PSP by the Bay

Just returned from the annual CurePSP International Research Symposium, held this year in on the campus of University of California San Francisco on October 27.  About 120 researchers attended, many from Europe and Japan.  The first keynote speaker was Bruce Miller from UCSF, perhaps the country’s leading behavioral neurologist, who gave an overview of PSP/CBD research with an emphasis on activity of the Tau Consortium, the multi-institutional research group that he directs.  The other keynoter was Robert Stern, a neuroscientist at Boston University who directs clinical research at BU’s Chronic Traumatic Encephalopathy Center.

Bruce’s talk touched on many topics — from the nosology and pathology of the various cognitive/behavioral syndromes in the tauopathies to the sleep disturbance in PSP (hyperarousal is common in PSP, while hypoarousal predominates in CBD).  Perhaps most interesting was his up-to-the minute summary of the state of tau PET imaging in PSP diagnosis (not nearly ready for prime time, though potential exists).

Bob’s lecture summarized the story of CTE.  He emphasized that the most important frequent cause isn’t concussions, but the continual sub-concussive blows to the head such as those suffered by football players during routine blocking and tackling.  He was too smart to speculate much about a relationship between the tauopathy of CTE and that of PSP, but I’m not:  I’ll say that in both cases, individuals with a genetic predisposition to tau aggregation are exposed to a precipitating factor – repeated brain tissue stretching for one, some sort of toxin for the other.  If we can find the genetic background for one, we may find it for the other.

Perhaps the genetic answers will emerge from the whole-exome sequencing project that is complete and in the writing phase or the whole-genome sequencing project that is well under way. But as pointed out in another Symposium talk by Jerry Schellenberg, the U Penn geneticist who heads those efforts for the PSP Genetics Consortium, there’s a lot of “genetic dark matter” in the form of genomic deletions, undetectable by mere sequencing.

Maybe in future posts I’ll get more into the other excellent CurePSP Symposium talks – and the 18 concomitant poster presentations.  Or maybe I’ll get distracted by a random shiny object I find in somewhere else.

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You’ve dragged me back, or: an anti-tau antibody update

Yeah, yeah.  I know it’s been 15 months since my last post.  I’m fine, thanks – just busy with other things.  But today a kind reader from Canada named Maureen posted a comment asking where I was all this time and threw in a little flattery just to get me going.  I’m a sucker for that.

So here’s the latest on the hottest topic in PSP-ology, the antibody trials.  Right now, five drug companies have anti-tau antibodies in the pipeline.  Two have started human trials and are in Phase 2.  The others are still in laboratory phases or in the early planning for human trials.  As a reminder, these antibodies are directed at the tau protein and are given by intravenous infusion, typically at intervals of a month.  The idea is to intercept the misfolded, abnormally aggregated tau as it passes through the intercellular fluid between brain cells. (We still don’t know if it’s between neurons or glia or both.)  The hope is to slow the spread of the disease process through the brain.  This treatment would probably not improve any existing deficits, just slow the rate at which they worsen going forward.

The first drug company out of the blocks was Bristol-Myers Squibb, which sold its neurodegeneration division to Biogen in April 2017.  This Phase 1 trial was designed only to assess safety and comprised only 48 patients, a quarter of whom received placebo.  The antibody passed with flying colors, with no important side effects over the 12 months of treatment.  The patients (including the 12 assigned to placebo for the first year) are continuing to receive active drug and are generating more data along the way.  The protocol did include measures of efficacy, the principal one being the ability of the treatment to slow the progression of the disease relative to placebo as measured by the PSP Rating Scale.  But that effect would have to have been huge to be statistically noticeable in so small a study.

That’s the purpose of the Phase 2 studies, of which 2 are in progress.  The one that Biogen bought from Bristol-Myers Squibb will include 396 patients, a third of whom will receive placebo infusions.  The study will take place at about 35 sites in the US, Europe and Japan.  Recruitment has begun and the double-blind phase will last a year for each patient, though it will probably take about 6 months to get all of the patients entered.  This study has been dubbed the “PASSPORT” study.  (It looks like an acronym, but it doesn’t really abbreviate anything, except that the letters “PSP” are in there somewhere.)

The one by AbbVie, another big drug company, comprises 180 patients, of whom 60 will get placebo.  It will take place at 18 sites around the US plus 3 in Canada, 2 in France and one in Australia.   About half of the sites are presently recruiting patients.  This study is called “ARISE.”  When I find out what that’s supposed to abbreviate, I’ll let you know.

You can find more info about both trials, including contact information for prospective participants, at clinicaltrials.gov.  Here’s the listing for the BMS/Biogen study and here’s the link for the AbbVie study.

Maybe a future post will regale you with my random thoughts about whether anti-tau antibodies are actually likely to help.

[Full disclosure: I consult on an hourly basis for both BMS/Biogen and AbbVie in matters regarding the PSP Rating Scale, which I published in 2007 and is the principal outcome measure for both studies.  I have no personal financial interest in the studies’ outcome.]

Express yourself, or better yet, don’t.

An original and interesting observation just appeared that might help explain how the known genetic variants associated with PSP might cause the disease.  It has to do with regulating gene expression.

Mariet Allen, PhD, a junior researcher at Mayo Clinic Jacksonville, and colleagues published the paper in the current issue of Acta Neuropathologica.  The senior author is Nilüfer Ertekin-Taner, PhD, who received a grant from CurePSP for this work.  The general idea of using such “endophenotypes” to assess the role of genetic variants in causing PSP has long been proposed by their mentor, Dennis Dickson, MD, a leading neuropathologist, who was also an author of this paper.

They used tissue from 422 brains from the CurePSP Brain Bank at Mayo that had been confirmed as having PSP.  They made three types of measurements in each brain: gene expression (measured as messenger RNA), epigenetic methylation of DNA (measured as CpG islands), and numbers of the various classic PSP micro-anatomic changes that have been known for decades.  They correlated those measurements with whether each case carried the major or minor allele of markers reported in  the genome-wide analysis (GWAS) of single-nucleotide polymorphisms published in 2011 by Hoglinger et al. (Disclaimer: I was a minor co-author on the 2011 paper.)

Without getting too much into the details, the results were that the genetic variants and increased DNA methylation at MAPT (the gene for tau protein) and/or MOBP (the gene for myelin-associated basic protein) were associated with increased expression levels of some proteins not previously associated with PSP.  One such protein was “leucine-rich repeat-containing protein 37A4” or LRRC37A, which is coded at chromosome 17q21.31-q21.32.  The genetic marker status at that location was also associated with increased expression levels and methylation levels in two other proteins encoded by genes at the same approximate location, ARL17A and ARL17B.  (Adenosine diphosphate ribosylation factor-like GTPases are involved in protein transcription and like LRRC37A, are located next to the MAPT gene on chromosome 17.)

LRRC37A appears to be involved in regulating interactions of proteins with other compounds.  Its upregulation is known to be harmful to cells.  Intriguingly, its gene produces a wide variety of alternatively spliced protein forms (where some exons’ protein products are included, others excluded, from the finished protein product) in different people and in different species.  This may suggest that this gene is unstable and could easily be induced to make an inappropriate variant of its protein by a subtle exposure to a toxin or a toxic effect of another gene.

Furthermore, the marker status at the MAPT locus correlated with more intense tau aggregates in the form of coiled bodies and tufted astrocytes, two of the standard diagnostic features of PSP.  This reinforces the idea that tau overexpression is part of the pathogenesis of PSP and that inhibiting that expression could provide prevention.

So as the authors modestly conclude, “MOBP, LRRC37A4, ARL17A and ARL17B warrant further assessment as candidate PSP risk genes.”  All of these associations may suggest new drug targets, but it’s a long slog from there to the clinic.  However, if someone screens a library of existing drugs for their ability to suppress overexpression of these proteins, the path to a treatment could be much, much shorter

Critique of pure prionopathy

If you follow the latest in neurodegenerative disease research, you’ve heard the “prion hypothesis” or “pathogenic spread hypothesis.”  For the past five or six years, it’s been widely claimed, and almost as widely accepted, that the proteins that mis-fold and aggregate in the brain cells in PSP, as well as in its big brothers Alzheimer’s and Parkinson’s and the rest, spread through the brain in a way similar to how prion protein spreads through the brain in the prion diseases such as Creutzfeldt-Jakob disease, mad cow disease and kuru.  A respectable body of experimental evidence supports — or at least is compatible with —  this idea.

But now a pair of highly respected Harvard neuroscientists, Dominic Walsh and Dennis Selkoe, have said not so fast.  In a very well-balanced and dispassionate review of the prion hypothesis in Nature Reviews / Neuroscience, they show that while the existing evidence is compatible with cell-to-cell spread of toxic protein aggregates, there is still plenty of room for a hypothesis that posits selective cell vulnerability with a more generalized toxic influence.  I won’t get into the technical weeds, but here are their major points:

  1. Even in the classical prion disorders, it is well-accepted that “cell-autonomous” factors, rather than just spread from nearby cells, determines which cells are and are not involved.  The salient example is that the asparagine-for-aspartate mutation at position 178 in the prion protein causes familial CJD when the person has a valine at position 129 in the same protein but causes fatal familial insomnia with there’s a methionine at 129.  (Neither of the latter substitutions by itself is pathogenic.)
  2. The “pathogenic spread” hypothesis rests in no small part on the observations of Braak and colleagues that early-stage Alzheimer’s or Parkinson’s pathology in people dying from other causes is confined to certain specific brain areas, suggesting that the process starts there and spreads.   But Walsh and Selkoe point out that those early sites of pathology may merely be the areas most sensitive to a generalized insult.  Furthermore, only about half of the cases of each of those diseases followed that pattern.
  3. Another buttress for the pathogenic spread hypothesis is the observation that 5-10% of fetal substantia nigra cells transplanted into the striatum of patients with Parkinson’s developed Lewy bodies themselves after a number of years. But this need not be the result of spread of pathogenic alpha-synuclein; it could be the result of a more generic insult such as inflammation in the injection site, where most of the injected cells have died.  They cite evidence that activation of microglia (the brain’s inflammatory cells) in other types of neural grafts can produce Lewy bodies in those grafts.
  4. The experiments showing that injected alpha-synuclein or tau protein can induce the formation of aggregates in host brain is incomplete because they do not adequately demonstrate actual cell loss or impairment of brain function in the host animal. We know from other lines of experiment that aggregates alone do not correlate well with neurological impairment in human or experimental neurodegenerative disease.
  5. The pathologic anatomy of rare, dominantly inherited forms of Alzheimer’s, Parkinson’s and frontotemporal dementia fits well within the spectrum of their corresponding sporadic conditions. A genetic cause, producing the same intense pressure for protein aggregation in many areas of the brain simultaneously, would not be expected to mimic the anatomic pattern of a single-anatomic-source process posited by the pathogenic spread hypothesis.
  6. There are still many questions left unanswered by the pathogenic spread hypothesis. This doesn’t directly contradict its other tenets, but it weakens its explanatory power. It cannot explain the initial protein misfolding; how the aggregates are released; how they remain aggregated in the interstitial fluid where the concentration of the protein is far less; why they don’t stick to the outsides of cells after being excreted, as their physical chemical characteristics suggest they should; how they choose only certain target cells to penetrate; and how the aggregates escape into the cytoplasm from the membrane vesicles that presumably would be the vehicles by which they penetrate their targets.

 

As a final point, Walsh and Selkoe make a case for avoiding the term “prion-like” or “prion-oid” with reference to neurodegenerative diseases unrelated to the prion protein itself.  They list several known features of prion protein spread in the known prion diseases that as far as we know are absent in PSP, Alzheimer’s, Parkinson’s, etc.  They also cite the absence of any known transmissibility of the non-prion-protein and point out that we don’t know enough about either group of diseases to equate them at that level of terminology.

Excellent scientists that they are, Walsh and Selkoe describe a set of experiments to undertake and new research tools to develop in order to strengthen or reject the pathogenic spread hypothesis.  Maybe I’ll get to that in another post.  But they end with the hope that the pathogenic spread hypothesis is true, for that would provide many potential therapeutic targets that would not otherwise exist.

A new definition of PSP

When you design a research project in PSP, it’s important to make sure that everyone in the subject group with PSP in fact has PSP. Otherwise, you degrade the statistical power of the trial to detect any benefit of the treatment. The standard diagnostic criteria for PSP (called the “NINDS-SPSP Criteria” and spearheaded by Irene Litvan, MD, now of UCSD) were published in 1996 and have worked well for that purpose; Their positive predictive value (the fraction of patients satisfying the criteria who actually have PSP) and specificity (the fraction of those without PSP who fail to satisfy the criteria) are close to 100%.
But as we now enter the new era of trials of experimental neuroprotective treatment for PSP, we would like to diagnose the disease at an earlier stage, when such interventions are most likely to be effective, and the NINDS-SPSP Criteria don’t do that so well, with a sensitivity of about 80% overall, certainly lower in early cases. Another shortcoming is that the various phenotypes of PSP that have been described since 2005 won’t in many cases satisfy the criteria, which were designed for the “original flavor,” now called PSP-Richardson’s syndrome.
So time has marched on and we need a new set of criteria. Günter Höglinger, MD, Professor at the German Center for Neurodegenerative Disorders in Munich and probably the world’s leading clinical researcher in PSP, organized an international effort to revise the criteria. I’m privileged to serve on the four-person Steering Committee. A year ago we started to hash things out by email and conference calls, using the published articles on clinical features of PSP that use either autopsy or the NINDS-SPSP Criteria as a gold standard. The group, comprising 33 people from 11 countries, met in Munich on March 9 and 10 to turn our rough draft into a final version suitable for submission to a journal for peer review.
The new criteria recognize the various phenotypes of PSP. They are PSP-Richardson syndrome (about 55% of all PSP), PSP-parkinsonism (30%), PSP-frontal dementia (5%), PSP-ocular motor (1%), PSP- pure akinesia with gait freezing (1%), PSP-corticobasal syndrome (1%), PSP-progressive non-fluent aphasia (1%), and PSP-cerebellar (<1%). The remaining few percent are combinations of these or still-unrecognized forms.
The new criteria also delineate various “oligosymptomatic” or “prodromal” (the wording remains unsettled) forms, which may or may not develop into one of the diagnosable phenotypes. For example, there is now evidence that someone in the PSP age group with gradually progressive gait freezing for several years and a normal MRI, even without other abnormalities, will almost always prove to have PSP. The same is true for someone with bilateral rigidity and bradykinesia who fails to respond to levodopa and has some sort of nonspecific, undiagnosable visual symptoms or dizziness. Neither of these patients would satisfy the proposed new criteria for any of the PSP phenotypes, but they may still be worth identifying for inclusion in a longitudinal cohort study of people who are at risk of developing PSP. Our new criteria do that.
I’ll keep you updated.

Stay in school and drink rain

The largest-ever environmental and occupational risk factor survey in PSP was just published. Irene Litvan of UCSD led a group of sites throughout North America with 284 patients and 284 controls who were friends or non-blood relatives of the patients.
The results corroborate the finding of all three previous such studies that lesser educational attainment is more common in people with PSP. Two of those studies were done by me and my colleagues in New Jersey (1988 and 1996) and the other was in France by Vidal et al (2009).
In this new study, the odds ratio for having earned a college degree was 0.585 (95% confidence interval 0.345 to 0.993, p = 0.047). The only other statistically significant result was that people with PSP reported having drunk well water for an average of 11.7 years, while for the controls, the figure was 7.4 years. That p-value after multivariate correction, was 0.032. They showed that these two findings were not correlated to each other in this subject group.
Interestingly, the well-documented tendency in Parkinson’s disease for non-smoking was not observed. In fact, there was a non-significant trend in the opposite direction, with the odds ratio of 1.096 (multivariate corrected p = 0.082) for smoking among the PSP group relative to controls.
So what’s the take-home? We’ve been saying for years that most of the diseases for which we have no clear cause (most cases of cancer, Alzheimer’s, atherosclerosis, schizophrenia, PSP, etc.) are the result of a genetic predisposition and an environmental trigger, with “environment” being broadly defined as anything other than the person’s genome. This study suggests that for PSP, the trigger (or one of the triggers) is something associated with the lifestyles, work places or home neighborhoods of people with lesser education. But the only clue the study provided beyond that is that the trigger may be something in well water. Furthermore, using well water may tend to correlate with other toxic exposures or experiences that the survey did not ask about.
This result may now stimulate researchers to study “environmental” causes of PSP more closely and may induce granting agencies to support such studies. Of course, this search will be guided in part by ongoing genetic studies of PSP: If a variant in a detoxification gene is found to be over-represented in PSP, then perhaps the corresponding toxin is the environmental trigger. If a gene variant that causes over-expression of a gene is found to be over-represented in PSP, then environmental agents that cause a similar effect would immediately become suspect.
Another point, just to make life more complicated: Environmental toxins may not only act directly, as, for example, lead in the drinking water affects childhood brain development. They may also cause epigenetic changes that affect the expression of genes. They may also affect the gut bacteria, the “endobiome,” which itself produces and alters a wide array of compounds, some of which could be pathogenic.
So we’ve got work to do, but Dr. Litvan and colleagues have taken an important step.