Prion diseases

Prions are proteins that are now thought to cause disease

Prion diseases, all of them fatal, include BSE - mad cow disease - and new variant Creutzfeldt-Jakob disease (nvCJD) - thought to be the human equivalent.

Prions are mutated proteins, but not all scientists accept they are the cause of disease.

However, Professor Stanley Prusiner, the Nobel prize winning scientists who first proposed that proteins could cause disease, says that today "a wealth of experimental and clinical data" proves his ideas were right.

Genetic factors

Scientists thought that fatal insomnia was due to a genetic disorder that was passed down the generations.

Professor Stanley Prusiner discovered prions

But the discovery of the sporadic form of the disease appears to show that it can also develop spontaneously. Prions have been blamed for both forms of the disease.

Researchers from the University of California in San Francisco described the new form in the New England Journal of Medicine.

An accompanying editorial said the existence of the disease was now established: "Sporadic fatal insomnia is unquestionably here to stay as a rare new member of the group of prion diseases."

Five diseases affect humans

There are five prion diseases known to affect humans - CJD, Gerstmann-Straussler-Scheinker disease, kuru, nvCJD, and fatal insomnia.

Prion diseases damage the brain

They were originally - and are also - known as "transmissible spongiform encephalopathies" - transmissible because they can be transmitted between humans and animals, and spongiform encephalopathies because they often leave the brain riddled with holes like a sponge.

Creutzfeldt and Jakob were the first to discover such a disease. They described the disorder that bears their names between 1920 and 1921.

It first appeared as dementia, and progressed rapidly to destroy brain tissue and cause death.

Cannibalism spreads disease

The next disease appeared between 1928 and 1936, and was described in the work of Gerstmann, Straussler and Scheinker.

Some believe the diseases can pass along the food chain

The disease characteristically leads to a lack of co-ordination and destruction of brain tissue.

Kuru was first described in 1957 by Vincent Zigas of the Australian Public Health Service and D Carleton Gajdusek of the US National Institutes of Health.

It exists only among a single tribe in Papua New Guinea. The afflicted tribe - the Fore Highlanders - describe it as the "laughing death", because it leads to loss of co-ordination accompanied by dementia.

The World Health Organisation, along with most doctors, associates the transmission of the disease with ritualistic cannibalism in Papua New Guinea.

Professor Prusiner says: "The affected individuals probably acquired kuru through ritual cannibalism - the Fore tribe reportedly honoured the dead by eating their brains. The practice has since stopped, and kuru has virtually disappeared."

Deadly insomnia

Familial fatal insomnia was first described in 1986 by Elio Lugaresi and Rossella Medori of the University of Bologna and Pierluigi Gambetti of Case Western Reserve University in Cleveland.

BSE was found in cattle in 1986

It starts with protracted insomnia, leading to dementia, eventually reaching a state where sufferers cannot tell reality from dreams. They can expect to die between seven and 25 months of developing the condition.

New variant CJD is described as a distinct disease, and, like kuru, is only found in one country - in this case the UK.

It was first described in 1995, nine years after the discovery of BSE, and some scientists believe that, also like kuru, it was transmitted through the food chain - in this case from beef.

A role in other diseases

Scientists looking into the workings of prion diseases believe the proteins may play in a role in other brain disorders.

Professor Prusiner says: "Ongoing research may also help determine whether prions consisting of other proteins play a part in more common neurodegenerative conditions, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis."

He points to similarities between these and prion diseases: "The more widespread ills mostly occur sporadically but sometimes run in families.

"All are also usually diseases of middle to later life and are marked by similar pathology - neurons degenerate, protein deposits can accumulate as plaques, and glial cells (which support and nourish nerve cells) grow larger in reaction to damage to neurons."

Treatment outlook

The outlook for treatments for genuine prion diseases is so far bleak.

Scrapie affects sheep

Spongiform encephalopathies are more common among animals - scrapie affects sheep and there is a mad cat disease - and the treatment approach to animal sufferers has been to cull them.

But now the effect on humans is better publicised, treatment ideas are coming forward.

The key is likely to lie in emerging gene therapies, and scientists hope they will be able to shut down unwanted proteins - prions being a prime candidate for this.