capitalized
on this idea in 1972 when they published Gene therapy for human genetic disease? In this paper
Friedmann and Roblin reflected on the issues revolving around the successful implementation of
genetic therapy. Citing the lack of understanding revolving around gene regulation and genetic
recombination in human cells, the relation between the disease state and the molecular defect and
the short term and long term effects of gene therapy. It was not until 1985 that research,
completed by a collaboration between the National Heart, Lung, and Blood Institute and the
National Cancer Institute, was able to show that adenosine deaminase (ADA) deficient cells were
able to be corrected using a retrovirus. As the experimentation progressed into the late 1980s the
team brought on Dr. Steven Rosenburg, who helped the team figure out which tumor infiltration
cells (TIL cells) were safe for use and by 1990 a 9 year old girl and 4 year old girl were the first
humans to be infused with the corrected cells. These girls and similar patient are still receiving
treatment for their defect, however, the infusion has proved a success thus far. With the success
of 1990 a slew of experiments were undertaken. This led to the curing of sickle cell anemia in
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mice and the use of stem cells as vectors in 1992. By 2010 gene therapy has been able to cure
retinal disease, color blindness, adrenoleukodystrophy and two cases of metastatic melanoma. In
the past five years scientists have made remarkable progress and there are now gene therapy
products and treatments available in many corners of the world.
Structure and Design
The current field of somatic genetic therapy is constantly being changed and redefined
but the central theme has remained the same. A victor, may it be viral or non-viral, is embedded
with a sequence of genes that it is meant to imbed in the desired cell. The difference between
these is that the implementation of a viral vector is known as transduction while the
implementation of a non-viral vector is known as transfection. The method used in somatic gene
therapy is relatively the same if the intention is to augment, inhibit or even terminate the cell.
The desired trait is acquired from a correct gene sequence and is then injected into the vector.
The vector is then injected into either the patient in an in vivo procedure or injected into a culture
of the patients cells in an ex vivo procedure. In an in vivo procedure the cells begin to produce
the desired product within the body. In an ex vivo procedure the cells are reinjected into the
body where it proceeds to develop the desired product. In many cases the ex vivo approach is a
lot less likely to cause an immune response due to the lack of a foreign vector entering the body.
As you can see in Figure 1 the adenovirus vector, in an in vivo treatment, binds with the cell
membrane of the target cell and is delivered to the nucleus of the cell where the new gene is
inserted in the cell’s genetic sequence.
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Figure 1, National Library of Medicine (US)
Viral Vectors Viral vectors utilizes the natural function of lysogenic viruses, which is to
replicate and infect multiple cells to integrate foreign genes into the cells’ DNA. The two main
groups of viruses used are retroviruses and adenoviruses. Retroviruses infect their host and
introduce an RNA sequence along with some enzymes that allow the sequence to be reverse
transcribed back into the host cell’s nucleus and fitted into the cell’s genome. Now as the cell
divides the new sequence will be replicated with the new cell and be expressed just the same.
The other major form of viral vectors is Adenoviruses. The Adenovirus carries its genetic
material in the form of double stranded DNA and does not bond its self with the genome of the
cell. The virus implants itself in the nucleus and is able to be transcribed just like any other
strand of DNA, however, since it does not bond with the genome it will not be replicated when
the cell divides.
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Non-viral Vectors Non-viral vector methods offer an alternative to viral vectors and
their subsequent limitations. Plasmids, a popular, circular DNA molecule is a non-viral vector
that is able to diffuse into the cell with a much larger gene strand than viral vectors. The
plasmids is held within a liposome which allows the plasmid to enter the cell by merging with
the membrane of the cell. Despite these benefits non-viral vectors such as this have a much lower
success rate than regular viral vectors.
Technique and Function
The three types of gene therapy techniques are gene augmentation, gene inhibition and
the killing of specific cells. In gene augmentation therapy a diseased cell with a dysfunctional
gene sequence is replaced by a functioning gene sequence that then allows the cell to function
normally and produce the protein it previously was unable to produce. Gene augmentation
therapy is best used for disease such as cystic fibrosis or ADA deficiency where the effects of the
disease are not permanent and are the result of the loss of the production of a specific protein.
Gene inhibition therapy is best suited for the treatment of diseases caused by gene activity that is
considered inappropriate, such as the over activity of cancerous cells. The two main goals of
gene inhibition is to either inhibit the expression of the dysfunctional gene or interfere with the
protein created by the dysfunctional gene. An example of this would be an oncogene. Under
certain circumstances an oncogene can cause normal cells to become diseased tumor cells.
Proper gene inhibition therapy would be able to implant a gene that possesses the ability to block
the expression of the oncogene. The killing of specific cells through gene therapy main target
are diseases that affect a certain area of the body, or type of cell, which can be treated by killing
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this groups of cells. This form of gene therapy is conducted by either inserting a suicide gene
into the diseased cell to terminate the cell or by marking the cell which will prompt the body’s
immune system to destroy the diseased cell. Another technique relating to this technique
involves genetically modifying immune cells to target and identify cells that need to be
terminated. This technique may also be used to help the immune cells to produce a specific
product that can aid in the curing of the ailment caused by the defective cells being targeted.
Success
Apart from the early success in treating ADA, and discovering the mean of using vectors
to rewrite cell genomes, scientist from china have created Gendicine
Gendicine is the first clinically approved, originally by the Chinese government, genetic
therapy used for the treatment of head and neck squamous cell carcinoma. Gendicine is an
adenovirus vector with the purposes to express wild-type p53, a tumor suppressor gene found in
humans. The trials conducted between November 2000 and May 2003 shoe that 64% of patients
showed complete regression of the tumors while 29% showed partial regression. Leaving only
7% of patients with no regression. Today several labs around the world are attempting to perfect
and repurpose this treatment as a means to eliminate other tumor growths throughout the body.
Glybera is a new treatment that just got approval in Germany, for the treatment of lipase
deficiency. This treatment works through an adeno-associated viral vector that targets the
muscle cells of the body to release the human lipoprotein lipase (LPL) gene which cures the
patient of lipase deficiency. In nearly all cases Glybera the fat concentration in the blood stream
was reduced in nearly all patients. Despite this being the first gene therapy technique being
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officially accepted in the United States and Europe It does not come without disadvantages. The
patients must undergo immunosuppressive therapy and pay a considerable amount of money for
the procedure, one million USD. Despite these setbacks this is a step in the right direction.
Limitations
As with every modern technology and medical feat there are limitations to what can be
accomplished. These limitations will differ depending on which technique, deliver method and
ailment being corrected. However, limitations such as knowledge of the human genome and the
commercial viability of pursuing certain cures due to the limited number of patients. Gene
augmentation is only successful if the new gene is able to produce the protein at a sufficient level
as required by the patient. Meaning if a high enough does is not given the amount of the product
produced will not be large enough to reverse the effect on the body. Obviously the effects of the
disease must be reversible for this to work as a cure. In gene inhibition It becomes difficult to
establish which gene will produce which proteins and if it will inhibit the actions of another gene
and protein when implemented. In gene specific killing of cells the obvious issue is that the
treatment will actually prompt the body to kill cells that are functioning normally instead of, or
in addition to, the dysfunctional cells. This issue is also a concern in regards to the genetically
modified immune cells. It becomes difficult to ensure that the cells being targeted are the
dysfunctional cells and the product being released does not have adverse effects on the patient.
Improvements:
This field is constantly being reinvented as more and more pieces of the genetic code are being
identified and the ways in which diseases and viruses attack and infect cells are being uncovered.
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This continued research has led to the prominent use of Lentiviruses and Virosomes, which
possess the abilities to overcome the flaws that older vectors suffered.
Lentiviruses are a subclass of retroviruses that possess the unique ability to infect nondividing cells where other retro viruses can only infect dividing cells. The lentivirus is
transcribed into the genome of the cell where the cell then produces the desired product. As the
vector diffuses the virus stays in the genome, now as a provirus, and is passed onto the new cell
when the old one divides. However, the location where it shows up is not well understood as it
seems it shows up in increasingly random locations. Several studies are currently in the works to
discern exactly where these genes end up and the genome. Until then lentiviruses will remain a
gamble of sorts.
Virosomes are a special kind of non-viral vector that combines the benefits of viral and
non-viral vectors by being a synthetic liposome covered with a viral protein. These virosomes
have the capability of a non-viral vector in the ability to carry much larger strands of genes while
not causing an immune response. They also have the benefits of a viral vector because of their
viral coating which helps diffuse into the cell membrane. This combination of attributes allows
the synthetic virosome to handle this process in a much easier manner.
Future technology
The most promising field of research is the current race for the cure to HIV. Currently
the method of treatment being researched, vectored immunoprophylaxis (VIP), has made
considerable progress in recent years. The first in 2013 when researchers at the University of
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California were able to cure, and immunize, mice utilizing a special adeno-associated virus
vector that uses muscle tissue to create the anti-body. In March of this year a research team
comprised of many notable universities was able to utilize a similar adeno-associated virus
vector that is able to cure, and immunize, monkeys of all known HIV strains. Researchers are
now hoping to not only use this method to begin human trials but to also cure similar diseases
that are currently untreatable. The future of the somatic gene therapy field lies within the
progress of this treatment.
Societal and Ethical Implications
Despite the many positives that are associated with somatic gene therapy there are many
societal and ethical questions to take into account. The largest concern with this treatment is the
apartment availability to the wealthy but not to the poor. This question comes up a lot when
discussing Glybera and its one million dollar price tag. Despite this being an extremely
expensive product at the moment this price will not remain at this high rate. As technology
improves and more products are allowed to be released to the market costs will go down as well.
This has been the way of economics for generations. The high demand accompanied by a low
supply will only result in a high price, unless government regulated. Another, more ethical
concern, is the ability of individuals to utilize this treatment for cosmetic and recreational uses.
With the increase in the use of genetic therapy there is a fear that genetic therapy will be used to
change perfectly healthy cells so that they can express more desirable traits. This could possibly
create a social climate that is more willing to look down on people with these undesirable traits
because it is possible to change them. This is especially true if somatic gene therapy has any
effect on the climate of germ line genetic therapy and the idea of creating so-to-speak designer
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babies. Obviously this concern has much racial connotation. Even though these concerns are
well founded the potential benefits from somatic gene therapy far outweigh the positives that are
well within reach.
Conclusion
This paper has discussed the history of somatic gene therapy from the mid-1960s to the
present day. And discussed the many different forms of treatment and vectors, highlighting the
use of adeno-associated vectors above all else and its ability to cure HIV and possibly a
multitude of other diseases. The current ethical question was discussed and highlighted in this
paper to show the disparity in wealth and the unavailability of a cure for the poor. The only hope
for the future of somatic gene therapy is the continuation of funding, especially in the current
studies being conducted for HIV.
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