SYNTHETIC PRIONS MAY HOLD THE KEY TO TREATMENT OF CREUTZFELDT-JAKOB DISEASE
MIAMI BEACH—Just as prions are the infective agent underlying the pathogenesis of Creutzfeldt-Jakob disease (CJD) and other forms of transmissible spongiform encephalopathy (TSE), synthetic versions of prions may hold the key to treating these diseases, according to Nobel Laureate Stanley B. Prusiner, MD, PhD, who delivered the Robert Wartenberg Lecture here in April at the AAN Annual Meeting.
“I think we can use synthetic prions to help us treat CJD, provided we also find a means of early diagnosis,” said Dr. Prusiner, Director of the Institute for Neurodegenerative Diseases at the University of California-San Francisco, where he is Professor of Neurology and Biochemistry. “I'm hopeful that identifying other proteins will help us in our quest to treat patients with CJD, and that identifying a treatment for this disease will lead to finding cures for other CNS disorders caused by proteins that undergo misprocessing.”
Dr. Prusiner received the 1997 Nobel Prize in Physiology or Medicine for his discovery of prions as the infective agent in CJD and other TSEs. He is also the founder of InPro Biotechnology, Inc., which is developing prion therapies for commercial availability.
Prions are proteins that can adopt at least two different conformations. The normal form of the protein called PrPC (cellular prion protein) can be degraded by proteases, while the aberrant form PRPsc (scrapie) cannot. The accumulation of PRPSC causes neurodegeneration. According to Dr. Prusiner, the development of synthetic prions results from prior research that established the following four concepts:
- Prions are infectious agents that contain no RNA or DNA genome;
- Prion diseases may occur as infectious, genetic, or sporadic disorders;
- Prion diseases result from the accumulation of PrPsc, an abnormal transformation from the normal PrPc prion protein;
- PrPsc can exist in several forms, each of which produces a unique disease phenotype.
“Prions occur by forcing a precursor protein to acquire a second conformation,” Dr. Pruisner said, noting that this event converts the prion protein from a highly helical form to a pathogenic, less helical beta form. “They are unique among pathogens in that they replicate without nucleic acid. Prions use a protein only.”
The second conformation causes the protein to misfold and accumulate, which in turn causes amyloid depositions that underlie prion-based diseases. In humans, kuru, Gerstmann-Sträussler-Scheinker syndrome, and familial fatal insomnia are also prion diseases as well as CJD.
REPLICATION WITHOUT RNA, DNA
Studies with transgenic mice show both how prions can replicate without RNA or DNA and the elusive nature of finding a treatment for prion disease. In 2004, Dr. Prusiner and co-investigators reported that they created a synthetic prion using genetically engineered bacteria – Escherichia coli – that was purified and folded into the abnormal configuration that would give it infectious properties. The investigators injected either the synthetic prion – recombinant PrP (89–230) – or a saline solution into mice engineered to make the normal prion protein or PrPC (Science 2004; 305:673–676).
The mice that had been injected with the recombinant PrP (89–230) all developed signs of neurological disease – they had the large vacuoles characteristic of prion disease – while those that received the saline solution did not. When they injected some of the prion-affected brain tissue into normal mice, they became sick within 90 to 150 days.
TESTING FOR BSE
“How to test for mad cow disease has been a challenge, because the assays used in transgenic mice take too long, and immunohistochemistry is not a good method” because it is not sensitive enough to detect prions, Dr. Prusiner said. He and his co-investigators developed a method tailored to prion disease known as a “confirmation-dependent immunoassay” (CDI). Although the United States Department of Agriculture had denied that BSE could be transmitted through muscle tissue, he and his co-investigators had used CDI to identify abnormal prions in the muscles and forelimbs of mice that had been given infected beef.
To illustrate his point, Dr. Pruisner related the cases of two patients with CJD without characteristic pathology at autopsy. When CDI was used, both were positive. In another instance, eight patients with sporadic CJD had negative immunohistochemistry tests but positive CDI.
ON THE HORIZON: TREATMENT BASED ON SYNTHETIC PRION
Because of the rapid progression of CJD, an effective treatment and an effective test for early diagnosis both have to be developed, Dr. Pruisner said. He and his co-investigators in the company he founded have focused efforts on identifying treatments.
“The drug discovery pipeline for treatment of prion diseases differs from the combination of serendipity and biology that is more typical,” he said. “We are looking for ways to disrupt the interaction between PrPC and the misfolded protein. We will be looking for bioactive compounds based on this approach.”
One such drug, quiacrine, blocks prion production. “Clinical trials of quiacrine are underway in London,” he said. “We believe that this is the first study of a treatment for CJD.” The study will analyze outcomes for survival, neurological examination findings, and cognitive testing, he said. Investigators at the University of California-San Francisco are enrolling patients so that a trial in the United States can begin, he said.
IMPORTANCE OF RESEARCH
“Dr. Prusiner's research shows that all you need is the prion protein to create other prions,” said Robert B. Petersen, PhD, Associate Professor of Pathology and Neuroscience at Case Western Reserve University in Cleveland, where he is a genetics consultant at the National Prion Disease Pathology Surveillance Center.
“Dr. Pruisner's research eliminates the question of other infectious material being involved,” Dr. Petersen said. “The prion protein, without anything else, causes the disease. The research puts to rest other schools of thought and show that you can take the prion protein and generate a prion without any other material to aid in the conversion.”
Dr. Prusiner's research may show that the future is bright for identifying a treatment for CJD, he added. “His research provides the framework for thinking more effectively about identifying a treatment for CJD.”
ARTICLE IN BRIEF
- ✓ Nobel Laureate Dr. Stanley B. Prusiner reported on research efforts to create synthetic prions, which he contends will be useful for treating Creutzfeldt-Jakob disease and other prion-related disorders.