Between 1901 and 1906, Alois Alzheimer began collecting data on the disease that would eventually bear his name. A patient with memory deficiency was autopsied after her death and her brain was found to contain amyloid plaques and neurofibrillary tangles. Around a half century prior in 1861, Guillaume-Benjamin-Amand Duchenne had described a disease that would bear his name, a form of muscular dystrophy, and like Alzheimer he had patient samples for study. In the next century and more both diseases would be studied and reported on, Duschenne Muscular Dystrophy was eventually linked to a single protein called dystrophin, and a number of FDA-approved treatments exist which target dystrophin and improve patient outcomes. Alzheimer’s disease was also linked to a protein, the amyloid plaques found by Alois contained a protein called amyloid beta. But while both diseases seem to have known causes, treatments for Alzheimer’s disease remain ineffective. What’s more, there is a growing body of evidence that the amyloid beta hypothesis for Alzheimer’s disease is on shaky ground. How is it that more than a century of study has not allowed us to even understand Alzheimer’s disease?
First, it must be said that the amyloid beta (Aβ) hypothesis for Alzheimer’s Disease (AD) didn’t come out of nowhere. Not only were the amyloid plaques found in Alzheimer’s patients coming from Aβ, but genetic evidence showed that the mutations associated with AD all seemed to affect the Aβ pathway. If the diagnostic criteria for AD included Aβ, and genetic evidence supported a role for Aβ, it seemed Aβ must surely be the cause of the disease. And further biochemical evidence supported a role for Aβ, for example when Aβ was shown to cause neuronal cell death in cultured nerve cells. The Aβ hypothesis even connects well with other diseases, Aβ acts as an aggregating prion and aggregating prions are known to cause other neurodegenerative diseases such as Creutzfeldt-Jakob Disease and Kuru. Note that some biochemists say a protein is only a prion if it comes from the prion gene of the human body, but like champagne this definition is expanding. So the Aβ hypothesis isn’t a hypothesis without support, it has strong biochemical evidence at the genomic and proteomic level, and fits in well with other brain diseases. It can certainly be said that Aβ proponents have ignored or downplayed evidence against the Aβ hypothesis, but that behavior is common in all disciplines. Science advances one funeral at a time.
Second, it should be recognized that AD is a difficult disease to study involving a difficult organ to study. AD affects memory and behavior by affecting the brain, those are processes and an organ that are still very opaque to us in general let alone in the context of AD. So AD is a disease we don’t understand affecting processes we don’t understand in an organ we don’t understand. Maybe we should feel grateful we even have drug candidates to begin with?
To bring this back to my own work, let me give you an example of the very small problem I am working on and the difficulties I am facing in getting data. We have a theory that there are different subtypes of AD. There is the rapid-onset (r-AD) subtype and the slow-onset or traditional (t-AD) subtype. We believe that this difference may be structural in nature, that the proteins causing r-AD and t-AD are the same but that they have different shapes. To this end, I am studying the structural variations of sarkosyl-insoluble proteins from AD patients.
OK what does that mean? I start by requesting patient samples from deceased AD patients matching either the r-AD or t-AD subtype. This is difficult because not everyone really agrees on the diagnostic criteria of these two subtypes (already we have problems!). Then once I have a patient sample, I perform a sarkosyl extraction. Sarkosyl is just a detergent like the one you wash your clothes with. A detergent can dissolve some things (like the dirt on your clothes) while not dissolving other things (like the pigments coloring your clothes). Previous studies have shown that the proteins causing AD are sarkosyl insoluble, so just like how laundry detergent will wash away dirt while leaving behind pigments, I can use sarkosyl to wash away non-AD proteins and keep the AD-causing proteins. These sarkosyl insoluble proteins include Aβ, but also include things like Tau and alpha-synuclein which some people hypothesize are the true cause of AD. The sarkosyl extraction is difficult, and I seem to fail at it as often as I succeed, am I just bad at my job or is this all really really hard? I hope it’s the latter but you never know. Then, once I’ve extracted the material I need from the patient’s brain, I use a variety of techniques to try to test our theory about AD. I can see if the extracts from r-AD and t-AD brains have different affects on neuronal organoids (artificial culture of cells that resembles an organ, in this case a brain), I can image the extracts with electron microscopy, I can take structural measurements with NMR, and so far all the data is frustratingly vague. I haven’t been at this job super long, but I can tell you I am not finding the One True Cause of Alzheimer’s disease any time soon.
And I think my struggles are fairly representative of the AD-researching community at large, or at least the ones I’ve talked to. It’s a disease that can only be studied biochemically post-mortem, the samples you get are both very limited and highly variable, it’s hard to relate the biochemistry back to the behavior and memory because we don’t have very good theories about that stuff to begin with, and we’re trying to use all the latest and greatest techniques to study this but we’re still struggling to get strong evidence to support our theories. After a century and more of study, we still don’t seem to be anywhere close to curing Alzheimer’s, we can’t really treat it, and we barely understand it. It can be frustrating and difficult work
Streams of Consciousness 