New info we can use this afternoon

If you like my research updates but need a break from the molecular stuff, here’s an interesting and unexpected clinical discovery, usable at routine visits for care of PSP.

Michelle Troche, PhD, is a speech/language specialist in the Laboratory for the Study of Upper Airway Dysfunction in the Department of Biobehavioral Sciences at Columbia University’s Teachers College.  She’s a past grantee of CurePSP’s

for a project published a year ago on training patients with PSP to protect their airways from aspiration. Now, she and her team, with James Borders as first author, have analyzed cough function in quantitative detail in patients with PSP and Parkinson’s.

The two groups of 26 patients were designed to be similar in terms of age, sex, disease duration and severity of swallowing difficulty.  Their cough was analyzed with a device called a pneumotachograph (“air-speed-writer’) that fed the data into a software system.  The procedure was performed during both coughing on request and coughing induced by a 2-second spray of capsaicin in 4 progressively increasing concentrations.  Capsaicin is an irritant found in hot peppers, so kudos to those volunteer patients!

After a sophisticated statistical correction for confounding factors, some results were as expected: that the patients with PSP were able to generate only a fraction of the expiratory flow rate and volume of those with PD.  However, another result was unexpected: patients with PSP were more bothered by increasing concentrations of the capsaicin than were those with PD, but were no more likely to respond by coughing. 

In the researchers’ words, “. . . it is interesting to note that although both groups exhibited blunted urge-to-cough slopes compared to prior research in healthy adults, patients with PSP demonstrated increased urge to cough compared to PD. This means that even though the participants with PSP were perceiving the increasing cough stimulus more than patients with PD, they were not coughing more to that stimulus.

So what does this mean?  We think of PSP as a motor and cognitive disorder, but this result shows that there’s also a sensory deficit.  The sensory input into the cough reflex takes place in the brainstem, the main location of pathology in PSP, so this result makes sense.  It’s just that no one had previously thought about it. (The same phenomenon accompanies many important scientific discoveries – they seem so obvious in retrospect.)

Another brainstem reflex, the auditory startle response, has long been known to be impaired in PSP but not in PD, so the results of Borders et al have that ex ante confirmation.

Like any pioneering work, this one has its limitations.  Although the PSP and PD groups were similar in terms of disease duration (both about 5 years) and swallowing function, the PSP group was much worse in terms of overall disability as measured by the widely-accepted Schwab and England Activities of Daily Living (SEADL) scale. With 100 being normal on the SEADL, the PSP group averaged 48, the PD group 79.  This is expected given PSP’s more rapid disease course from onset to death — the 5-year disease duration is a far greater fraction of the average survival in PSP (7 years) than in PD (15 years).  So perhaps the authors should/can add the SEADL score to their statistical model.  Another issue is that the experimental procedure did not consider any effect on the cough function of PD medication, which dramatically helps general motor function of PD but not of PSP.

Now, how can we use this information?  Dr. Troche and colleagues suggest that patients with PSP be monitored closely for cough deficit and that their own, previously published protocol for sensorimotor training in PSP, referred to above, could be instituted sooner rather than later.

Short stuff

Most of my posts are long — maybe too long. The charitable explanation is that I can’t resist my instincts as a professor to explain stuff so my learners can understand it. The less charitable explanation is that I’m just a windbag. So here are a bunch of very brief items of news, ideas and opinion about PSP and CBD in the style of Twitter. In fact, I’ll even limit my character count to 280, including spaces. Here goes.

A group in Bologna did skin biopsies to look for a phosphorylated form of α-synuclein in PD, PSP or CBD, and controls – 26 subjects in each group. They found it in all 26 with PD, in no controls, and in 24 with PSP/CBD. (The other two had PD-like features.) Now: how about MSA?

You’ve noticed that CurePSP’s publicity materials call PSP, CBD and MSA “prime of life” diseases because those conditions’ usual decades, the 50s, 60s and 70s, are when life can otherwise be lived to the fullest. Do you agree? Let me know.

A group called the PSP Research Roundtable was formed in 2017 to help speed the process of testing promising drugs. It’s run by CurePSP and has membership from academia, the FDA, the NIH, drug companies, biotech, philanthropy and patient advocacy groups.

Transposon Therapeutics has started a Phase 2 trial of TPN-101 in PSP at private clinical trial sites in Boca Raton, FL and Farmington Hills, MI. Like many available HIV drugs, TPN-101 inhibits the enzyme reverse transcriptase, but otherwise, details are sparse.

About the mechanism of action of TPN-101: I can tell you that another reverse transcriptase inhibitor routinely used for HIV called efavirenz (trade names Sustiva and Stocrin) reduces tau aggregation. CurePSP is currently supporting a study of it in a mouse tauopathy model in The Netherlands.

Enough for now. No windbag, I.

Marker development: anarchy vs plutocracy?

You’ve heard me whining that we need a diagnostic “trait marker” for PSP. In other words, we need to be able to accurately distinguish PSP – during life — from such mimics as Parkinson’s, multiple system atrophy, Alzheimer’s, corticobasal degeneration, normal-pressure hydrocephalus and others. Only in that way can we create “pure” groups of patients in which to study the disease and test specific treatments.
Right now, the best diagnostic test we have is the MDS-PSP Diagnostic Criteria, which requires only traditional history-taking and a hands-on neurological exam. Those criteria work well for PSP-Richardson syndrome after the first couple of years but not quite well enough for earlier-stage PSP-RS nor for the “minority” or “atypical” types, which together account for 60 to 75 percent of PSP.

The most promising markers using laboratory or imaging data are levels of phosphorylated tau and neurofilament light chain (NfL) in the spinal fluid and blood; and perhaps MRI measurements of the size of the midbrain, pons and related areas at the base of the brain. But these are far from ready for prime time. NfL is a protein component of brain cells that has been shown to occur at about a two-fold higher level, and to increase faster over time, in PSP and MSA than in PD, Alzheimer’s and other neurodegenerative diseases. MRI changes don’t occur in the early stages. Positron emission tomography is coming along, but won’t be ready for use in PSP for another few years, and even then will be costly and not widely available.

Last week, my routine surveillance of new PSP-related research papers in the literature yielded two interesting hits — both about PSP trait markers, both using new lab techniques, and both from Italy.

Corinne Quadalti and colleagues at the University of Bologna measured NfL and alpha-synuclein in spinal fluid and blood. They found that plasma NfL alone worked very well in distinguishing PD from PSP, with an accuracy of 0.94. (“Accuracy” in this context is the area under the receiver operating characteristic curve, which compares sensitivity with specificity. Perfect accuracy is 1.0 and a useless test’s accuracy is 0.5, where a coin flip would work as well.)

Alpha-synuclein is the main protein aggregating in Parkinson’s, dementia with Lewy bodies and multiple system atrophy. It is to those diseases what tau is to PSP and CBD. To measure it, they used a new technique called “real-time quaking-induced conversion” (RT-QuIC; pronounced, “R-T quick”), which measures that protein in its misfolded and aggregated forms. This prevents that abundant protein in its normal form from swamping the measurement. The result was positive in 91% of their patients with PD and in none of their 58 patients with PSP or CBD.

Now, if you have a nose for statistics, you’ll raise your hand and say, “But those 9% of PD patients with a negative test comprise more people in the general population than all the patients with PSP or CBD, so a negative test doesn’t mean much.” and you’d be right. So, while the sensitivity of the test for PD is excellent, the specificity is low, rendering the overall accuracy in a real-world situation insufficient.

For that reason, the authors combined two measurements – spinal fluid NfL and serum alpha-synuclein, with a resulting improvement in distinguishing PD from PSP/CBD to a sensitivity of 97.4% and specificity of 100%. That’s more like it, but keep in mind a few issues: They combined PSP and CBD into one group, and we don’t know if the results apply as well to each disease alone. They had no autopsy confirmation of the diagnoses, which means that these patients were already at a stage that was possible to diagnose using traditional clinical criteria; this means that patients with earlier-stage illness will be needed in a follow-up study. Finally, and as always, the results have to be confirmed at other centers using other techniques.

The other eye-catching paper was from Ida Manna and colleagues at the University Magna Graecia in Catanzaro, Italy. They use exosomal micro-RNA (miRNA) in blood to distinguish among PSP, PD and healthy controls.

Exosomes are tiny bubbles of brain cell membrane enclosing whatever cell contents were there when the bubble pinched itself off and floated free. They often find their way into the bloodstream. MicroRNAs are stretches of RNA averaging only about 22 nucleotides. They do not encode proteins as messenger RNA (mRNA) does, but instead bind to mRNAs to regulate their translation into protein. They are specifically encoded in the DNA of the genome and about 2,000 of them are known to exist.

Dr. Manna et al measured levels of 188 miRNAs for which there is evidence of association with some neurodegenerative disease. They found a set of 6 miRNAs that together yielded an accuracy in distinguishing PSP from PD of 0.91. The accuracy for distinguishing PSP from controls was 0.90.
Of course, many of the same caveats that I listed for the other paper apply to this one. Plus, PSP mimics other than PD were not included in the analysis. Just as important is that there were only 25 patients with PSP and they were a mixed bag of 20 with PSP-Richardson and 5 with PSP-Parkinson. In applying a marker for the purpose of excluding patients with PD from a study of PSP, it is critical to be able to distinguish PD from PSP-P. It is unlikely that those 5 patients with PSP-P constituted a statistically valid sample for that purpose. That will be a project for another day.

What do I take away from these two papers? Neither of them alone provides a marker just yet, and each has its drawbacks given the current early stage of work. But perhaps, with some refinement, combining them with other non-invasive markers could create a diagnostic panel with enough accuracy to distinguish PSP from all of its mimics. After all, in medicine in general, multiple diagnostic tests (several tests of body fluids, some imaging, a physiologic test such as an EKG) must be combined to produce an actionable diagnosis. Why should PSP be any different?

I think the problem (and it’s a good problem to have) is that new candidate markers are being identified all the time, as are ever more sophisticated technology for measuring them, with RT-QuIC, miRNA and exosomes as prime examples. That means as researchers turn their attention to early-phase development of newer ideas using newer technology, ideas that looked potentially useful if pursued further may be neglected and not developed into practical tests. What to do? Do we just let scientific nature take its course in its traditional, anarchical way, waiting for research groups to take techniques with good initial data to the next level? Or should a group of experts with an iron fist issue some sort of “white paper” listing which markers with good preliminary evidence, perhaps like the ones I describe here, should be nurtured with funding and collaborations? If so, who chooses those experts? And once the experts are chosen, how can we prevent them from favoring the ideas in which they’ve invested their own time, resources and reputations?

You know where the “comment” button is.

An RNA surprise

As usual, some background may be helpful here: You’ve heard of genomics, the analysis of large numbers of gene variants as a clue to disease causation.  But genomics doesn’t deal with the actual proteins produced by those genes.  That led to proteomics, the analysis of proteins in a tissue or fluid sample, as a more relevant window into disease mechanisms and possibly as a path to diagnostic tests.  But proteins in samples can be influenced by many things such as breakdown of proteins by enzymes or by the cells’ normal garbage disposal mechanisms.  A solution is “transcriptomics,” which identifies and quantifies messenger RNA (mRNA) on its way from being encoded (“transcribed”) by DNA to being translated into protein.  Although every cell in our bodies has our entire genetic endowment in the form of DNA, only a fraction of those genes is actually transcribed into mRNA.  The types and amounts of mRNA produced depend on the job of the specific cell and its protein needs at the moment.  Click here if you’d like to chew on a highly technical but excellent, current review of the topic.

In the current issue of the prestigious Journal of Clinical Investigation, a team mostly from the Mayo Clinic Jacksonville has compared the transcriptomes of tissue from cerebrum and cerebellum with autopsy-confirmed PSP to those with Alzheimer’s disease and to controls with no autopsy evidence of neurodegenerative disease.  I’ll cut to the chase:  To the researchers’ surprise, they found the four types of tissue to be quite similar in their transcriptomic profiles.  This result suggests that a treatment or a diagnostic test directed at one or more of the protein (or mRNA!) abnormalities in either disease could work for the other disease as well.  We already suspected that, but without a lot of supporting evidence beyond the superficial observation that both diseases involve tau aggregation.  This new paper provides some better evidence.

I’ll return to the paper’s practical implications in a minute.  But first, back to biology class you go: “O-omics” results (Wikipedia lists 45 kinds of “omics” in biomedicine to date) are by their nature shotgun approaches, typically yielding a long, inscrutable list of statistically significant but questionably meaningful differences between subject groups.  So, seeking some sort of useful pattern in the data, researchers divide the genes, proteins or mRNAs yielding statistically significant “hits” into categories based on their known functions.  These categories are called “networks” because they’re based on interactions or on commonalities of function.  The process of creating and using such standardized, defined categories in genomics is called “gene ontology.”

The authors of the new paper point out that previous transcriptomic work in AD has revealed differences from controls in many such networks, most prominently “immune function, myelination, synaptic transmission and lipid metabolism.”  They also point out that it’s usually difficult to know if these mRNA differences are the cause of the cellular damage in AD or merely the brain’s reactions to that damage.

Now back to this project:  As I said above, the transcriptomic work analyzed not only the cerebral cortex, where the researchers knew ample pathology exists in both AD and PSP, but also, as a kind of control group, the cerebellar cortex, where the standard autopsy shows little or no damage in AD or PSP.  Another feature of the study design was to use the temporal lobe as the source of its cerebral samples.  That part of the brain is heavily involved in AD by standard methods but little or not at all in PSP. 

Surprisingly, the results were that the transcriptome abnormalities (relative to non-neurological controls) were quite similar in all four types of tissue – AD temporal cortex, PSP temporal cortex, AD cerebellar cortex and PSP cerebellar cortex.  In their words, “The DEG [differentially expressed gene] changes between AD and PSP in two regions of the brain demonstrate a striking conservation [consistency] of transcriptomic changes across these different neurodegenerative diseases.”

Because the degree of traditionally measurable cell damage differed markedly across those four sets of samples, they infer that the changes are “upstream,” meaning at an early step in the disease process, rather than “downstream,” in reaction to damage.  That would mean that even in the early stages of AD and PSP, the disease process is already at work in areas that have not started to show physical signs of damage as assessed by microscope, MRI or PET scan.

What were the networks most affected by the transcriptomic changes?  In the authors’ words: “Up-regulated in both AD and PSP were gene networks serving chromatin modification, gene expression, chromosome organization and metabolism of nucleotides. In the cerebellum the shared upregulated genes link to biological processes relating to RNA and RNA transcription, cell-cell junctions, and heart, kidney, gland, and circulatory system development. Shared down-regulated genes in AD and PSP are associated with gene ontology cell compartment terms related to mitochondrial and ribosomal functions in both the temporal cortex and the cerebellum.”  

The paper’s first author is Xue Wang, PhD, a bioinformatician at Mayo.  The last two authors, sharing credit as senior authors, are Todd Golde, MD PhD of the University of Florida and Nilufer Ertekin-Taner, MD PhD.  I emailed Dr. Taner for her take on the results.  She replied in part, “These findings can be leveraged to develop multifaceted therapies and biomarkers that address these common, complex and ubiquitous molecular alterations in neurodegenerative diseases.”  I’d agree.

So, this unexpected discovery suggests that it may prove fruitless to look for causes of specific neurodegenerative diseases in their gene expression profiles.  Abnormal gene expression may not be the true origin of the disease, but only the cell’s reaction – not to downstream physical damage visible with standard tools, but to some other, far more upstream causation.  Yet, interrupting that reaction at the level of its mRNA might be the key to halting the progression of multiple, of not all, neurodegenerative diseases.  Adding to the appeal of that approach is that the treatment targets are available at a very early stage of the disease process.

Testing that suggestion will have to start with analyzing more than just AD and PSP, and I’m guessing that this effort is already in progress thanks to the excellent collection at the Mayo Brain Bank.  I’ll keep you informed.

A shock to the system

Researchers at McGill University in Montreal have reported improvement in gait speed in a woman with PSP using transcranial direct current stimulation (tDCS). 

The research findings that I pass along here are generally of high scientific quality.  This one is only a single case report and was published only as a letter to the editor, which typically meets a lower standard in the peer-review process.  But it’s in a good journal and from a well-reputed group with a long record of accomplishment in a closely related field.  Plus, it’s about a low-risk potential treatment of PSP — a disease without much other treatment.  So – good enough for me.

The paper’s first author is Carlos Roncero, MD, PhD, a psychiatrist and psychologist and the senior (i.e., last-named) author is Howard Chertkow, MD.  Both McGill professors have long and distinguished research records.  Dr. Chertkow has worked extensively on tDCS for Alzheimer’s disease and is perhaps most famous for having developed, along with two colleagues, the Montreal Cognitive Assessment (MoCA), a quick test of general cognition that works very well in PSP and other frontal lobe conditions and is used world-wide. 

There has been research before in both tDCS and in transcranial magnetic stimulation for movement disorders, including a bit in PSP.  But the previous work has used arcane physiological variables or speech as their outcome measures rather than gait or balance.

The methods

The procedure consists of passing a weak electric current through the brain, in this case from two electrically negative electrodes (“anodes”) on the skin, one over each deltoid muscle (at the shoulder), to a single positive electrode (“cathode”) atop the center of the head, where the left and right “primary motor cortices” nearly touch.  Nothing pierced the skin – these were just wires ending in 5 x 7-cm (2 x 3- inch) patches held by adhesive. Each deltoid received 2 milliamps of current over 20 minutes per day for 4 consecutive days per week for 3 weeks.  The patient’s gait was tested during the fourth stimulation of the third week and then monthly for 5 months after the stimulation sessions had ended.  As a placebo control, before the first week of stimulation, they gave the patient a week of sham stimulation, with the apparatus in place but the switch off, and considered the gait result at that point to be her baseline.  The nature of the gait test was the time required to walk 24 meters (26 yards) using the same walker that the patient was using at home.  Three clinicians flanked her to prevent falls but did not touch her or the walker.

Here are the results:

Note that the “interval time” on the vertical axis is the time to cover 3 meters, calculated by timing the patient for the 24 meters and dividing by 8.  The average healthy woman of that age (61) walking at maximum speed covers 3 meters in about 1.6 seconds and walking comfortably in 2.3 seconds. (I calculated those times from the reference data in this publication.)  With only sham stimulation, the patient’s time for 3 meters was 11.92 seconds.  It sped up to 9.46 seconds by the end of the third week. The time improved further a month later and further still a month after that, to 7.47 seconds.  After another month, it started to return to baseline and returned a bit further a month later, but then stabilized at about 9.8 seconds for 3 weeks.  So they resumed the stimulation and the next week brought improvement to 8.72 seconds. 

There are some methodological issues. 

Unfortunately, the gait was not tested pre-sham and the patient was not asked if she knew that she had received a sham treatment and when it was given.  If the real stimulation had produced a bit of an electrical sensation in her skin, that could have had a placebo effect with a resulting false-positive result.

Secondly, we don’t know how much, if at all, the speed would have improved beyond that 8.72 seconds if after 5 months had they had given the treatment for the same 3 weeks as the first time.  We also don’t know if this degree of improvement made a difference to the patient’s activities of daily living; nor if it was accompanied by an increased risk of falling not observable over the short time sample of the tests. 

Another caveat is that the gait was assessed using a simple timing of gait speed with a walker rather than with an automated gait analysis system.  Such devices are available commercially and typically have the patient placed in a harness to prevent falls and monitor dozens of variables transduced through electronic contacts embedded in a long walking mat. 

The clinicians with the patient during her walking tests were aware of whether she was receiving sham or real treatment and could have unconsciously influenced her performance. 

Bottom line

In summary, this harmless electrical stimulation procedure may eventually prove to give moderate improvement in gait speed in people with PSP, with long-term retention of benefit.  This result could serve as justification for a grant to study the issue in a larger group of patients and using more rigorous procedures and an assessment of improvement in the patients’ activities of daily living.

The methodologic informalities I’ve complained about are standard in exploratory research, which I’m sure is why this prestigious journal’s editor accepted the manuscript.  This is a good example of how science doesn’t just come up with new knowledge, “eureka!”-style.  The process is full of fits and starts, blind alleys, disagreements, human error, and lots of sweat, with one piece providing a toehold for the next until something useful emerges and is confirmed by others.   

OK, so maybe we do have a marker.

You may recall a post from last week lamenting the state of diagnostic markers for PSP.  But now I’m happy to report that things are starting to look up. 

A paper in the current issue of Movement Disorders is from a group at Fudan University in Shanghai led by Dr. Ling Li.  Two of the 17 authors work at Taiwan-based Aprinoia Therapeutics.  Last on the author list is the “Progressive Supranuclear Palsy Neuroimage Initiative” (not to be confused with the 4-R Tau Neuroimaging Initiative based at UCSF under Adam Boxer).  I don’t know if the PSPNI is an academically-based research group or a consortium created by Aprinoia.  I’ll try to find out.  In any case, Aprinoia is developing a PET tau ligand called [18F]-APN-1607, formerly known as [18F]-PM-PBB3. 

The new report in the current issue of Movement Disorders is from group from Fudan University in Shanghai led by Dr. Ling Li.  Two of the 17 authors work at Taiwan-based Aprinoia Therapeutics.  Last on the list of authors is the “Progressive Supranuclear Palsy Neuroimage Initiative” (not to be confused with the 4-R Tau Neuroimaging Initiative based at UCSF under Adam Boxer).  I don’t know if the PSPNI is an academically-based research group or a consortium created by Aprinoia.  I’ll try to find out.  In any case, Aprinoia is developing a PET tau ligand called [18F]-APN-1607, formerly known as [18F]-PM-PBB3. 

First, a little background:

What’s a “PET ligand”? What’s “PET”? Positron emission tomography is a way of mapping the locations of a specific compound (called the “target”, typically a protein of some sort) in the body. First, a compound (the “ligand”) that can bind to the target, and hopefully only to that target, is formulated, and that’s the hard part scientifically. Then the ligand is attached to an atom that emits radiation, specifically positrons, for a time that’s short enough to avoid poisoning the patient or the environment. The most common positron-emitting atom is fluorine-18, but carbon-11 is another common one you’ll see. The resulting compound is injected intravenously into the patient. In about an hour or so, the ligand has bound to its target molecule. After a positron has traveled about a millimeter, it has lost enough energy that when it next hits an electron, the two annihilate each other, emitting two photons (in this case also called gamma particles) in opposite directions. The patient is precisely positioned next to a type of camera that can detect these, and when it detects two photons at exactly the same time, it calculates their common point of origin and puts a dot on its software map accordingly. The result is a series of 2-dimensional slices showing the locations of the positron emitter with its ligand. PET images are initially just shades of gray but for ease of eyeball interpretation are typically displayed in an arbitrarily chosen array of colors, with the “cool” blue colors signifying low ligand uptake and “hot” reds the highest uptake.

Although the FDA approved Tauvid (flortaucipir; [18F]-AV-1451; [18F]T807) in May 2020 as a tau-directed PET ligand for Alzheimer’s, neither that compound nor several other candidates have proven adequate in PSP.  The main reasons have been that the “tau burden” in PSP is only 1% of that in AD, which makes the PET signal insufficiently distinguishable from the normal brain’s background.  Also, PET in general has a much lower spatial resolution than MRI or even CT, so the small size of PSP’s specific areas of involvement makes it hard for PET to distinguish PSP from other disorders.  Another issue has been non-specific binding. That is, some candidate tau PET ligands bind less to tau than to other compounds that tend to occur in the same set of brain cells but may not be affected much in PSP.  A good example has been [18F-THK-5351, which distinguishes PSP from healthy people, but was found to bind mostly to monoamine oxidase B, an enzyme important in dopamine metabolism.

Another ligand, [18F]-PI-2620, has avoided that pitfall and distinguishes PSP from healthy controls.  It has not yet been shown to distinguish PSP from other atypical parkinsonisms, though adequate studies of that question have not been published.  Nor has [18F]-PI-2620 been tested in patients with early PSP, where there is greatest need for a diagnostic marker – the average PSPRS score of the patients in the one published diagnostic study was 38 (0 normal, 100 worst possible), by which time PSP is usually easily diagnosable at the “bedside.” (  Nor has that ligand been tested for its ability to distinguish PSP-RS from other subtypes or to track disease progression over time.

Another ligand,[18F]-PI-2620, has avoided that pitfall and distinguishes PSP from healthy controls.  But it has not yet been shown to distinguish PSP from other atypical parkinsonisms, though adequate studies of that question have not been published.  Nor has [18F]-PI-2620 been tested in patients with early PSP, where there is greatest need for a diagnostic marker – the average PSPRS score of the patients in the one published diagnostic study was 38 (0 normal, 100 worst possible), by which time PSP is usually easily diagnosable at the “bedside.” (  Nor has that ligand been tested for its ability to distingish PSP-RS from other subtypes or to track disease progression over time.

This week’s development

The news flash is that [18F]-APN-1607, has leapt ahead of [18F]-PI-2620, at least for now.  (Not that we shouldn’t have multiple tau PET ligands for PSP with slightly different properties for different clinical situations – that would be great!)  The paper of Li et al included 20 patients with PSP (a lot for an early-phase PET study), of whom 16 had probable PSP-Richardson syndrome, 2 had PSP-parkinsonism, 1 had PSP-progressive gait freezing and 1 had “suggestive of” PSP.  Their average PSP Rating Scale score was 31.6, which is toward the milder end of the range typical of PSP drug trials and milder than the patients in the l[18F]-PI-2620 trial.  There were also 7 with MSA-parkinsonism, 10 with Parkinson’s disease (both of which are alpha-synucleinopathies, not tauopathies) and 13 healthy controls.  The results were corrected for any effects of age, sex, or disease duration and for multiple comparisons.   

The study found that [18F]-APN-1607 PET shows major differences between PSP and healthy controls in 12 brain regions known from autopsy studies to be affected most in PSP. The same could be said for the comparisons of PSP with Parkinson’s or MSA-P, although when only the putamen (part of the basal ganglia) was considered, 4 of the 7 patients with MSA-P had as much binding as those with PSP.  So the authors combined the measurements from the substantia nigra (part of the midbrain, which is part of the brainstem) with those of the putamen, achieving much better separation.  Still it was far from perfect: The standard measure of diagnostic accuracy at an individual patient level, as opposed to merely comparing two groups’ average measurements, is the area under the receiver operating curve (AUC).  That statistic, where perfect is 1.0 and useless is 0.5, takes into account both sensitivity and specificity.  The AUC based on [18F]-PI-2620 uptake in putamen and midbrain for PSP vs the synucleinopathies was 0.811 and for PSP vs. controls, 0.909.  Good but not great.

When they homed in on the subthalamic nucleus, a tiny area that may be where PSP starts in the brain, the AUC was an excellent 0.935 (0.975 for MSA-P and 0.908 for PD).  But that nucleus is so small relative to the spatial resolution of PET that it could be a problem to train large numbers of radiologists and technicians to measure it in the real world using real-world hardware and software.

Li L, Liu F-T, Li M, et al. Movement Disorders 36: 2314-2323, 2021.

In the figure above, the first and fifth columns are the MRI images used as templates on which the PET images (the colored areas in the other columns) are superimposed. The group of images on the left are axial images through the planes of (from left to right) the pons, midbrain and putamen. On the right are sagittal images through planes a bit left of midline, midline and a bit right of midline. Each row is one patient with the condition listed at the far left. (HC means healthy control.) Note that all three subtypes of PSP show strong uptake of the tracer in the putamen and midbrain and none of the other patients shows this combination. The brain area with the greatest difference between PSP and non-PSP, the subthalamic nucleus, is too small to appear to the naked eye as a clear and separate dot in these images.

Flies in the ointment

A major pitfall for [18F]-PI-2620 is its sensitivity to light, which renders it inactive.  A solution to this problem would require not only opaque containers, but also opaque IV tubing.  This can be achieved by wrapping transparent tubing in foil, a standard procedure in hospitals for other photosensitive drugs, but one with obvious drawbacks.

The study of Ling et al did show, for several brain regions, a weak correlation of PSPRS score with [18F]-PI-2620 uptake.  The association was best for the raphe nuclei, an area of the pons (in the brainstem) with widespread connections that use serotonin as their neurotransmitter and are most closely associated with control of sleep.  Weaker, but still statistically significant associations were found also for 5 other areas.  Another selling point for [18F]-PI-2620 is that the PET signal did not correlate with the subject’s age, suggesting that the uptake is related to the severity of the illness and not some effect of aging in the context of illness. However, the duration of illness did not correlate with [18F]-PI-2620 uptake, suggesting that this technique might not be able to document PSP progression or its slowing in response to treatment in a drug trial.

Another issue left untouched by the new publication is whether [18F]-PI-2620 can distinguish PSP from CBD.  That would require subjecting patients with corticobasal syndrome (CBS) to amyloid scanning to rule out Alzheimer’s disease as the cause of their CBS, leaving a tauopathy as the most likely, but not the only, explanation.  Nor were non-Richardson PSP subtypes evaluated, other than in those 2 patients with PSP-P. 

A possible flaw in the methodology is the relatively slow progression of disability in this group of patients (on average, 0.70 PSPRS points per month, compared with about 0.92 in other studies), suggesting some sort of atypicality (or a difference of definitions of the date of onset).  Another is that the PET measurements were obtained at one point in time, which may not have been the best point given the rate of brain uptake and metabolic breakdown of the [18F]-PI-2620.  Using a rate of uptake over time rather than an absolute maximum would have been preferable and is the current state of the art.

Ling et al emphasize that their study is only the beginning of the clinical evaluation of [18F]-PI-2620 in PSP.  Future studies should include larger numbers of patients, more non-Richardson types, CBD, and a repeat scan in each patient after 6 months or more in order to assess the ability of the technique to document disease progression in individuals.

But it’s progress!

My last two posts summarized the portions of the PSP Study Group’s October 4 meeting on imaging, markers and longitudinal observational studies.  This one’s on the current state of neuroprotective clinical trials.  The information is from presentations by Adam Boxer and Günter Höglinger and from informal contributions by other attendees.

First, some background

“Neuroprotective” means slowing or maybe halting the progression of the underlying disease process without improving the current symptoms or disability.  It is to be distinguished from “symptomatic” treatment, which only helps the symptoms or disability, typically transiently, while the underlying process continues. 

The four most recent failed neuroprotective treatments have been davunetide, a neurotrophic (i.e., neuron growth-promoting and repair) agent; tideglusib, a kinase inhibitor (that works by preventing abnormal attachment of phosphate groups to tau), tilavonemab and gosuranemab (both monoclonal antibodies directed against the “first,” or N-terminal, end of the tau molecule). None of these four slowed PSP progression as measured by the PSP Rating Scale or any other bedside test, although there’s controversial evidence that tideglusib slowed the progression of atrophy in relevant brain areas on MRI.  

Other hopeful PSP neuroprotective agents that have failed to work in double-blind trials in recent years.  These, in no particular order, are salsalate, an approved non-steroidal anti-inflammatory drug that reduces tau phosphorylation; TPI-287, an anti-cancer drug that improves microtubule function; coenzyme Q-10, a nutraceutical that enhances mitochondrial energy production; Juvenon, an antioxidant; pyruvate, creatine and niacinamide, other antioxidants; riluzole, a drug with multiple mechanisms that is approved for neuroprotection in ALS, where its benefit is minimal; rasagiline, an inhibitor of monoamine oxidase-B, an enzyme that produces toxic free radicals from dopamine; lithium, an approved drug in psychiatry that reduces tau phosphorylation; valproate, an approved drug in psychiatry and for epilepsy that does the same; and methylene blue, an approved drug for multiple medical problems that inhibits tau aggregation.

Monoclonal antibodies

We don’t know why the antibodies have failed to date.  Maybe tau’s the N-terminal isn’t consistently present or accessible to antibodies in whatever form of tau is relevant to the spread of PSP through the brain.  Maybe the trials started too late in the course of the disease.  Maybe not enough of the antibody was able to cross the blood brain barrier, even though the tau content of the spinal fluid as measured in the lumbar space (not near the brain) was dramatically reduced.  Maybe tau is protected from antibodies as it moves between neurons by some sort of bubble-like or bridge-like membrane structure.  Maybe the cell-to-cell transmission of tau isn’t the most critical or rate-limiting step in the pathogenesis of PSP. 

A promising bit of support for N-terminal antibody treatment comes from three patients who participated in the gosuranemab trial’s site at the University of Pennsylvania who later died and were autopsied.  Their brains showed changes in the glial cells suggesting that the antibodies had incited a clear anti-tau reaction that was absent in untreated patients with PSP.  Although the neurofibrillary tangles and other visible, insoluble tau deposits were unchanged by the antibody, the authors of the paper (and I) conclude that maybe all that’s required for clinical efficacy is some tweaking to the antibody, to its dosage, to its ability to cross the blood-brain barrier, or to the stage in the course of PSP when it’s given.               

Despite the failure of the two antibodies so far and our shortage of explanations, drug companies have continued to develop monoclonal antibodies against tau.  These are being tested (almost) exclusively in Alzheimer’s for the near future.  Zagotenemab (LY3303560, from Lilly) and semorinemab (RO7105705, from Roche) are both directed against tau’s N-terminal.  BIIB076 (from Biogen) and JNJ-63733657 (from Johnson & Johnson) are directed against tau phosphorylated at position 217.  Bepranemab (UCB0107 from UCB) and E2814 (from Eisai) target the mid-portion of tau.  Lu AF87908 (from Lundbeck) targets phosphorylated amino acid 396, near the C-terminal.  The lone PSP trial of any of these is a Phase 1b (i.e., double-blind but designed to test safety, not efficacy) trial of beprenamab at one center in Germany.  Even if the drug does well in that trial, further efforts are planned only for Alzheimer’s for the time being.

Anti-sense oligonucleotides

A Phase 1, double-blind trial of NIO752, an ASO from Novartis, is in progress at 7 sites in the US, 2 in the UK, 2 in Canada and 5 in Germany.  The 48 patients on active drug will be divided into three groups, each with a different dosage level, and 12 patients will receive placebo.  The lowest dosage level will start first and the next will start only if there is no immediate safety issue with the first. The drug must be given by intrathecal injection, which means directly into the spinal fluid by injection into the thecal sac at the base of the spine.  The procedure is identical to a diagnostic “spinal tap” except that that’s a fluid removal for diagnosis and this is a fluid administration for treatment.  This will be performed 4 times at 1-month intervals followed by another 3 months of observation.  More info is here.

ASOs are short strands of RNA with multiple mechanisms of action, each at a different step in the process of translating information from the MAPT (microtubule-associated protein tau) gene into the tau protein.  Many experts feel that this approach, being far “upstream” in the pathogenetic process, is the most promising of the current neuroprotective ideas for the tauopathies.  Obviously, the issues of safety and convenience of monthly spinal taps are potential obstacles.  ASO neuroprotection against Huntington’s disease, where the aggregating protein is “huntingtin,” was reported in June 2021 to have failed, but so little is known of the mechanisms of ASOs that this is not necessarily bad news for the tauopathies.

OGA inhibitors

To self-plagiarize from a 2015 post, a class of experimental drugs for the tauopathies “reduce tau aggregation by inhibiting OGA (O-GlcNAcase; pronounced “oh-GLIK-na-kaze”). That enzyme removes the sugar N-acetyl-beta-D-glucosamine from either serine or threonine residues [amino acids] of proteins. The opposing reaction, catalyzed by O-GlcNAc transferase, like other post-translational modifications, is a common way for cells to regulate proteins. In the case of tau, having that sugar in place reduces aggregation.”  Got all that? A major plus for the OGA inhibitors is that they, like most enzyme-inhibiting drugs, are small molecules, which means they can be taken orally.

Trials of OGA inhibitors for PSP have not yet begun and there’s no clue in the grapevine as to when that might happen.  But a first-in-human study of ASN-51 (from Asceneuron) in 40 patients with Alzheimer’s is under way in Australia. 

My sources tell me that Merck has another OGA inhibitor that has not yet started clinical testing.  It’s not even listed as a pre-clinical candidate in the latest revision of Merck’s publicly available, on-line pipeline info, which was last updated on July 27, 2021.

Although salsalate failed to slow PSP progression, another approved non-steroidal anti-inflammatory called tolfenamic acid reduces tau production. A single-center, Phase 2a trial had planned to start enrolling 24 patients with PSP at the Cleveland Clinic in Las Vegas in early 2021, but the trial start is delayed indefinitely.  The drug is available by prescription for migraine in the UK and some other countries but not in the US. 

Finally, AZP2006 (from AlzProtect) activates secretion of progranulin in the brain, reducing inflammation, and also has an independent action as a tau anti-aggregant.  It is given as an oral solution.  A Phase 1 trial in progress at three centers in France and a Phase 2 trial at the same three sites is planned.

For more technical details on neuroprotection (and symptomatic treatment) in PSP, see the excellent recent review by Lawren VandeVrede and colleagues from UCSF.

Our CurePSP Centers of Care review is mostly on symptomatic PSP treatment but includes a section on neuroprotection.

Markers: the longitudinal approach

We got plenty of candidate PSP treatments.

We got drug companies willing to risk their resources on trials for a rare disease.

We got clinical trial sites with proven records of efficiency. 

We got patients willing to make the sacrifices demanded by clinical trials. 

What ain’t we got? 

We ain’t got markers. 

(Deepest apologies to Rodgers and Hammerstein.)

Markers in this context are simply diagnostic tests, and there are two kinds – trait markers and state markers.  Trait markers allow us to distinguish people with from those without the disease, preferably in a very early stage, where treatments designed to prevent further decline would be most likely to occur and most useful to the patient.  Trait markers also allow us to exclude from a PSP trial any people who don’t actually have PSP.  State markers, on the other hand, quantify the amount of damage that’s already occurred and the degree of benefit of the experimental treatment. 

The best trait marker for PSP to date is purely clinical, meaning that it does not require any sort of imaging, automated measurements of movement, gene testing or chemical testing of body fluids.  That’s the MDS-PSP Criteria, published in 2017. Other types of tests can help exclude from consideration other conditions such as Alzheimer’s, Parkinson’s, MSA, normal-pressure hydrocephalus and vascular parkinsonism, but they are only helpful in those cases where a specific alternative diagnosis is plausible.  They don’t positively diagnose PSP; they only rule out other things.

Two up-and-coming trait markers for PSP are spinal fluid levels of tau with a phosphate group on amino acid 181 (Ptau181) and neurofilament light chain (NfL).  Ptau181 levels are below normal, on average, in all forms of PSP except for the gait-freezing type (PSP-PGF).  This contrasts with Alzheimer’s disease, where that marker is elevated, on average.  The average level of neurofilament light chain (NfL) in the spinal fluid is much higher in PSP and CBS than in controls or Alzheimer’s but is also elevated in many other neurodegenerative disorders.  So the ratio of NfL divided by Ptau181 in the spinal fluid is an good marker for PSP, but cannot distinguish it from CBD, and for PSP-Richardson, it may not be as accurate as the bedside clinical criteria. For ordinary clinical use, a blood test would be easier than a spinal tap, and the utility of these levels as a state marker has not been adequately studied, even for CSF.  That requires a longitudinal study over a period of at least a year.  So the NfL/Ptau181 ratio isn’t ready for prime time as a PSP trait marker, much less as a state marker.

The most widely used state marker for PSP is still the PSP Rating Scale, which is also purely clinical. (Disclaimer: I developed the PSPRS starting in 1995 and published it in 2005 along with my statistician colleague Pam Ohman-Strickland.)   It takes 15 minutes to administer and requires no equipment other than an armless chair, a cup of water to test swallowing — and the apparatus between the neurologist’s ears.  In recent years, modifications of the PSPRS have been shorter, easier to administer by laypersons, or more directly reflective of the patient’s daily activities.  Although all of these revisions are valid and have been shown to correlate well with the full, original PSPRS, none has been widely tested in the field, and the PSPRS remains the standard for now.  But it’s not good enough.  Its score is affected by common non-PSP conditions such as injuries, arthritis or strokes, or by PSP-related conditions; for example, orientation testing can be affected by apathy, gait testing by muscle rigidity, blepharospasm by Botox and everything by dehydration or malnutrition.  So there’s a lot of variance in the PSPRS as measured from one visit to the next.  This dictates that trials be large enough and long enough to cancel out the “statistical noise,” and that costs money.

A longitudinal study is observational – it includes no treatment.  It enrolls patients with the disease of interest, or sometimes also healthy people with histories suggesting a high risk of developing that disease.  Many longitudinal trials also enroll control subjects with no apparent risk for the disease — typically spouses, relatives or friends of those in the first two groups.  All of the subjects undergo tests at entry using whatever diagnostic procedures are being evaluated as markers, some of which are repeated periodically.  The study follows the patients through their course, at least with interim histories and physical exams.  If feasible and appropriate, autopsies are obtained to verify the diagnosis and to correlate specific autopsy features with diagnostic test results during life.  The goal is to identify which, if any, of the diagnostic tests prove able to accurately identify people with the disease in the earliest stages and which can track their subsequent course with precision.

There are presently at least 8 PSP longitudinal studies in progress: 2 in Germany and 1 each in India, Italy, Japan, Luxembourg, the US/Canada and the UK.

At the PSP Study Group meeting on October 4, James Rowe of Cambridge updated the group on the longitudinal PROSPECT-M-UK study, which is headed by Huw Morris of University College London. (“M” is for MSA, a late addition.) It now includes 21 academic clinic sites in the UK and about 700 patients, of whom about 100 have made more than the initial visit.  They have found that using MRI measures of atrophy of certain regions of the cerebrum is more precise than the PSPRS, reducing the number of patients needed for a treatment trial by nearly half.  The measures were atrophy of frontal and temporal lobes and enlargement of the lateral ventricles, an indirect sign of diffuse cerebral atrophy.  This confirms and extends the findings in the two trials of monoclonal antibodies that failed to help PSP, where MRI at the start and end of the studies provided a sharper picture of the patients’ progression than the PSPRS.  The reduction of the sample size was even more marked for CBD, but in fairness, the PSPRS was not designed for that disease.  One of the PROSPECT-M-UK study’s specimen collections is skin biopsies.  These can be used to look for tau aggregation in nerve endings, a potential early-stage, only slightly invasive trait marker.  Skin biopsies can also be used to create stem cells, which are then converted into neuronal cultures in which experimental treatments can be tested.  In this case, each such “brain in a dish” will come with a detailed, standardized clinical record.  Even more important, that lab model is not a mouse with a PSP-like condition, but a human being with real PSP.

LK Prashanth of Vikram Hospital in Bangalore described the longitudinal PSP study being conducted by the Parkinson Research Alliance of India. The Pan-India Registry for PSP (PAIR-PSP) includes 15 centers with 68 patients, with a goal of 1,000 over the next 2 years.  They are performing whole genome sequencing along with more conventional measures.  They have found that PSP-Richardson syndrome, the classic form, exists in only 25% of their group.  Next is PSP-parkinsonism with 22% and PSP-CBS with 18%.

Martin Klietz of Hannover Medical School updated the group on the two German studies, DESCRIBE and ProPSP.  The first has enrolled 400 patients, the second, 276.  Each study covers the entire country, although one is based in the north, at Hannover and the other in the south, at Munich.

Rejko Krüger of the University of Luxembourg mentioned that his institution’s longitudinal Parkinsonism study, which recruits from that small country as well as nearby areas of France, Germany and Belgium, has recruited 80 patients to date, and is collecting skin biopsies and spinal fluid in addition to the usual imaging and clinical markers.

Takeshi Ikeuchi of Niigata University, Japan, described the Japanese Longitudinal Biomarker Study in PSP and CBD (JALPAC).  It has accumulated 337 patients with at least one visit, of whom 257 have had at least two.  They found PSP-Richardson in a slightly higher percentage, 35%, than did the study in India.  They found a good correlation of the PSPRS with disease duration but, as expected, wide range of velocities of progression across patients. 

No one at the meeting provided an update on the US/Canada study, which focuses not specifically on PSP or CBD, but on a much more inclusive disease category called frontotemporal dementia (FTD).  PSP and CBD are often classified within the category of the FTD’s because they usually feature dementia of frontal lobe origin.  The protein aggregating in the brain cells is different in the various FTD diseases – tau, TDP-43 and FUS are the most common.  The study, called “ALL-FTD,” is headed by Brad Boeve at the Mayo Clinic Rochester and Adam Boxer and Howard Rosen at UCSF.  It presently includes 21 sites in the US and 2 in Canada. The longitudinal arm has a goal of 1,100 patients and the biofluid-focused arm, with just one visit apiece, aims for 1,000 patients.  I’ll let you know about current PSP enrollment once I can squeeze that out of someone, but for more info, try their website.

Gabor Kovacs of the University of Toronto described a project based in Japan to study “incidental PSP.”  This is early brain changes of PSP that had not yet started to cause symptoms by the time of death.  It is found in specimens donated by families whose loved one died without known neurological illness.  One such collection, at Banner Health in Arizona, found very mild PSP pathology in 5% of their autopsied brains.  This means that 5% of the elderly population may be incubating PSP.  Of course “may” is the critical word, but analyzing the medical, genetic, and toxin exposure backgrounds of such a large group of people, even in retrospect, could provide valuable clues to the cause of PSP.

Solving our image problem

Günter Höglinger of Hannover University in Germany is probably the world’s most productive PSP researcher right now.  A few years ago, he organized a PSP Study Group as part of the International Parkinson and Movement Disorder Society.  Most of the 51 members are European – I’m one of the 11 US members.  The PSPSG’s main accomplishment to date is developing and publishing a new set of diagnostic criteria for PSP.  The group meets for a couple of hours in person every year in conjunction with the IPMDS’s conference, but of course, the past two meetings have been on Zoom.  The agenda is to informally discuss our recent research activities and ideas.

This year’s meeting was held on October 4, 2021.  Here’s a boiled-down, edited and explicated version of the proceedings.  The topics were classified into imaging, longitudinal studies, fluid markers and treatment.  As per last week, it will be the installment plan: Each of those four topics will be a separate post here on PSP Blog.


James Rowe of Cambridge University, a legitimate rival to Günter as the world’s current leading PSP researcher, described the value that 7-Tesla MRI brings to PSP research.

Most standard MRI scans for medical care use a magnetic field strength of 1.5 Tesla and a growing number use 3 T for additional resolution.  But about 100 research MRI machines world-wide are capable of 7 T imaging.  This provides, for the first time, a clear image of the locus ceruleus (LC), a cylindrical cluster of blue pigmented cells in the brainstem that uses noradrenaline as its neurotransmitter.  It averages 14.5 mm in length but only 2.0 mm in diameter, making it difficult to see with conventional 1.5 T or 3.0 T MRI.  It supplies input to many other brain areas and degenerates in PSP and other neurodegenerative disorders.  Dr. Rowe hopes that the rate of worsening of atrophy of the LC on 7 T MRI may be usable as an outcome measure in PSP neuroprotective treatment trials.

A technique called magnetic resonance spectroscopy (MRS) uses existing MRI machines to provide not an anatomical image, but a measure of levels of some kinds of chemicals in specified areas of brain tissue.  It’s currently used mostly in brain tumor diagnosis. (Side note: MR spectroscopy long antedates MR imaging, which essentially takes MRS measurements of multiple pencil-shaped volumes of tissue sharing a slice of brain and then uses a computer to reconstruct those numbers into a two-dimensional image.) 7T MRI provides greater resolution here as well.  Dr. Rowe reported that he is studying the effects on circuits in the cortex of tiagabine (brand name Gabitril), an approved epilepsy drug that increases levels of the inhibitory neurotransmitter gamma-amino-butyric acid (GABA).  A similar drug is atomoxetine (brand name Strattera), which is approved for attention-deficit hyperactivity disorder.  Dr. Rowe is leading a clinical trial of that drug for disinhibited behavior, apathy and impulsivity in PSP.  A secondary outcome measure in that PSP trial, i.e., one that will not be critical to the study’s conclusions because it’s still an experimental test, is using 7T MRS to assess GABA levels in selected brain areas.   

Adam Boxer of University of California San Francisco, yet another very prolific PSP researcher, described the progress of his NIH-supported project, “4-repeat tau neuroimaging initiative,” or 4RTNI (pronounced “Fortney”).  The study is following patients with PSP or CBS every 6 months using MRI to track atrophy and tau PET to track tau aggregate accumulation.  The study also includes clinical evaluations, plasma levels of Ptau217 (tau with a phosphate group attached at amino acid number 217) and PET scans for beta-amyloid to detect Alzheimer’s disease (AD), which in an atypical form is the pathology underlying many cases of CBS.  The goal is to develop better diagnostic tests and progression markers for use in future PSP and CBD treatment trials.  While the Richardson syndrome clinical picture is almost always explained by underlying PSP pathology, an especially pressing issue is to distinguish CBS caused by CBD pathology (CBS-CBD) from CBS caused by AD pathology (CBS-AD).  Plasma levels of Ptau217 are very high in people with AD pathology, either as classic clinical AD or as CBS-AD, but normal in CBS-CBD and CBS with other pathologies.  A commonly used statistical measure of accuracy, the “area under the receiver operating characteristic (AUC),” for plasma Ptau217 in distinguishing CBS-AD from CBS-CBD is 0.96, very close to the theoretical ideal of 1.0.  However, that’s for advanced cases.  The test’s utility in early cases, where it’s likely to be needed most, is much less so far.  Dr. Boxer tentatively concludes that in distinguishing CBS-CBD from CBS-AD, plasma Ptau217 is almost as accurate as amyloid PET, the current standard, a much more difficult and costly procedure.

Dr. Boxer discussed another project in progress within the 4RTNI umbrella to help distinguish CBS-AD from CBS-CBD or CBS-PSP (i.e., to allow patients with CBS to participate in anti-tau treatment trials).  His research group combined a measure of cortical atrophy with one of midbrain atrophy using a Bayesian logistic regression.  That’s a technique that allows one to create a statistical “model” of a phenomenon, or to dissect its component parts, by successively trying different solutions and tweaking each based on the previous result.  This is different from traditional statistical models, which use event frequencies rather than successive refinements of an a priori hypothesis.  Look it up.  They were able to achieve an AUC for CBS-CBD vs. CBS-PSP to 0.95 for patients presenting with motor signs and 0.91 for those presenting with non-motor signs. 

Next post: Longitudinal PSP studies

The tao of tau: Part 2

As promised, here’s the second of two installments on the latest in tau-ology, at least as of the Tau2020 conference, held in February 2020. Yesterday’s post covered treatment, and today’s, everything else. There was no Tau2021, but Tau2022 is planned for February.

Tau structure and function

More is becoming known about the N-terminal domain of tau, where exons 2 and 3 are alternatively spliced (i.e., some forms of tau have the amino acid sequence encoded in the MAPT gene’s exon 2, others have those of exons 2 and 3).  This end of the protein is now suspected of controlling the spacing between microtubules, which comprise the cell’s internal skeleton and transport system.  Next to that is the “proline-rich domain,” which interacts with enzymes and with other proteins that include the WW domain.  That’s where a protein includes the amino acid tryptophan occurring in a string that regulates signaling between proteins. This is part of the new body of evidence that tau is partly a signaling protein.  (Fun fact: The single-letter amino acid abbreviation system (allegedly) assigned “W” to tryptophan because T was taken and part of the tryptophan’s molecule has a W shape.)  

The highlight of the conference was probably the keynote talk from Dr. Michel Goedert describing his group’s work on high-resolution imaging of tau using cryo-electron microscopy. (See my recent post, “A frozen treat” for details and an update.)  The bottom line was that each tauopathy has its own, very different, pattern of tau misfolding that is uniform across patients with that disorder.  That’s true even for disorders with the same isoforms, like PSP and CBD, which are both 4R (i.e., 4-repeat; having 4 microtubule-binding areas) but have radically different tau misfolding patterns.  Better understanding of these structures may point to disease-specific diagnostic and therapeutic innovations.  This starts to undermine the hope that PSP will provide the key to all tauopathies, including Alzheimer’s. For this and his other work, Dr. Goedert was awarded the $250,000 Rainwater Prize at this conference.

Post-translational modifications of tau

You know about tau phosphorylation, and the textbooks say that it can result from “stress.”  But more recent research has identified some specific causes such as brain ischemia, as from atherosclerosis; brain trauma; and excessive sodium intake.  The last works through activation of the immune system.

A recently-identified role for tau is to protect DNA in the cell’s nucleus from oxidative stress, which can result from certain toxins or from mitochondrial dysfunction.  Tau seems to help the histone proteins in the nucleus do their job of regulating access to the genome.

Abnormally phosphorylated tau in the cytoplasm can indirectly affect the function of DNA in the nucleus in many ways.  These include deranging the function of actin and microtubules, which maintain the structure of the nucleus, damaging DNA, damaging the protein portion of chromosomes, affecting RNA handling, reducing ribosome stability and encouraging DNA code rearrangements.  The next problem is to determine exactly which PTM’s do what and to develop drugs specifically targeted at those that actually cause neurodegenerative diseases. In fact, we don’t yet even know for sure that hyperphosphorylated tau causes tau aggregation in humans.

Although we know of 50 different mutations in the MAPT (i.e., the tau) gene that cause or increase risk of tauopathies, none of them is known to act via causing PTM’s.  This confirms that the pathogenesis of PSP is multifactorial and that the combination of factors differs across individuals.

Tau genetics

It has been known since 1999 that PSP is associated with a genetic variant called the H1 haplotype.  This consists of a section of chromosome 17, comprising the MAPT gene and about 15 others, that is reversed relative to the rest of the chromosome.  More recently, variants, especially the H1c subhaplotype, have been discovered that associate more strongly and specifically with PSP. The mechanism may be to allow the protein transcription machinery access to other areas of the genome where the handful of other genes associated with PSP have been identified. 

A project under the aegis of the Tau Consortium and the scientific leadership of Celeste Karch, Alison Goate and Sally Temple has created a collection of cells from 140 (and counting) volunteers with pure tauopathies such as PSP and some of their family members.  Some of the cells are fibroblasts taken from skin biopsies, others are stem cells (i.e., induced pluripotent stem cells, IPSCs) made from such fibroblasts and still others are neural cells made from those stem cells.  Some of the volunteers had single-gene causes of their illness and others had a single genetic variant that increases risk for PSP but is insufficient to cause it.  Some of the cells from the latter group with a single, identifiable mutation have had that mutation corrected using CRISPR, leaving a cell culture with only the poorly-understood genetic “background” necessary to cause the disease.  This represents a valuable tool for studying garden-variety, “sporadic” (i.e., apparently with no familial clustering) PSP. The cells are offered as a resource to carefully vetted researchers worldwide.

Mutated versions of a gene called LRRK2 (“lark-two”) are known as a group to be the strongest genetic contributors to Parkinson’s disease.  But strangely, a few people with some of LRRK2 mutations, including the most common one, develop PSP rather than PD. Now, Drs. Edwin Jabbari, Huw Morris and colleagues used samples from the UK’s Parkinson’s Disease Society Brain Bank to find that a genetic marker close to LRRK2 is associated with more rapid progression of PSP.  LRRK2 is known to affect disposal of dysfunction or excessive protein, possibly including tau. It also is involved in neuroinflammation. Both mechanisms are at or near the top of the current list of contributors to the pathogenesis of PSP.  The logical next step is to develop drugs suppressing the toxic activity of the enzyme produced by the LRRK2 gene.

The significance of the extreme predominance of 4-repeat tau in the tangles of PSP and CBD remains unclear.  (Most other tauopathies have an equal combination of 3-repeat and 4-repeat tau “isoforms,” mimicking tau in normal human brain, and a few tauopathies have predominantly 3-repeat tau.)  At Tau2020, The Rainwater Charitable Foundation awarded its early-career award to Dr. Patrick Hsu of the Salk Institute for a new technique that allows researchers to control the number of repeats in tau.  It’s based on the same general type of RNA-manipulating technology as CRISPR-Cas9, but in this case it’s called CRISPR-CasRx. It can be adapted to manipulate “alternative splicing” not only of tau, but also that of many other proteins that, like tau, have multiple isoforms. So far, the technique is only for neuronal cell cultures, but it opens up a world of potential experiments to fix the molecular variation in tau underlying PSP and CBD.

Prion-like tau propagation

We know that in cells growing in a researcher’s dish or in a mouse’s brain, misfolded tau introduced into the system can travel cell to cell, templating new copies of itself along the way.  But the details of the process and its relevance to the human tauopathies remain unclear.  In fact, in no human tauopathy has such a process been conclusively demonstrated, although it has been clearly observed in the prion protein disorders such as Creutzfeldt-Jacob disease or mad cow disease.  So, as we try to confirm the prion-like hypothesis in the tauopathies, we do have to remain open to alternative ideas to explain the spread of the disease within the brain.

A clue to why tau is secreted by brain cells could be its recently-discovered role as a local hormone or signaling molecule, to regulate the activity of brain cells and the sleep-wake cycle. However, even here the mechanisms are unclear.  There is recent evidence that tau is released from cells encapsulated in tiny membrane bubbles called vesicles.  In that case, the tau may be protected from therapeutic antibodies designed to slow the spread of tauopathies.  Additional recent evidence has found tau receptors on brain cells consisting of heparan sulfate proteoglycans, low-density lipoprotein receptor-related protein and even amyloid precursor protein.  For now, the science of tau spread remains, like most embryonic sciences, a collection of disconnected observations.

Independent of the specific molecular form or transit vehicle used by tau, imaging studies have recently shown that a prediction of the next brain region to become involved in a progressing tauopathy can be based on the involved area’s concentration of abnormal tau, its synaptic connections and its areas of direct non-synaptic contact with other cells.  Both of the latter two routes operate via active mechanisms; passive diffusion is no longer considered a factor.

Why are tau aggregates toxic?

This is a large, complicated issue, but one recently discovered clue is that brain cells containing tau aggregates, when finding themselves under stress for some other reason, signal the brain’s immune cells to come and engulf them, but without killing them. If this has the effect of protecting the tau-containing cells without preventing them from secreting their tau, then it could mean that a new tau-directed treatment could work better if coupled with a drug that inhibits the brain’s immune function.

Tau-based brain imaging

Positron emission tomography (PET) imaging using a radio-labeled glucose analog as a tracer for energy production is a standard way to help distinguish the frontotemporal disorders such as PSP, CBD and FTD from Alzheimer’s disease.  PET using a dopamine analog can distinguish the atypical parkinsonisms from Parkinson’s disease but is only available for research use.  But we have no PET-based technique to specifically identify PSP or CBD.  PET using a tau tracer is showing excellent results in distinguishing AD from other dementias, but it works poorly for PSP.  One problem is that the spatial resolution of PET is insufficient to show the tau deposits because it sticks to some other normal and abnormal molecules in the same set of neurons.  But many drug companies are developing many tau tracers and a few of them are starting to show more validity for PSP.  One, called [3H]CBD-2115, has shown good accuracy but doesn’t cross the blood-brain barrier.  However, some tweaks to the molecule or to the BBB might solve that problem.  Hopefully, it will be safe to administer and once broken down in the brain, won’t leave any radioactive remains behind for any length of time.

An exciting but early-stage development is that Genentech and AC Immune are developing an anti-tau antibody called semorinemab in tandem with a tau-directed PET tracer called [18F]-GTP1.  The PET tracer, in addition to tracking any slowing of tau aggregation, may also be useful in allowing a measurement of tau aggregation at the study baseline to predict the individual’s disease progression absent any intervention.  That, in turn, could allow a more individualized interpretation of the neuroprotective drug’s benefit.  Although that seems a good model for drug testing, it was announced in September 2021 that in prodromal and mild AD, semorinemab failed to slow progression in most of the outcome measures but did slow progression by 44% in one critical bedside test, the ADAS-Cog11.  The sister trial in moderate AD continues.  The companies have not announced plans for testing in PSP.

The NIH and the Rainwater Charitable Foundation have each created consortia to develop tau-based PET tracers for non-AD tauopathies.  The RCF effort involves the Michael J. Fox Foundation in its efforts to distinguish PD from PSP and CBD.