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Genetic Disorders - Gene Therapy

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Genetic disorders have been plaguing people for ages and causing fatalities. However, with new information and research, and something called gene therapy, hope now exists for these unfortunate individuals. Gene therapy is a technique for correcting defective genes responsible for disease development. It has been around for a while now and is getting more advanced with time. Experimentation is an ongoing process with gene therapy. Ethical issues are something that has been accompanying the procedure since it has been used. New facts on gene therapy continue to be uncovered as we speak.

To start off, an overview of why people need gene therapy should be covered. Each of us carries about half a dozen defective genes. However, we remain ignorant to this fact unless we are among the millions of people who have a genetic disorder. About one in ten people has, or will develop some time later in life, an inherited genetic abnormality. And approximately two thousand eight hundred specific conditions are known to be caused by defects in just one of the patient's genes. Some single gene disorders are pretty common, such as cystic fibrosis.

Most people do not suffer harmful effects from our defective genes because we carry two copies of nearly all genes. One is inherited from our mother and the other from our father. The only exceptions to this rule are the genes found on the male sex chromosomes. "Males have one X and one Y chromosome, the former from the mother and the latter from the father. So each cell has only one copy of the genes on these chromosomes. In the majority of cases, one normal gene is enough to avoid all the symptoms of disease. If the gene that may be harmful is recessive, then its normal counterpart will carry out all the tasks assigned to both. A disease will develop only if someone inherits two copies of the recessive gene from their parents." (Web source #3) In other terms, if the gene is dominant, it alone can produce the disease, even if the counterpart is normal. Finally, there are the X chromosome-linked genetic diseases. Because males have only one copy of the genes from this chromosome, there are no others available to fulfill the defective gene's function. Hemophilia is a common result of this.

To continue, how gene therapy works, should be explained. There are several different approaches scientists may use to correct faulty genes with therapy. "One method is that a normal gene may be inserted into a nonspecific location within the genome to replace a nonfunctional gene. This happens to be the most common approach utilized. Another method is an abnormal gene could be exchanged for a normal gene through homologous recombination." (Web source #2) Also, the abnormal gene could be repaired through selective reverse mutation, which would restore the gene to its original function. Finally, the regulation or degree to which a gene is turned on or off, of a particular gene could be altered.

In the majority of 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 agents and insert therapeutic genes.

There are a series of different types of viruses used as gene therapy vectors. One is adenoviruses. These are 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. Another type is retroviruses. This is a class 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. HIV is a retrovirus. A third type is adeno-associated viruses, which are a class of small, single-stranded DNA viruses that can insert their genetic material at a specific site on chromosome nineteen. One more type

is herpes simplex viruses. This is a class of double-stranded DNA viruses that infect particular cell type, neurons. Herpes simplex virus type one is a common human pathogen that causes cold sores. 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. Another nonviral

approach involves the creation of an artificial lipid sphere with an aqueous core. (Web source #2)

Today, the current status of gene therapy is unofficial. In fact the Food and Drug Administration has not yet approved any human gene therapy product for sale. Gene therapy is currently experimental and has not quite been proven completely successful in clinical trials. For example, little forward progress

has been achieved since the first gene therapy experimental trial began in 1990. "A major setback was suffered in 1999 when the death of an 18-year-old

named Jesses Gelsinger occurred. He was participating in a clinical trial for ornithine transcarboxylase deficiency, or OTCD. Jesse's death is believed to have been triggered by a severe immune response to the adenovirus carrier. FDA's Biological Response Modifiers Advisory Committee 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 guide lines." (Web source #2)

There are certain factors that have kept gene therapy from becoming an effective treatment for genetic diseases. One is the 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." (Web source #2) 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 numerous rounds of gene therapy. Another factor is 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



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