Re: Do prions affect tissue other than nervous tissue?

Prusiner first identified the disease causing protein, and labelled it a 
prion, in 1982. He derived the term "prion" from the words protein and 
infectious.Prions are a relatively newly discovered disease causing 
agent. While most disease causing agents are themselves organisms 
(bacteria, fungi, parasites) or, in the case of viruses, at least use DNA 
or RNA to replicate themselves, prions are only protein molecules. 

Protein molecules are one of the basic building blocks of living things. 
Other basic molecules include carbohydrates and fats. Each protein 
molecule has a unique shape or conformation. The uniqueness of the 
conformation is due to the sequence of amino acids that join together to 
make the protein molecule. The shape or conformation has a direct bearing 
on the protein's function. Basic biology students are all familiar with 
the metaphor of a lock and key, which is used to describe protein 
function. The notion of the unique fit between a lock and key describes 
the way proteins interact with particular other molecules.

The discovery of prions has thrown a new twist to our understanding of 
protein chemistry, because prions can have different conformations. It 
may be that there are many kinds of proteins that have the capacity to 
exhibit this "prion behaviour". To date only one has been identified, and 
this one has only been isolated from mammals. In its "normal" 
conformation, the prion protein is not disease causing. However, prion 
protein can change its conformation. In the different conformation, it 
becomes disease causing. 

Prions of the disease-causing conformation are unusually stable and 
cannot be broken down easily, thus bypassing the normal recycling 
processes of the body. It is not known why the protein changes 
conformation, but once it does so, it can induce other "normal" proteins 
to change too, causing a chain reaction. In sufficient numbers, the 
stable, changed, proteins clump together, forming particles that are 
capable of disrupting normal body functioning. The family of diseases 
caused by this prion is characterised by the fact that they are all 
caused by degeneration of the brain.

Prion diseases are all ultimately fatal. There is a relatively long 
incubation time while sufficient numbers of the protein with the disease-
causing conformation accumulate. This is followed by the manifestation of 
neurological symptoms of increasing severity. These symptoms are due to 
nerve cell death in the brain. Because the disease particles accumulate 
in the brain, it is difficult to diagnose and treat patients suffering 
from prion disease. Indeed, the diagnosis can only be verified after 
death, when a brain autopsy can be conducted. 

The earliest recognised disease caused by this prion was scrapie in 
sheep. It was first documented in Iceland during the 18th Century and is 
now know to affect other animals including mink, cats, deer and moose. 

Bovine spongiform encephalopathy (BSE), or mad cow disease, first became 
evident in England in 1985. Due to its long incubation time, the epidemic 
did not reach its peak until 1992.

There are several prion diseases that affect humans. Kuru is a disease 
affecting the Fore-people in New Guinea. This tribe of relatively 
isolated, primitive peoples engaged in a ritualistic, cannibal rite 
following death. This was the mechanism for directly transmitting the 
prion. Gertsmann-Strässler-Scheinker (GSS) is a hereditary prion disease, 
which results from a mutation in the gene coding for the prion protein. 
Fatal Familial Insomnia (FFI) is also a hereditary prion disease, 
resulting from a different mutation in the prion gene. 

The disease process of the Creutzfeldt-Jakob Disease (CJD) involves two 
mechanisms. It occurs spontaneously in 85-90% of the cases. In the 
remaining 10-15% of the cases, it is due to a gene mutation. It affects 
about one in a million people. There are rare cases of CJD being 
transmitted, for example, in growth hormone preparations. Another example 
of transmission followed the BSE epidemic, and a new variant of CJD 
(nvCJD) has also been identified, which is believed to be connected to 
the BSE epidemic.