Tag Archives: mutations

You gotta know when to fold ’em

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The last few posts have been about things at the macro level, from clinical trials to government action.  Now, let’s dive back into some molecular biology — if you’re nerd enough for it.

Yesterday, a paper appeared from researchers at the University of Alberta, in Canada, led by Drs. Kerry T. Sun and Sue-Ann Mok, comparing the folding structure of normal and abnormal versions of the tau protein. 

First, some background.  You all know that proteins are strings of amino acids. The healthy adult human brain has six forms of the tau protein ranging in size from 352 to 441 amino acids.  Tau’s normal job is to maintain brain cells’ internal structure and some other housekeeping tasks.  Tau unattached to something else normally flops around in the cell’s fluid like a piece of overcooked spaghetti in boiling water.  In PSP and the other tau-related disorders, tau becomes abnormally folded onto itself and forms toxic clusters that eventually clump further into neurofibrillary tangles.  Those are visible through a microscope and are critical in the diagnosis of the “tauopathies” although the details of how misfolded or aggregated tau actually causes loss of brain cells remain unknown.

Some more background: Although over 99% of people with PSP have no mutations in the tau gene, there are 50 different mutations in tau that do cause neurodegenerative diseases, many of which closely resemble PSP.  The most widely used experimental animal model for PSP has received a copy of a human tau gene with one of these 50 mutations. 

The new project analyzed the folding structure of normal tau protein and samples of abnormal tau protein, each with one of the 37 most important tauopathy-causing mutations.  It found that, at least as far as this lab technique could determine, no structural difference between normal tau and two of the most popular abnormal versions of tau used in research, the P301S mutation (where the amino acid proline at position 301 is replaced by the amino acid serine) and the R406W (arginine to tryptophan).  Another mutation commonly used in animal models, P301L (proline to leucine) does alter the structure.  That’s the form of tau addressed by the two monoclonal antibodies that AbbVie and Biogen, respectively, recently found did not help PSP. 

Of the other 34 mutations tested, 12 produced no structural change and the location of the mutation had no discernible effect on the folding structure.  Nor did the rate of aggregation influence the resulting structure. 

Interestingly, one of those 12 producing detectable misfolding is the A152T (alanine to threonine) mutation, which is the only single-amino-acid substitution tau mutation we know of that increases the risk of “sporadic” (i.e., non-familial) PSP.   

There are some caveats:

  • This study does not examine the effects of post-translational modifications (PTMs) on the folding structure of tau.  Nor did it study the effects of the various mutations on the ability to accept PTMs.  PTM’s are small molecules such as phosphate, acetate, methyl groups, sugars, and ubiquitin that can be attached to the protein in health to regulate its function, or as an effect of disease processes like PSP. 
  • The study restricted itself to only one of the six adult human tau isoforms, called 0N4R.
  • The 0N4R form of tau has 383 amino acids (the others range from 352 to 441) and locations that can alter the folding pattern occur in only about 45 of those.  So, as you’d guess, an amino acid substitution can change the chemical properties of a protein without changing its folding pattern.  Another major issue is that many of those 45 misfolding spots are hidden inside the folded structure, obscuring them from the researchers’ analysis.

Despite these limitations, we can conclude that the various amino acid substitutions affect the misfolding pattern of tau in different ways.  Any explanation of the cause of ordinary, sporadic PSP at its most profound molecular level can be guided by studying all of those misfolding patterns for hereditary PSP but will also have to take account of whatever bad thing the A152T mutation is doing – and that thing, according to this paper, is NOT to directly cause tau to misfold.

Genetic testing advice

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I’ll plagiarize myself again. CurePSP asked me to write up a piece in response to someone asking if a person with PSP or CBD (or suspected PSP or CBD) should have genetic testing. The short answer is mostly “no,” but here are the details:

People with an established diagnosis of PSP or CBD and a close relative with one of those conditions (or a strong suspicion thereof) should consider having their MAPT gene sequenced.  That’s the gene encoding the tau protein.  But the vast majority of those with PSP or CBD have no similarly affected relatives, and for them, genetic testing is neither necessary nor useful.

The test should sequence the entire MAPT gene and not merely check for the ten mutations, all of them very rare, that have been associated with PSP (seven with CBD) in the past.  If the person with the disorder (called the “proband”) proves to have a mutation in MAPT, then other family members with similar symptoms can be tested as well. 

Variants in eight genes other than MAPT have been associated with PSP and five with CBD, but each of these raises the disease risk only slightly and testing for them is not useful in an individual or even in a family.  That’s because these are only “marker associations,” not specific mutations altering a protein known to be involved in the PSP process. The markers are called “single-nucleotide polymorphisms” or “snips” and unlike the MAPT mutations, they incriminate a span of about 100 or so genes, not a specific gene, much less a specific mutation that can be tested for.

We must keep in mind that many mutations, even in genes like MAPT, are harmless and do not cause disease.  Sixty MAPT mutations have been reported in humans so far, and only ten of them are known to cause PSP. So having a mutation in MAPT and having PSP doesn’t necessarily mean that one caused the other.  If a relative with the same symptoms has the same mutation, it may still not be cause-and-effect, but if that relative is distant, or the proband and two or more relatives share symptoms and a mutation, then the likelihood of cause-and-effect is greater. 

Even if there’s a good statistical likelihood that the proband’s mutation is the cause of their disease, and a healthy relative proves to have the same mutation, one still would not be able to predict how soon the relative might start to develop symptoms.  That information could be available soon from some other type of “pre-symptomatic” or “predictive” testing, but no such test has yet proven to be sufficiently accurate in such a situation.

People with suspected PSP or CBD with no relatives with similar symptoms have no need for genetic testing.  Even if one of the known PSP/CBD-causing mutations were found, it would not contribute much to the likelihood of PSP or CBD as the diagnosis explaining the symptoms.  The same is true for the healthy relatives of someone with established PSP/CBD.

So, as you can tell, estimating disease risk from genetic testing can be complicated.  That’s why a professional genetics counselor or a physician with expertise in adult neurogenetics should advise anyone considering having family genetic testing for PSP or CBD. Don’t just rely on the simple report supplied by 23andMe.

I’ll update this as necessary.

Is PSP genetic? An update

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Last week, someone wrote to CurePSP asking if PSP was genetic. I took a look at what I had previously provided CurePSP on that topic to post on its website, and decided it wasn’t nearly detailed enough. So I decided to write up the following. A version of it appears, or will soon appear, at http://www.curepsp.org.

PSP only very rarely runs in families.  Fewer than one in 20 people with PSP knows of even one other family member with PSP, even counting distant cousins. 

But when multiple genetic variants confer only small risks of developing a disease and some sort of non-genetic factor is also necessary, it will be rare for more than one member of a family to have the unlucky co-occurrence of enough of those factors to produce outward signs of the disease. 

That’s basically how PSP works, but then things get a little more complicated:

The gene on chromosome 17 that encodes the tau protein is called MAPT, for “microtubule-associated protein tau.”  The MAPT gene has two variants that are more common in PSP than in the rest of the population.  One of them is called the H1 haplotype and actually consists of a section of the chromosome that is reversed relative to adjacent sections.  About 95 percent of people with PSP have this variant on both of their copies of chromosome 17, while this is true for only about 60 percent of the rest of the population.  So the H1 haplotype is (nearly) necessary but far from sufficient to cause the disease. 

We’re still not quite sure how the H1 haplotype increases PSP risk.  It may simply increase the amount of tau produced, which causes that protein to stick together, even if it’s structurally normal.  But more recent work shows that it causes too many methyl groups to stick to the MAPT gene, altering its function. This is exciting because drugs can be developed to alter DNA methylation. Other recent evidence supports the idea that the H1 haplotype reduces the fraction of tau molecules that include the fragment encoded by the MAPT gene’s exon 2. 

The other MAPT variant associated with PSP is statistically independent of the H1 haplotype and its function is unknown.

Over the past two decades a handful of other gene variants not on chromosome 17 have been found to be slightly more common in people with PSP than in those without PSP.  These genes help control a variety of critical processes such as disposal of damaged proteins, inflammatory mechanisms, operation of synapses, and integrity of the brain cells’ insulating sheaths.  However, the effect of these genes, individually or together, is too small to serve as a diagnostic test for the disease or to produce more than one case in a family.   

A gene called LRRK2 has been found to influence (in a rough way) not the likelihood of PSP, but the age at which it starts.  CurePSP is presently supporting a project to pursue this clue to try to find a blood test that might predict the individual’s rate of progression.  As it happens, mutations in LRRK2 are the most common cause of familial Parkinson’s disease and the occasional person with that mutation will have the pathology of PSP at autopsy despite having had the outward appearance of PD during life.  Wonders never cease.  Drugs that suppress the action of abnormal (and normal) LRRK2 are in trials for Parkinson’s.

Despite all I’ve said about the genetic component of PSP being subtle, a small fraction of people with PSP do have a relative with the same diagnosis, raising questions about the risk to their siblings and children.  A few points of advice about that:

  • When a disease occurs in several members of a family in a pattern consistent with either a dominant or a recessive mechanism, it’s easy nowadays to identify that gene.  Despite the dozens of families alleging multiple members with PSP, such a gene has never been reported in the literature. 
  • False-positive diagnoses of PSP are common.  This may account for most of the reports of multiply-affected families, even if one of them had autopsy confirmation.  However, in most situations where two or more relatives have been diagnosed with PSP, there have been no autopsies.
  • A strongly familial disorder called frontotemporal dementia with parkinsonism (FTDP) can mimic PSP, even at autopsy, but the special features of PSP such as balance loss and trouble with downgaze are mild or absent.  Many of the mutations causing this disorder are in the MAPT gene, but those mutations do not occur in non-familial PSP.  Furthermore, FTDP is associated with the MAPT’s H2 rather than H1 haplotype.  Both of these points cast additional doubt on FTDP being real PSP.  The FTDP-associated mutations can be detected by a commercially available blood test with a doctor’s prescription, but they are very rare, with only about 100 such families having been reported in the medical literature world-wide.
  • Despite those caveats, there actually are two or three families world-wide having several members with ordinary PSP (i.e., not FTDP) both during life and at autopsy, with no mutations in the MAPT gene.  Such families can be highly valuable for PSP research, as the gene causing their disease could be encoding a protein that might be key to all PSP.

Bottom line: 

Familial PSP is so rare that people with that condition need not be concerned for their children or siblings.  This advice even accounts for the possibility that what has been diagnosed as PSP may in fact be its rare, familial imitator, FTD with parkinsonism.  Most PSP experts advise their patients’ healthy relatives to make no changes to plans for career, children or finances because of one person with PSP in the family. 

However, when there is a clear indication of two or more close relatives with PSP, one should consider testing one affected person for FTDP by sequencing either the MAPT gene or a battery of genes associated with various dementing neurodegenerative diseases.  This should be done only with the guidance and participation of a genetics counselor or neurologist well-versed in interpreting genetic testing.  If the affected patient has one of those mutations, then another affected relative can be tested as confirmation and healthy relatives can be tested for the same specific mutation if they so choose.  However, a positive result would not predict the age of symptom onset, so there is little or no actionable information to be gained through testing healthy relatives.

Further research results in the near term could change these recommendations, so keep an eye on http://www.curepsp.org for updates.  But if you want me to speculate right now, take a look at the next post.