To elaborate on Halcyon&on’s good but potentially misleading write-up, it should be pointed out that not all victims of prion diseases are found to have the plaque deposits in their brains. When these amyloid plaques were first discovered, they were thought to be the cause of death, but this theory was quickly discarded when it was established that only about 15% of all CJD victims had them to a severe degree. Subsequently, it became apparent that there was an inverse relation between the extent of the plaques and the infectiousness of the brain tissue found in post-mortem samples. Furthermore, those with the most severe spongiform encephalopathy were the least likely to have plaque deposits.
These plaques consist of PrP27-30, a fragmented form of the abnormal prion mutation (known as PrPsc). The term PrP, which refers to the abormal protein, stands for protease-resistant protein, meaning that the proteolytic enzyme Protease K has no effect on it. However, where these plaques are concerned, this apparently isn’t the case. The presence of these plaques indicate that the Protease K enzyme has had some effect on the PrPsc, reducing it to the smaller, less infectious PrP27-30, which has the tendency to accumulate in amyloid plaques. These plaques are found most commonly among victims of genetically inherited prion diseases, and are in many cases believed to be part of a sort of coping mechanism, actually enabling the patient to live longer while suffering from the disease. Post-mortem analysis of these plaques have shown that they often consist of even smaller protein fragments than PrP27-30, such as PrP11, which has a molecular weight of 11 kilodaltons.
CJD variants, such as Gerstmann-Straussler-Scheinker disease, which can take anywhere from 1 to 15 years to kill its victims, is always indicated by the presence of PrP plaques, but not always by spongiform encephalopathy. By the same token, non-variant CJD lasts around 3-12 months, and spongiform encephalopathy is always present, while PrP plaques are less common. In cases such as the former, the cause is usually famillial.