GENE THERAPY, NEW HOPE FOR HUMANKINDS’ LIFE

ü  Imagine that you accidentally broke one of your neighbor's windows. What would you do? You could:

1. Stay silent: no one will ever find out that you are guilty, but the window doesn't get fixed.
2. Try to repair the cracked window with some tape: not the best long-term solution.
3. Put in a new window: not only do you solve the problem, but also you do the honorable thing.

ü  You can think of a medical condition or illness as a "broken window." Many medical conditions result from flaws, or mutations, in one or more of a person's genes. Mutations cause the protein encoded by that gene to malfunction. When a protein malfunctions, cells that rely on that protein's function can't behave normally, causing problems for whole tissues or organs. Medical conditions related to gene mutations are called genetic disorders.
So, if a flawed gene caused our "broken window," can you "fix" it? What are your options?

1. Stay silent: ignore the genetic disorder and nothing gets fixed.
2. Try to treat the disorder with drugs or other approaches: depending on the disorder, treatment may or may not be a good long-term solution.
3. Put in a normal, functioning copy of the gene: if you can do this, it may solve the problem!

If it is successful, gene therapy provides a way to fix a problem at its source.
Gene therapy
is the insertion of genes into an individual's cell and biological tissues to treat disease, such as cancer where deleterious mutant alleles are replaced with functional ones.
Gene therapy may be classified into the two following types:

Germ line gene therapy
In the case of germ line gene therapy, germ cells, i.e., sperm or eggs, are modified by the introduction of functional genes, which are ordinarily integrated into their genomes. Therefore, the change due to therapy would be heritable and would be passed on to later generations. This new approach, theoretically, should be highly effective in counteracting genetic disorders and hereditary diseases. However, many jurisdictions prohibit this for application in human beings, at least for the present, for a variety of technical and ethical reasons.
Somatic gene therapy
In the case of somatic gene therapy, the 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 patient's offspring or later generations.
Gene Therapy: requirements

• The gene must be identified and cloned.This has been done for the ADA gene.

ADA deficiency is a rare genetic disease. The normal ADA gene produces an enzyme called adenosine deaminase, which is essential to the body's immune system. Patients with ADA deficiency do not have normal ADA genes and do not produce functional ADA enzymes. ADA-deficient children are born with severe immunodeficiency and are prone to repeated serious infections, which may be life-threatening. Although ADA deficiency can be treated with a drug called PEG-ADA, the drug is extremely costly and must be taken for life by injection into a vein.
ADA deficiency was selected for the first approved human gene therapy trial for several reasons:

1. • The disease is caused by a defect in a single gene, which increases the likelihood that gene therapy will succeed.
   • The gene is regulated in a simple, “always-on” fashion, unlike many genes whose regulation is complex.
   • The amount of ADA present does not need to be precisely regulated. Even small amounts of the enzyme are known to be beneficial, while larger amounts are also tolerated well.

Many human diseases are caused by defective genes. A few common examples:

Disease	Genetic defect
hemophilia A	absence of clotting factor VIII
cystic fibrosis	defective chloride channel protein
muscular dystrophy	defective muscle protein (dystrophin)
sickle-cell disease	defective beta globin
hemophilia B	absence of clotting factor IX
severe combined immunodeficiency (SCID)	any one of several genes fail to make a protein essential for T and B cell function

• It must be inserted in cells that can take up long-term residence in the patient. So far, this means removing the patient's own cells, treating them in tissue culture, and then returning them to the patient.
• It must be inserted in the DNA so that it will be expressed adequately; that is, transcribed and translated with sufficient efficiency that worthwhile amounts of the enzyme are produced.

How does gene therapy work?
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. the most common vector is a virus that has been genetically altered to carry normal human DNA.

• Adenovirus: A class of viruses with double-stranded DNA genomes that cause respiratory, intestinal, and eye infections in humans. The virus that causes the common cold is an adenovirus.
• [edit] Retroviruses: A class of viruses that can create double-stranded DNA copies of their RNA genomes. These copies of its genome can be integrated into the chromosomes of host cells. Human immunodeficiency virus (HIV) is a retrovirus
• [edit] Adeno-associated viruses: A class of small, single-stranded DNA viruses that can insert their genetic material at a specific site on chromosome 19.
• [edit] Herpes Simplex Virus: A class of double-stranded DNA viruses that infect a particular cell type, neurons. Herpes simplex virus type 1 is a common human pathogen that causes cold sores.

Non-viral methods
Non-viral methods present certain advantages over viral methods, with simple large scale production and low host immunogenicity being just two. Previously, low levels of transfection and expression of the gene held non-viral methods at a disadvantage; however, recent advances in vector technology have yielded molecules and techniques with transfection efficiencies similar to those of viruses.
Naked DNA
This is the simplest method of non-viral transfection. Clinical trials carried out of intramuscular injection of a naked DNA plasmid have occurred with some success; however, the expression has been very low in comparison to other methods of transfection.
Hybrid methods
Due to every method of gene transfer having shortcomings, there have been some hybrid methods developed that combine two or more techniques. Virosomes are one example; they combine liposomes with an inactivated HIV or influenza virus. This has been shown to have more efficient gene transfer in respiratory epithelial cells than either viral or liposomal methods alone. Other methods involve mixing other viral vectors with cationic lipids or hybridising viruses.
Diseases threated
Cancer, cystic fibrosis, Haemophilia, muscular dystrophy, sickle cell anemia, diabetes, etc.
Problems and ethics

• Short-lived nature of gene therapy – 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 –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 that it has seen before makes it difficult for gene therapy to be repeated in patients. Other concerns include the possibility that transferred genes could be “overexpressed,” producing so much of the missing protein as to be harmful; that the viral vector could cause inflammation or an immune reaction; and that the virus could be transmitted from the patient to other individuals or into the environment.
• Problems with viral vectors – Viruses present toxicity, immune and inflammatory responses, and gene control and targeting issues. Viruses cause disease, infect more than one type of cell. They might infect healthy cells as well as cancer cells.In addition, when viruses or liposomes are used to deliver DNA to cells inside the patient's body, there is a slight chance that this DNA could unintentionally be introduced into the patient's reproductive cells. If this happens, it could produce changes that may be passed on if a patient has children after treatment.
• Multigene disorders – Conditions or disorders that arise from mutations in a single gene are the 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. Multigene or multifactorial disorders such would be especially difficult to treat effectively using gene therapy.
• Chance of inducing a tumor (insertional mutagenesis) - If the DNA is integrated in the wrong place in the genome, for example in a tumor suppressor gene, it could induce a tumor.