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  • For the first time in medical history scientists have corrected the cause of Huntington’s disease (HD)
  • HD is a fatal congenital neurodegenerative disorder that causes uncontrolled movements, emotional challenges, and loss of cognition
  • Current treatments only help symptoms rather than slow the progression of the disease
  • Researchers from University College London (UCL)havesafely lowered the levels of toxic proteins in the brain that cause HD
  • Experts say this is the biggest breakthrough in neurodegenerative research for 50 years
  • Earlier, an American animal study successfully used a similar technique to “silence” the mutant huntingtin gene in mice brains
  • Gene silencing stops the gene from making any mutant protein but does not eradicate the mutant HD gene
  • More studies are necessary to show whether the UCL study will effectively change the course of HD
  • Gene editing is a game-changer in biomedical research, but it faces significant technical and ethical challenges

Huntington’s disease and gene silencing
 
In December 2017, scientists completed the first human genetic engineering study that targeted the cause of Huntington’s disease (HD) (also known as Huntington’s Chorea), and successfully lowered the level of the harmful huntingtin protein that irreversibly damages the brains of patients suffering from this incurable degenerative condition. Current treatments for HD only help with symptoms, rather than slow the disease’s progression. The study’s leader, Professor Sarah Tabrizi, director of the Huntington’s Disease Centre at University College (UCL) London’s Institute of Neurology, says, “The results of this trial are of ground-breaking importance for Huntington’s disease patients and families”. Tabrizi’s research followed an earlier American study, which successfully used a similar technique to “silence” the mutant huntingtin gene in mice brains.
 
This Commentary describes Huntington’s disease, the 2 studies to silence the huntingtin gene, and also the gene silencing technology, which underlies both studies.
 

Huntington's disease
 
Huntington’s disease (HD) is a fatal congenital neurodegenerative disorder caused by a mutation in the gene of a protein called huntingtin, which triggers the degeneration of cells in the motor control regions of the brain, as well as other areas. HD is one of the most devastating neurodegenerative diseases, which some patients describe as Parkinson’s, Alzheimer’s and Motor Neurone disease rolled into one. HD leads to loss of muscle co-ordination; behavioural abnormalities and cognitive decline. Generally if one parent has HD then each child has a 50% chance of inheriting the disease. HD affects both sexes and about 12 people in 100,000, but appears to be less common in people of Japanese, Chinese, and African descent. If a child does not inherit the huntingtin gene, s/he will not develop the disease and generally cannot pass it to subsequent generations. Although there is a wide variation in its onset age, the majority of HD patients are diagnosed in middle age. Currently there is no cure for the disorder: although drugs exist, which help manage some symptoms they do not influence the progression of the disease.
 
 Signs and symptoms
 
The characteristic symptoms of HD include, cognitive impairment, mood shifts, irritability, depression and behavioural changes. As the disease develops symptoms get progressively worse and include uncontrolled movements, cognitive difficulties and issues with speech and swallowing. HD typically begins between the ages of 30 and 50. An earlier onset form called juvenile HD occurs in people under 20.  Symptoms of juvenile HD differ somewhat from adult onset HD and include unsteadiness, rigidity, difficulties at school, and seizures.  
 
Diagnoses
 
A genetic test, together with a medical history and neurological and laboratory tests, support doctors in their diagnosis of HD. Genetic testing, which costs between US$250 and US$350, is both cost-effective and diagnostically precise, and is important to establish whether HD is present in a family because some other illnesses may be misdiagnosed as HD. The disorder is a model for genetic testing because HD is relatively common, its etiology is understood, and there is significant experience with its management. There are 3 main types ofHD genetic testing: (i) to confirm or rule out the disorder, (ii) pre-symptomatic testing, and (iii) prenatal testing. Persons at risk of HD often seek pre-symptomatic testing to assist in making decisions about marriage, having children, and career. Positive results can evoke significant adverse emotional reactions, so appropriate pre- and post-test counselling is important.
  
Treatment
 
Current treatments can only alleviate the symptoms of HD, and do not delay the onset or slow the progression of the disease. Until the findings of the Tabrizi study there was no treatment that could stop or reverse the course of the disorder. Tetrabenazine and deuterabenazine are drugs prescribed for treating the chorea associated with HD.  Antipsychotic drugs may also help to alleviate chorea and can be used to help control hallucinations, delusions, and violent outbursts associated with the disease. Drugs may be prescribed to treat depression and anxiety, which are relatively common among HD sufferers. Drugs used to treat HD may have side effects such as fatigue, sedation, decreased concentration, restlessness, or hyper-excitability, and only should be used when symptoms create problems for the individual.
 
The Emory Study

In June 2017 scientists from the Emory University School of Medicine in Atlanta, USA, published findings of an animal study in the Journal of Clinical Investigation, which used the gene editing technique CRISPR-Cas9 to “silence” the mutate huntingtin gene (mHTT) in mice brains.

Study leader Xiao-Jiang Li, professor and expert in molecular mechanisms of inherited neuro-degeneration, used adult mice engineered to have the same mutant Huntington's-causing gene as humans, and were already showing signs of the disease. Using CRISPR-Cas9 Xiao-Jiang introduced genetic changes in an afflicted region of the brain that prevented further production of the faulty huntingtin gene. After 3 weeks, researchers noted that the brain region where the vector was applied, the mice brains showed that the aggregated proteins had almost disappeared, and there was a concomitant improvement in their physical functions; although not to the levels of the control mice.

The Emory research team’s findings showed that CRISPR-Cas9 successfully silenced part of a gene that produces toxic protein aggregates in the brains of mice, and demonstrated that the technique holds out the possibility of a one-time solution for HD.
 
The UCL study
 
What the Emory study achieved in mice the UCL study achieved in humans. The UCL study of the huntingtin-lowering drug Ionis-HTTRx led by Tabrizi and sponsored by Ionis Pharmaceuticals, a US$6bn NASDAQ traded company based in Carlsbad, California, used a similar technique as the Emory study to “silence” the mutated huntingtin gene. The study, which had been in pre-clinical development for over a decade, enrolled 46 human patients with early HD in 9 study centers in the UK, Germany and Canada. Each patient received 4 doses of either Ionis-HTTRx or a placebo, which were given one month apart by injection into the spinal fluid to enable the drug to reach the brain. As the study progressed, the dose of Ionis-HTTRx was increased several times according to the ascending-dose study design.
Orphan drug

Ionis-HTTRx is a so-called antisense drug, which means that it inhibits the expression of the huntingtin gene and therefore reduces the production of the mutant huntingtin protein (mHTT) in patients with HD.  In January 2016 Ionis-HTTRx received orphan drug designation from the US Food and Drug Administration (FDA), and the European Medicines Agency. This is a special status given to drugs that are not developed by the pharmaceutical industry for economic reasons but which respond to public health need.
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UCL study extended

Ionis-HTTRx was found to reduce the amounts of the mutant huntingtin gene that caused HD in the patients tested. It was also found to have an acceptable safety and tolerability profile.  It is too early to call Tabrizi and her colleagues’ findings a “cure” for HD, as the study was too small and not long enough to demonstrate whether patients’ clinical symptoms improve over time. Long-term data are necessary to show whether lowering the mHTT will effectively change the course of the disease. Notwithstanding the study’s findings point to the prospect of effective future treatments.
 
As a result of the study’s success, Ionis’s partner, Roche, a Swiss multinational healthcare company, has exercised its option and paid US$45m to license Ionis-HTTRx and assume responsibility for its further development, regulatory activities and commercialization. A future open-label extension study is expected to assess the effect of Ionis-HTTRx on the progression of HD, and Ionis Phamaceuticals announced that all patients in the completed study would be offered a place in the extension study.
 
Gene silencing

Gene silencing, the technique used in the both the UCL and Emory studies, relies on the fact that cells do not directly copy DNA into protein, but instead make a rough copy from a chemical called RNA, which acts as a “messenger” carrying instructions from DNA that control proteins. Gene silencing techniques target the RNA message: cutting it up, and thereby stopping the cell from making the mutant protein. However, even if gene silencing works to reduce the level of the harmful huntingtin gene, as it did in both the UCL and Emory studies, it does not change the DNA, and a HD mutation carrier still has the mutant HD gene. The “silencing” simply stops the gene making any mutant protein. Rather than silencing the mutant huntingtin gene it would be more efficacious if scientists could cut out the extra copies of the mutation that causes the disease.
 
CRISPR

CRISPR allows scientists to easily and inexpensively find and alter virtually any piece of DNA in any species. The technology potentially offers a cure for a number of incurable diseases, but its use in humans is not only ethically controversial, but also challenged by a need to find efficacious ways to deliver gene editing techniques inside the human body. Notwithstanding, there is a global race to push the technique to its limits.
 
Despite the potential of gene editing technology, scientists have encountered significant delivery challenges in using CRISPR techniques in humans for HD. Because CRISPR therapies are based on big protein molecules, they cannot be taken as a pill, but have to be delivered into the brain using injections, packaged into viruses, or similar technology. This presents delivery challenges, and the efficacy of gene editing therapies for neurodegenerative disorders is predicated upon effective delivery.

 
Takeaways

The UCL study significantly reduced the relevant protein levels in the cerebrospinal fluid of patients with Huntington’s. CRISPR’s success with HD raises the possibility that the technique might work for other neurodegenerative disorders such as Alzheimer’s. However, the genetic causes of Alzheimer’s and other neurodegenerative disorders are less well understood and more complex than Huntington’s, which makes them potentially more challenging. Further there are still significant scientific and ethical challenges to be overcome before gene-editing technology becomes common practice.
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