The Physics of Prion Diseases

Mad cow disease is one of a class of neurodegenerative illnesses caused by misshapen proteins known as prions. Recent outbreaks of the ailment in European cattle and an increase in occurrences of the related Creutzfeldt-Jacob disease (CJD) in humans have turned prion diseases into growing public health threats.

Although the international medical community has made impressive advances in understanding the deviant proteins, new work by physicists at the University of California, Davis (D. Cox, cox@rilke.ucdavis.edu, 530-752-1789; R. Singh, singh@physics.ucdavis.edu, 530-752-4710), suggests that a fairly simple statistical mechanics model may help explain prion disease progression. Among other things, the researchers found motivation for their model in the statistically uniform, low-level incidence of CJD worldwide, and in the highly reproducible incubation time relative to infection dose, both characteristics consistent with the physical and chemical processes described by statistical mechanics.

Through a simulation of the infection on a two-dimensional lattice, the researchers discovered that a handful of prions can serve as seeds for the growth of more prions on infected neurons. Once the prion aggregates are big enough, various processes can break them up and cause the prions to leap to other neurons, thus seeding new crops of prions as the diseases progresses---ultimately leading to death of the infected victim.

The model, however, offers much more than a bleak image of a prion disease death march through the neurological system. The researchers found cause for hope in their attempt to incorporate an asymmetry in the cross-species virulence of prion infections. For example, prions that normally infect mouse neurons can also effectively attack hamster neurons, but related prions that are deadly to hamsters are ineffective in mice. The new model suggests that an injection of harmless hamster prions into an infected mouse might lead to competition between the mouse and hamster prions that could dramatically slow the progress of the disease.

It is a startling concept: fighting prions with prions. Although such a treatment doesn't promise a cure for prion diseases, it could extend the incubation time to the point that the onset of an ailment like CJD is imperceptible in a human lifetime.

In addition to suggesting novel prion disease therapies, a statistical mechanics model of the infections might aid in predicting the course of outbreaks such as the string of CJD cases that struck England in the late 1990s. (A. Slepoy et al., Physical Review Letters, 30 July 2001; text at Physics News Select)