Their study, reported in the Oct. 16 issue of Nature, provides important clues to understanding the role of prions, unorthodox infectious agents whose ability to transmit disease has confounded physicians and scientists. The work, by Dr. Surachai Supattapone, assistant professor of biochemistry and of medicine, opens new avenues of exploration for diagnosis and treatment of a perplexing group of neurodegenerative disorders called prion diseases.
Prions lack RNA or DNA, the nucleic acids that contain genetic information to replicate. No one knows what spurs conversion of a normal prion protein to a disease-causing counterpart. Supattapone, with coauthors Nathan Deleault and Ralf Lucassen, has discovered that RNA may be a catalyst for transformation.
"It has been well proven that nucleic acids, including RNA, are not part of the infectious agent, so it's an ironic twist that a catalyst for the reaction may be RNA," said Supattapone. He emphasized, however, that the findings are consistent with the "protein-only" hypothesis of prion diseases because the nucleic acids are in the host and are not contained in the disease spreading particle.
Prions related to infectious brain diseases such as Cruetzfeld-Jakob disease in humans, chronic wasting or scrapie in animals have long been known, but these diseases often develop over years, so research to piece together the process has moved slowly.
The discovery more than a quarter century ago that prions were proteins, devoid of nucleic acids, upended what scientists assumed not only about disease transmission, but about life itself. All mammals have a gene to make a prion protein, but the normal prion is a different shape than the infectious prion. Somehow, this normal protein is modified into an abnormal counterpart that accumulates exponentially in the brain until death.
"It's a curious thing, because this protein is able to stimulate its own formation and change, without nucleic acids. It's been a fascinating question for people to come to grips with scientifically in addition to being the central reaction in an important medical problem," Supattapone said.
Now, his team has discovered that specific RNA molecules are required to transform prion proteins into their abnormal shapes. They devised a technique to observe how the normal prion protein, called PrPC, efficiently converts into the abnormal infectious protein, PrPSc (scrapie), in a test tube. This biochemical assay enabled the researchers to pinpoint the conversion mechanisms in the cell.
They found that adding enzymes that slice RNA blocked accumulation of abnormal prion protein. Taking it further, the researchers purified RNA and reconstituted the activity they had abolished with the RNA-cleaving enzymes. Adding enzyme knocked down scrapie protein and adding RNA brought it back up, indicating that RNA is really a stimulator. Moreover, RNA appears to boost the signal even of dilute amounts of scrapie protein.
The existence of RNA-converting factors could aid early detection of prion diseases, now incurable and invariably fatal. Ideal treatment time would be in the nascent stages of symptoms, before development of permanent brain damage. Current diagnosis requires a small brain biopsy, that may not provide much abnormal protein, but a technique where RNA can be added to amplify the signal may provide a more sensitive early diagnosis.
The next step is to pinpoint the RNA molecule, which seems to be only in mammals. "This stimulatory RNA appears to be a specific one, which makes it exciting to study. If we can identify, clone and produce this specific RNA, it may be useful as therapeutic target or a diagnostic tool. In addition, it may offer clues to the mechanism of conversion," Supattapone said. More studies are required to confirm the results in animals, he cautioned, but he is optimistic that the process in the test tube will contribute important insight into prion diseases.
The work was supported by the Burroughs Wellcome Fund Career Development Award, the Hitchcock Foundation and an NIH Clinical Investigator Development Award. For more information, contact Supattapone at: Surachai.Supattapone@dartmouth.edu or 603-650-1192.
Journal
Nature