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, like 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.

PSP treatments in or near human trials

When a patient or caregiver asks me if anything can be done for PSP aside from palliative measures, my ready answer is that there’s a lot of research now into specific treatments that might slow or halt disease progression. I never have time to get into details in the time available, so I’m not sure my assurance is credible. So, putting my keyboard where my mouth is, here is a pretty thorough list of treatments that are in human trials for PSP or will enter such trials this year:

Anti-tau antibodies: BMS-986168 (Phase 1), C2N-8E12 (Phase 1). Both are in early stages of recruitment at multiple North American sites. The rationale is to bind and destroy abnormal tau en route between brain cells. (Disclosure: I’m a consultant to Bristol-Myers Squibb and a site investigator .) Other drug companies and academic labs are also working on anti-tau antibodies, but at an earlier stage.

Tau anti-aggregants: Leucomethylthioninium (LMTX). This is a derivative of methylene blue in Phase III for Alzheimer’s and frontotemporal dementia; If successful, PSP could be next. But beware the hype that has accompanied methylene blue and its derivatives.  The results from earlier-phase trials have not been published, which is curious.

Microtubule stabilizer: TPI-287 (Phase I). This is closely related to the taxane group of cancer drugs. In cancer, stabilizing microtubules helps prevent cells from dividing. In the brain, it compensates for the loss of tau, which normally stabilizes microtubules as the cells’ transport and skeletal system.

Tau acetylation inhibitor: Salsalate (Phase 1); This is being tested at UCSF, UCLA and UCSD in an open-label “futility” design. In other words, the study will determine not if the drug works, but if it deserves to be tested further. The same drug is being tested for multiple other disorders and has long been on the market as a non-steroidal anti-inflammatory drug.

Tau aggregation inhibitors: ASN-561, an O-GlcNAcase inhibitor. This will probably enter Phase I in 2016. It acts by promoting the attachment of a sugar molecule, N-acetyl glucosamine, to the tau protein, thereby inhibiting its aggregation. Such “OGA” inhibitors are also being tested for other conditions, including cancer.

Anti-sense oligonucleotides: These are RNA molecules designed to inhibit the production of 4-repeat tau, which is over-produced in PSP relative to 3-repeat tau. That imbalance could be contributing to tau aggregation. These have not reached human trials.

Anti-microglial agent: FK506 reduces the activity of microglia, inflammatory cells in the CNS. Evidence is increasing that such inflammation is a cause, rather than an effect, of cell loss in many of the neurodegenerative diseases. In fact, several immune-response-related genes were among the top 10 “hits” in the 2011 study of genetic risk factors in PSP.

Young plasma: Only in 10 patients, non-controlled and only at UCSF, this study will give plasma from healthy men younger than 30 to patients with PSP. The primary outcome issue is safety and tolerability, but efficacy measures will also be applied. Recruitment is under way. The theory is that some unknown blood-borne molecule in young people prevents them from developing PSP and could slow the process in someone with the disease.

Mitochondrial nutrient: Coenzyme Q-10 (Two small double-blind studies, one published and one unpublished) show similar modest improvement in PSP Rating Scale scores. This is a symptomatic treatment but the above items on this list are all potentially neuroprotective.

For more information on any of these, see http://www.clinicaltrials.gov.

Is PSP the route to not just Alzheimer’s but also Parkinson’s?

We’ve known for many years that Parkinson’s disease, which the textbooks call an α-synucleinopathy, has some aggregated tau as well. It appears that each of the two proteins, once misfolded, not only induces its own normal brethren to misfold, it also induces copies of the other to misfold.
The first demonstration of this synergistic misfolding and resulting aggregation came in a series of three papers between 2002 and 2004 from the lab of John Trojanowski and Virginia Lee at Penn. The first authors were John Duda, Bernard Giasson and Paul Kotzbauer. (Disclaimer: I was one of their co-authors on all three.)

Now, Julia Gerson, a grad student in the lab of Rakez Kayed at the University of Texas Galveston, has presented work at the Society for Neuroscience that harnesses that finding of a PD/tauopathy overlap for therapeutic purposes. (Another disclaimer: Kayed has a related grant from CurePSP, where I chair the grant review.)

Gerson and friends created antibodies directed at oligomeric tau, which is tau in small aggregates of maybe 20 or 30 molecules, which are still small enough to remain in solution in the cytoplasm and therefore invisible to light microscopy, unlike mature neurofibrillary tangles. They didn’t want to target normal, non-aggregated tau for fear of disrupting the normal function of that protein, which is to stabilize microtubules.

They injected those anti-tau antibodies into mice that had a copy of a variant of the human gene encoding α-synuclein. The variation was an G209A mutation, producing an A53T alteration in the resulting protein. This is the mutation that my colleagues and I found in 1997 as the cause of PD in a large Italian-American family with autosomal-dominant PD, a finding that first linked PD with α-synuclein. (That’s Disclaimer #3. You’re starting to see why I’m so interested in this new finding.)

The antibody protected the mice against the loss of dopaminergic neurons that the α-synuclein mutation caused in the untreated mutant mice. Mice that received antibody against normal tau did even more poorly than the controls.

So here’s the take-home: Developing an anti-tau antibody for treatment of PSP may also help Parkinson’s. We already expect that it may help Alzheimer’s because that’s clearly a tau disorder. But now, the synergistic toxicity of tau and α-synuclein could also allow a single anti-tau antibody to protect against both Parkinson’s and dementia with Lewy bodies (which also has aggregation of both proteins).

If I were the drug companies, I’d be sitting up and taking notice. Two companies, Bristol-Myers Squibb and AbbVie (licensing an antibody from C2N) have already started anti-tau antibody trials in people with PSP. Others have anti-tau programs in progress.

This new report, which may extend the utility of those products to Parkinson’s, should give that snowball an extra push.