A NEW form of fatal dementia has been discovered in 16 Americans, 10 of whom have already died of the condition. It resembles Creutzfeldt-Jakob disease - with patients gradually losing their ability to think, speak and move - but has features that make it distinct from known forms of CJD.

No one yet knows how the disease originates, or under what conditions it might spread. Nor is it clear how many people have the condition. "I believe the disease has been around for many years, unnoticed," says Pierluigi Gambetti, director of the US National Prion Disease Pathology Surveillance Center at Case Western Reserve University in Cleveland, Ohio. Cases may previously have been mistaken for other forms of dementia.

Since Gambetti's team wrote a paper describing an initial 11 cases referred to his centre between 2002 and 2006 (Annals of Neurology, vol 63, p 697), another five have come to light. "So it is possible that it could be just the tip of the iceberg," Gambetti says.

As in other spongiform encephalopathies, such as CJD and mad cow disease (BSE), the brain tissue of victims is full of tiny holes. This damage is thought to be caused by the accumulation of prions, misfolded versions of a brain protein called PrP that can convert normal PrP molecules into their own misshapen form.

Some features of the new disease are different, however. All known disease-causing prions resist degradation by proteases - enzymes which digest the normal form of PrP. But prions from patients with the new disease are broken down by the enzymes.

Some very rare forms of CJD run in families and are caused by mutations in the gene for PrP. Six of the cases described in Gambetti's paper were from families with a history of dementia, suggesting a genetic cause. However, these people had no mutations in their PrP genes. "Maybe there are other genes that have an influence on the disease," suggests James Ironside of the UK's National CJD Surveillance Unit in Edinburgh.

Most forms of CJD develop spontaneously, for unknown reasons, but can be spread if someone is exposed to brain material from people with CJD, for instance, by neurosurgery using inadequately sterilised instruments.

One variant of CJD has been linked to the consumption of contaminated meat from cattle with mad cow disease. If the new condition is similarly caused by something in the victims' diet, or another environmental cause, new measures might be needed to protect public health.

Gambetti is now conducting experiments in mice to see how the disease is transmitted. He suspects that there is no cause for alarm. "I believe the disease occurs naturally, and is not due to environmental causes," he says.

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New Scientist Reporter: Andy Coghlan

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