One goal of recombinant-DNA technology is the cure of genetically based human diseases by the use of gene transfers from healthy individuals. Therapeutic genes can be introduced directly into a cell through various chemical or physical processes that make the cell membrane temporarily permeable to foreign DNA. An indirect transfer method incorporates a beneficial gene into the genetic material of a virus, which is then allowed to infect the target cell. The most efficient method of transferring genes is to use a type of RNA virus called a retrovirus. After infecting a host cell, the retrovirus makes a DNA copy of itself, which is then inserted into the genetic material of the host cell.
The first gene therapy trial was in 1990, involving a child that could not produce the enzyme adenosine deaminase (ADA), which is crucial for the normal development of the immune system. Lymphocytes were removed from the child's blood and retrovirally altered by the addition of an ADA gene. These cells were then returned to the child's bloodstream.
A different gene-transfer procedure was developed for use in a group of patients with malignant melanoma skin cancer. It involved encasing copies of genes for a protein called HLA-B7 in droplets of fat (liposomes) that facilitate the passage of DNA into cells. The liposomes were injected directly into a tumor, allowing the genes to enter the cancerous cells. When the cells produce the HLA-B7 protein, it becomes incorporated into their membranes where it attracts the T cells of the patient's immune system to the malignant cells, which are then destroyed.
Since the first trial in 1990, more than 100 patients have undergone various gene transfer procedures, involving a number of genetic diseases. These include hemophilia, cystic fibrosis, and familial hypercholesterolemia. All reports indicate that there has been a measure of success in treating the particular disease but no claim of an absolute cure.