From ovine to bovine to human, the chain of incrimination runs likethis: People in Britain who contract Creutzfeldt-Jakob disease (CJD)ostensibly catch the prion infection from cattle afflicted with madcow disease _ bovine spongiform encephalopathy (See BioWorldToday, April 8, 1996, p. 1.)
Those animals picked up the pathogenic prion from eating foddercontaining the body parts of sheep with scrapie _ the ovine form ofprion disease. Whereupon, authorities banned ovine ingredients fromanimal feed.
That was like locking up a suspected serial killer, then finding thathis brand of homicide continued to claim victims. Despitepurification of their provender, British cows continued to come downwith the encephalopathy.
This apparent non sequitur pointed toward other gaps in the chain ofevidence: Where did the mad sheep acquire their scrapie in the firstplace? Might that infectious agent prove to be a common-denominator reservoir of prion disease in sheep, cow and peoplealike?
In the North Atlantic island republic of Iceland, sheep outnumber thehuman population of a quarter million by more than two to one. Andscrapie among those Icelandic sheep is endemic _ or more correctly,enzootic.
"Years ago," American virologist Richard Carp told BioWorld Todayin an interview, "Icelanders attempted to eradicate scrapie, from theirisland. From fields where this disease was endemic, they took all thesheep out and slaughtered them, then left those fields empty for up tothree years."
Carp continued: "When they put sheep back into those fields, some ofthem became infected, and although the eradication program hasproven to be fairly successful, they are still getting cases." Carpheads the virology department at the New York State Institute forBasic Research in Developmental Disabilities, on Staten Island, N.Y.Its director is Henryk Wisniewski.
The outcome of the Icelandic scrapie eradication program, heobserved, "obviously brings to mind the potential for a reservoir foran infectious agent. Because it's a little hard to understand,considering Icelandic winters, how agents could get washed away, yetsurvive, in that kind of weather over a three-year period."
So Carp and his Staten Island colleagues involved in these scrapiestudies thought about grasses and hay. Meanwhile in Reykjavik, aleading scrapie specialist at the University of Iceland, SigurdurSigurdarson, "was pushing the idea," Carp said, "that it hadsomething to do with mites, particularly hay mites."
Mites, closely related to ticks, are ubiquitous, and number at least20,000 species. In all human beings, they colonize the hair folliclesof eyebrows. "Pull out an eyebrow hair," Carp suggested; "there areDermatophagoides mites at the root."
Sigurdarson's lab in Reykjavik and a team at the Staten Islandinstitute, headed by Wisniewski, have mounted a joint investigationto indict Icelandic hay mites as the vectors that infect sheep, andother susceptible mammals, with prion disease.
They report their preliminary results in the current issue of TheLancet, dated April 20, 1996, in a letter titled: "Mites as vectors forscrapie."
To begin their detection, the Icelanders removed all scrapie-affectedsheep from five farms, then collected and froze half a dozen speciesof hay mites. These alleged perpetrators went to Staten Island forcentrifuging, pellet pulverization and supernatant purification.
This active ingredient presumably harbored the pathogenic prionprotein.
Late in 1994, the team injected it into the brains and peritonealcavities of white mice, and waited to see if the infection would turnthem into mad mice.
Mice don't develop anything like a sheep's scrapie symptoms, Carpsaid. "Their version of the disease has entirely differentsymptomatology. We look at their competency in walking on a non-electrified gridwork. As animals develop the infection, theircompetency in walking decreases. Their feet begin to miss the rungs.
"When a normal mouse misses a rung, he immediately puts his footback up on it, whereas as the disease progresses, an affected animalwill leave the foot down longer and longer. Toward the terminalstage, sometimes all four legs will dangle through the grid, and themouse won't be able to pull them up. In fact, they have developedataxia."
Of 71 mice injected with mite extract, the authors report in TheLancet, ten were clinically positive for scrapie, with the scrapie prionprotein detectable in their brain tissues. And since submitting theirpaper to the journal three months ago, two more mice turned positive.
"It is possible," Carp and his co-authors conclude, "that hay mitesacting as vector and/or reservoir have played a part in the continuingoccurrence of bovine spongiform encephalopathy in the U.K. afterthe ban on the use of sheep and cattle products for cattle food."
Possibly but not positively, Carp emphasized, "It's a preliminaryfinding; we need to repeat it. We're getting more mites in fromIceland, and repeating the basic infectivity finding could take us overa year. We're also looking for the nucleic acid that encodes prionprotein _ the normal version of the scrapie form.
Prion-Deprived Mice Wilder Than Wild Type
Nobody yet knows what that normal prion protein does in themammalian brain. Loss of its function, whatever it may be, appears totrigger the neurodegenerative prion diseases, notably fatal familialinsomnia. (See BioWorld Today, Aug. 10, 1995, p. 1.)
This circumstance prompted sleep researcher Irene Tobler, at theUniversity of Zurich, to wonder whether the proper role of prionprotein is to regulate mammalian sleep patterns.
In two strains of transgenic mice with prion genes knocked out oftheir genome, she put this question to the test. Instead of observingthe animals' footwork on a grid, she measured theirelectroencephalographic (EEG) brain waves and circadian rhythmperformance.
Tobler is first author of a paper in the current Nature, dated April 18,1996, titled: "Altered circadian activity rhythms and sleep in micedevoid of prion protein."
"Any mammalian species that you sleep-deprive," Tobler toldBioWorld Today, "will react with an increase of slow waves in theEEG during recovery. This goes for humans, dogs, cats, any primate,hamster, you name it. And it's true for the mouse as well."
"Our prion-knockout mice," she said, "react more strongly to sleepdeprivation than do wild-type controls. And as a separate finding, Iwould say that their sleep is twice as fragmented as that of the wildtype."
Besides sleep, she clocks the effect of circadian rhythms on prion-null mice. "We measure their activity on the running wheel when theydon't have a synchronizing light-dark cycle. There, wild types runwith a rhythm quite a bit shorter than 24 hours, whereas the null micehave a consistently longer rhythm."
From this, Tobler concluded that "the prion protein may have a kindof stabilizing effect on the circadian rhythm. But we don't know atwhat point the prion protein would be involved in generating orchanging these rhythms."
She sees two ways of looking at the cause of fatal familial insomnia:"One possibility is lost function of the normal protein; the other couldbe the change of the protein to the scrapie version.
"My work," she concluded, "has brought the first possibility closer."n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.