Professor John Collinge on tackling prion diseases

What are prions, why are they important, and how might they help us develop treatments for neurodegenerative conditions like dementia?

Prions are lethal pathogens that cause neurodegenerative diseases of humans and other mammals.

The best-known human prion disease is sporadic Creutzfeldt-Jakob disease (sCJD), a rapidly progressive dementia which accounts for around 1 in 5000 deaths worldwide. In sharp distinction to all other infectious agents, prions lack their own DNA or RNA genome and consist of polymers of a misfolded form of a normal cellular protein (the prion protein or PrP) which form amyloid fibrils.

These fibres grow by addition of PrP molecules at their ends and they eventually fragment producing more prion particles which continue this process and spread throughout the brain. The final proof of the once controversial “protein-only hypothesis” of prions came with the determination of the structure of infectious prions at near atomic resolution by cryogenic electron microscopy by ourselves and US colleagues in the last few years.

The normal cellular prion proteins are very similar between different species of mammals and therefore a prion infection from one species can sometimes infect another species. This is what happened with the prion disease of cattle, bovine spongiform encephalopathy (BSE) in the 1990’s which caused a new human prion disease known as variant Creutzfeldt-Jakob disease (vCJD) and led to the BSE crisis in the UK, EU and other countries.

While human prion diseases are thankfully rare, there are common prion diseases of other species, for example scrapie in sheep and goats worldwide and chronic wasting disease in deer in North America. While prions were first thought to be unique to these rare neurological diseases, it became clear that the molecular process was of far wider relevance with for example the recognition of several different proteins in yeast that could form prions.

Most importantly with respect to neurodegeneration and dementia in humans, it has been established that similar so-called “prion-like” mechanisms are involved in much commoner conditions including Alzheimer’s and Parkinson’s diseases. In Alzheimer’s disease (AD) for example, two proteins in the brain, amyloid-beta and tau can form self-propagating assemblies which spread in the brain. Indeed, we reported in two articles in Nature that the amyloid-beta pathology seen in AD can be transmissible between humans in rare circumstances causing the newly recognised condition iatrogenic cerebral amyloid angiopathy.

There is accumulating evidence also for iatrogenic AD. Understanding prion biology, and in particular how propagation of prions leads to neurodegeneration, is therefore of central research importance in medicine. Many years ago, we demonstrated that targeting the production of the normal cellular prion protein completely halted the progression of neurodegeneration (and indeed even reversed early pathological changes) in laboratory mice. This work has underpinned multiple efforts to develop rational treatments for prion and other neurodegenerative diseases.

What first attracted you to the area of prion diseases?

I first became involved in this field while working as a graduate student applying early molecular genetic methods to study neuropsychiatric diseases and was involved in the first description of mutations in the prion protein gene in the late 1980s in what are now known as the inherited prion diseases.

As it was already known that brain tissue from patients who died from some of these genetic conditions could transmit disease when inoculated into laboratory animals, it seemed to me highly likely that some version of the then intensely controversial “protein-only hypothesis” was likely to be correct: this had major implications in pathobiology.

I went on to show that being heterozygous for a common human prion protein polymorphism had a profound effect on susceptibility to CJD; I considered this entirely consistent with a protein-only agent and this led to further work studying the genetics of prion disease.

It seemed to me at the time that these early genetic insights, albeit in a rare disease, provided a powerful way in to study the fundamental basis of neurodegeneration. Of course, the evolving concerns about BSE in the early 1990’s also focussed my mind on the specific public and animal health risks posed by prions.

You led the UK’s first clinical trial in CJD, the largest yet conducted internationally. Can you tell us about this? 

I was asked in 1997 by Medical Research Council (MRC) at the request of UK Government to establish and lead an MRC Unit to focus on understanding prion diseases and to ultimately develop treatments for them.

At the time it was unknown how many people would develop vCJD following the widespread dietary exposure of the UK population to BSE prions and the possibility that this may eventually affect hundreds of thousands could not then be excluded.

An early proposal (by Dr Prusiner at UCSF) for a treatment for CJD was the anti-malarial drug quinacrine based on early work in prion-infected cell cultures. We were asked by the Chief Medical Officer to establish a clinical trial and did so in collaboration with the MRC Clinical Trials Unit also based at UCL.

While the MRC PRION-1 trial, as is was called, did not show any benefit of quinacrine, we did learn a great deal about how best to conduct a clinical trial in CJD in conjunction with patients and families affected by these terrible conditions.

This lead on to the formation of the National Prion Monitoring Cohort (NPMC) to study the natural history of prion diseases and to develop better clinical scales and biomarkers, and earlier diagnosis, to facilitate future clinical trials. In particular, we reasoned that having a large longitudinal data set would allow us to conduct adequately powered efficacy trials by comparison of treated patients with historical controls rather that using a more classical placebo-controlled study which was understandably unacceptable to patients and their families given the rapid and invariably fatal progression of these diseases.

The NPMC has been extremely successful with the strong support of the patient community and has recruited over 1100 patients to date, by far the largest dataset worldwide, and has enabled development and validation of multiple clinical scales and blood and CSF biomarkers.

What in your opinion have been some of the most important findings of your research to date?

Our early work established and characterised the inherited prion diseases and genetic susceptibility to acquired and sporadic prion disease, and pioneered diagnostic and presymptomatic genetic testing of neurodegenerative disease.

Many further genetic advances followed. Prions exist in multiple strain types and we developed molecular strain typing of prions which we applied in 1996 to first demonstrate that vCJD was caused by the same prion strain as cattle BSE, a finding of critical public and animal health significance at the time.

We characterised the pathogenesis of vCJD to inform public health risk assessments, developed the first blood test for vCJD and effective means to prion sterilise surgical instruments. We proposed the now widely accepted “conformational selection hypothesis” to explain the relationship between prion strains and intermammalian transmission barriers and proposed that prion strains constitute a “cloud” under host selection rather than a molecular clone.

Importantly, we described subclinical prion infections in which animals lived a normal lifespan despite harbouring high levels of prions and went on to study the kinetics of prion propagation in vivo and showed that propagation and neurotoxicity occur in two distinct mechanistic phases with pathology only developing after prion levels had plateaued in the brain.

We subsequently confirmed that prions themselves are not directly neurotoxic. These insights may be fundamental to understanding other diseases involving propagation and spread of assemblies of misfolded proteins, notably amyloid-beta and tau in AD.

Our discovery of human transmission of amyloid-beta pathology, mentioned above, in individuals treated many years earlier in childhood with human cadaver-derived pituitary growth hormone (c-hGH) accidentally contaminated with amyloid-beta seeds (prions) has wide implications for understanding, preventing and treating neurodegenerative diseases.

We defined iatrogenic cerebral amyloid angiopathy as a new disease, with relevance to Alzheimer’s disease and public health. Iatrogenic AD is likely to be recognised in the cohort of c-hGH recipients as they age further. Our demonstration that reducing prion expression during neuroinvasive prion disease in laboratory mice prevented onset, and reverses early pathology, produced a proof of principle of therapeutically targeting prion protein.

This led to our development of a biopharmaceutical which we have used to treat CJD. Recently, we have described the elusive structural basis of prion strain diversity: how prions can encode information in a non-Mendelian manner by determination of near atomic resolution structures of multiple prion stains by cryogenic electron microscopy.  

In addition, we are proud of our long term field studies on the epidemic human prion disease kuru in the Eastern Highlands of Province of Papua New Guinea (PNG), in collaboration with the PNG Institute for Medical Research and the affected communities, which led to major insights including establishing the range of possible incubation periods of human prion infections (documenting cases with incubations over 50 years) and discovery of a novel prion protein variant selected by the epidemic which we demonstrated provides complete protection against prion infection and disease and the molecular structural basis of which we have recently characterised.  

To what extent do you think we are entering a new era when it comes to developing drugs that could be used to prevent, or even reverse, neurodegenerative diseases?

Thankfully we are entering a time when disease-modifying treatments for neurodegenerative diseases are becoming feasible and indeed first-generation agents have arrived, but we cannot yet prevent, halt or reverse neurodegeneration.

Our own work validating cellular prion protein as a therapeutic target led us to develop a humanised monoclonal antibody with high affinity for cellular PrP and this has been used to treat six patients with CJD at UCLH. We consider the encouraging results justify a formal clinical trial and are seeking funding support for this at present.

Our therapeutic strategy has been to target normal cellular PrP itself, the substrate for prion propagation, and not the disease-related assemblies of misfolded PrP that accumulate during disease. We reasoned, given the diversity of these species, that drugs binding prions themselves would lead to the rapid development of resistance and indeed this has been shown to be the case with drugs developed elsewhere.

There may be important lessons here for other neurodegenerative diseases. For example, this may be critical in determining whether monoclonal antibody drugs targeting amyloid-beta fibrils or other assemblies, which also exist as structural polymorphs, have a sustained therapeutic effect or result in strain selection and evolution of resistant sub-strains as in prion diseases.

A number of pharmaceutical and biotech companies are however developing gene targeting methods, conceptually analogous to those we demonstrated many years ago block prion pathogenesis, to reduce expression of proteins implicated in various neurodegenerative diseases. Given the complexity and diversity of AD, in which multiple proteinopathies are involved, it is likely that effective treatments are going to require a cocktail of drugs hitting multiple targets.

Another key consideration is the importance of accurate diagnosis and early treatment, not only for the obvious need to intervene before irreversible brain cell loss has occurred, but because at the stage where significant cell death (with release of toxic materials) is occurring, these secondary non-specific neurodegenerative processes may dominate and be unresponsive to the specific targeted therapies. The ultimate aim must be to identify these pathogenic processes very early (ideally pre-clinically) and intervene to delay, and eventually prevent, clinical progression or onset.

Related

• Professor John Collinge's academic profile
• MRC Prion Unit and Institute of Prion diseases