GENE THERAPY – An Overview of the Future Panacea_by Dr Mansi Arya

GENE THERAPY – An Overview of the Future Panacea
Contributed by:- Dr. Mansi Arya, R & D Officer – SBL Pvt. Ltd., Consultant – SBL Clinic
Credential info: dr.mansiarya333@gmail.com, mansi.arya@sblglobal.in, Category: Genetics
Genes, which are carried on chromosomes, are the basic physical and functional units of heredity. Genes are specific sequences of bases that encode instructions on how to make proteins. Although genes get a lot of attention, it’s the proteins that perform most life functions and even make up the majority of cellular structures. When genes are altered so that the encoded proteins are unable to carry out their normal functions, genetic disorders can result.
Altered Genes
Each of us carries about half a dozen defective genes. We remain blissfully unaware of this fact unless we, or one of our close relatives, are amongst the many millions who suffer from a genetic disease. About one in ten people has, or will develop at some later stage, an inherited genetic disorder, and approximately 2,800 specific conditions are known to be caused by defects (mutations) in just one of the patient’s genes. Some single gene disorders are quite common – cystic fibrosis is found in one out of every 2,500 babies born in the Western World – and in total, diseases that can be traced to single gene defects account for about 5% of all admissions to children’s hospitals.
Diseases of Genetic Origin
Most of us do not suffer any harmful effects from our defective genes because we carry two copies of nearly all genes, one derived from our mother and the other from our father. The only exceptions to this rule are the genes found on the male sex chromosomes . In the majority of cases, one normal gene is sufficient to avoid all the symptoms of disease.
If the potentially harmful gene is recessive, then its normal counterpart will carry out all the tasks assigned to both. Only if we inherit from our parents two copies of the same recessive gene will a disease develop. On the other hand, if the gene is dominant, it alone can produce the disease, even if its counterpart is normal. Clearly only the children of a parent with the disease can be affected, and then on average only half the children will be affected. Huntington’s chorea, a severe disease of the nervous system, which becomes apparent only in adulthood, is an example of a dominant genetic disease. Finally, there are the X chromosome-linked genetic diseases. As males have only one copy of the genes from this chromosome, there are no others available to fulfill the defective gene’s function. Examples of such diseases are Duchenne muscular dystrophy and, perhaps most well known of all, hemophilia.
What is Gene Therapy ?
Gene therapy is a technique for correcting defective genes responsible for disease development. Researchers may use one of several approaches for correcting faulty genes:

• A normal gene may be inserted into a nonspecific location within the genome to replace a nonfunctional gene. This approach is most common.
• An abnormal gene could be swapped for a normal gene through homologous recombination.
• The abnormal gene could be repaired through selective reverse mutation, which returns the gene to its normal function.
• The regulation (the degree to which a gene is turned on or off) of a particular gene could be altered.

How does Gene Therapy  work ?
1.In most gene therapy studies, a “normal” gene is inserted into the genome to replace an “abnormal,” disease-causing gene. A carrier molecule called a vector must be used to deliver the therapeutic gene to the patient’s target cells. Currently, the most common vector is a virus that has been genetically altered to carry normal human DNA.
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 virus genome to remove disease-causing genes and insert therapeutic genes.Target cells such as the patient’s liver or lung cells are infected with the viral vector. The vector then unloads its genetic material containing the therapeutic human gene into the target cell. The generation of a functional protein product from the therapeutic gene restores the target cell to a normal state. See a diagram depicting this process. Some of the different types of viruses used as gene therapy vectors are Retroviruses, Adenoviruses , Adeno-associated , Herpes simplex viruses

2. Besides virus-mediated gene-delivery systems, there are several nonviral options for gene delivery. The simplest method is the direct introduction of therapeutic DNA into target cells. This approach is limited in its application because it can be used only with certain tissues and requires large amounts of DNA.
3. Another nonviral approach involves the creation of an artificial lipid sphere with an aqueous core. This liposome, which carries the therapeutic DNA, is capable of passing the DNA through the target cell’s membrane.
4. Therapeutic DNA also can get inside target cells by chemically linking the DNA to a molecule that will bind to special cell receptors. Once bound to these receptors, the therapeutic DNA constructs are engulfed by the cell membrane and passed into the interior of the target cell. This delivery system tends to be less effective than other options.
Researchers also are 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 mutations. It would be a large vector capable of carrying substantial amounts of genetic code, and scientists anticipate that, because of its construction and autonomy, the body’s immune systems would not attack it. A problem with this potential method is the difficulty in delivering such a large molecule to the nucleus of a target cell.
What is the current status of Gene Therapy Research ?
The Food and Drug Administration (FDA) has not yet approved any human gene therapy product for sale. Current gene therapy is experimental and has not proven very successful in clinical trials. Little progress has been made since the first gene therapy clinical trial began in 1990. In 1999, gene therapy suffered a major setback with the death of 18-year-old Jesse Gelsinger. Jesse was participating in a gene therapy trial for ornithine transcarboxylase deficiency (OTCD). He died from multiple organ failures 4 days after starting the treatment. His death is believed to have been triggered by a severe immune response to the adenovirus carrier.
Another major blow came in January 2003, when the FDA placed a temporary halt on all gene therapy trials using retroviral vectors in blood stem cells. FDA took this action after it learned that a second child treated in a French gene therapy trial had developed a leukemia-like condition. Both this child and another who had developed a similar condition in August 2002 had been successfully treated by gene therapy for X-linked severe combined immunodeficiency disease (X-SCID), also known as “bubble baby syndrome.”
FDA’s Biological Response Modifiers Advisory Committee (BRMAC) met at the end of February 2003 to discuss possible measures that could allow a number of retroviral gene therapy trials for treatment of life-threatening diseases to proceed with appropriate safeguards. In April of 2003 the FDA eased the ban on gene therapy trials using retroviral vectors in blood stem cells.
What factors have kept  Gene Therapy from becoming an effective treatment  for  Genetic Diseases ?

• 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 the 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.

• Immune response – Anytime a foreign object is introduced into human tissues, the immune system is designed to attack the invader. The risk of stimulating the immune system in a way that reduces gene therapy effectiveness is always a potential risk. Furthermore, the immune system’s enhanced response to invaders it has seen before makes it difficult for gene therapy to be repeated in patients.

• Problems with viral vectors – Viruses, while the carrier of choice in most gene therapy studies, present a variety of potential problems to the patient –toxicity, immune and inflammatory responses, and gene control and targeting issues. In addition, there is always the fear that the viral vector, once inside the patient, may recover its ability to cause disease.
• Multigene disorders – Conditions or disorders that arise from mutations in a single gene are the best candidates for gene therapy. Unfortunately, some 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. Multigene or multifactorial disorders such as these would be especially difficult to treat effectively using gene therapy.

What are some recent developments in  Gene Therapy  Research?

• A team of British doctors from Moorfields Eye Hospital and University College in London conduct first human gene therapy trials to treat Leber’s congenital amaurosis, a type of inherited childhood blindness caused by a single abnormal gene. The procedure has already been successful at restoring vision for dogs. This is the first trial to use gene therapy in an operation to treat blindness in humans.

• A combination of two tumor suppressing genes delivered in lipid-based nanoparticles drastically reduces the number and size of human lung cancer tumors in mice during trials conducted by researchers from The University of Texas M. D. Anderson Cancer Center and the University of Texas Southwestern Medical Center.
• Researchers at the National Cancer Institute (NCI), part of the National Institutes of Health, successfully reengineer immune cells, called lymphocytes, 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.
• Gene therapy is effectively used to treat two adult patients for a disease affecting nonlymphocytic white blood cells called myeloid cells. Myeloid disorders are common and include a variety of bone marrow failure syndromes, such as acute myeloid leukemia. The study is the first to show that gene therapy can cure diseases of the myeloid system.
• University of California, Los Angeles, research team gets genes into the brain using liposomes coated in a polymer call polyethylene glycol (PEG). The transfer of genes into the brain is a significant achievement because viral vectors are too big to get across the “blood-brain barrier.” This method has potential for treating Parkinson’s disease.
• RNA interference or gene silencing may be a new way to treat Huntington’s. Short pieces of double-stranded RNA (short, interfering RNAs or siRNAs) are used by cells to degrade RNA of a particular sequence. If a siRNA is designed to match the RNA copied from a faulty gene, then the abnormal protein product of that gene will not be produced.

• Researchers at Case Western Reserve University and Copernicus Therapeutics are able to create tiny liposomes 25 nanometers across that can carry therapeutic DNA through pores in the nuclear membrane.

• Sickle cell has been successfully treated in mice.

Ethical Considerations……

• Preliminary attempts at gene therapy are exorbitantly expensive. Who will have access to these therapies? Who will pay for their use?
• What is normal and what is a disability or disorder, and who decides?
• Are disabilities diseases? Do they need to be cured or prevented?
• Does searching for a cure demean the lives of individuals presently affected by disabilities?
• Is somatic gene therapy (which is done in the adult cells of persons known to have the disease) more or less ethical than germline gene therapy (which is done in egg and sperm cells and prevents the trait from being passed on to further generations)? In cases of somatic gene therapy, the procedure may have to be repeated in future generations.

The Future
The Human Genome Project in the U.S. will provide about $200 million each year to scientists in multidisciplinary research centers who are attempting to determine the makeup of all human genes. Together with similar programs in Europe, it is hoped that in 15 years time we shall be able to identify and treat all diseases to which humans are susceptible. This will revolutionize modern medicine, and hopefully improve the quality of life of all men, women, and children. Already, the genes for Duchenne muscular dystrophy, cystic fibrosis, and retinoblastoma have been identified, and more such information is emerging all the time.
REFERENCES

1. Doctors Test Gene Therapy to Treat Blindness at www.reuters.com (May 1, 2007).
2. Dual Gene Therapy Suppresses Lung Cancer in Preclinical Test at  (January 11, 2007).
3. New Method of Gene Therapy Alters Immune Cells for Treatment of Advanced Melanoma at www.cancer.gov (August 30, 2006).
4. DNA Nanoballs Boost Gene Therapy at NewScientist.com
5. Murine Gene Therapy Corrects Symptoms of Sickle Cell Disease from March 18, 2002, issue of The Scientist.
6. See Undercover Genes Slip into the Brain at (March 20, 2006).
7. See Gene Therapy Appears to Cure Myeloid Blood Diseases In Groundbreaking International Study at www.cincinnatichildrens.org (March 31, 2006).
8. See Gene Therapy is First Deafness ‘Cure’ at  (February 11, 2005).
9. See Gene Therapy May Switch off Huntington’s