In the human form of mad cow disease, called Creutzfeldt-Jakob, a person's brain deteriorates—literally developing holes that cause rapidly progressing dementia. The condition is fatal within one year in 90 percent of cases. The culprits behind the disease are prions—misfolded proteins that can induce normal proteins around them to also misfold and accumulate. Scientists have known that these self-propagating, pathological proteins cause some rare brain disorders, such as kuru in Papua New Guinea. But growing evidence suggests that prions are at play in many, if not all, neurodegenerative disorders, including Alzheimer's, Huntington's and Parkinson's, also marked by aggregations of malformed proteins.
Until recently, there was no evidence that the abnormal proteins found in people who suffer from these well-known diseases could be transmitted directly from person to person. The tenor of that discussion suddenly changed this September when newly published research in the journal Nature provided the first hint such human-to-human transmission may be possible. (Scientific American is part of Springer Nature.)
For the study, John Collinge, a neurologist at University College London, and his colleagues conducted autopsies on eight patients who died between the ages of 36 and 51 from Creutzfeldt-Jakob. All the subjects had acquired the disease after treatment with growth hormone later found to be contaminated with prions. The surprise came when the researchers discovered that six of the brains also bore telltale signs of Alzheimer's—in the form of clumps of beta-amyloid proteins, diagnostic for the disease—even though the patients should have been too young to exhibit such symptoms.
These observations suggest that the tainted hormone injections might have carried small amounts of beta-amyloid proteins that triggered the formation of more such proteins. Neither Alzheimer's nor any known human prion diseases are contagious through direct contact. Yet human transmission of prion diseases has occurred through certain medical procedures and, in the case of kuru, cannibalism. The new study therefore raises the possibility that Alzheimer's is a transmissible disease with an etiology akin to prion diseases.
The new finding is provocative, but experts advise caution in interpreting the results. For instance, neuroscientist John Trojanowski of the University of Pennsylvania points to the small size of the study and lack of direct evidence for transmission in support of causality. But if it is eventually shown that Alzheimer's and other neurodegenerative diseases indeed share the same basic pathological pathway and mechanism, treatments could target one and all.
“Transmission may occur in only a small percentage of human cases,” says Claudio Soto, a professor of neurology at the University of Texas Health Science Center at Houston. “But the underlying principle is the most important thing that could lead to new opportunities for therapeutic interventions and diagnostics.” Investigators such as Soto and Collinge are working on ways to detect in body fluids the presence of small clumps of the transmissible proteins now thought to be involved in Alzheimer's and other neurodegenerative diseases, which could represent a diagnostic advance.
Such detection will likely be difficult. A study published online in September in the journal Nature Neuroscience by Mathias Jucker of the University of Tübingen in Germany and his colleagues required extremely sensitive methods to find minuscule clumps of beta-amyloid proteins, referred to as seeds, in mice brains. These seeds appear to be able to regain pathological properties even after six months of lying dormant. These possibly prionlike proteins might therefore exist in the brain long before symptoms develop, at levels too low to be found by routine tests.
One potentially prionlike protein may cause several diseases, according to a study published this summer by Nobel laureate Stanley Prusiner, who discovered prions in the 1980s. Prusiner and his colleagues found that a “strain” of alpha-synuclein—the misfolded protein involved in Parkinson's—can cause a similar but rare neurodegenerative disease, called multiple-system atrophy. Understanding how variants of these disease-causing proteins differ in shape and how the particular configuration influences their pathogenic nature is destined to become a focus of future research. “There's evidence that both prions and beta-amyloid exist as different strains and have very different biological effects,” says Lary C. Walker of Emory University, who was involved in the Nature Neuroscience study. “I think understanding this will give us insight into what's happening in disease.”
As the evidence increases, more scientists now suspect that prionlike processes probably underlie all neurodegenerative disorders. Prusiner expected the current change in thinking: in his 1997 Nobel Prize lecture, he predicted that the understanding of prion formation could “open new approaches to deciphering the causes of and to developing effective therapies for the more common neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS).”