A provocative new paper published in Nature suggests that neurodegenerative disorders such as Alzheimer’s and Parkinson’s may be transmissible through certain medical procedures. It’s an alarming claim—but one that will require more proof if it’s to be accepted by the scientific community.
To understand how the researchers came to this conclusion it’s important to consider the way that Creutzfeldt-Jakob disease (CJD) develops in humans. To date, more than 200 individuals have contracted the neurological disease as a result of treatment with human growth hormone (HGH). Prior to 1985, these patients were administered a growth hormone derived from the pituitary glands of human cadavers, some of which were contaminated with prions—abnormal infectious proteins that can fold in structurally variable ways (today, HGH is synthetic so this is no longer a concern). Prions can cause proteins to unfold in unwieldy ways, including some that result in serious health problems, such as CJD.
Similarly, neuroscientists have learned that Alzheimer’s, Parkinson’s, and motor neurone diseases such as prion disease result from the spread of misfolded proteins that kill brain cells, which causes the brain to shrink. These disorders are driven by the spread of two particular misfolded proteins, namely amyloid plaques and tau. And it’s here where the new study, headed by John Collinge and Sebastian Brandner from the University of College London, comes in.
While performing an autopsy on eight individuals who died between the ages of 36 and 51, and who caught their CJD from contaminated HGH injections, the researchers unexpectedly discovered severe to moderate grey matter and vascular amyloid beta pathology in four of them. This was a surprise because of the relatively young age of the subjects, and because none of these patients had problematic mutations or predispositions to Alzheimer’s.
The researchers suspect that, when these individuals were administered their HGH treatments, the growth hormone was also contaminated with the amyloid beta protein, which then spread through their brains. This would suggest that the “seeds” responsible for certain neurodegenerative diseases can be transmitted during certain medical procedures or via contaminated surgical instruments.
It’s a conclusion that makes sense, and it’s being taken very seriously. The researchers are urging an investigation into other possible routes of prion infection, including blood transfusions and tissue transplantation (this despite the fact that no evidence exists to support such a claim—at least not yet). The concerns of the researchers noted, there are an array of shortcomings to the study.
For one, it’s a purely observational study—one conducted on an excruciatingly small sample of subjects. As The Economist bluntly points out, “this study cannot prove that deposits of amyloid beta were caused by seeds of the protein in contaminated hormone injections.” Moving forward, the researchers will have to determine if old stocks of HGH are indeed contaminated with amyloid proteins (and in fact, that’s exactly what they plan to do).
Again, it’s important to point out that, since 1982, human-derived hormone injections are no longer in use. And just so that we’re exceptionally clear on the matter, people cannot contract Alzheimer’s or Parkinson’s through person-to-person contact.
David Allsop, a professor of Neuroscience at the University of Lancaster, points out some other problems with the study:
I can imagine that this might result in a lot of misleading headlines. What the paper shows is that some people treated with human growth hormone who subsequently went on to develop CJD also show evidence of [amyloid beta] deposits, a key feature of Alzheimer’s disease, in their pituitary glands. What the paper does NOT demonstrate is whether these people would have gone on to develop Alzheimer’s disease had they lived long enough (they died of CJD) or that their pituitary β amyloid deposits were caused by contamination of growth hormone with a ‘rogue’ form of β amyloid. One possible (and indeed likely) explanation is that deposition of the ‘prion protein’ in CJD can result, in some cases, in the co-accumulation of β amyloid.
And indeed, it’s well known from other studies, including animal studies, that certain types of rogue proteins can predispose to accumulation in another.
Allsop concludes thusly:
There is no evidence that Alzheimer’s disease can be transmitted from one person to another, or through use of contaminated surgical instruments, and these results should be interpreted with a great deal of caution.
Read the entire study at Nature: “Evidence for human transmission of amyloid-β pathology and cerebral amyloid angiopathy”.