Biotechnology in medicine

In medicine, recombinant DNA technology can make medicines that are safe and effective. Proteins made this way are identical to human ones, so they do not cause unwanted immune reactions, as proteins taken from animals can. About 30 such medicines have been approved worldwide; 12 are sold in India.

Human insulin. People with diabetes need insulin. It used to be taken from the pancreas of slaughtered cattle and pigs, but some patients developed allergies to this animal insulin. Making human insulin by genetic engineering was difficult because of how insulin is built:

• Insulin has two short chains, A and B, joined by disulphide bridges.
• In the body it is first made as a longer molecule, pro-insulin, which has an extra stretch, the C peptide. The C peptide is removed to give mature insulin.

In 1983, the American company Eli Lilly found a way round this. They made DNA sequences for the A and B chains and put them into plasmids of E. coli, which then made the two chains separately. These were extracted and joined by disulphide bonds to form human insulin.

Gene therapy tries to cure an inherited disease by putting a correct gene into a person's cells to make up for the faulty one. The first clinical gene therapy was given in 1990, to a 4-year-old girl with ADA deficiency. The enzyme adenosine deaminase (ADA) is essential for the immune system to work; without it, the child has almost no immune defence.

• Lymphocytes are taken from the patient's blood and grown in culture.
• A working ADA gene (as cDNA) is put into them, using a retroviral vector.
• The cells are returned to the patient.

But lymphocytes do not live forever, so the treatment must be repeated. A permanent cure would need the gene to be put into cells at an early embryonic stage, or into the bone-marrow cells that make lymphocytes.

Early diagnosis. Many diseases can be treated best if found early, before symptoms appear, when there are very few pathogens. Molecular methods make this possible:

• PCR can amplify a tiny amount of a pathogen's DNA so it can be detected. It is used to find HIV in suspected AIDS patients, and mutations in genes of suspected cancer patients.
• A radioactive probe (a labelled single strand of DNA or RNA) binds to its matching DNA in a sample, and the result is detected by autoradiography.
• ELISA works on the principle of antigen-antibody interaction: it detects either the pathogen's antigens or the antibodies the body makes against it.