Tag Archives: gene therapy

Proof of principle and cause for hope

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The gene therapy company uniQure announced today that its has succeeded in slowing the rate of progression of early-stage Huntington’s disease (HD) by 75 percent.  Although the specific treatment would not work for PSP, the general principle successful in HD could be relevant to all neurodegenerative diseases.

The new research is not yet peer-reviewed nor published.  In writing this post, I used information from the company’s press release,  a news article from the BBC, and Old Reliable, ClinicalTrials.gov.

Unlike PSP, HD is a purely genetic disease.  It works on an autosomal dominant mechanism with full penetrance, which means that anyone inheriting one copy of the disease-causing version of the relevant gene from either parent will develop the disease.  The gene’s technical name is IT15 and it encodes a protein called huntingtin or HTT (notice the “-in” ending indicating a protein).  The gene defect is extra copies of a span of the three nucleotides C, A, and G. This “CAG repeat expansion” directs the cell’s protein factories (the ribosomes) to build into the HTT protein an excessively long string of the amino acid glutamine.  The normal span is 7 to 35 CAG repeats, but in people with HD, one of the person’s IT15 genes has at least 36 repeats. In people with HD, the normal version of the IT15 gene continues to make normal HTT, which means that half of their HTT is normal and half isn’t. The new treatment suppresses the brain’s production of the abnormal half.

Here’s how the trial worked: The researchers started with a kind of virus routinely used in research called AAV, which readily enters brain cells but by itself causes no harm.  They made short stretches of DNA designed to encode a type of micro-RNA corresponding to the abnormal HTT protein.  They inserted that DNA into the viruses and dubbed the result, “AMT-130.” In a 12-18-hour neurosurgical procedure, they injected the AMT-130 viruses into the caudate and putamen, the parts of the brain where HD does its main damage. The viruses released their DNA into the brain cells, which started transcribing it into RNA.  In this case the RNA was actually a “microRNA” designed to bind and disable the cells’ own abnormal RNA that would have gone on to be translated into abnormal HTT protein.   

In that way, the researchers hoped to reduce the cells’ production of abnormal HTT protein.

The trial included 29 people with HD at four study sites (Two in Warsaw, Poland and one each in London, UK and Cardiff, Wales.) Seventeen of the participants received a high dose of the virus, 12 received a low dose and all were observed for 3 years.  They were examined using the standard Unified Huntington’s Rating Scale (UHRS) and other measures of neurological function as well as spinal fluid sampling to measure levels of proteins associated with neurodegeneration.  As a control group, the trial used records of people with HD from an unrelated study of the natural history of the disease called “Enroll-HD.”

The result in the high-dose group was far better than anyone dreamed of. 

The “primary outcome measure,” the rate of worsening in the UHRS, was only 25 percent of that of similar patients from the control group.  Subsidiary measures of clinical efficacy gave similar or even better results.  Levels of neurofilament light chain (NfL), a protein released into the spinal fluid by degenerating brain cells, actually declined, while increasing in the control population. 

The low-dose group gave much less impressive results, which in a way is good because it suggests that the improvement was actually from the treatment rather than from some statistical fluke.

So, is this relevant to PSP?  Yes and no.

It’s relevant to PSP because:

  1. PSP and HD are both neurodegenerative diseases with an abnormally aggregating protein playing a critical but incompletely understood role in the loss of brain cells: tau for PSP, huntingtin (HTT) for HD.
  2. The anti-sense oligonucleotide treatment presently under development in PSP, NIO-752, works by the same principle as the AMT-130 virus.  But it’s injected into the spinal fluid and engages the tau messenger RNA directly, whereas AMT-130 releases DNA, which encodes RNA acting as the equivalent of an anti-sense oligonucleotide.

It’s not so relevant to PSP because:

  1. The tau protein aggregating in PSP is not defective from a genetic standpoint.  Yes, it’s misbehaving, but as far as we know, PSP has no common, specific, mutated form of the tau gene that could make its RNA susceptible to a targeted attack like that provided by AMT-130.  Rather, the misbehavior of tau in PSP is caused by other abnormalities in the brain cells resulting from the cumulative effect of multiple mild genetic mutations, probably along with some sort of toxic environmental exposure. 
  2. The ASO under development for PSP simply reduces the production of normal tau, and since tau has essential functions in the healthy brain, we would not want to completely eliminate its production as AMT-130 could potentially do for HTT in HD.  This means that any benefit provided by the ASO in PSP would have to be moderate at best.
  3. The damage in early HD (the stage recruited by this trial) is almost entirely in the caudate and putamen, the targets of the injections.  But in PSP, by the time a patient is diagnosed, the damage has involved many more than just two areas on each side of the brain.  This would make injecting all the involved areas extremely difficult.

Despite these reservations, the news is good for PSP because like the monoclonal anti-beta-amyloid antibodies for Alzheimer’s disease, AMT-130 sets a precedent for slowing the course of a neurodegenerative disease by attacking an aggregating protein.  But unlike the AD results, the patients receiving AMT-130 for HD suffered only mild side effects and enjoyed a dramatic benefit.

Even if this technique can’t help PSP because its tau is not genetically defective, other proteins are likely to be mutated in at least a few people with PSP.  We do know of 22 genes with some sort of genetically-related defect, but we don’t know if any are encoded into defective proteins like the HD mutation is. 

But we can hope that before too long, there will be diagnostic markers to detect PSP before it spreads beyond two or three small brain areas; and the results of genetic testing in a lone individual with PSP will allow their neurologist to order up a cocktail of injectable gene therapies to fit their own combination of mild gene mutations.  We can dream.

Re-programming

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This morning I received an email from a CurePSP support group leader in Texas forwarding a local newspaper clipping about a young girl in Taiwan with a genetic metabolic defect of the brain who had received a form of gene therapy.  She asked if that approach could be of potential use against PSP.

Here’s my answer:

For decades, a routine neuroscience laboratory tool has been to inject the brain with a harmless virus, called a “vector,” carrying a gene to induce brain cells to manufacture that gene’s protein product.  This has been useful in PSP research. Before long, the same idea could become a treatment for patients with neurodegenerative diseases.  The main drawback is that it requires a neurosurgical procedure to inject the virus with the therapeutic gene into the specific spot(s) in the brain where it’s needed. 

This approach has worked in early-phase trials in people with Parkinson’s disease, where cells that make dopamine are degenerating, and is continuing safety studies in PD.  The gene in those trials encodes the enzyme AADC (aromatic amino acid decarboxylase), which controls dopamine’s rate of production.  AADC mutations do not occur in PD, but the girl in Taiwan who received the gene therapy was suffering from an inherited deficiency of AADC, causing delayed neurological development. 

This sort of gene therapy, but using MAPT, the gene for the tau protein, has been used in PSP research to produce a rat model for use in testing new treatments.  The company sponsoring the AADC deficiency trial in Taiwan is developing an MAPT gene therapy for the rare form of frontotemporal dementia caused by mutations in MAPT, called FTDP-17.  Unfortunately, PSP, unlike AADC deficiency or FTDP-17, is not caused by a single mutation in a known gene, so it would not be amenable to having that gene replaced by this sort of gene therapy.  It’s true that PSP, like PD, includes a dopamine deficiency, but PSP would not respond to AADC gene therapy for the same reason it doesn’t respond to L-DOPA (which is converted by the body into dopamine): the brain cells on which dopamine acts degenerate in PSP. 

The hopeful note, however, is that if a compound such as a growth factor protein or an anti-sense oligonucleotide (ASO) is found to help PSP, a gene for that compound could, in theory, be inserted into a viral vector and injected into the brain.  That could provide a steady, lifetime supply of the compound.

Larry