Resistance to prion disease in humans

prion conversionTransmissible spongiform encephalopathies (TSEs) are rare human neurodegenerative disorders that are caused by infectious proteins called prions. A naturally occurring variant of the human prion has been found that completely protects against the disease.

A protective variant of the prion protein was discovered in the Fore people of Papua New Guinea. Beginning in the early 1900s, the prion disease kuru spread among Fore women and children as a result of ritual cannibalism of the brains of deceased relatives. When cannibalism stopped in the late 1950s, kuru disappeared.

Survivors of the kuru epidemic are heterozygous for a prion protein gene (prnp) with a unique amino acid change not seen in other populations, a change at position 127 from glycine to valine (G127V). The G127V change was always seen together with methionine at 129. Heterozygosity for M and V at amino acid 129, which is protective against prion disease, is found in humans all over the world.

Transgenic mice were used to determine if the G127V change in prion protein protects against disease. These mice lack the murine prnp gene (which encodes the normal prion protein) and contain a copy of either the wild type human prnp gene, or one with changes at amino acids 127 and 129. The mice were then inoculated intracerebrally with brain extracts from individuals who died of kuru. Mice with wild type human prnp were susceptible to infection. In contrast, transgenic mice heterozygous for the variant prnp (G127M129/V127M129) were completely resistant to infection. The mice were also resistant to infection with prions from cases of another human TSE, Creutzfeldt-Jacob disease.

Prnp transgenic mice were also challenged with variant Creutzfeldt-Jacob disease prions. This novel TSE arose after consumption of beef from animals with the prion disease bovine spongiform encephalopathy (BSE). These mice were susceptible to infection with vCJD prions, not a surprising result given that the Fore people were never exposed to BSE prions. However, mice homogygous for the altered prnp (V127M129/V127M129) were completely resistant to infection with vCJD prions – as resistant as mice with no prnp genes.

The protective effect of the M129V polymorphism is thought to be a consequence of inhibition of protein-protein interactions during prion propagation (i.e. the conversion of normal prion to pathogenic prion). How the G127V change confers protection is unknown.

These results show that the G127V change confers resistance to kuru and was likely selected as a consequence of the epidemic. If kuru had not been stopped by the abolition of cannibalism, it likely would have been self-limiting, as individuals with resistance to the disease, caused by the G127V change, repopulated the Fore people.

A mad cow in America

spongiform encephalopathyA dairy cow in California is the fourth known American case of mad cow disease, which is caused by prions, infectious agents composed only of protein (the story hit the press the day after my lecture on this type of illness). Unlike viruses, prions have no nucleic acid and no protective coat. But virologists know all about them because, as Stanley Prusiner once said, there was a time when only virologists believed that they existed.

Prions are found in mammals and in fungi, but only in mammals are they infectious and pathogenic. All mammals make normal forms of the prion protein (PrPc) which is found in many tissues including the nervous system. The pathogenic form, called PrPSc, is a structurally altered form of PrPc. The PrPSc protein, named after the first prion disease studied, scrapie in sheep, causes PrPc to undergo a structural transformation to the pathogenic form. The PrPSc protein becomes deposited in amyloid fibrils in the brain, leading to neurodegenerative diseases known as transmissible spongiform encephalopathies (TSE), after the sponge-like appearance of the brain observed in afflicted animals (image).

There are three different ways to acquire a TSE. One is by infection: a human consumes meat that contains PrPSc, or receives a corneal transplant from a donor with an undiagnosed TSE . The PrPSc proteins make their way to the brain where they cause the host’s PrPc to misfold and become the pathogenic PrPSc. The more PrPSc that is made, the more the normal PrPc is converted to the pathogenic form. After an incubation period of many years, the host develops an invariably fatal neurodegenerative disease characterized by dementia in humans. There is also a familial form, in which mutations in the gene encoding PrPc are inherited; these cause the PrPc protein to misfold to form the pathogenic form. In the sporadic form PrPc spontaneously converts to PrPSc without any known mutation or infection.

TSEs occur in different forms with varied symptoms and pathology. There are TSEs of humans (Creutzfeld-Jacob disease, fatal familial insomnia, Gerstmann-­Sträussler syndrome, Kuru) cows (bovine spongiform encephalopathy or mad cow disease), sheep and goats (scrapie), deer, elk, and moose (chronic wasting disease), and of a variety of other mammals.

This brings us back to the mad American cow, the first in the US since 2006. It died on a dairy farm and was tested for BSE as are 40,00o other cows each year in this country. The reason why this is big news is that back in the 1990s there was an outbreak of human TSE in the United Kingdom caused by consuming beef from animals with BSE. The cows acquired BSE by being fed processed animal byproducts as protein supplements, which unknowingly contained pathogenic prions. Bt the time the disease was detected in cows, contaminated meat had already entered the human food chain. Cows are routinely tested for BSE precisely to avoid a similar outbreak of human TSE.

The dead cow apparently had atypical BSE – that is, it was not a consequence of eating contaminated meat and it was not an inherited disease. Atypical BSE is caused by strains of prions distinct from other forms. This is good news because it means that the feed that the cow was receiving was not contaminated with pathogenic prions. Furthermore, the cow was not destined for meat production; it was a dairy cow that had died and was selected for random sampling.

Could the milk produced by this cow and consumed by humans pose a risk for transmission of a TSE to humans? It is known that ewes with scrapie shed infectious and pathogenic prions in their milk. However cows with BSE have  much less PrPSc accumulation in peripheral tissues, and in particular lymphoid tissues which include the mammary glands. It seems unlikely that cow milk contains prions, but it is a question worth revisiting. Pathogenic prions are highly resistant to heat, ultraviolet irradiation and other extreme conditions, so would certainly survive the pasteurization process.

D. Carleton Gajdusek, 85

Virologist D. Carleton Gajdusek, who was awarded the 1976 Nobel Prize in medicine for unraveling the nature of the prion disease Kuru, has died, as reported by the New York Times.

Gajdusek’s work on Kuru, a fatal encephalopathy found in the Fore people of New Guinea, proved that human transmissible spongiform encephalopathies (TSEs) could be transmitted among humans. This group of rare, slow diseases now includes Creutzfeldt-Jakob disease, Gerstmann-Straussler syndrome in humans, as well as bovine spongiform encephalopathy (BSE, or mad cow disease) and scrapie in sheep and goats. These disorders are rare, but always fatal. Gajdusek’s contribution was to understand that the fatal encephalopathy affecting the Fore people was spread among women and children by ritual cannibalism of the brains of deceased relatives. The disease stopped when cannibalism was discontinued. 

Subsequent work by others showed that the infectious agents of TSEs are highly unconventional because they lack nucleic acid genomes. Stanley Prusiner called the agent of scrapie a ‘prion’ and suggested that an misfolded form of the protein causes the fatal encephalopathy. The genomes of many animals, including humans, carry a gene called prnp (encoding the PrP protein) which is essential for the pathogenesis of TSEs. In the simplest case, the misfolded PrP protein (PrPsc) converts normal PrP protein to more copies of the pathogenic form. The altered protein may be acquired by infection, by inheritance of the prnp gene with an autosomal mutation, or sporadically. It is believed that Kuru was established in the small Fore population when the brain of an individual with sporadic CJD was eaten.

I met Dr. Gajdusek briefly in the 1980s when he presented a seminar at Columbia. We were in a room alone for about 5 minutes, awaiting the arrival of other participants in the post-seminar dinner. In that short time it became clear that he was extremely intelligent, but not at all interested in my work on poliovirus, much to my disappointment.