What are the prospects of stamping out Huntington’s disease?

Huntington’s disease (HD) is, without doubt, one of the most dreaded neurological disorders. It is named after George Huntington, but the first description is probably by Charles Oscar Waters in 1842. It is dominantly inherited, each child carrying a 50% chance of acquiring the faulty gene. The genetics is slightly tricky because HD is also a tricnucleotide repeat expansion disorder, similar to some other neurological diseases such as Friedreich’s ataxia (FA), Kennedy disease, myotonic dystrophyspinocerebellar ataxia (SCA), and oculopharyngeal muscular dystrophy (OPMD). In these diseases, a section of the genetic code duplicates itself repeatedly, producing abnormally long segments; worse still, these segments get longer which each transmission down the family line. This is called genetic anticipation, and it leads to later generations of the family developing the disease at an earlier age, and manifesting it more severely.

By Zephyris from en.wikipedia.org, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2118354
By Zephyris from en.wikipedia.org, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2118354

HD is not a nice disease. It is accompanied by chorea, probably the most distressing abnormal movement to torment the human body. This is a continuous, writhing muscle activity which involves all the body, and generating very grotesque and painful postures. As if this wasn’t enough, dementia eventually sets in, as does almost every other neurological symptom one could imagine. HD is a problem neurology needs to solve. And thankfully there is some activity in that direction. Here are 4 recent hope-raising developments.

1. Gene silencing with ISIS-HTTRx

RNA molecules. NIH Image Gallery on Flikr. https://www.flickr.com/photos/nihgov/24148252722
RNA molecules. NIH Image Gallery on Flikr. https://www.flickr.com/photos/nihgov/24148252722

The manufacturers of ISIS-HTTRx must surely be rueing the unfortunate choice of name for their gene silencing drug. But they will take comfort in its promise to crush HD. It is the first trial of a new drug for HD, and it is touted as probably ‘one of the most important developments since the gene for Huntington’s disease was discovered‘. ISIS-HTTRx neutralises huntingtin, the toxic product which accumulates in, and damages, the nerves of people with HD. The only snag…it has to be delivered directly into the spinal fluid. I’m sure an oral tablet will eventually follow, but ISIS-HTTRx is still a long way off; it has to be tested in human volunteers first. One eye then on Sarah Tabrizi, the trial lead, and the other eye on the drug’s name; ISIS pharmaceuticals is now IONIS.

2. Suppressing Huntingtin by enhancing PPAR-δ

By Emw - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8820973
By EmwOwn work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8820973

PPAR-δ stands for peroxisome proliferator-activated receptor delta, and it is a good guy. Researchers have shown that enhancing the activity of PPAR-δ in mouse models of HD has a beneficial effect on mitochondrial function, motor activity, neurodegeneration, and survivalHuntingtin, the infamous bad protein in HD, suppresses PPAR-δ activity. But the wily researchers found a way to reverse this suppression by using an agent called KD3010. They announced their findings in Nature Medicine under the refreshingly self-explanatory title, PPAR-δ is repressed in Huntington’s disease, is required for normal neuronal function and can be targeted therapeutically. (OK, it could be a little shorter). The question now is whether this can be translated to humans. We don’t have too long to wait to find out because the Food and Drug Administration (FDA) has just approved KD3010 human trials

3. Removing cholesterol by boosting CYP46A1

By Jynto (talk) - Own workThis chemical image was created with Discovery Studio Visualizer., CC0, https://commons.wikimedia.org/w/index.php?curid=37702275
By Jynto (talk) – Own workThis chemical image was created with Discovery Studio Visualizer., CC0, https://commons.wikimedia.org/w/index.php?curid=37702275

CYP46A1 is an enzyme which regulates the breakdown of cholesterol. And what has cholesterol got to do with HD? Well…wait for this…cholesterol accumulates in the nerve cells of people with HD, and may contribute to nerve damage. The good news is that CYP46A1 helps to get rid of cholesterol, and some researchers postulate that medicines which enhance the activity of CYP46A1 will improve HD. This all comes from a paper in the journal Brain titled CYP46A1, the rate-limiting enzyme for cholesterol degradation, is neuroprotective in Huntington’s disease. We are still at the proof of concept stages, but it will help if the CYP46A1-enhancing drugs come as handy pills! 

4. Controlling chorea with deutetrabenazine

By (bencbartlett (talk)) - I (bencbartlett (talk)) created this work entirely by myself., CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=27611647
By (bencbartlett (talk)) – I (bencbartlett (talk)) created this work entirely by myself., CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=27611647

Neurologists are familiar with tetrabenazine, the best treatment for chorea. And Star Trek fans are familiar with the heavy hydrogen atom, deuterium. Put the two together and, voila, you get deutetrabenazine (SD809). The heavy hydrogen of deuterium makes deutetrabenazine a more stable drug. This should make it last longer in the body, and also cause less side effects. Considering that the adverse effects of tetrabenazine include depression and parkinsonism, this is not an insignificant advantage (pardon the double negative…I couldn’t help it).

How well does deutetrabenazine translate to clinical practice? Sufficiently well enough it seems, going by the trial published in JAMA Neurology titled Effect of Deutetrabenazine on Chorea Among Patients With Huntington Disease. The authors compared the drug to placebo and showed that deutetrabenazine effectively improved chorea at 12 weeks. It is not surprising that the trial compared deutetrabenazine to placebo rather than the existing alternative; head-to-head drug trials are as rare as hen’s teeth in medicine (I wonder why that is). Anyway, deutetrabenazine may be coming to a pharmacy near you soon…we hope. 

Hope. Sign pointing to the village of Hope, Derbyshire UK. Paul Sifter on Flikr. https://www.flickr.com/photos/polsifter/4047982682
Hope. Sign pointing to the village of Hope, Derbyshire UK. Paul Sifter on Flikr. https://www.flickr.com/photos/polsifter/4047982682

There is still a long way to go yet, but each  small step is a glimmer of hope for a neurodegenerative disease such as HD.

Remember, you can have everything HD at your fingertips with neurochecklists (and pardon the shameless pitch).

Addendum

Shortly after posting this blog I came across these articles on HD prospects

  • From Huntington’s Disease News comes Pridopidine. One more to add to the hope for neuroprotection against HD.
  • From the Hazard Gazette comes SIRT2 as a future treatment target for HD

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