By Dr Ananya Mandal, MD
A prion is a small infectious particle composed of abnormally folded protein that causes progressive neurodegenerative conditions. These mis-folded proteins do not multiply in the host organism that they infect. Instead they affect the brain structure by acting as a template, inducing proteins with normal folding to convert to the abnormal prion form.
These newly formed mis-folded proteins, in turn, act as further templates for the conversion of more normal proteins. There is therefore an exponential accumulation of the prions in the tissue of the central nervous system.
Prions cause several diseases, with two of the most notable being Bovine spongiform encephalopathy (BSE or mad cow disease) seen in cattle and livestock and Creutzfeldt-Jakob disease (CJD) seen in humans.
The abnormally folded proteins lead to brain damage and symptoms of prion disease. They have a very stable structure that is not denatured by chemicals meaning it is difficult to dispose of or contain them. The abnormal fold that prions induce is called an amyloid fold, in which the protein is aggregated into densely packed beta sheets.
These polypeptide aggregates form plaques which are thought to cause "entanglement" of neurofibrils and interfere with synapse function, leading to loss of neurones and dementia.
Prion-like behavior can also be seen in some types of fungi. These fungal prions have been studied extensively to provide clues as to how prions affects mammals. Fungal prions, however, are not harmful to their hosts.
Prions were first discovered in the 1960's by radiation biologist Tikvah Alper and the mathematician John Stanley Griffith who found that the spongiform encephalopathies in cattle were mainly caused by protein infective agents. In 1982, Stanley B. Prusiner purified the hypothetical infectious agent and coined the name "prion" for this protein molecule.
Reviewed by Sally Robertson, BSc