Some scientists and researchers hold the opinion that CJD is caused by an unusual form of virus or other organism, however such a virus has never been isolated in a patient suffering from the disease. CJD also has certain characteristics which are not usually noted in known bacteria or viruses – the infectious agent which causes the disease is very difficult to kill, has a very long incubation period which can be up to 40 years and does not seem to have any DNA or RNA (the nucleic acids which form genetic information). The most popular current theory is therefore that CJD and other forms of TSE are caused by a malfunctioning protein called a prion.
Normal, functioning prion proteins are harmless, and are found in body cells, however there is a form of prion protein which causes disease. Although both forms of prion have identical sequences of amino acids (which make up proteins) the infectious variety of prion takes on a different shape – that of a beta pleated sheet, while the normal prion will have an alpha-helical shape. In the case of sporadic CJD, an individual’s normal prions may spontaneously alter to the infectious type, which then begins a chain reaction that creates more infectious prion proteins.
Once the abnormal prion proteins appear, they group together, and it is believed that these ‘protein aggregates’ or groups of prion proteins lead to the brain damage which occurs in patients of CJD. It is not yet understood exactly how this damage happens however.