Seeds of Alzheimer’s disease can potentially be transferred from one person to another by certain medical procedures that could include dental treatments, a study unveiled in Bradford suggests.
While not proving that dementia can be “caught”, the explosive findings provide the first evidence of its transmission in humans via microscopic protein molecules.
Blood donations are not considered a meaningful risk, but should be investigated as a precaution, say the researchers.
British scientists stumbled on the discovery while investigating a rare form of “iatrogenic” Creutzfeldt Jakob Disease (iCJD), a brain-destroying condition known to be spread by contaminated surgical instruments and procedures.
They inspected the brains of eight patients who died from the disease after receiving pituitary growth hormone extracted from cadavers.
Q&A: How Alzheimer’s is transmitted - what we know now:
Unexpectedly, six bore a clear molecular hallmark of Alzheimer’s - sticky clumps of fragmented protein called amyloid beta that form among neurons and on the walls of blood vessels. In four cases, the amyloid deposits were widespread and only one patient was not affected at all.
All eight individuals were relatively young, aged 36 to 51, and none had genetic variants associated with early-onset Alzheimer’s disease.
The fact that each of these patients was a growth hormone recipient cannot be a coincidence, scientists believe. The evidence points to the hormone carrying the “seeds” of Alzheimer’s pathology as well as iCJD.
Since the prion proteins responsible for iCJD can be transmitted in other ways - for instance, by neurosurgery - experts fear the same could also be true for the Alzheimer’s molecule.
Lead scientist Professor John Collinge, director of the Medical Research Council Prion Unit at University College London, said there was increasing evidence that neurodegenerative diseases including Alzheimer’s might, in rare circumstances, be “acquired”.
Speaking on a phone-link to the British Science Festival taking place at the University of Bradford, he said: “What we need to consider is that in addition to there being sporadic Alzheimer’s disease and inherited or familial Alzheimer’s disease, there could also be acquired forms of Alzheimer’s disease - by analogy to what we’ve seen for many years with CJD.
“You could have three different ways you have these protein seeds generated in your brain. Either they happen spontaneously, an unlucky event as you age, or you have got a faulty gene, or you’ve been exposed to a medical accident. That’s what we’re hypothesising.”
Writing in the journal Nature, the scientists pointed out that, like CJD prions, amyloid beta protein fragments stuck to metal surfaces and resisted conventional sterilisation.
Previous experiments on laboratory mice and monkeys had already shown that transmission of the Alzheimer’s protein is at least theoretically possible.
When liquified brain tissue from deceased Alzheimer’s patients was injected into the central nervous systems of the animals, they developed the brain changes associated with the disease.
The scientists do not believe CJD prions somehow triggered the development of amyloid beta deposits in the brains of the growth hormone patients. But they say the process of extracting pituitary growth hormone might “purify” both the prions and amyloid beta particles.
Brains of 116 patients with prion diseases who had not received pituitary growth hormone did not have the Alzheimer’s hallmark.
However, other research showed that amyloid beta could form in the pituitary gland, a pea-sized body at the base of the brain.
The patients’ brains did not show another key change associated with Alzheimer’s, twisted strands of protein within nerve cells called tau tangles. This could be because their lives were cut short before this hallmark of the disease had developed, said the scientists.
Treatment of people of short stature with pituitary growth hormone taken from dead donors began in the UK in 1958. It was stopped in 1985 after confirmed reports of CJD among recipients.
Because growth hormone from different cadavers was mixed up before being distributed, a treatment given to an individual patient may have originated from a large number of donors. This greatly increased the risk of disease transmission.
In total, 1,848 men and women in the UK underwent the procedure for stunted growth, and of these 77 have so far died from CJD.
As of 2012, 450 cases of iatrogenic CJD have been identified around the world linked to cadaver-derived growth hormone and to a lesser extent other medical procedures, including corneal transplants and neurosurgery.
Alzheimer’s protein seeds could follow similar transmission pathways, Prof Collinge believes. Questioned specifically about dentistry, he said: “The seeds will potentially stick to metal surfaces whatever the instrument is. With prions, we know quite a lot about that. Certainly, there are potential risks with dentistry where it’s impacting on nervous tissue, for example root canal treatments.
“If you are speculating that amyloid beta seeds might be transferred by instruments, one would have to consider whether certain types of dental procedure might be relevant.”
He stressed there was no epidemiological evidence at all to suggest that Alzheimer’s could be transmitted via blood transfusions, but added: “I think it’s not unreasonable to have a look. My concerns would be more to see if there is a risk of seeding from metal surfaces. I think that is something we ought to prioritise.”
Meanwhile, he urged people not to be concerned about planned medical procedures, and to dismiss any notion of Alzheimer’s being “contagious” in the same way as flu, for instance.
“No way is this suggesting that Alzheimer’s is a contagious disease,” he said. “You can’t catch it by living with someone who has Alzheimer’s disease or being a carer.
“I don’t want to cause any alarm. No-one should consider cancelling or delaying any kind of surgery. But I think it would be prudent to do some research in this area going forward.”
Dr Eric Karran, chief scientist at the charity Alzheimer’s Research UK, said: “Previous research has suggested that the amyloid protein may behave in a similar way to the prion protein responsible for CJD, but this study provides evidence that amyloid could also be passed between humans through contaminated brain tissue.
“Current measures in place to limit contamination with the prion protein and minimise CJD risk from hospital procedures are very rigorous and the risk of developing CJD from surgical contamination is extremely low.
“The biggest risk factor Alzheimer’s is age, along with genetic and lifestyle factors. If further research was to confirm a link between historical tissue contamination and Alzheimer’s, it would only likely be relevant to a tiny proportion of the total number of people affected.”
Dr Doug Brown, director of research at Alzheimer’s Society, said: “While these findings are interesting and warrant further investigation, there are too many unknowns in this small, observational study of eight brains to draw any conclusions about whether Alzheimer’s disease can be transmitted this way.
“Notably, while seven of the eight brains studied had beta-amyloid deposits - a protein found in the brains of people with Alzheimer’s disease - the presence of this alone does not mean that they would have gone on to develop the disease.
“Injections of growth hormone taken from human brains were stopped in the 1980s. There remains absolutely no evidence that Alzheimer’s disease is contagious or can be transmitted from person to person via any current medical procedures.”
Neuroscientist Professor John Hardy, from University College London, said: “With the previous mouse data, I think we can be relatively sure that it is possible to transmit amyloid pathology by the injection of human tissues which contain the amyloid of Alzheimer’s disease. Does it have implications for (for example), blood transfusions: probably not, but this definitely deserves systematic epidemiological investigation. Does it suggest Alzheimer’s disease is infectious through contact? Almost certainly not.”