Causes of Genetic Defects

Each human being carries normal as well as some defective genes. Usually, the individual does not become aware of the presence of a defective gene until a disease associated with the gene is manifested in him or her or in a relative. More than 4,000 medical disorders caused by defective genes have been identified, each with varying degrees of seriousness. About 10 percent of the human population will evidence, sooner or later, some type of disorder. Although genes are responsible for predisposition to disease, the environment, diet, and lifestyle can affect the onset of the illness.

An example of a genetic disease is cystic fibrosis, which frequently becomes evident in the first years of life for the child carrying the defective gene. The mutant gene causes the development of cysts and fibrous tissue in the patient's pancreas and the production of thick and viscous lung mucous. The mucous makes breathing very difficult and, in many cases, is fatal. On average, in Western countries, about 1 child in 2,500 has the disease. If the child receives two defective recessive alleles of the gene named CF (one from each parent), he or she will develop the disease. Patients with cystic fibrosis can reduce the symptoms of the disease with drugs developed through genetic engineering. A cure for cystic fibrosis may come through gene therapy. One possibility is a genetically engineered virus, carrying the corrective gene, which after being introduced into the patient's lung cells would allow the lungs to function properly. The introduced gene would allow the lung cells to produce a protein that eliminates the mucus.
Most people do not manifest genetic diseases because, most of the time, they are carriers of just a single defective copy of the CF gene. As most of the defective genes are recessive, meaning two copies are needed for expression of the disease, most people do not have the disease. This is the reason for the larger incidence of genetic diseases in children from related parents.

If the defective gene, however, is dominant, the disease is expressed in any people that carry the defective gene. Huntington's Disease, a disorder of the nervous system that usually occurs after the age of 45, is an example of a genetic disease caused by a dominant gene.

Having a defective gene does not make disease development a certainty. Besides the large effect from genetics, the environment is also important to the onset of many illnesses. Diseases such as heart disease do have a genetic component, but are largely dependent on diet and lifestyle. Some genetic diseases also have benefits. A classic example of a genetic disease that has a beneficial effect on human survival is sickle cell anemia. There exists in the human population a defective b-hemoglobin gene and individuals carrying two copies of the defective gene develop sickle cell anemia, a blood problem caused by defective hemoglobin and consequently misshapen red blood cells. The genetic mutation in the defective allele of this disease is a single nucleotide change, from an A in normal genes to a T in the mutant. This single nucleotide mutation results in a mutant b-hemoglobin that possesses the amino acid valine instead of glutamine. The mutant b-hemoglobin has less affinity to oxygen, becoming a poor oxygen transporter in the blood.

However, carriers of a single copy of the defective allele do not have the disease, and they are also resistant to malaria. There is an obvious advantage of carrying a single allele of the defective hemoglobin gene, especially in regions where malaria is endemic, as in tropical regions of Africa.
The first case of gene therapy occurred in 1990, at the NIH in Bethesda, Maryland. On that occasion, a four-year-old patient with a severe immune system deficiency (adenosine deaminase enzyme [ADA] deficiency or bubble-boy disease) received an infusion of white blood cells that had been genetically modified to contain the gene that was absent in his genome. Since then, gene therapy has been studied and experimentally tested for several medical conditions.
Diseases caused by the absence of an enzyme or the presence of an inactive enzyme are potential targets for gene therapy. Cystic fibrosis, ADA deficiency, and many other genetic diseases are among the candidates for gene therapy.

Medical Conditions for Which Gene Therapy Is Being Studied are as under
ADA deficiency > Hemophilia
AIDS > Liver cancer
Asthma > Lung cancer
Brain tumor > Melanoma
Breast cancer > Muscular dystrophy
Colon cancer > Neurodegenerative conditions
Diabetes > Ovarian cancer
Heart diseases > Prostate cancer

Tags: Bio Technology, Bio Genetics, Gene Therapy

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