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Biotechnology and Society---Part
25
Gene therapy - 2
The light which experience gives is a lantern on the stern which shines only on the waves behind us. - Samuel Taylor Coleridge, poet (1772-1834)
In part-1 of this topic we examined the basics of gene therapy and the issues to be tackled. Let us now look at some of the developments which refine the methodology and apply the technique in some genetic maladies, including a product that has been approved for use in China.
Background: From 1990 onwards several clinical trials of gene therapy have been attempted with more failures than successes. It is now obvious, with the benefit of hindsight, that many of the trials were premature, guided by the enthusiasm of the researchers based on laboratory successes without any solid foundation of answers to “what if” questions. Most of the trials were based on successes of the experiments on laboratory mice. Bridging the gap between the experiments in mice and human trials is still very challenging. While the methodology was perfect, it is the vectors, initially thought to be safe, which went awry in either causing organ failures or causing leukemia in children by ending up in wrong targets in the two alarming instances which made the regulatory authorities take a step-back approach and halt all trials for a period in order to re-evaluate the entire process.
The pre-clinical work essentially rests on three features. First the gene has to be flawless, second, the vector used must be incapable of causing harm but capable of honing in on the right target, and third, better animal models other than mice must be available before human trials can be undertaken. As for better animal models, breeding colonies of cats and dogs (with induced diseases) must be established and trials must be conducted on these large animals to gain a better understanding of the effects of gene therapy.
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Vectors:
By now, most researchers are using modified viruses as predominant vectors,
although other vehicles such as liposomes and plasmids are also used to a
limited extent. Among the viruses used are: retroviruses (also known as RNA
viruses, among them the most well-known is the HIV virus which causes AIDS),
Adenoviruses (which cause respiratory, intestinal and ocular infections), Adeno-Associated
viruses (which inhabit the human host without causing any detectable pathology),
and Herpes Simplex viruses (these cause cold sores). The important element in
using these viruses is that these viruses must be made replication-deficient
(i.e., the viruses should not be able to multiply thereby causing the diseases
they are capable of) but must maintain the ability to attach themselves to the
target cells.
Parkinson’s Disease (PD)
affects 10 million people worldwide. The primary manifestation of this disease
is the inability to control limb movements. Weill Cornell Medical College
scientists used a viral vector containing the gene GAD (gultamic acid
decarboxylase) and delivered it to the brain of a 55-year-old man suffering from
PD in August 2003. So far no ill-effects have been observed. The gene product
makes a chemical called GABA (gamma amino butyric acid) which calms overactive
nerve cells. They plan to treat 12 more patients soon.
At the Salk Institute (La
Jolla, California) gene therapy trials on mice which were induced with
amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig disease, were
conducted in mid-2003. This disease kills the neurons (nerve cells) that connect
the spinal cord to body’s muscles, causing paralysis and death when breathing
becomes impossible. The researchers coupled a gene called IGF-1 (insulin-like
growth factor) to a virus and infected the affected muscles. The treated mice
lived about 25 to 40 days longer than the untreated mice. The gene product
instructs the cells not to die. More trials with larger animals are needed
before human trials can take place for this disease. Gene therapy research also
goes on for other central nervous system diseases such as Alzheimer’s, PD, and
Huntington Chorea.
Another disease that has been
garnering the attention of medical researchers for a long time is sickle cell
disease. In a study supported by the National Institutes of Health and conducted
at MIT and Harvard Medical School, mice were bioengineered to contain the
defective sickle cell gene. The bone marrow containing the defective gene was
removed and genetically “corrected” with the normal gene. The corrected
marrow was then transplanted into other mice with sickle cell disease whose bone
marrow had been removed by radiation. The newly-implanted gene started making
the correct haemoglobin for at least 10 months in all mice. The lentiviral
vector used to deliver the therapeutic gene is based on the HIV virus. In human
trials, when conducted in this manner, the patient’s own bone marrow cells
would be removed and genetically corrected and transplanted back. Two safety
issues have to be addressed before human trials. One is the safety of the
lentiviral vector which is based on the HIV. In addition, when the bone marrow
in the patient is destroyed to get rid of the defective gene by radiation, the
treatment should not be toxic to other tissues in the body.
Oxford Biomedica (Oxford, UK)
uses Equine Infectious Anaemia Virus which is not linked to any human disease.
Pieces of the virus are fused into plasmid vectors (lentivector). Transfection
(infection by transfer into the cells) of rat neuronal cells line was effective.
The company is planning to use this vector for Parkinson’s disease (PD). They
are expecting approval for human trials based on studies on monkeys. Genetix
(Cambridge, MA, USA) is also using a lentipak ® vector for treating sickle cell
anaemia and beta-thalassemia. The whole lentivirus (an RNA virus) was engineered
to be safe for human use. The gene is a modified beta-globin gene to transfect
mouse bone marrow cells which carry the defective gene and introduce the
modified cells back into the animal. Human trials are expected soon.
Cell Genesys (Foster City,
California) uses lentiviral vectors in their work on haemophilia A (this disease
causes uncontrolled bleeding upon a cut). The gene that is introduced is the
gene which makes Factor VIII. The gene is large by normal standards. The safety
of the lentiviral vector is enhanced by using only 3 of 9 genes present in the
parent virus. The replication and pathogenesis genes have been removed.
Transgene (Strasbourg, France) has seven gene therapy trials that are in
progress. The target diseases are cancer and cystic fibrosis.
Two recent developments offer
greater hope for gene therapy. In Glasgow (Scotland), approval was granted for a
large clinical trial of a treatment for patients with an aggressive brain tumour
called malignant glioma. The first patient to receive the experimental treatment
seven years ago is still alive. The vector to be used in this trial is the
Herpes Simples Virus, modified suitably so that it can target only the cancer
cells and kill them.
Researchers at Brigham &
Women’s Hospital (Boston) and Duke University Medical Center (Durham, North
Carolina) coupled a therapeutic gene with a genetic sensor that detects and
responds to oxygen deprivation in heart tissue (caused by obstructed blood flow
in coronary arteries). The sensor switches the therapeutic gene “on” which
would be useful in incidents of stroke, trauma and sepsis patients. The gene was
tested on rats successfully. This was the first successful demonstration of a
therapeutic gene with a sensor that responds to the problem.
China
at the forefront: In October 2003 the State FDA of China granted
Shenzhen SiBiono Gene Technologies Inc. (Shenzhen, China) approval to produce
the world’s first commercially licensed gene therapy for head and neck
squamous cell carcinoma (HNSCC). Five years of clinical trials preceded the
approval. The product is now marketed in China as Gendicine. An adenoviral
vector incorporating the p53 tumour suppressor gene was injected once a week for
8 weeks into patients during the clinical trials. Sixtyfour per cent of the
patients had complete regression of tumour and 32 per cent had partial
regression which is considered very significant. Those patients who want this
medicine must go to China to get treatment.
Gene therapy still has a long way to go
before it can become a practical tool to fight diseases currently
considered incurable. Genetic diseases cannot be “treated”
effectively. They have to be cured. The way to cure is to rectify the
defective gene. The field is beckoning and the promises are plenty but
the advice is to proceed as one would upon seeing a yellow light.
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