By David N. Leff

Science Editor

As of last week's news, France and Britain remain at loggerheads over the continuing French ban on British beef. This shutout of cattle from the United Kingdom goes back to the onset of bovine spongiform encephalopathy (BSE) - better known as "Mad Cow Disease." More than 175,000 cattle, primarily dairy cows, have died of this prion-inflicted disorder over the past decade, and untold numbers of animals have been put down preventively, in efforts to halt the spread of BSE.

How many more cattle were exposed to BSE prions, but slaughtered before developing clinical signs, is uncertain.

Meanwhile, the number of human victims of prion-caused Creutzfeldt-Jakob disease (CJD) has jumped alarmingly in Britain, from a scant 13 deaths two years ago to half a hundred cases now - most of them teen-agers and young adults. Before the disorder emerged in cows, CJD was a rare, one-in-a-million affliction, which took years to strike down its victims. Latterly, death comes in a few months to what is recognized as an atypical "new variant"- nvCJD. (See BioWorld Today, March 19, 1999, p. 1.)

This apparent bovine-human contagion proved hard to prove; most persons who succumbed had never been near dairy animals, and several were vegetarians. So epidemiologists - and the public - entertained a "species-barrier" hypothesis. This held that CJD could not be communicated from beast to man. The term "species-barrier" refers to the relative lack of susceptibility of one species to prions derived from another species. That comforting belief has just suffered a chilling blow, with publication today of a report in the Proceedings of the National Academy of Sciences (PNAS), dated Dec. 21, 1999. Its title lays it on the line: "Compelling transgenetic evidence for transmission of bovine spongiform encephalopathy prions to humans."

The paper's co-senior authors are neurologist Stanley Prusiner and neuropathologist Stephen DeArmond, both at the University of California, San Francisco. Prusiner won a Nobel prize in 1997 for discovering that the neurodegenerative diseases known as spongiform encephalopathies are caused by ubiquitous infectious prions (proteinaceous infectious particles). These weird particles, far smaller than viruses, contain no DNA, and, according to Prusiner, acquire their pathogenicity by altering the shape of their structures, by mutation or inheritance.

Transgenic Mice Precisely Mimic BSE Hallmarks

To establish that the particular strain of prions responsible for BSE is the same strain that causes nvCJD, the co-authors created transgenic mice as intermediaries between cows and people. They engineered these animals to carry genes expressing the bovine prion protein, which is not in itself lethal. Then they injected the mice with prions from diseased cows. Of 73 animals inoculated, 72 came down 250 or so days later with what might be called mad-mouse disease. That is, neurologically, symptomatically and in typical BSE-like brain perforations, they had contracted the infection.

Those Swiss cheese-like cerebral holes clustered primarily in the brainstem, periaqueductal gray and habenula. "That distribution in rodents," DeArmond observed, "is highly similar to BSE."

Next, the co-authors inoculated another group of transgenic mice with prions from the initially infected animals. Sure enough, 250 days later, all of those developed the neurological disorder. This second passage confirmed that the animals transmitted mad cow disease prions, with no species-specific properties attributable to the mice themselves.

Finally, the team switched from bovine to human prions, using brain tissues from victims dead of nvCJD, supplied by Britain's National CJD Surveillance Unit in Edinburgh, Scotland (of which two researchers are co-authors of the PNAS paper). Of 24 transgenic mice inoculated with prions from these human autopsies, 23 had the same incubation period and pattern of brain damage as did the animals that received BSE prions. The co-authors cite as one neuropathological difference the presence of prion-immunopositive amyloid plaques in the subcallosal brain region.

"BSE and new-variant CJD produce the identical pattern of disease in transgenic mice," DeArmond said. "The characteristics were different than those found with inocula from other CJD cases, or scrapie from sheep. These findings argue unequivocally that BSE and new-variant CJD are the same strain of prion." (Scrapie is a prion disease of sheep and goats.)

Before these transgenic mice came on the scene, the only sure-fire way to determine if a live cow harbored BSE was to wait out its incubation period - four years minimum. To short-circuit this delay, scientists inject cattle tissue into wild-type, normal mice. But these non-transgenic animals take nearly a year - if at all - to develop signs of the disease. Hence, DeArmond pointed out, "The newly developed, transgenic mouse model should provide a sensitive and timely test for detecting BSE prions."

Mice Make Best Mad Cow Model - For Now

"In the future, "the PNAS paper suggested, "it may prove possible to create transgenic mice that express modified bovine prions, eliminate the species barrier, and provide even shorter incubation periods. However, until such mice are available, transgenic bovine-prion mice represent the best bovine model for BSE."

The PNAS article closed on a scarier note: "Although earlier proposals of an etiological link between BSE and nvCJD were disquieting, the investigations reported here raise greater concern that a large section of the United Kingdom population may be at considerable risk."

What about the U.S. population?

To protect public health, the FDA in 1997 banned the use of most mammalian protein in the production of animal feeds for cows, sheep and goats. And companies that produce drugs from animal tissues must conduct a purification process designed to destroy prions. These moves followed similar regulations adopted by Great Britain in 1989.

However, the FDA ban excludes processed products produced for human consumption from collagen, blood and gelatin. The PNAS paper's co-authors are concerned that, under some unusual circumstances, these materials might harbor prions. Moreover, that the technique for screening cattle products is not sensitive enough.