Hi everyone! I’m Beena, a first-year PhD student at the MRC Prion Unit at UCL. Whilst my research focuses on Alzheimer’s disease (AD), it is also heavily influenced by prion diseases. I’m sure most of you reading this have probably heard of the BSE cattle outbreak that occurred between 1986 and 2000. Over 190,000 animals were killed because of a rare degenerative disease commonly known as mad cow disease. A common symptom of this disease in cows is incoordination. Even though it usually takes four to six years from the time of infection to first show symptoms, the infected animals succumb to the disease very quickly and usually die within two weeks to six months.
We now know that this “contagious” neurodegenerative disease is caused Prions; an abnormal variant of a prion found on the surface of every neurone in the mammalian brain. Unlike viruses which inject its DNA into cells to replicate, rogue prions can transform normal prion proteins by altering their shape. This allows misfolded prions to spread through the brain relatively quickly and leads to eventual neuronal cell death and brain tissue loss.
Whilst the chances of contracting a prion disease is rarer than catching a virus, it’s important to remember that almost 200 people in the UK disease of a prion disease called variant Creutzfeldt Jakob disease (vCJD) after eating beef that was contaminated with cattle prions (BSE beef). Prion diseases can also be inherited as is the case of Fatal Familial Insomnia or can randomly (e.g. sporadic CJD). Whilst inherited prion disease as exceptionally rare, sporadic prion disease account for 85% of all documented cases.
This statistic is strikingly similar to the percentage of people that developed sporadic AD. We associate AD with memory loss and personality changes. It affects millions of people around the globe but mostly in the later stages of life. Both AD and prion disease are characterised by the misfolding of protein aggregates in the brain at post-mortem (misfolded amyloid-beta and tau in the cerebrum at AD patients). It is hypothesised that because both AD and CJD are caused by misfolded versions of normal brain proteins, they might share properties with prions such as the ability to “seed” in the brain. Whilst results from mouse studies have shown that both rogue proteins can induce pathology following injection, data from patients who died from iatrogenic CJD following injection of contaminated growth hormone has led to an interesting hypothesis. Specifically, the amyloid plaques that were present in these patients may have been because the growth hormone may have been contaminated with amyloid seeds.
However, it is important to remember that Alzheimer’s disease was not transmitted in the mouse models and there is a cohort of healthy older people who die with high levels of amyloid and show no signs of AD. Overall, AD still cannot be described as an infectious disease like prion disease but studies have hinted that amyloid pathology could be seeded in humans under exceptionally rare conditions.