Download Human Gene Therapy : A Brief Overview of the Genetic

© JAPI • february 2013 • VOL. 61 41
Review Article
Human Gene Therapy : A Brief Overview of the
Genetic Revolution
Sanjukta Misra*
Abstract
Advances in biotechnology have brought gene therapy to the forefront of medical research. The prelude to
successful gene therapy i.e. the efficient transfer and expression of a variety of human gene into target cells has
already been accomplished in several systems. Safe methods have been devised to do this, using several viral
and no-viral vectors. Two main approaches emerged: in vivo modification and ex vivo modification. Retrovirus,
adenovirus, adeno-associated virus are suitable for gene therapeutic approaches which are based on permanent
expression of the therapeutic gene. Non-viral vectors are far less efficient than viral vectors, but they have
advantages due to their low immunogenicity and their large capacity for therapeutic DNA. To improve the function
of non-viral vectors, the addition of viral functions such as receptor mediated uptake and nuclear translocation of
DNA may finally lead to the development of an artificial virus. Gene transfer protocols have been approved for
human use in inherited diseases, cancers and acquired disorders. In 1990, the first successful clinical trial of gene
therapy was initiated for adenosine deaminase deficiency. Since then, the number of clinical protocols initiated
worldwide has increased exponentially. Although preliminary results of these trials are somewhat disappointing,
but human gene therapy dreams of treating diseases by replacing or supplementing the product of defective or
introducing novel therapeutic genes. So definitely human gene therapy is an effective addition to the arsenal of
approaches to many human therapies in the 21 st century.
Introduction
J
ames Watson was quoted as saying “we used to think that our
fate was in our stars, but now we know, in large measures,
our fate is in our genes”. Genes, the functional unit of heredity,
are specific sequences bases that encode instructions to make
proteins. Although genes get a lot of attentions, it is the proteins
that perform most life functions. When genes are altered,
encoded proteins are unable to carry out their normal functions,
resulting in genetic disorders. Gene therapy (use of genes as
medicines) is basically to correct defective genes responsible for
genetic disorder by one of the following approaches-1,2
•
14%
Other
diseases
Historical Perspectives
Since the earliest days of plant and animal domestication,
about 10,000 years ago, humans have understood that
characteristics traits of parents could be transmitted to their
offspring. The first to speculate about how this process worked
were ancient Greek scholars, and some of their theories remained
in favor for several centuries. The scientific study of genetics
began in 1850s, when Austrian monk Gregor Mendel, in a
series of experiments with green peas, described the pattern
of inheritance, observing that traits were inherited as separate
units we know as genes. Mendel’s work formed the foundation
Cancer
Fig. 1 : Proportion of protocol for human gene therapy trials relating
to various types of diseases52
*
Asst. Professor, Dept. of Biochemistry, Institute of Medical Sciences
and SUM Hospital, Bhubaneswar 751 003, Orissa
Received: 09.06.2011; Revised: 29.02.2012; Accepted: 29.02.2012
© JAPI • february 2013 • VOL. 61 An abnormal gene could be repaired through selective
reverse mutation
Other gene therapy projects are targeted at conditions such
as heart disease, diabetes mellitus, arthritis and Alzheimer’s
disease, all of which involve genetic susceptibility to illness.5
Table 1 shows a summary of approved current clinical gene
therapy protocols
Monogenic
diseases
6%
•
Gene therapy states and remains an experimental discipline
and many researches remain to be performed before the
treatment will realize its potential. Majority of the gene therapy
trials are being conducted in United States and Europe, with
only a modest number in other countries including Australia.
Scope of this approach is broad with potential in treatment of
diseases caused by single gene recessive disorders (like cystic
fibrosis, hemophilia, muscular dystrophy, sickle cell anemia
etc), acquired genetic diseases such as cancer and certain viral
infections like AIDS,3,4 as shown in Figure 1.
Infectious
diseases
70%
An abnormal gene could be swapped for a normal gene
homologous recombination
• Regulation (degree to which a gene is turned on or off) of
a particular gene could be altered
A normal gene could be inserted into a nonspecific location
within the genome to replace the Nonfunctional gene (most
common)
10%
•
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© JAPI • february 2013 • VOL. 61
Table 1 : Summary of approved and published current clinical gene therapy protocols50
Disorder
ADA deficiency
Objective
ADA replacement
Alpha-1-antitrypsin
deficiency
AIDS
Cancer
Cystic fibrosis
Familial
hypercholesterolemia
Fanconi’s anemia
Gaucher’s disease
Hemophilia B
Rheumatoid arthritis
Blood
Target cells
Mode of delivery
Retrovirus
Alpha-1-antitrypsin
replacement
Antigen presentation HIV
inactivation
Immune function
enhancement
Respiratory epithelium
Liposome
Countries with protocols
Italy, the Netherlands,
United States
United States
Blood, marrow Blood,
marrow
Blood, marrow, tumour
Retrovirus
United States
Tumour ablation
Tumour
Austria, China, France,
Germany, Italy, the
Nethelands, United States
United States
Chemoprotection
Stem-cell marking
Blood, marrow
Blood, marrow, tumour
Retrovirus, liposome,
electroporation, cellmediated transfer
Retrovirus, non-complexed
DNA, cell-mediated transfer
Retrovirus
Retrovirus
Cystic fibrosis
transmembrane regulatory
enzyme replacement
Replacement of low-densitylipoprotein receptors
Complement group C gene
delivery
Glucocerebrosidase
replacement
Factor IX replacement
Cytokine delivery
Respiratory epithelium
Adenovirus, liposome
Liver
Retrovirus
United States
Blood, marrow
Retrovirus
United States
Blood, marrow
Retrovirus
United States
Skin fibroblasts
Synovium
Retrovirus
Retrovirus
China
United States
for later scientific achievements that heralded the era of modern
genetics. But little was known about the physical nature of genes
until 1950s, when American biochemist James Watson and British
biophysicist Francis Crick developed their revolutionary model
of double stranded DNA helix. Another key breakthrough
came in the early 1970s, when researchers discovered a
series of enzymes that made it possible to snip apart genes at
predetermined site along a molecule of DNA and glue them back
together in a reproducible manner. Those genetic advances set
the stage for the emergence of genetic engineering, which has
produced new drugs and antibodies and enabled scientists to
contemplate gene therapy. A few years after the isolation of genes
from DNA, gene therapy was discovered in 1980s.6
Process of Gene Therapy
Approach
The process of gene therapy remains complex and many
techniques need further developments. The challenge of
developing successful gene therapy for any specific condition is
considerable. The condition in question must be well understood,
the undying faulty gene must be identified and a working copy
of the gene involved must be available. Specific cells in the
body requiring treatment must be identified and are accessible.
A means of efficiently delivering working copies of the gene to
the cells must be available. Moreover diseases and their strict
genetic link need to be understood thoroughly.
Types of gene therapy
There are 2 types of gene therapy.
1.
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Germ line gene therapy:7 where germ cells (sperm or egg) are
modified by the introduction of functional genes, which
are integrated into their genome. Therefore changes due to
therapy would be heritable and would be passed on to later
generation. Theoretically, this approach should be highly
effective in counteracting genetic disease and hereditary
disorders. But at present many jurisdictions, a variety of
United States
Canada, France, Sweden,
United States
United Kingdom, United
States
technical difficulties and ethical reasons make it unlikely
that germ line therapy would be tried in human beings in
near future.
2.
Somatic gene therapy:8 where therapeutic genes are transferred
into the somatic cells of a patient. Any modifications and
effects will be restricted to the individual patient only and
will not be inherited by the patients offspring or any later
generation.
Gene delivery
In most gene therapy studies, a normal gene is inserted into
the genome to replace an abnormal, disease causing gene. Of
all challenges, the one that is most difficult is the problem of
gene delivery i.e. how to get the new or replacement gene into
the patient’s target cells. So a carrier molecule called vector
must be used for the above purpose.9 The ideal gene delivery
vector should be very specific, capable of efficiently delivering
one or more genes of the size needed for clinical application,
unrecognized by the immune system and be purified in large
quantities at high concentration. Once the vector is inserted
into the patient, it should not induce an allergic reaction or
inflammation. It should be safe not only for the patient but also
for the environment. Finally a vector should be able to express the
gene for as long as is required, generally the life of the patient .
Two techniques have been used to deliver vectors i.e. ex-vivo
and in-vivo.10 The former is the commonest method, which uses
extracted cells from the patient. First, the normal genes are cloned
into the vector. Next, the cells with defective genes are removed
from the patient and are mixed with genetically engineered
vector. Finally the transfected cells are reinfused in the patient
to produce protein needed to fight the disease. On the contrary,
the latter technique does not use cells from the patient’s body.
Vectors with the normal gene are injected into patient’s blood
stream to seek out and bind with target cell (Figure 2).
Some of the vectors used in gene therapy include:
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Also this has been tried to treat SCID due to ADA
deficiency with relative success. As researchers have
grown more confident, they have begun injecting
altered retroviruses directly into tissues where the
corrected genes are needed. For example- in cystic
fibrosis (mutated gene impairs lung function), healthy
genes are inserted directly to the lining of bronchial
tube. Experimental animal studies are being conducted
to establish the effect in muscular dystrophy.
Adenovirus16
•
To avoid problem of inserting genes at wrong sites,
some researchers have turned to other types of viruses.
A class of virus with double stranded DNA genome
that can cause respiratory, intestinal and eye infection
(especially the common cold).When these viruses infect
a host cell, they introduce their DNA molecule into
the host. The genetic material of the adenovirus is not
incorporated into the host cell’s genetic material. The
DNA molecule is left free in the nucleus of the host
cell, and the instructions in this extra DNA molecule
are transcribed just like any other gene (Figure 1).
Adenovirus also can infect a broader a broader variety
of cells than retrovirus, including cells that divide more
slowly, such as lungs cells. However, adenovirus also
are more likely to be attacked by the patient’s immune
system, and the high levels of virus required for
treatment often provoke an undesirable inflammatory
response. Despite these drawbacks, this vector system
has been promoted for treating cancer of liver and
ovaries and indeed the first gene therapy product to be
licensed to treat head and neck cancer is Gendicine, p53
based adenoviral product.17 Concern about the safety
of the above vectors was raised after the 1999 death of
Jesse Gelsinger while participating in a gene therapy
trial.18 Since then, work using adenovirus vector has
focused on genetically crippled version of the virus.
Adeno-associated viruses [AAVs]
Fig. 2 : Gene therapy using an adenovirus vector51
A. Viral Vector
One of the most promising vectors currently being used
is harmless viruses. Viruses have evolved a way of
encapsulating and delivering their genes to human cells in
a pathogenic manner. Scientists have tried to take advantage
of this capability and manipulate the viral genome and
replace them with working human gene.11 This altered
virus can then be used to smuggle genes into cells with
great efficiency. Some of the viruses insert their genes into
the host genome, but do not actually enter the cell. Others
penetrate the cell membrane disguised as protein molecule
and enter the cell. Once the transplanted gene is ‘switched
on’ in the right location within the cell of an infected person,
it can then issue instructions necessary for the cell to make
the protein, that was previously missed or altered .
Some of the different types of viruses used as gene therapy
vectors:
•
Retrovirus12
First viruses to be used as vectors in gene therapy
experiments were retroviruses. They belong to a
class of viruses (RNA as genetic material ) which can
create double stranded DNA copies with the enzyme
reverse transcriptase. These copies of its genome can be
integrated into the chromosome of host cell by another
enzyme carried the virus called integrase. Now the
host cell has been modified to contain a new gene. If
such modified host cells divide later, their descendants
will contain the new genes. Although retroviruses
have been used in most gene therapy experiments
so far, they present problems.13 One such problem is
that integrase enzyme can insert genetic material of
the virus into any arbitrary position in the genome of
the host, which can lead to insertional mutagenesis (if
insertion is in the middle of the gene) or uncontrolled
cell division (if gene happens to be one regulating cell
division) leading to cancer. This problem has recently
begun to be addressed by utilizing zinc finger nuclease14
or by including certain sequences such as beta globin
locus control region to direct the site of integration to
specific chromosome.
Gene therapy trial using retroviral vector to treat
X-linked severe combined immune deficiency represent
the most successful application till date.15
© JAPI • february 2013 • VOL. 61 •
One of the most promising potential vectors is a recently
discovered virus called the AAV, which infects a broad
range of cells including both dividing and non dividing
cells. AAVs are small viruses from the Parvovirus
family with a genome of single stranded DNA. It can
insert genetic material at a specific site on chromosome
19 with near 100% certainty. Researchers believe that
most people carry AAV which do not cause disease
and do not provoke an immune response. Scientists
have demonstrated the animal experiments using
AAV to correct genetic defects.18 It is now being used
in preliminary studies to treat hereditary blood disease
hemophilia, muscle and eye disease. Also clinical trials
have been initiated to use AAV vectors to deliver genes
to brain as the virus can infect nondividing cells like
neurons in which their genome are expressed for a long
time.
The chief drawback of AAV is that it is small, carrying
only 2 genes in its natural state. Its payload therefore
is relatively limited. It can produce unintended genetic
damage because the virus inserts its genes directly into
host cell’s DNA. Researchers have also had difficulties
in manufacturing large quantities of the altered virus.
The production problem has recently being solved by
Amsterdam Molecular Therapeutics.19 The recombinant
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© JAPI • february 2013 • VOL. 61
DNA, which does not contain any viral genome and
only the therapeutic gene, does not integrate into
the genome, instead fuses at its end to form circular,
episomal forms which are predicted to be the primary
cause of long term gene expression.
•
Herpes simplex virus [HSV]
It is a human neurotropic virus, which is mostly used for
gene transfer in nervous system. It has a large genome
compared to other viruses, which enable scientist to
insert more than one therapeutic gene into a single
virus, paving the way for treatment of disorders caused
by more than one gene defect. HSV makes an ideal
vector as it can infect a wide range of tissues including
muscle, liver, pancreas, and nerve and lung cells. The
wild type of HSV-1 virus is able to infect neurons which
are not rejected by immune system.20 Antibodies to
HSV-1 are common in humans, however complications
due to herpes infections are somewhat rare.
B. Non-Viral Methods
Simplest method of non-viral transfection is direct DNA
injection.21 Clinical trials to inject naked DNA plasmids
have been performed successfully. There have been trials
with naked PCR products, which have had greater success.
Research efforts have yielded several non-viral methods
gene transfer such as electroporation (creation of electric
field induced pores in plasma membrane), sonoporation
(ultrasonic frequencies to disrupt cell membrane),
magnetofection (use of magnetic particle complexed with
DNA), gene guns (shoots DNA coated gold particles into
cells by using high pressure) and receptor mediated gene
transfer are being explored.22 Each method has its own
advantages and disadvantages.
Among the several nonviral approaches, receptor mediated
gene transfer currently holds the most promise. This
application involves the use of DNA conjugated with
specific proteins (viral structural protein), or with liposome,
or both.22 Under experimental ex-vivo conditions, liposomes
containing DNA have been shown to undergo cellular
uptake through endocytosis, with subsequent transient
exogenous gene expression. Nevertheless, the application
of this approach will likely be limited until methods for the
stable integration of the endocytosed DNA are devised and
improvement in target ability, transfection efficiency and
DNA carrying capacity are developed.22,23
Recently several chemical methods like use of synthetic
oligonucleotides (to inactivate defective genes by using
antisense specific to target gene), lipoplexes (made up of
anionic and neutral lipids) and polyplexes (complex of
polymers with DNA) have been used to facilitate delivery of
the DNA into cell.24 Recently there have been some hybrid
methods developed that combine two or more techniques.
For example- vibrosomes that combine liposomes with an
inactivated HIV or influenza virus. This has been shown to
have more efficient gene transfer in respiratory epithelial
cell than either viral or liposomal method alone. Other
hybrid methods involve mixing viral vectors with cationic
lipids. Researchers are also experimenting with introducing
a 47th (artificial human) chromosome into target cells. This
chromosome would exist autonomously alongside the
standard 46, not affecting their workings or causing any
mutation. It would be a large vector capable of carrying
substantial amount of genetic code, and scientists anticipate
130
that, because of its construction and autonomy, the body’s
immune system would not attack it.
The advantage of direct transfers of non-complexes
or protein- complexed DNA by chemical, mechanical,
electrical, particle bombardment method or artificial
chromosome method include the possibility of transferring
relatively large DNA fragments. However, these processes
are still inefficient, are limited to ex-vivo gene transfer and
have undefined cytotoxic effects.21
Target tissues for gene therapy
Hematopoietic cells derived from bone marrow (BM) may
be readily obtained and manipulated ex-vivo in a variety
of tissue culture system. 25 Furthermore, therapies based
on transfer of genetically modified hematopoietic cells are
potentially applicable to a wide range disorders, including
the hemoglobinopathies, AIDS and cancer. For these reasons,
BM cells are attractive targets in gene therapy research.25
Gene transfer into human hematopoietic stem cells capable
of proliferating in-vivo for long period of time, giving rise
to large number of progeny expressing the desired product,
remains an elusive goal. The main limitation is related to the
fact that hematopoietic cells greatest potential for proliferation
are generally quiescent and resistant to retrovirus mediated
gene transfer.26 Meanwhile, the utility of genetically modified
hematopoietic cells, with limited but predictable proliferative
potential continues to be elevated.25
Research into the usefulness of other cell types as gene
transfer vehicle remains very active. The known regenerative
properties of the liver make it an attractive gene transfer
target. Disorders theoretically amenable to this form of therapy
include familial hypercholesterolemia, hemophilia, urea cycle
defect, -1-antitrypsin deficiency and phenylketonuria.27 Other
potential targets for therapy include skeletal muscle cells for
amelioration of muscular dystrophies, respiratory epithelial
cells for the treatment of respiratory insufficiency in cystic
fibrosis, and central nervous system tissue for degenerative
neurologic disorders.20,28 The utility of carrier cells expressing
exogenous genes in nonantigenic porous microcapsules has also
been demonstrated, and this method promises long term gene
delivery using “universal donor” cell lines.29 In a more radical
experimental approach, implantable “neo-organs” made up
of genetically modified carrier cells bound to collagen coated
synthetic fibres have been shown to have the capacity to become
vascularized intraperitoneally and to secrete proteins.30
Journey of Clinical Trials in Gene
Therapy [1980 – 2010]
There are several early speculations on the method of gene
therapy.31 In 1966, Tatum predicted that viruses could be used
to convert genes in theoretical studies in somatic-cell genetics.
The first attempt in genetics was done in Pecking ducklings
which were injected with DNA extracts from Khaki Campbell
ducks and expressed some of the characteristics of the said duck
but another trial on albino rat by DNA extracts of pigmented
rat did not produce any significant result. In 1970, American
doctor Stanfield Rogers tried to treat two sisters, suffering from
arginemia (that lacks enzyme arginase, a type of protein) by
injecting shope papilloma virus containing an arginase gene but
this gene therapy was unsuccessful to raise the above protein
levels higher. In 1977, scientists were able to use gene therapy
technique to deliver a gene into cells of mammals.
© JAPI • february 2013 • VOL. 61
© JAPI • february 2013 • VOL. 61 In 1980, Mercola and Cline32 undertook the first human gene
therapy trial to treat β thalassemia patients by transfecting
β globin gene into human bone marrow cells. This protocol
lacked appropriate ethical review, was widely reviewed as
premature on scientific grounds and was eventually stopped.
Two important points emerged from this study. One was the fact
that highly regulated and coordinated expression of both ά-like
and β-like globin genes would likely be required for successful
gene therapy of the hemoglobinopathies. Other was the need
to address the safety and ethical concern adequately in clinical
trials of gene therapy.
because viral vectors are too big to get across the blood brain
barrier. This method has potential for treatment for Parkinson’s
disease.
Scientists at the National Institute of Health (Bethesda,
Maryland) have successfully treated metastatic melanoma in two
patients using killer T cells genetically retargeted to attack the
cancer cells. This study constitutes one of the first demonstrations
that gene therapy can be effective in treating cancer. In another
development, gene therapy may be used to Huntington’s disease.
Short interfering RNAs (siRNAs) are designed to match the RNA,
copied from a faulty gene and to produce abnormal protein
product of that gene. This RNA interference or gene silencing
may be used in gene therapy to switch off Huntington’s disease.39
In 1990, American doctor Anderson performed one of the
first successful gene therapy study on a 4 year old girl named
Ashanti DeSilva, with a rare genetic immune system disorder
called severe combined immuno-deficiency (SCID). The lack
of production of adenosine deaminase (ADA), had made her
immune system weak, so she had become susceptible to many
severe diseases. Anderson and his colleagues extracted her
WBCs, implanted genes producing ADA into WBCs and then
transferred the cells back to her body. The WBCs strengthened
the girl’s immune system made it possible for her to survive.
The effects were only temporary, but successful.33
In 2005, scientists were able to repair deafness in guinea pig
by using adenovirus vector.40 Atoh1 gene (which stimulates hair
cell’s growth) was delivered to cochlea resulting in regrowth of
hair cells and so regaining 80% of original hearing threshold.
This study may pave the way to human trials of gene therapy
in such cases.
In 2006 (March), an international group of scientists
announced the successful use of gene therapy to treat two adult
patients for a disease affecting myeloid cells.41 Study published
in Nature Medicine, is believed to be the first to show that gene
therapy can cure disease of myeloid system. In May 2006, a
team of scientists from Italy, reported a breakthrough for gene
therapy in which they developed a way to prevent immune
system from rejecting a newly delivered gene with the use of
micro RNAs, whose natural function could be used to selectively
turn off the identity of the therapeutic gene.42 The researchers
were successful in mice experimentation. This work will have
important implication for the treatment of hemophilia and other
genetic disease by gene therapy. In August 2006, researchers
successfully reengineered immune cells, called lymphocyte,
to target and attack cancer cells in patients with advanced
metastatic melanoma. This is the first time that gene therapy
is used to successfully treat cancer in humans. In November
2006, Preston Nix from the University of Pennsylvania School of
Medicine reported on VRX496, a gene-based immunotherapy for
the treatment of human immunodeficiency virus(HIV) that used
a lentiviral vector for delivery of an antisense gene against the
HIV envelope. Patients responded to the above therapy showing
stable and increased immune response (CD4 T cell count). This
was the first evaluation of a lentiviral vector administered in
U.S. Food and Drug Administration-approved human clinical
trials for any disease.43 Data from an ongoing clinical trial were
presented at CROI (conference on retrovirus and opportunistic
infection)
In 1992, Claudio Bordignon of Italy performed the first
procedure of gene therapy using hematopoietic stem cells as
vectors to deliver genes intended to correct hereditary disease.34
In 1993, a new born baby Andrew Gobea, with SCID, was
treated by gene therapy technique using retrovirus vector
carrying ADA gene.35 Blood was removed from his placenta and
umbilical cord immediately after birth, containing stem cells.
Retrovirus (carrying ADA gene) and stem cells were mixed,
after which they entered and inserted the gene into stem cells’
chromosome. Stem cells containing the working ADA enzyme
were also given weekly. For next few years, WBCs (produced
by stem cells), made ADA enzyme using ADA gene. After then
further treatment was needed.
In 1999, gene therapy suffered a major setback with the death
of 18 year old Jesse Gelsinger who participated in a gene therapy
trial for ornithine transcarboxylase deficiency.18 He died from
multiple organ failure 4 days after starting the treatment. His
death was believed to have been triggered by a severe immune
response to the adenovirus carrier.
In 2002, French researcher Alain Fischer tried to cure children
suffering from X- linked SCID (also known as bubble boy)
by inserting retrovirus carrying normal gene into children’s
blood stem cell. This clinical trial was questioned when 2 of
them developed a leukemia-like condition.36 However another
major blow came in Jan 2003, when the “FOOD and DRUG
ADMINISTRATION” [FDA] placed a temporary halt on all
gene therapy trials using retrovirus vector in blood stem cells.
Then in April 2003, FDA eased the ban after regulatory review
of the protocol in USA, UK, France, Italy and Germany37 since
the treatment had benefitted a large number of children.
In 2007, a team of British doctors from Moorefield’s Eye
Hospital and University college of London, announced the
world’s first gene therapy trial to test a revolutionary gene
therapy treatment for a type of inherited retinal disease i.e.
Leber’s congenital amaurosis, which is caused by mutation
in the RPE65 gene. Sub-retinal delivery of recombinant AAV
carrying RPE65 yielded positive result, with patient having
modest increase in vision, and more importantly, no apparent
side-effect.44
Researchers at Case Western Reserve University and
Copernicus Therapeutics have been able to create DNA nanoballs
(tiny liposomes 25nm) that can carry therapeutic DNA through
pores in nuclear membrane. Moreover gene therapy approach
repairs errors in messengerRNA derived from defective genes.
This technique has the potential to treat thalassemia, cystic
fibrosis, and some forms of cancers.
In 2009 (March), the School of Pharmacy in London tried
nanotechnology based gene therapy (which delivers genes
wrapped in nanoparticles) to target and destroy hard-to-reach
cancer cells.45 In September 2009, journal Nature reported that
researchers at the University of Washington and University of
Florida were able to give trichomatic vision to squirrel monkeys
In 2003, Los Angeles research team inserted genes into brain
using liposome coated in a polymer called polyethylene glycol.38
The transfer of gene into brain is a significant achievement
© JAPI • february 2013 • VOL. 61 45
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46
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using gene therapy.46 This could have a significance on future
treatment for colour blindness in humans. In November 2009,
the journal Science reported that researchers succeeded at halting
a fatal brain disease, adrenoleukodystrophy, using a vector
derived from HIV to deliver the gene for the missing enzyme.47
In 2010, a paper by Komaromy et al. published in April 2010,
deals with gene therapy for a form of achromatopsia (complete
colour blindness) in dogs. It is presented as idle model to develop
gene therapy directed to cone photoreceptor. Cone function
and day vision have been restored for at least 33 months in two
young dogs with achromatopsia. However, the therapy was less
efficient for older dogs.48
Disadvantages of Gene Therapy
No therapy, established or experimental, is without some
associated risks. As human gene therapy is still a relatively new
procedure, there are still many risks associated with it. Scientists
do not yet understand all of the risks and there has not been
enough time to complete detailed studies on how gene therapy
works and the problems that it poses. Safety will appropriately
remain an important consideration as the field of gene therapy
evolves.13 Some of the problems of gene therapy include:
• Short-lived nature of gene therapy: Before gene therapy can
become a permanent cure for any condition, the therapeutic
DNA introduced into target cells must remain functional
and cells containing the therapeutic DNA must be long-lived
and stable. Problems with integrating therapeutic DNA
into the genome and the rapidly dividing nature of many
cells prevent gene therapy from achieving any long-term
benefits. Patients will have to undergo multiple rounds of
gene therapy. Moreover, the new gene fails to express itself
or the virus does not produce the desired response.
• Immune response: Anytime a foreign object is introduced into
human tissues, the immune system has evolved to attack
the invader. The risk of stimulating the immune system in
a way that reduces gene therapy effectiveness is always a
possibility. Furthermore, the immune system’s enhanced
response to invaders makes it difficult for gene therapy to
be repeated in patient.12
• Problem with viral vectors: Viruses, while the carrier of
choice in most gene therapy studies, present a variety of
potential problems to the patients- toxicity, immune and
inflammatory response and gene control and targeting
issues. In addition, there is always the fear that viral vector,
once inside the patient, may recover its ability to cause
disease.
• Multigenic disorders: Conditions or disorders that arise
from mutation in a single gene are best candidates for
gene therapy. Unfortunately, some of the most commonly
occurring disorders, such as heart disease, high blood
pressure, Alzheimer’s disease, arthritis and diabetes, are
caused by the combined effects of variations in many genes.
Multigenic or multifactorial disorders would be especially
difficult to treat effectively using gene therapy.11
• Insertional mutagenesis: The main problem that geneticists
are encountering is the virus may target the wrong cells. If
the DNA is integrated in the wrong place in the genome,14
for example in a tumor suppressor gene, it could induce a
tumor. This has occurred in clinical trials for X-linked SCID
patients in which hematopoietic stem cells were transduced
with a corrective transgene using a retrovirus, and this led
to the development of T cell leukemia in 3 of 20 patients.
Ethical and Social Consideration
Gene therapy is a powerful new technology that might have
unforeseen risks, scientists first develop a proposed experiments
i.e. protocol, that incorporates strict guidelines. After the approval
from FDA, the organization continues to monitor the experiment.
In the course of a clinical trial, researchers are required to report
any harmful side effects. Critics and proponents all agree that
risks of gene therapy must not be substantially larger than the
potential benefit. Gene therapy poses ethical considerations for
people to consider.49 Some people are concerned about whether
gene therapy is right and it may be used ethically.
Some of the ethical considerations for gene therapy include:
• Deciding what is normal and what is a disability;
• Deciding whether disabilities are diseases and whether they
should be cured;
• Deciding whether searching for a cure demeans the live of
people who have disabilities;
• Deciding whether somatic gene therapy is more or less
ethical than germ line gene therapy
Initial experiments using gene therapy have been conducted
primarily in patients for whom all other treatments have failed,
so that the risks are small. Many people feel that because gene
therapies use altered genes and potentially dangerous viruses,
those treatments should be tested more extensively.
Conclusion
Most scientists believe the potential for gene therapy is the
most exciting application of DNA science, yet undertaken.
How widely this therapy will be applied, depends on the
simplification of procedure. As gene therapy is uprising in the
field of medicine, scientists believe that after 20 years, this will
be the last cure of every genetic disease. Genes may ultimately
be used as medicine and given as simple intravenous injection
of gene transfer vehicle that will seek our target cells for stable,
site-specific chromosomal integration and subsequent gene
expression. And now that a draft of the human genome map
is complete, research is focusing on the function of each gene
and the role of the faulty gene play in disease. Gene therapy
will ultimately play Copernican part and will change our lives
forever.
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