Some years ago one of the dogmas of biology was overthrown—it was long believed that diseases could only be transmitted by structures that contained nucleic acids, that is, by viruses or by microorganisms. It is now clear that a group of brain diseases can be transmitted by a protein. These are the diseases called spongiform encephalopathics: bovine spongiform encephalopathy in cows (mad cow disease), scrapie in sheep, and Creutzfeldt-Jacob disease and kuru in humans. These diseases are rare in humans but have recently increased in farm animals.
The infectious agent is called a prion and is a protein. It is coded for by a gene the animals have as part of their genome. In healthy individuals the gene is expressed in the cells of the nervous system and generates an innocuous protein called PrPc (prion-related protein cellular). PrPc has a globular region at the C-terminal end but the N-terminal region seems to be unstructured.
Sometimes the same polypeptide chain folds up differently with the disordered N-terminal section instead folding into a structure rich in beta sheet. This is called PrPse (prion-related protein scrapie). The disease arises because PrPse can cause normal PrPc protein molecules to fold up into the abnormal form. This keeps happening and lumps of PrPse form that damage nerve cells. The evidence is that it spread in cattle herds through food that contained recycled meat from infected animals. It is not yet known how one PrPse molecule triggers the aberrant folding of a normal PrPc molecule.
Before the problem was fully comprehended, infected animals had been used for human food. There have been a number of cases in people that are thought to have come from eating contaminated beef—these arc new variant Creutzfeldt-Jacob disease. It remains to be seen how many people will develop this fatal disease.