Diseases such as mad cow disease (bovine spongiform encephalopathy or BSE), which are caused by faulty proteins called prions, could be treated by adding closely related, noninfectious prions from other species. This sort of therapy could increase the incubation period of the disease well past a human lifetime, according to a theoretical study by physicists at the University of California, Davis.

Prion diseases like BSE are caused when a common protein in the brain, PrP, does not fold properly. Somehow, this faulty protein makes other PrP proteins fold in the same way. The misfolded proteins build up and stick together, forming mats and tangles in the brain.

Graduate student Alex Slepoy and colleagues built a mathematical model of prion growth. They set rules for how a few infectious prions convert normal PrP proteins around them. As more proteins are altered and can affect their neighbors, the damage spreads at an increasing rate.

The change in protein folding takes less than a billionth of a second, but the disease that results can take decades to develop, said Daniel Cox, who with fellow UC Davis physicist Rajiv Singh is one of the senior authors on the study. The model was intended to bridge the gap between these time scales, Cox said. Predictions from the model were a good fit with data from the BSE epidemic in cattle in England, he said.

Prions can jump from one species to another. Some cases of human disease in England were likely caused by meat contaminated with BSE prions. But in some cases, the prions are incompatible. For example, mouse prions can attack hamsters, but hamster prions do not thrive in mice.

The UC Davis team used their model to see what would happen if incompatible, foreign prions in their correctly folded form were added to a prion infection in progress. They found that if the dose were high enough, the foreign prions would block infectious prions from converting normal proteins and forming aggregates. This would slow down or stop the spread of the disease.

Although this treatment is only a theoretical possibility at the moment, this sort of therapy could increase the disease incubation time well past a human lifetime. Cox and Singh are now hoping to use a similar model to study Alzheimer's disease, based on data collected by the UC Davis Medical Center.

The study is published in a recent issue of Physical Review Letters.