SUMMER 1997, Vol 6, No. 3

Mad Cow Disease
and Prion Proteins:
New Challenges in Brain Infections

Almost 50 years ago, while traveling in Papua, New Guinea, a young American physician, Dr. Carleton Gajdusek, was asked to evaluate a most unusual problem. Upon reaching early middle age, female members of a local aboriginal tribe developed progressive unsteadiness and shaking of the limbs, then died of relentless neurological deterioration. Neither the cause of this disease (called kuru, the tribe's word for "shaking") nor a treatment were known.

Dr. Gajdusek embarked on a remarkable series of experiments which led, more than a decade later, to the cause of kuru and ultimately to its eradication. He was awarded the Nobel Prize in Medicine in 1976. His studies, undertaken more than 40 years ago, paved the way to understanding a contemporary, frightening public health problem often described as "mad cow disease."

The story of these two diseases and the research that relates them is one of the most remarkable medical narratives of the latter half of this century. The culmination of these studies is the discovery of a novel type of infectious particle: the "prion," an entity completely unlike any known infectious agent.

The answer to kuru came from two lines of investigation: analysis of tribal culture among the Fore tribe of aboriginals in New Guinea, and careful, long-term laboratory experimentation. One of the most striking tribal customs of the Fore was cannibalism. It was traditional for women to eat the brains of the deceased as a ritual of respect. Dr. Gajdusek and others speculated that this custom might, in some manner, transmit an infectious agent triggering brain deterioration.

(Adapted by Leigh Coriale Design and Illustration, with permission, Science [July 12], 1996, American Association for the Advancement of Science.)

Under the microscope, the brains of the kuru patients and injected chimps both showed many "vacuoles" or small holes. Perhaps the most compelling support for Dr. Gajdusek's hypothesis is the fact that almost no children born following the elimination of cannibalism among the Fore people (in 1959) developed kuru, ruling out heredity and suggesting that the disease is infectious.

The pathological findings of kuru resembled distinctive changes in brain structure seen in at least two other disorders: an aggressive, lethal, dementing illness in adults known as "Creutzfeld-Jacob disease" (CJD), and a brain sickness in sheep called "scrapie," known in Great Britain since the first half of the 18th century. All of these conditions are associated with a similar vacuolar brain degeneration commonly referred to as "spongiform encephalopathies." The name scrapie comes from the tendency of affected sheep to scrape against fences and trees, presumably because of itching. The sheep are stripped bare in spots, their limbs shake, they lose balance, and neurological deterioration is inexorable. That scrapie was infectious had been recognized for years. These features suggested that scrapie, Creutzfeld-Jacob disease, and kuru might well share a common, infectious mechanism.

In the early 1980's, a research team headed by Dr. Stanley Prusiner at the University of California in San Francisco obtained the first evidence that the agent that caused scrapie was a protein. This astonishing hypothesis ran counter to dogma in that nucleic acids (DNA and RNA), not proteins, were thought to be essential for "replication" of all known viruses and bacteria. Combining the concepts of protein and infection, Prusiner coined the evocative term "prion" to describe the offending agent.

The suggestion that an infectious agent might be composed purely of protein was heretical, but a large body of data has accumulated to support this view.

The abnormal (scrapie) protein is called PrP(SC); it is an altered form of a protein, PrP(C) , that is present in normal brain cells. The infectious PrP(SC) arises from PrP(C) because of mutations that alter the amino acid sequence and the three-dimensional shape of PrP(C). It is thought that PrP(SC) binds to PrP(C) and captures more and more protein molecules in large aggregates or clots called "amyloid". This type of amyloid is toxic to neighboring nerve cells.

The amyloid of prion disease looks like the amyloid of Alzheimer's disease when examined by electron microscopy, but the resulting brain pathologies are quite different. Prion-induced spongiform changes are not seen in brains of patients with Alzheimer's disease.

In recent years, a number of mutations have been discovered in the PrP gene. In humans, these mutations cause four different diseases: kuru, Creutzfeld-Jacob disease (CJD), fatal familial insomnia, and Gerstmann-Straussler-Scheinker (GSS) disease. As noted, CJD patients die with profound dementia, often with dramatic uncontrolled twitching of the arms and legs. Most individuals with CJD first become ill in mid-life (typically at age 60 or so).

By contrast, fatal familial insomnia usually strikes young adults, often as a sleep disorder with changes in mentation, hallucinations, stupor, and perhaps even coma. There may also be marked incoordination and spontaneous, rapid muscle twitches (myoclonus). Insomnia may be due to involvement of a large group of nerve cells deep within the cerebral hemispheres called the thalamus, which probably regulates the balance between consciousness and sleep.

In GSS, individuals who inherit a mutant form of PrP develop memory loss and limb clumsiness, also as young adults. This progresses to involve limb rigidity, slowing of movements and sometimes tremor. The latter is suggestive of Parkinson's disease and, indeed, GSS patients may respond to anti-Parkinson medications.

Animals models of prion disease have been established in primates, goats, cats and rodents after inoculation of PrP(SC) protein directly into the brain. In recent years, no research tool has been more important to molecular analyses of disease than genetically altered mice.

At about the time when these properties of PrP were identified, a new chapter began in the history of prion-related diseases. In 1986, an epidemic of a new disease in cattle broke out in England. This disease, characterized by the development of confusion, anxiety and weakness, came to be called "mad cow disease." It was quickly recognized that the brains of these cattle develop spongy brain deterioration. Accordingly, this disorder was designated "bovine spongiform encephalopathy" or BSE.

In reviewing the histories of the BSE cattle, it became clear that most had been fed the internal organs of infected scrapie sheep, prepared as a "meat and bone meal" (MBM) food supplement. Thus, it seemed highly likely that BSE was a prion disease that had "jumped" species from sheep to cows through an oral route. Moreover, until 1988, MBM obtained from cattle was also used as a food supplement for cattle, raising the possibility that once it started in cows, BSE may have been propagated among cows by this route.

At this time, more than 160,000 British cattle have been slaughtered because of proven or suspect BSE. While it is not yet clear how many British cattle will develop the illness, some estimates are as high as one million.

Emergence of the BSE epidemic has prompted many safety concerns. Perhaps most important, if the infectious prion was able to cross a species barrier from sheep to cows, can it move from cows to humans? Is there a risk to humans who eat beef from herds that have had cows affected with BSE? Is there a risk to consuming other products of cattle in prion-affected herds? Moreover, many repackaged bovine derivatives (e.g. gelatin) are used in a wide range of products, including mints and other candies, lipstick, and shampoo. These questions prompted some 15 countries to ban the importation of British beef.

To date, there has been no proven instance of transmission of prion disease from animals to humans. However, the fear of a human epidemic related to BSE was fueled in 1995 and 1996 when it was reported that 11 relatively young individuals in Britain succumbed to a progressive, dementing illness that resembled an early-onset form of CJD. At autopsy, these brains showed evidence of spongiform encephalopathy and molecular studies indicated that they were indeed prion diseases.

Further and most significantly, the prion proteins isolated from these atypical CJD brains were structurally related to those found in BSE. Reports of these cases in the scientific and popular press occasioned an outburst of concern among British citizens, tourists in Britain, and farmers in France and elsewhere who urged that all of the British cattle herds should be destroyed to prevent further cases of BSE in humans. Underscoring these questions was apprehension about whether the British government had been aggressive enough in investigating the problem of BSE and its public health risks.

As of spring, 1997, it remained unclear whether there will be other cases of early adult-onset, atypical CJD in Great Britain. Thus, from an epidemiologic and public heath perspective, it will be extremely important to continue to monitor the status of BSE and new variant CJD. From medical and scientific perspectives, such vigilance is amply warranted, as are continued basic investigations both of prions and their proliferation within the brain and their mechanism of neurotoxicity. Ultimately, such research will tell whether the BSE - CJD relationship is a fluke coincidence or the first glimpse of a major public health problem.

By understanding the mechanisms of transmission of the prion protein and interaction between PrP(SC) and PrP(C), we will be able to determine whether other diseases, perhaps neurodegenerative diseases, arise either from defective protein or protein interactions that take on a slowly propagating, deadly quality. This should facilitate the establishment of new diagnostic tools for these proteins and new standards for safety in prophylaxis of these diseases, hopefully making organ transplantation and surgical procedures more safe. In the long-term, the true fruit of such research will be new strategies to arrest and perhaps even reverse the prion diseases once they are initiated. *

Dr. Brown is Associate in Neurology at Massachusetts General Hospital and Assistant Professor at Harvard Medical School; Dr. Fischbach is Chairman, Department of Neurobiology, at Harvard Medical School and President of the Harvard Mahoney Neuroscience Institute.