Gene silencing takes a targeted step forward
Targeting the mutant Huntington's disease gene for silencing, while leaving the healthy gene untouched
Most Huntington’s disease researchers agree silencing the huntingtin gene is one of the most promising treatments in the pipeline. But we don’t know whether switching off the gene is safe. Now a Canadian team has shown that ‘alleleAllele one of the two copies of a gene-specific’ gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein – targeting just the mutant copy of the gene, and leaving the healthy copy active – works and is safe in an HD mouse.
Gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein recap
It’s clear what causes Huntington’s disease: a ‘spelling mistake’ in the gene that tells cells how to make a protein called huntingtin. The mutant gene itself causes no harm: it’s the protein built using the gene’s instructions that causes problems.
If we could tell cells not to make the harmful protein, in theory the damage it inflicts could be avoided. That’s the idea behind gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein therapies, also known as huntingtin silencing.
When a cell uses a gene to make a protein, the first step is to make ‘working’ copies of the gene. They’re made from a DNA-like chemical called ‘messenger RNAmessenger RNA A message molecule, based on DNA, used by cells as the final set of instructions for making a protein.’ or mRNAmessenger RNA A message molecule, based on DNA, used by cells as the final set of instructions for making a protein.. This mRNAmessenger RNA A message molecule, based on DNA, used by cells as the final set of instructions for making a protein. message is the target of gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein treatments.
Scientists can make molecules of DNA, RNARNA the chemical, similar to DNA, that makes up the 'message' molecules that cells use as working copies of genes, when manufacturing proteins. and similar chemicals, that are precisely designed to stick to the huntingtin mRNAmessenger RNA A message molecule, based on DNA, used by cells as the final set of instructions for making a protein. message. When this happens, cells stop reading the message and protein production stops. In fact, cells remove the message. Effectively, this aims to turn off the huntingtin gene, halting production of the harmful protein.
When the drug molecule is made of DNA, it’s called an anti-senseanti-sense the half of the DNA double-helix that is mostly used as a backup, but sometimes produces message molecules oligonucleotide, or ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene.
You can read more background on gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein in our ‘Gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein primer’ article at HDBuzz.net.
AlleleAllele one of the two copies of a gene-specific silencing
Gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein therapies sound very attractive in HD. So why not test them in patients straight away? One problem is that we don’t know if it’s safe to switch off the huntingtin gene.
Each cell has two copies of every gene – one inherited from each parent. A single copy of a gene is called an alleleAllele one of the two copies of a gene. In the case of the Huntington’s disease gene, nearly everyone who has the disease, or who will develop it, has one normal alleleAllele one of the two copies of a gene – called the ‘wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein.’ alleleAllele one of the two copies of a gene – and one mutant alleleAllele one of the two copies of a gene. One mutant alleleAllele one of the two copies of a gene is enough to cause all the problems of HD.
We don’t fully understand what the huntingtin proteinhuntingtin protein The protein produced by the HD gene. does, but it certainly seems essential for many different aspects of normal cell function. Disabling both alleles of the huntingtin gene entirely might be dangerous – and could cause more harm than good.
So, one option that’s being looked at for gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein in HD is to selectively target the mutant alleleAllele one of the two copies of a gene, without touching the wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein. alleleAllele one of the two copies of a gene.
Work by Prof Michael Hayden’s group in Vancouver, Canada, addressed how this might be done. The work was just published in the journal Molecular Therapy. To target the mutant alleleAllele one of the two copies of a gene, Dr Jeff Carroll and Dr Simon Warby from Hayden’s lab looked for parts of the mutant alleleAllele one of the two copies of a gene that were different from the wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein. alleleAllele one of the two copies of a gene.
“Mutant protein levels fell by 80%, while the wild-type protein fell by only 3%”
(If the name Jeff Carroll seems familiar – yes, it’s the co-founder of HDBuzz. He didn’t have any input into this HDBuzz article, though.)
Targeting the mutant alleleAllele one of the two copies of a gene with SNPs
Hayden’s team made use of different patterns of single-letter spelling differences in the mutant and the wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein. huntingtin alleles. These single-letter differences are called ‘single nucleotide polymorphismssingle nucleotide polymorphisms a single-letter spelling difference in a gene. SNPs, pronounced 'snips', are common and most don't change the function of the gene.’ or SNPs – pronounced ‘snips’.
We each have thousands of SNPs that make our genes different from other people’s. Most SNPs are ‘silent’ – they don’t make any difference to the functioning of our genes.
The researchers had a lucky break. It turned out to be quite easy to distinguish between the mutant and wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein. huntingtin alleles using SNPs. They found 50 SNPs that were seen more frequently in the mutant alleleAllele one of the two copies of a gene than the wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein..
Next, they made ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene drugs that would target the SNPs they’d found. Using skin cells grown in the lab, they evaluated how well they recognized their targets and how well they suppressed the mutant huntingtin message, while preserving the message from the wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein. alleleAllele one of the two copies of a gene. More than half of the candidate ASOsASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene failed at this stage.
The next question was how many HD patients carried each target SNPsingle nucleotide polymorphisms a single-letter spelling difference in a gene. SNPs, pronounced 'snips', are common and most don't change the function of the gene.. That’s really important, because if a SNPsingle nucleotide polymorphisms a single-letter spelling difference in a gene. SNPs, pronounced 'snips', are common and most don't change the function of the gene. is rare, not many patients will benefit from an ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene treatment that targets it.
By asking this question, Hayden’s team narrowed the search down to four ASOsASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene.
Testing the ASOsASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene in neuronsneuron Brain cells that store and transmit information
The next step involved evaluating how well those four ASOsASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene would reduce the production of mutant huntingtin in neuronsneuron Brain cells that store and transmit information.
They used neuronsneuron Brain cells that store and transmit information from HD mice, grown in the lab, to select the best-performing ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene.
After making some chemical changes to maximize the ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene’s effects, they went on to test how well the treatment worked in living HD mice, when injected into the brain areas that are worst affected in HD.
They found that their ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene was effective in selectively blocking the message from the mutant HTTHTT one abbreviation for the gene that causes Huntington’s disease. The same gene is also called HD and IT-15 alleleAllele one of the two copies of a gene. It caused levels of mutant protein to be reduced by 80%, while levels of the wild-typewild-type the opposite of 'mutant'. Wild-type huntingtin, for example, is the 'normal', 'healthy' protein. protein dropped by only 3%. In the mice, the ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene treatment was safe and well tolerated.
In these mice, at least, this ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene looks like a good treatment.
What’s next?
The obvious question is whether such a treatment may be an option in people.
Hayden’s team claims that if their original list of SNPs was whittled down to the top three, they would be able to target the mutant alleleAllele one of the two copies of a gene selectively in 85% of HD patients. The best SNPsingle nucleotide polymorphisms a single-letter spelling difference in a gene. SNPs, pronounced 'snips', are common and most don't change the function of the gene. they tested is present in about half of HD patients.
Obviously, mice are still quite different from human beings. One of the challenges that remains is how to give the treatment in human beings, with their large brains. In addition, treatment may need to be given repeatedly, because humans live much longer than mice and the effects may wear off. There may be side effects in humans that the HD mice just didn’t predict – for example, switching off other important genes by mistake that humans have but mice don’t. Another issue is how tell whether this treatment is working – you can’t go chopping out bits of people’s brains to look at under the microscope. Finally, there’s the question of the 15% of patients who aren’t lucky enough to have the SNPs that these ASOsASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene target.
So, there’s lots of work to be done, but this is an exciting proof that alleleAllele one of the two copies of a gene-specific gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein can work safely in HD mice and that, in theory at least, a small number of SNPs could treat the majority of patients.
We try to avoid being too specific when it comes to timelines, because we know there’s lots that can go wrong along the way. So, we might look back and kick ourselves for saying this, but HDBuzz predicts that the first trial of gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein in human patients will begin in the next two years. Needless to say, whenever it happens, you’ll read about it here.
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