Gene therapy is a technique for replacing a
defective gene with a normal one. In simple terms gene therapy involves isolating a
good gene from normal
cells, and
cloning it.
Genetic engineers have been using viruses as a mode of transporting the cloned gene into the patient’s cells. This, however, has only been used in gene therapy in animals. The process is not yet considered safe for humans. Rapid progress in genetic technology should soon make this technology a reality.
First a good gene is inserted into a virus. The virus then invades the patient’s cells, serving as a vector, a vehicle that transfers the needed gene to the patient. The viruses used to build vectors consist of a core or RNA surrounded by a protein coat. To build a vector, geneticists first extract just a few of the viral genes, specifically the promoter and packaging genes. The promoter genes switch on adjacent genes and make them active. The packaging genes direct the entry of the viral RNA into the protein coat.
Genetic engineers next splice the good human gene in place between the promoter and packaging genes. Then many copies of this hybrid DNA are produced through cloning. The cloned DNA is then added to a culture of animal cells, which produce hybrid RNA.
The last step in producing the vector, is the packaging of this hybrid RNA in a protein coat. Without such a coat, it could not leave the animal cells and invade the human cells. This problem is solved by adding to the cell culture an RNA virus from which the packaging genes have been removed. Without these genes, the modified virus can produce protein coats but cannot use them. The hybrid RNA, however, does have packaging genes. Thus it enters the protein coat and becomes the needed vector.
When the vector is given to a patient, it injects the hybrid RNA into the cells while leaving the protein coat outside. Once inside the patient’s cells, the RNA can never leave because it lacks the coat genes. Then through the process of reverse transcription, the cell produces a DNA copy of the hybrid RNA that becomes incorporated into one of the patient’s chromosomes. Thus, the good gene becomes a permanent part of the patient’s genetic material.