One grim difference between Alzheimer's disease (AD) and treatablesenile dementias is that a diagnosis of AD can be definitelyconfirmed only by post-mortem brain autopsy.
The same goes for the prion-caused transmissible spongiformencephalopathies. In humans, the best known of these fatal braininfections is Creutzfeldt-Jakob disease (CJD). Even better knownthese last few months, especially in Britain, is bovine spongiformencephalopathy (BSE) _ mad cow disease.
Both of these prion infections track back to scrapie in sheep, in whichprion diseases were first discovered. (See BioWorld Today, April 8,1996, p. 1.)
The mutant prion proteins that cause scrapie, as well as CJD andBSE, carry the molecular license-plate PrPSc. The superscript standsfor "scrapie."
Sheep are suspected of causing BSE in British cattle, because ovinebody parts were added to bovine fodder. Cows, in turn, are suspectedof passing the PrPSc infection to humans, who then came down withCJD.
Prion disease is enzootic in European sheep, but strikes only certainanimals in an infected herd. Scrapie develops in a lengthy, silentincubation period, before symptoms erupt, and death soon follows.
At the Animal Science and Health Institute in Lelystadt, theNetherlands, veterinary epidemiologist Bram Schreuder hascoordinated a five-year project to achieve presymptomatic diagnosisof scrapie in sheep. Today's issue of Nature documents his group'ssuccess in a one-page report titled: "Preclinical test for priondisease."
A more detailed account appears in the May 1996 Journal of ClinicalMicrobiology, under the heading: "Immunohistochemical detectionof prion protein in lymphoid tissues of sheep with natural scrapie."
Tonsils Pinch-Hit For Brain
What Schreuder and his co-authors discovered is that instead ofautopsying an ovine brain for mutant prion deposits, it's sufficient tobiopsy a small fragment of tonsil from the soft palate at the back ofthe living animal's throat.
Tonsils, as parents and their children knew all too well a generationor two ago, are patches in the soft palate that sometimes swell upduring a respiratory infection. Which is why doctors routinelyremoved tonsils preventively from small children.
Of course, in so doing, they were depriving the body of a first-lineimmune-defense outpost. Tonsils consist of lymphoid tissue, loadedwith lymphocytes and macrophages to fight off infection.
Of 55 scrapie-dead sheep the Dutch scientists initially examined, theyfound infective PrPSc in the lymphoid tissues of 54, for a score of 98percent. Tonsils, with 93 percent, led all other lymphoid tissues, towit: spleen (11 percent), and various lymph nodes, 7 to 9 percent.
They used a battery of five PrPSc antibodies programmed againsteight antigenic targets to locate the infective miniparticles throughoutthe animals' carcasses.
None of a dozen healthy sheep showed any trace of the telltalemutant prion protein in brain or lymphoid tissue.
What makes this finding diagnostically useful is the fact that bysubsequently following 10 live sheep for five years, the Dutch teamhas been able to time the advance notice of infection afforded bytonsillar biopsy.
"We started off with disease-susceptible animals," Schreuder toldBioWorld Today. "They were born in an infected herd, and wereinfected naturally, not experimentally. We knew they should getscrapie when two years old."
Susceptibility to mad sheep disease varies in time with the animal'sgenetic inheritance of microsatellite repeating triplet codons. In this,they resemble some forms of Alzheimer's disease, which is alsothought by many to have a prion component.
A sheep that has acquired the iterative triplet gene variant from bothparents is homozygous, Schreuder pointed out. "They go down withthe disease in a fairly short time, say two years. Resistant animals,[without the mutant allele], can remain healthy even in an infectiveenvironment. And those in between, the heterozygotes [who inheritthe mutation from one parent only] can go down anywhere betweenthree and five years, usually."
He and his co-authors then bred 10 lambs, six homozygous _ i.e.,susceptible _ and four resistant. They took tonsillar biopsies at 10months after birth, "and to our surprise, the very first round of tissuefrom the six homozygous sheep already stained positive to specificantibodies against the prion protein." That is, the workers couldforetell the infection as long as a year before overt scrapie symptoms_ frantically scraping necks against trees and fence-posts _appeared.
The four genetically resistant animals showed no such antibody signsor pathological symptoms.
Saving Lambs From Slaughter
The Dutch veterinarian foresees being able to certify sheep as prion-free, thus avoiding preventive overkill of all animals in an infectedherd. But he envisages "another year-and-a-half of finalexperimentation before offering the technology to this market."
As for human application to CJD, Schreuder said, "Dutch humanpathologists have expressed interest. We have contacts, for the timebeing only in the Netherlands, with which we are interested incooperating. But even without our cooperation," he added, "anybodyhaving access to human material could immediately try it. Theantisera are published and known, and it's not a big deal to take alittle tonsillar biopsy."
Extending the method from ovine to bovine is a quite different story."Mad cow disease is, of course, a major issue," he observed. "Ourtest detects a marker protein for infection. If there is no infection,there will be no marker protein. So far, infectivity has not beendetected in the cattle tissues, so the chance one can detect this PrPScis remote."
But he doesn't rule out possible future application to the Britishscourge. "It's also true," he surmised, "that there must be prionsmoving between the bovine gut and the central nervous systemtissues. So somewhere along this line, you should be able to find atrain station where it changes trains. It doesn't necessarily have to bethe tonsillar lymphoid tissue." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.