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Gene therapy—fiction or reality?
They are the basis of life and hold the keys to unlock the
code to counter genetic diseases. Vanessa Mahapatra charts the successes
and failures of gene therapy and explores its potential in checking various
gene-linked disorders.
 Genes
have since long been considered the units of all life within our body. But now,
scientists are viewing them as the root cause of innumerable disorders. Thus
gene therapy comes into existence heralding an answer to numerous genetic woes.
"Gene therapy can be applied to any disease where you can identify that
some faulty gene causes the disease. There are several diseases like that,"
opines Srikumar Suryanarayan, President, R&D, Biocon. Gene therapy follows
a process whereby a functional gene replaces an absent or faulty gene, resulting
in the restoration of protein action, consequently eliminating the root cause
of the disease. Simple as it sounds, gene therapy has been under the limelight
for both its notable successes and concurrent failures.
Memoirs of Genes
Srikumar Suryanarayan
President- R&D
Biocon |
It all began with W French Anderson, the father of gene therapy,
who evolved the concept in a big way leading to the first clinical trial on
gene therapy in 1990. Gene therapy was executed on two girls suffering from
adenosine deaminase deficiency (ADA), a form of Severe Combined Immunodeficiency
Syndrome (SCID). The disease had plagued the immune system of both the girls
making them susceptible to repeated infections. Their bodies had extremely low
levels of specialised white blood cells, also known as T cells, which are the
immune system's instruments against invading organisms.
Scientists considered that replacing the defective ADA gene would trigger the
production of ADA, creating a permanent cure. Therefore, initiating gene therapy
on these patients, researchers induced the T cells from their blood to replicate
in culture. ADA-bearing retroviral vectors were then transferred into the cultured
T cells, which in turn integrated into the DNA and transferred the gene. The
enhanced T cells were then reintroduced into the girls. To the researcher's
delight, reported results were remarkable as their immune functions improved
progressively. However, one of the patients had to be subjected to continuous
treatment as the genetically treated WBCs work for only a few months. She therefore
has to be given repeated transfusion of blood containing the ADA gene. The results
of the second girl were welcomed more enthusiastically as after a review in
1995 and till date, it has been observed that the other patient has white blood
cells bearing copies of the replaced ADA gene.
This optimistic chapter is only one side of the gene therapy story. For this
positive, there have been many negatives. There have been quite a few retreats
in the research process that have caused caution in proceeding clinical trials.
A decade after the first clinical trial, there was a French clinical trial involving
17 children who were suffering with SCID deficiency caused by a defective gamma
C gene. Typically called 'bubble-babies', these children unlike the other two
girls, didn't have an immune system at all. To counter this, researchers introduced
the required gamma C gene into their system with the help of a viral vector.
As with the first trial, the results of this clinical trial too were positive,
but unfortunately only for the first couple of years. In 2002 one of the children
involved in the trial developed leukaemia, followed by another in 2003 and one
in 2005. The reason—correct gene reached the wrong target. A general misconception
with gene therapy is that the functional gene that is fed into the system is
exchanged for the dysfunctional one. However, contrary to this belief the accurate
gene is generally not swapped for the defective gene. Instead it just replaces
it within the system by lodging itself somewhere in the chromosome and still
being effective. "So it is a functional replacement rather than a physical
replacement," explains G Padmanaban, Distinguished Biologist, Indian Institute
of Science. In this case, the introduced gene lodged itself with another gene
called LMO2, a proto-oncogene that can cause cancer. This gene then activated
LMO2 resulting in the augmentation of leukaemia. According to Padmanaban, "This
may not happen in other diseases. But it is a setback to the field."
In 1999 too, gene therapy suffered a major hurdle with the
death of 18-year-old Jesse Gelsinger. Jesse was participating in a gene therapy
trial for ornithine transcarboxylase deficiency (OTCD). Instead of being cured
he died from multiple organ failures, four days after the onset of the treatment.
A severe immune response to the adenovirus carrier is believed to have triggered
his death. Till date this factor is a major hurdle in all gene therapy studies
and it has led to apprehensions in ongoing trials. Padmanaban remarks, "Gene
therapy is an area that goes one step forward and two step backwards."
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In most gene therapy studies, a normal gene is inserted into the genome
to replace an abnormal or defective gene. A carrier molecule called a
vector is generally used to deliver the gene into the target cells, the
most commonly used vector being a virus or viral vector. The vector is
introduced into the target cells following which, it unloads the gene
into the cell, hence restoring the generation of the functional protein.
The rectified protein action then brings the cell to its normal state
and the cause of the disease is eliminated. This is the general procedure
for gene therapy. Apart from this, there are a variety of other methods
of gene therapy, for instance:
- An abnormal gene could be swapped instead of
being replaced functionally, through homologous recombination
- The abnormal gene could be repaired through selective reverse mutation
- The regulation of a gene could be altered
- The expression of a particular gene could be repressed
These procedures are still being studied under
clinical trials. Another question in mind is the delivery of the gene
to the desired target, for which many options have come to the fore. Viruses
have evolved a way of encapsulating and delivering genes to human cells
in a pathogenic manner. Taking advantage of this capability, scientists
have conveniently been using viruses as the preferred option of gene delivery.
Viral vectors like retroviruses, adenoviruses, adeno-associated viruses
and hepes simplex viruses form the typical option in clinical trials,
depending on the type of sites they target.
Besides virus mediated approaches, another simple
method is the direct introduction of therapeutic DNA into the target cells.
However, this effort has its limitations as it can only be used with certain
tissues and requires large amounts of DNA. Yet another delivery system
is through liposome delivery. A liposome carrying the required DNA is
capable of passing through the target cell's membrane. A novel approach
called electroporation has recently held researchers interests for gene
delivery. According to this technique, a gene can be pushed into a cell
through the application of an electric pulse that forms pores in the cell
membrane, creating a way for the gene to enter. The seemingly simple methods
are largely theoretical and are still under experimentation.
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Hurdles
It has been more than two decades since the study of gene therapy commenced.
Yet it hasn't been approved as a clinical practice. There are many factors that
have raised eyebrows and kept gene therapy from becoming a conclusive and absolutely
effective treatment for countering genetic diseases. Apart from the obvious
failures there are many inherent loopholes that hindered any kind of progress.
One of the factors is the short-lived nature of gene therapy, which was observed
in the first clinical trial. If the therapeutic DNA that is introduced into
the cell does not remain functional for a long time, then all efforts are nullified.
In addition to this, the cells containing it must be sustainable before gene
therapy can become a permanent cure with long-term benefits; else, most patients
under-going it will have to be subjected to multiple rounds of treatment.
An additional hindrance, common to such therapies that involve the introduction
of a foreign body, is the response generated by the immune system in opposition
to the alien substance. Suryanarayan says, "One major side effect for gene
therapy is immunogenic reactions, which caused several trials to halt."
The insertion of a virus into the body could stimulate intense immune response,
something that caused Jesse Gelsinger's death in 1999. Furthermore, this would
raise a question to the acceptability of the vector in the body, deterring an
important method of gene delivery. Viruses also pose other potential problem
to the host in terms of toxicity, inflammatory responses, gene regulation and
targeting issues. A virus can lodge itself at a wrong site or may alter the
regulation of a gene, creating unwanted side-effects. "We still cannot
direct a gene to a particular site. That is physically impossible till now,"
says Padmanaban. He believes that the answer to targeting issues lies in a natural
phenomenon, homologous recombination. He says that this issue can be countered
if there could be some strategies by which one could force the system to undergo
homologous recombination.
Last but not the least, gene therapy cannot be developed for multigene disorders.
So far, disorders arising from mutations or defects of a single gene have been
the best candidates for gene therapy. Unfortunately, some of the most commonly
occurring diseases such as heart diseases, high blood pressure, Alzheimer's,
arthritis and diabetes are caused by the combined effects of variations in multiple
genes. Treating such diseases would add to the existing complications.
A new approach
Gene therapy is being tested in various forms and for various diseases like
SCID, Huntington's, Parkinson's. Lesch-Nyhan syndrome and phenylketonuria, among
many others. However, scientists have found that this therapy holds a lot of
promise for cancer as the treatment for this disorder involves the prevention
of the expression of a gene. This can be achieved through the revolutionary
anti-sense mechanism for gene regulation or RNAi, which is another form of gene
therapy. Even as the research initiatives progress, newer forms and techniques
are coming to the fore. "Despite all the setbacks, there are tremendous
numbers of research and clinical trials going on worldwide, especially for cancers.
Sixty percent of the trials are in some form of cancers," says Padmanaban.
Recently researchers have found a new channel to focus their energies upon.
This phenomenon is called the DNA vaccine, an alternative form of gene therapy.
Suryanarayan observes, "I have recently noticed that people are countering
immunogenic reactions by actually using DNA vaccines." According to this
method, instead of delivering the disease-causing gene into the body, one needs
to introduce an artificially copied and multiplied gene from the disease-causing
pathogen. The pathogen's gene expression ultimately leads to the synthesis of
proteins and hence the natural production of antibodies in the host's bodies.
Padmanaban explains, "Instead of introducing engineered proteins from a
malarial or influenza parasite, I can introduce the gene itself. This is a DNA
vaccine." A vaccine of this sort would usher a life long immune protection.
Since, gene therapy hosted many challenges, researchers at IIS turned to this
newfound method. Prof Rangarajan has already developed a DNA vaccine for rabies,
which has proved positive. This has now been transferred to the Indian Immunological
Institute in Hyderabad for the final stages of trial.
What lies beyond
Though clinical trials are on across the globe, not much work is being done
in gene therapy in India. Only Tata Cancer has initiated gene therapy studies
specifically for oral cancer. "One of the problems that drug companies
in India face is that the regulatory framework and the exposure level of our
regulators is still not geared up to international standards so as to easily
allow researchers in India to do cutting edge science, which involves taking
some risk, balanced with potential benefit to patients and society," remarks
Suryanarayan. "Gene therapy still has some risk associated with it and
trials will have to be approved after a lot of scientific consideration,"
he adds.
Hundreds of clinical trials are going on all over the world for testing potential
methods of developing effective gene-transfer strategies, tailoring them to
the dynamics of various cells and tissues, maintaining long-term cell survival
and establishing reliable gene expression. The road to the development of gene
therapy has been rocky and fraught with controversy. "Yet many researchers
still continue work because it has got promise for a permanent cure for certain
diseases," comments Suryanarayan. "It will happen very gradually.
But as one of the major alternatives to cure genetic diseases, it is very encouraging,"
he adds. Only further research can unravel the secrets to out do the complications
and develop the therapy into a miraculous remedy. Suryanarayan stresses on the
need to continue studies despite the roadblocks and says, "It hasn't been
a complete disaster. People will not understand how to develop it until they
move forward."
editorial@expresspharmaonline.com
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