(1) Introduction(2) History and development(3) Types of gene therapy(4) Methods used for gene therapy(5) Factors that have kept gene therapy ineffective for treatment

of genetic diseases(6) Advantages of gene therapy(7) Disadvantages of gene therapy(8) Conclusion

Gene therapy is a technique that involves insertion of normal genes to correct defective genes responsible for disease development.

It is the use of DNA as a pharmaceutical agent to treat disease. It derives its name from the idea that DNA can be used to

supplement or alter genes within an individual's cells as a therapy to treat disease.

The most common form of gene therapy involves using DNA that encodes a functional, therapeutic gene in order to replace a mutated gene.

Other forms involve directly correcting a

mutation or using DNA that encodes a therapeutic protein drug (rather than a natural human gene) to provide treatment.

Transcription

Translation

Therapeutic protein

Vector for efficient gene delivery

Therapeutic gene (Transgene)

The first FDA-approved gene therapy experiment in the United States occurred in 1990, when Ashanti DeSilva was treated for ADA-SCID. Since then, over 1,700 clinical trials have been conducted using a number of techniques for gene therapy.

2006: Scientists at the National Institutes of Health (Bethesda, Maryland) have successfully treated metastatic melanoma in two patients. This study constitutes one of the first demonstrations that gene therapy can be effective in treating cancer.

2007-2011: Research is still ongoing and the number of diseases that has been treated successfully by gene therapy increases.

Retinal disease Bin colour dness2011: Medical community accepted that it

can cure HIV as in 2008, Gero Hutter has cured a man from HIV using gene therapy.

Nanotechnology + gene therapy yields treatment to cancer. March, 2009. The School of Pharmacy in London is tested a treatment in mice, which delivers genes wrapped in nanoparticles to target and destroy hard-to-reach cancer cells

Results of world's first gene therapy for

inherited blindness show sight improvement. 28 April 2008. UK researchers from the UCL Institute of Ophthalmology and Biomedical Research Centre announced results from the world’s first clinical trial

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

Somatic Gene Therapy

In somatic gene therapy, the therapeutic genes are transferred into the somatic cells or body of a patient.

Any modifications and effects will be restricted

to the individual patient only and will not be inherited by the patient's offspring or later generations.

Somatic gene therapy represents the mainstream line of current basic and clinical research, where mRNA is used to treat a disease in an individual.

TYPES OF SOMATIC GENE THERAPY1. Ex vivo, which means “outside the body” Cells from the patient’s blood or bone marrow are

removed and grown in the laboratory.

They are then exposed to a virus carrying the desired gene. The virus enters the cells, and the desired gene becomes part of the DNA of the cells.

The cells are allowed to grow in the laboratory before

being returned to the patient by injection into a vein or bone marrow.

2. In vivo, which means “inside the body” No cells are removed from the patient’s body. Instead,

vectors are used to deliver the desired gene to cells in the patient’s body.

3. In Situ Vector is placed directly into the affected tissues.

In germ line gene therapy, Germ cells, i.e., sperm or eggs, are modified by the introduction of functional genes, which are integrated into their genomes.

This would allow the therapy to be heritable and passed on to later generations (offsprings).

Although this should, in theory, be highly effective in counteracting genetic disorders and hereditary diseases, many jurisdictions prohibit this for application in human beings for a variety of technical and ethical reasons

(In vivo)

(ex vivo)

DELIVERY MECHANISMS

Two methods are involved

(i)Non-viral mediated gene delivery method(ii)Viral mediated gene delivery method

(i)Direct injection of therapeutic DNA into target cellsThis approach is limited in its application because it can be used

only with certain tissues and requires large amounts of DNA

(ii) Liposomes Creation of an artificial lipid sphere with an aqueous core In this method, the liposome, which carries the therapeutic

DNA, is capable of passing the DNA, through the target cell's membrane

(iii) Inorganic nanoparticles.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

(iv) 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.

A problem with this potential method is the difficulty in delivering such a large molecule to the nucleus of a target cell.

There are other several methods for non-viral gene therapy, including, electroporation, the gene gun, sonoporation, magnetofection etc.

E.g In 2003 a University of California, Los Angeles research team inserted genes into the brain using liposomes coated in a polymer called polyethylene glycol. 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.

This method employs viruses as vehicles to transport the desired genes to the target cells These are recombinant viruses sometimes called biological nanoparticles

Different types of viruses used as vectors for gene therapy

Retroviruses A class of viruses that can create double-stranded DNA copies of their RNA genomes

Adenoviruses - A class of viruses with double-stranded DNA genomes that cause respiratory, intestinal, and eye infections in humans

Adeno-associated viruses - A class of small, single-stranded DNA viruses that can insert their genetic material at a specific site on chromosome 19

Herpes simplex viruses A class of double-stranded DNA viruses that infect a particular cell type, eg. Neurons

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

Scientists have manipulated the virus genome to remove disease-causing genes and insert therapeutic genes.

When target cells are infected with the viral vector. The vector then unloads its genetic material containing the therapeutic human gene

Then, generation of a functional protein product from the

therapeutic gene restores the target cell to a normal state.

(i) Short-lived nature of gene therapy -Problems with integrating therapeutic DNA into the genome and

the mortal nature of many cells prevent gene therapy from achieving any long-term benefits. Patients will have to undergo multiple rounds of gene therapy

(ii) Immune response - Anytime a foreign object is introduced into human tissues, the

immune system is designed to attack the invader

Furthermore, the immune system's enhanced response to invaders it has seen before makes it difficult for gene therapy to be repeated in patients

(iii) Problems with viral vectors -Though viruses are the carriers of choice in most gene

therapy studies, there some possible potential problems to the patient; toxicity, immune and inflammatory responses and the fear that the viral vector, once inside the patient, may recover its ability to cause disease.

Since gene therapy is relatively new and at an experimental stage, it is an expensive treatment.

This explains why current studies are focused on illnesses commonly found in developed countries, where more people can afford to pay for treatment.

It may take decades before developing countries can take advantage of this technology.

Scientists currently know the functions of only a few genes. Hence, gene therapy can address only some genes that cause a particular disease.

Worse, it is not known exactly whether genes have more than one function, which creates uncertainty as to whether replacing such genes is indeed desirable.

Give a chance of a normal life to baby born with genetic disease.

Give hope of healthy life to cancer patient.

For certain disease that do not have any cure except gene therapy, it could save many lives

ADVANTAGES OF GENE THERAPY

The genetic testing, screening and research in finding the availability of certain gene is very controversy.

May increase rate of abortion if prenatal test regarding baby with genetic disease is done.

The cost is very high and the patient might need an insurance to cover the treatment.

Cosmetic industry may monopolized this gene therapy if it is used in enhancing beauty and in vanishing the aging effect, rather than used for treatment of a disease.

DISADVANTAGES OF GENE THERAPY

Genes carried in nanoparticles used to treat cancer in mice.

Cured inherited blindness in humans. Successful in treating adenosine

deaminase deficiency an auto-immune disorder in humans.

Most usefull for the treatment of many diseases

Theoretically, gene therapy is the permanent solution for genetic diseases.

But it has several complexities. At its current stage, it is not accessible to most people due to its high cost.

A breakthrough may come anytime and a day may come when almost every disease will have a gene therapy

Gene therapy have the potential to revolutionize the practice of medicine.