Detecting Prions in Blood

Infectious prions are the likely cause of diseases like mad cow disease and variant Creutzfeldt-Jakob disease (vCJD). Currently, prion diseases are mostly detected by analyzing brain samples after death. Bioassays--in which live animals are exposed to tissue from potentially infected animals and then monitored for illness--have been used to detect prions in blood, but they take months or years and are thus impractical for routine use.

Neurology professor Claudio Soto and coworkers at the University of Texas Medical Branch, Galveston, have now devised a faster means of analyzing blood (Nat. Med., published online Aug. 28, dx.doi.org/10.1038/nm1286). Their work could lead to a technique for diagnosing prion diseases in live animals and people and could help keep infected beef out of the food chain, protect the blood supply, and diagnose vCJD at an early stage, when drugs might have a better chance of working.

The test "has the potential advantage of being developed into a large-scale screening procedure with a rapid turnaround time for results," says James W. Ironside, professor of clinical neuropathology at Western General Hospital, Edinburgh, Scotland. However, "the difficulties of this work should not be underestimated," he says, noting that it is not yet known whether the procedure will work in organisms and conditions different from those in which it has been tested.

Prions are too scarce in blood to be analyzed directly. In 2001, Soto and coworkers developed protein misfolding cyclic amplification (PMCA), in which sonication is used to amplify prions. They have now developed a serial, automated form of PMCA that amplifies prions more than 10 million-fold, so they're concentrated enough to be detectable by Western blot analysis. Altogether, the procedure takes several days. Soto's group detected prions in the blood of 16 of 18 infected hamsters, a sensitivity of 89%. There were no false positives--that is, no prions were detected in any of 12 healthy control hamsters.

"The next step, which we're currently working on, will be detecting prions in the blood of animals before they develop clinical symptoms and applying the technology to human blood samples," Soto says.

Another technique being developed to detect prions in blood was reported in 1999 by Mary Jo Schmerr, now at Ames Laboratory of Iowa State University, and Andrew J. Alpert of PolyLC, Columbia, Md. This technique, which combines chromatography, fluorescence immunoassay, and electrophoresis, has recently been improved (Anal. Chem. 2005, 77, 4489) and has been tested on about 500 sheep in Britain.