Creutzfeldt-Jakob disease (CJD) is caused by an infectious protein in the brain called a prion.
Role of proteins in the brain
Proteins are molecules, made up of amino acids, that help the cells in our body to function.
Proteins begin as a string of amino acids which then fold themselves into a three-dimensional shape. This 'protein folding' allows them to perform useful functions within our cells.
Prion proteins (which are not the same as the infectious prions that cause CJD) are a type of protein found in the brain and several other tissues of the nervous system. The exact role of prion proteins in our brain is unknown, but it is thought they may have something to do with our long-term memory.
Protein folding
Sometimes, mistakes happen during protein folding and the prion protein cannot be used by the body. These misfolded prion proteins are normally recycled by the body, but sometimes they can build up. This can cause problems, such as Alzheimer's disease.
Prions are misfolded prion proteins that enter brain cells and cause normal proteins to misfold as well. This causes the brain cell to die, releasing more prions to infect other brain cells.
Eventually, clusters of brain cells are killed and replaced with deposits of prions, called plaques.
These plaques produce small holes in the brain, causing it to become sponge-like. The damage to the brain causes the mental and physical impairment and eventual death associated with CJD.
Prions can survive in nerve tissue, such as the brain or spinal cord, for a very long time, even after the person or animal has died.
Sporadic CJD
Sporadic CJD is the most common type of CJD, although it is still very rare.
It is not known what triggers sporadic CJD, but it may be that a normal protein spontaneously changes into a prion or a normal gene spontaneously changes into a faulty gene that produces prions.
At present, nothing has been identified that increases your risk of developing sporadic CJD.
Variant CJD
There is clear evidence that variant CJD (vCJD) is caused by the same strain of prions that causes bovine spongiform encephalopathy (BSE, also known as mad cow disease).
A government inquiry in 2000 concluded that the prion was spread through cattle that were fed meat-and-bone mix containing traces of infected brains or spinal cords. The prion then ended up in processed meat products, such as beef burgers, and entered the human food chain.
Strict controls have been in place since 1996 to prevent BSE from entering the human food chain and the use of meat-and-bone mix has since been outlawed.
It appears that not everyone who is exposed to BSE-infected meat will go on to develop vCJD.
All definite cases of vCJD occurred in people with a gene called codon 129 which affects how the body makes a number of amino acids.
It is estimated that around one half of the UK population have this gene.
Cases of vCJD peaked in the year 2000, in which there were 28 deaths from vCJD. There were only five confirmed deaths in 2011. Some experts believe that the food controls have worked and that further cases of vCJD will continue to decline. As of January 2012 there were no patients with vCJD, but this does not rule out the possibility that other cases may be identified in future.
There is considerable uncertainty about how many people in the UK population are incubating vCJD.
A recent study based on random tissue samples suggested that around 1 in 4,000 might be infected with vCJD, but show no symptoms to date.
Familial CJD
This very rare form of CJD is caused by an inherited mutation (abnormality) of the gene that produces the prion protein. The altered gene seems to produce misfolded prions that cause CJD.
Everyone has two copies of the prion protein gene, but the mutated gene is dominant. This means you only need to inherit one mutated gene to develop the illness.
Most forms of familial CJD are inherited as dominant diseases, with a 50% chance of being passed on to the children of an affected individual. However, as the symptoms of familial CJD do not usually begin until a person is in their 50s many patients with familial CJD are unaware that their children are also at risk of inheriting this condition.
Iatrogenic CJD
Iatrogenic CJD (iCJD) is when the infection is spread from someone with CJD through medical or surgical treatment.
Most iatrogenic CJD cases have occurred through the use of human growth hormone, which is used to treat children who have restricted growth. Between 1958 and 1985, thousands of children were treated with the hormone, which, at the time, was extracted from the pituitary glands (a gland at the base of the skull) of human corpses.
A minority of those children developed CJD, as the hormones they received were taken from glands infected with CJD. Since 1985, all human growth hormone in the UK has been artificially manufactured, so there is now no risk.
A few other cases of iCJD occurred when people received transplants of infected dura (tissue that covers the brain) or came into contact with surgical instruments that were contaminated with CJD. This happened because prions are tougher than viruses or bacteria, so the normal process of sterilising surgical instruments had no effect.
Once the risk was recognised, the Department of Health tightened the guidelines on organ donation and the reuse of surgical equipment. As a result, cases of iCJD are now extremely rare.
