In September 2015, a study published in the journal Nature significantly stirred the scientific community and made people around the world understandably alarmed. The research suggested that Alzheimer’s disease might be transmittable through injections of the disease-driving protein amyloid-β.
Researchers all over the world now race to further investigate these claims, but meanwhile, the debate rages on. A feature, published in the same journal last week, discussed what the issue is really about.
The study that made headlines last year had identified amyloid plaques in brain of patients that had died of the prion-related Creutzfeldt-Jakob disease. These people caught the deadly illness when they received human growth hormone from diseased individuals, making John Collinge at The National Hospital for Neurology and Neurosurgery in London, one of the senior authors of the study, suspect that seeds of amyloid-β protein could also have been passed on with the human hormone preparations.
While the work did not imply that Alzheimer’s could be caught like a cold from sick relatives — it is not a contagious disease — it would make blood transfusions, organ donations, and surgical procedures high-risk activities in terms of transmission.
So how, exactly, do researchers think that Alzheimer’s might be transmittable? The amyloid-β plaques – as well as other types of protein aggregates in Parkinson’s and Huntington’s diseases – are made up of misfolded proteins, collectively called amyloids. The past decade has seen accumulating evidence that when misfolded amyloid-β comes into contact with normal protein, they can induce changes in their healthy counterparts, making them misfold and misbehave, much like prions do.
Ten years ago, German scientists showed that if you inject brain extracts containing misfolded amyloid-β into the brain, or even muscle, of healthy mice, it triggers the accumulation of misfolded protein and plaque formation. At the time, that seemed a highly unlikely scenario in humans since researchers could not imagine how the protein would be transmitted from one person to another.
The observations of amyloid-β plaques in the brain from Creutzfeldt-Jakob patients, which have since been observed in other patients with the disease, provide only circumstantial evidence that the seeds of amyloid-β were actually transmitted during treatment. At this time it is not possible to rule out that either the treatment itself or the presence of prion disease caused the changes.
Researchers the world over are currently re-analyzing brain specimens from Creutzfeldt-Jakob patients, and while conclusions have not yet been published, scientists report preliminary confirmatory findings. More convincing evidence would be provided if the original growth hormone or tissue samples that were transplanted into the brains of these patients would hold infectious amyloid seed. Most of these original samples are long gone, but Collinge is now examining the few still remaining for the presence of amyloid-β seeds. If found, they plan to inject them into mice.
This might turn out to be an arduous task since no one really knows what amyloid-β seeds might look like. Mathias Jucker at the University of Tübingen in Germany, who performed the initial experiments with injected amyloid-β in mice, is now combinig through surgically acquired brain samples from 700 epileptic patients for anything that might look like tiny clusters of amyloid-β. He also has access to medical records of the patients, indicating if they had cognitive problems before or after the surgery, allowing him to link any potential findings of amyloid seeds with symptoms.
Some argue that if Alzheimer’s disease was really transmittable via transplanted tissue or surgical procedures, epidemiological studies would have spotted it long ago. But Roy Anderson at Imperial College London told Nature that studies able to identify such a connection have not yet been performed. The lack of very large and well-curated databases containing information about symptom development and autopsy data has prevented such research, but the availability of high-quality databases is on the rise, and future epidemiological studies might contribute answers to this topic.
In the midst of these rather sparse research findings, researchers are divided into two camps. Some maintain there is no conclusive evidence that amyloid seeds can spread disease much in the same way prions do. These scientists claim that the recently published results likely have other biological explanations. Others are more cautious, stating that all amyloids should be considered dangerous until proven safe.
And the danger might be real also to the researchers working with the proteins since amyloids stick to surgical instruments and are not removed by standard sterilization techniques.
Adriano Aguzzi at University Hospital Zurich in Switzerland is among those who are concerned, stating in the Nature feature “that funding agencies should put out calls now to researchers to develop cheap and simple sterilization methods.”
Still, much of the ongoing discussions are contained to semantics surrounding the prion comparison; some contend that comparing amyloid-β to prions sets off ungrounded ideas and fears.
Brad Hyman at Harvard Medical School in Boston, Massachusetts, told Nature of one such misinterpretation: “One of my patients told me that she wasn’t getting any hugs any more from her husband who had read about the case in the media – that made me sad,” he said.
And while both camps do not want to raise an alarm of transmittable amyloids before more is known, the scientists who are open to the idea of transmittable Alzheimer’s believe that considering the similarities between prions and amyloids might help to answer research questions about the foundations of this devastating disease.