Discovery and characterisation of prions
Stanley B. Prusiner won the 1997 Nobel Prize for his discovery of a multiply by copying themselves. Prions are unusual as they are simply small protein molecules.
In 1972, one of Prusiner’s patients died of Creutzfeldt-Jakob disease (CJD), and this moved him to study the disorder. CJD is a dementia-related disease that causes nerve cells in the brain to die, leaving the brain with a characteristic spongy appearance. Very similar diseases had been observed in other animals. Most of these diseases appeared to occur spontaneously, and some seemed to be inherited. However, it was also known that these fatal diseases could be ‘caught’ by eating diseased brain tissue. Prusiner investigated what could be spreading the disease that was present in brains.
Prusiner experimented on infected mice and hamsters and by 1982 he had isolated a protein, which he named a ‘prion’, that he believed to be the infectious agent. This was an unexpected discovery. As proteins cannot make new copies of themselves, Pusiner had to learn more in order to convince the scientific community that a protein could transmit disease.
Prusiner found that the gene that encodes the prion was found in many animals, including humans, and that the protein this gene produced is a normal component of white blood cells and is especially abundant in the brain. Prusiner showed that the prion existed in two forms that differed only in the way the proteins were folded. The ubiquitous form found in all humans is harmless, however, the other form, when found in large enough quantities, binds together to create thread-like structures that rapidly destroy brain cells. Inherited forms of prion disease arise from mutations in the prion gene that result in the harmful prion being produced.
Most cases of CJD arise from a spontaneous change of the prions in the body, and a small proportion of cases result from ingestion of infected offal. The key to these cases is that, when they encounter each other, the stable harmful prion form transforms the non-harmful into the harmful form. This is how the harmful prions ingested in diseased brains, or the couple of spontaneously mis-folded prion proteins in the body, are able to multiply and cause fatal diseases like CJD.
Prusiner’s work revolutionised our understanding of infectious disease, and by identifying prions, he has allowed researchers to target the causative agent directly when developing treatments for CJD, and provided possible insights into the development of other dementias such as Alzheimer’s disease.