Applications in medicine
Genetically engineered insulin, gene therapy and molecular diagnosis.
The essentials
Recombinant DNA technology allows safe and effective drugs to be made in large amounts. Recombinant therapeutics do not cause unwanted immunological reactions as often as similar products isolated from non-human sources.
Genetically engineered insulin: Insulin used earlier was extracted from the pancreas of slaughtered cattle and pigs, and some patients developed allergies to it. Human insulin is made of two short polypeptide chains, chain A and chain B, linked by disulphide bridges. In the body it is synthesised as a prohormone, pro-insulin, which has an extra stretch called the C-peptide; this is removed during maturation. In 1983, Eli Lilly, an American company, prepared two DNA sequences for the A and B chains, introduced them into plasmids of E. coli to produce the chains separately, then extracted the chains and combined them by forming disulphide bonds to make human insulin.
Gene therapy corrects a gene defect diagnosed in a child or embryo by inserting a normal gene into the person's cells or tissues. The first clinical gene therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) deficiency, an enzyme crucial for the immune system. Lymphocytes from her blood were grown in culture, a functional ADA cDNA was introduced using a retroviral vector, and the cells were returned to her. These cells are not immortal, so periodic infusion is needed. Introducing the ADA gene into cells at early embryonic stages could be a permanent cure.
Molecular diagnosis: Early detection is difficult by conventional methods (serum and urine analysis). PCR can detect very low amounts of a pathogen's DNA, for example in suspected AIDS patients or to find mutations in suspected cancer patients. A single-stranded DNA or RNA probe tagged with a radioactive marker hybridises to its complementary DNA in a clone of cells and is detected by autoradiography; a mutated gene does not appear on the film because the probe does not match. ELISA works on antigen–antibody interaction and detects infection through antigens or through antibodies made against the pathogen.
Key points
- Recombinant insulin avoids the allergic reactions some patients had to animal insulin.
- Other ADA treatments: bone marrow transplantation and enzyme replacement therapy, which are not complete cures.