MIAMI BEACH—Finding a sensitive and specific test for the early detection of mad cow disease and other prion disorders in asymptomatic individuals and animals is a major focus of current research.
Scientists reported here in April at the AAN Annual Meeting that they are one step closer to achieving that goal, using MRI to detect the presence of abnormal prion proteins in the spleens of asymptomatic mice.
If the approach can be replicated in humans, the test could offer a means of identifying asymptomatic carriers who are a potential source of transmission, said chief investigator Marcin Sadowski, MD, PhD, Assistant Professor of Neurology and Psychiatry at New York University School of Medicine.
“Asymptomatic carriers are still a reservoir for infection through blood transfusion or organ transplantation,” said Dr. Sadowski, who estimated there are probably more than 10,000 people worldwide, mostly in Europe, who fall into this category.
Prion diseases – fatal transmissible neurodegenerative disorders of animals and humans associated with prolonged incubation periods – include scrapie, bovine spongiform encephalopathy (BSE), and Creutzfeldt-Jakob disease (CJD).
To develop the new test, Dr. Sadowski and colleagues took advantage of the fact that in scrapie, BSE, and variant CJD – the human form – prions first gain a foothold in the lymphatic system, before traveling to the CNS and the brain, where they do their real damage.
“If you eat contaminated beef, for example, prions accumulate in the lymphatic system for months or years,” he said. “After a few years, prions start spreading to the brain and actually cause disease.”
First, the researchers attached the contrast agent gadolinium to ligands that bind to PrP-scrapie, the abnormal prion protein that accumulates in the lymphatic system of an infected person.
Then, two months after infection with scrapie, mice were injected with the labeled ligand. “At this time, the mice have no symptoms and no prions are found in the brain – which is equivalent to the state one to two years after human infection,” Dr. Sadowski said.
The animals were sacrificed and their spleens imaged using MRI. “There was a significant change in the MRI signal in the spleens of infected mice but not in control mice,” he said. “This is a way to directly image the presence of abnormal prion protein in the lymph system.”
Now that they have showed proof of principle, Dr. Sadowski's group is using MRI on live mice to determine how early they can pick up the signal changes, he said. And they would like to embark on human clinical trials within two to three years.
In addition to identifying carriers who could unwittingly transmit the disease, early detection might someday be critical for prevention and treatment, Dr. Sadowski said.
“While all prion diseases are currently considered untreatable, animal studies suggest passive immunization with monoclonal antibodies may prevent neurological symptoms of the disease if administered early, before prions spread to the brain,” he explained (Nature 2003; 422:80–83).
Although some researchers are working on blood tests, Dr. Sadowski thinks the lymphatic approach has more promise. “Prions are present in blood, but at a low level – 1,000-fold below the level of detection,” he explained. “You'd need a lot of blood.”
Also, while a blood test would reveal whether a person had been infected, it would be more difficult to follow response to therapy, Dr. Sadowski said. “Prions may still be present in the lymph system even if they are not in the blood,” he explained.
Patrick Bosque, MD, Assistant Professor of Neurology at the University of Colorado Health Sciences Center in Denver, said the approach was innovative. Dr. Bosque was not involved in the study.
“They made peptides for the prion protein, labeled them, and MRI showed that the peptides seemed to accumulate preferentially in the spleen,” he said. Identification of asymptomatic carriers is currently a major issue, especially in Britain, where nearly all variant CJD cases have developed, he added.
Dr. Bosque pointed to a case in which a British person had a blood transfusion from an asymptomatic infected donor as reason for concern. At autopsy, “they looked at his spleen and there were lots of PrP-scrapie particles, so they worried that he might have eventually developed the disease himself,” Dr. Bosque said.
The transfusion occurred in 1996, one year before safeguards were applied to the blood supply in Britain. Nevertheless, “the disease has a long latency period and we're only 10 years out from the peak of cases. People can take decades to get sick,” Dr. Bosque said.
But while the MRI approach for detection of prion disease is promising, it is still in the earliest stages of testing, he stressed. “My big concern,” he said, “is whether other conditions could result in the same MRI signal. Humans with other diseases might also have an abnormal signal on this test.”
PRIONS: ONE OF MEDICINE'S BIGGEST MYSTERIES
For years, prions have been one of medicine's biggest mysteries. English farmers noticed the first of the prion-related diseases – scrapie – in sheep and goats in the mid-eighteenth century.
In the 1920s, two German neurologists Hans Gerhard Creutzfeldt, MD, and Alphons Maria Jakob, MD, described the rare, fatal brain disorder that bears their names, classic Creutzfeldt-Jakob disease. The disease, which causes a rapid, progressive dementia and associated neuromuscular disturbances, is often referred to as a subacute spongiform encephalopathy because it usually produces microscopic vacuoles in neurons that appear “sponge-like.”
But the work of an NIH virologist Carleton Gajdusek, MD, and Stanley Prusiner, MD, of the University of California-San Francisco, have paved the way for a new understanding of these diseases. In 1976, Dr. Gajdusek was awarded the Nobel Prize for his work showing that both CJD and kuru – another human spongiform encephalopathy – were transmissible, slow-acting viruses. Dr. Gajdusek discovered kuru among the people of Papua, New Guinea, who practiced cannibalism and contracted the disease after eating the infected dead.
In 1997, Dr. Prusiner was awarded the Nobel Prize for his work on prions – championing the theory that infectious proteins can cause a range of neurodegenerative brain diseases by misfolding and causing other proteins to do likewise.
In the 1980s, an epidemic of bovine spongiform encephalopathy (BSE) began in the United Kingdom, peaking in 1993 with almost 1,000 new cases per week. No one is sure how the animals caught BSE, but scientists believe they became infected after eating feed that had been made from scrapie-infected animals.
In 1996, variant Creutzfeldt-Jacob disease was detected in humans and linked to mad cow disease. It is believed that humans contracted the disease after eating cattle products contaminated with BSE-infected CNS tissue.
ARTICLE IN BRIEF
- ✓ Investigators reported that they were able to use MRI to detect the presence of abnormal prion proteins in the spleens of asymptomatic mice.