What is Gene Therapy?

Gene therapy is the transfer of one or more normal or modified genes into an individual's body cells to correct a genetic defect or boost resistance to disease. It also includes the use of genes to treat various other human illnesses such as cancer and cardiovascular disease. There are two main methods of gene therapy:

(a) Ex vivo Method

(b) In vivo Method

Gene therapy of adenosine deaminase deficiency

Ex Vivo Method

During ex vivo therapy i.e. outside the living organism, the cells are removed from a patient treated and returned to the patient. A retrovirus, which has an RNA genome, is often used as a vector to carry normal genes into the cells of the patient. The recombinant RNA from the retrovirus enters a human cell, such as a bone marrow stem cell, reverse transcription occurs. It is the resulting DNA that carries the normal gene into human genome (as shown in the figure). A four year old girl suffered from adenosine deaminase deficiency (AD). It is a lethal genetic disorder that severely limits the ability of the body to fight disease. This disorder is caused by a mutation to a single gene that is expressed in our white blood cells. It is called severe combined immunodeficiency syndrome (SCID). SCID is often called the “bubble baby disease” after David, a young person who lived under plastic dome to prevent infection. AD is involved in maturation of T and B cell. White blood cells were removed from her blood and infected with a retrovirus that carried a normal gene for the enzyme. Then the cells were returned to the girl. This experiment was performed on Sept 14, 1990. A few months later similar experiment was performed on an 8 years old girl.

Since white blood cells do not reproduce, for several years these two girls were given genetically engineered white blood cells every few months. Genetically engineered stem cells are preferred because they are long lived and their use may result in a permanent cure.

Gene therapy is being used for treatment of hypercholesterolemia (excess cholesterol in blood) a condition that develops when liver cells lack a reception for removing cholesterol from the blood. The high levels of blood cholesterol make the patient subject to fatal heart attacks at a young age. In a newly developed procedure, a small portion of the liver is surgically excised and infected with a retrovirus containing a normal gene for the receptor.

Chemotherapy in cancer patients often kills off healthy cells as well as cancer cells. In clinical trials, researchers have given genes to cancer patients that either make healthy cells more tolerant of chemotherapy or make tumors more vulnerable to it. In one trial, bone marrow stem cells from about 30 women with late-stage ovarian cancer were infected with a virus carrying gene for multiple-drug resistance.

In Vivo Method

Other gene therapy procedures use viruses, laboratory grown cells, or even synthetic carriers to introduce genes directly into patients. If in vivo (inside the living organism) therapy is used, no cells are removed from the patient. For example, liposomes are microscopic vesicles that spontaneously form when lipoproteins are put in a solution. Liposomes have been coated with healthy cystic fibrosis genes and sprayed into patient's nostrils as a possible treatment for cystic fibrosis (It is characterized by an excessive secretion of very thick mucus from the lungs, pancreas, liver, digestive tract. This mucus can interfere with breathing, digestion and liver function, and make the person vulnerable to pneumonia and other infections). Cystic fibrosis patient lack a gene that codes for transmembrane carrier of the chloride ion. Patient often die due to numerous infection of the respiratory tract. Retroviruses can be used to carry genes for cytokinin, soluble hormones of the immune system, directly into the tumors of patients. It has been observed that the presence of cytokinin stimulates the immune system of the body to remove cancer cells. During coronary artery angioplasty a balloon catheter is sometimes used to open up a closed artery. Unfortunately the artery has a tendency to close up once again. But investigators have come up with a new procedure. The balloon in coated with a plasmid that contains a gene for vascular endothelial growth factor. The expression of the gene, which promotes the proliferation of blood vessels to bypass the obstructed area, has been observed in at least one person.

Perhaps it will be used one day for in vivo therapy to cure hemophilia, diabetes, Parkinson disease, or AIDS. To treat hemophilia, patient could get regular dose of cells that contain normal clotting-factor genes. Or such cells could be placed in organoids, artificial organs that can be implanted in the abdominal cavity. To cure Parkinson disease, dopamine-producing cells could be grafted directly into the brain. These procedures will use laboratory-grown cells that have been stripped of antigens (to decrease the possibility of an immune system attack).

Prenatal Screening for Genetic Defects

Genetic screening has become widespread for developing fetus (foetus) a process known as prenatal (pre-before, natal birth) screening. It provides the parents information about diseases, genetic disorders before birth. Scientists use a variety of methods in prenatal screening.

Amniocentesis

To perform amniocentesis physician first determines the position of the fetus with the help of ultrasound imaging in which sound waves are used to produce an image of the fetus. Then he carefully inserts a needle through the mother's abdomen into her uterus, avoiding the fetus. The physician extracts about 10 ml of the amniotic fluid, (the fluid which surrounds the fetus). The cells are cultured in the laboratory for several weeks. By then enough dividing cells can be harvested so that karyotyping can be done and chromosomal abnormalities can be detected. Amniotic fluid can also be used for DNA testing.

Amniocentesis

Genetic Counselling

Genetic Counselling is the giving of information and advice about the risk of genetic diseases and their outcome. Tests are now available for a large number of genetic diseases. For example chromosomal tests are available for cystic fibrosis, neurofibromatosis (nodules all over the body surface) and Huntington disease (inherited and fatal disease marked by progressive mental deterioration causing shaking of body). Blood can identify carriers of thalassemia (hereditary disorder of blood causing anemia) and sickle cell disease. By measuring enzyme levels in blood, tears or skin cells, carriers of enzyme defects can also be identified for certain inborn metabolic errors, such as Tay-sachs disease (Lipid accumulation in brain cells, mental deficiency, blindness, death in childhood). From this information, the physician can sometimes predict the chances of a child having the disorder. Blood tests of the couple is performed for Rh factor. Chorionic villus sampling and amniocentesis is done to determine genetic disorder. If so treatment may be available even before birth or parents may decide whether or not to end the pregnancy.