Shooting The Messenger: The End of Neurodegenerative Disease?
Updated: Jul 4, 2018
The effective treatment of neurodegenerative disease poses one of the most significant challenges for contemporary Neuroscience. Generally speaking, these debilitating pathologies are marked by progressive neuronal death, and the emergence of neurologically disabling symptoms which have the potential to disrupt relationships, and turn someone’s life upside down.
At present, our therapeutic interventions are only able to modestly improve the symptoms that these conditions bring, and are unable as of yet to halt the degenerative process. With that said, we are in an exciting time where substantial advancements are being made in research fields attempting to do just that, by silencing the expression of specific genes presumed to underlie the pathology we see in various degenerative conditions. RNA interference (RNAi) is one such example, which is gaining increasing recognition as a novel and powerful therapeutic means to tackle neurodegenerative disease. This term was originally coined when in the 1990’s, a group of scientists used double-stranded RNA (dsRNA) to silence gene expression, by the disruption of messenger RNA (mRNA) . By injecting dsRNA into a roundworm, these scientists observed a sequence-specific gene silencing, and subsequently identified dsRNA molecules as the crucial components of a post-transcriptional silencing pathway.
So, if we can exploit this pathway, then there exists the possibility to down-regulate the expression of specific genes thought to play a role in neurodegenerative disease?
In short, yes. We won’t spend a great deal of time talking about the ins and outs of the RNAi silencing pathway, and if you’re interested you can read a detailed account in the Dykxhoorn paper referenced below . The need to know elements of this pathway is the splitting of dsRNA into short-interfering RNA (siRNA), which is then assimilated into a silencing complex. An antisense strand then directs this complex to the complementary target mRNA, which results in its endonucleolytic cleavage.
While we have the knowledge and the means to silence gene products, this leaves important questions left unanswered. How do we get these RNAi molecules to the brain, and how can we sustain any therapeutic effects? Can we target specific neuronal populations?
Recombinant viral vectors may in part address some of these questions, by representing a mode of delivery which is able to sustain the prolonged delivery of genetic instructions . A recombinant adeno-associated virus (rAAV) is a particularly favourable vector to introduce foreign molecules to the mammalian brain, given it is not pathogenic to humans, nor does it trigger an immune response upon administration to the brain . Even so, these vectors present their own set of difficulties - it may be that astrocytes, instead of neurons, are more affected by this technique. Given the positioning of astrocytic end-feet, they may have superior access to exocytosed molecules, relative to neurons .
However, relatively recent results from transgenic mice experiments suggest that RNAi may still be quite effective. These experiments, often examining the effect of therapeutic RNAi in Huntington’s and Alzheimer’s disease, can tell us a little more about the utility of RNAi. Huntington’s Disease (HD) is a dominant inherited neurological disease, characterised pathologically by the gradual loss of striatal neurons, prior to the selective degradation of cortical neurons . Involuntary movements, executive dysfunction, and impaired emotional recognition are some among the many cognitive and behavioural impairments a sufferer may experience. Conversely, some common components of the pathological profile of Alzheimer’s disease includes the presence of amyloid-beta (Aβ) containing plaques, synaptic depletion, and tau-containing neurofibrillary tangles . Similar to HD however, AD is also characterised by progressive cognitive decline, and behavioural and neuropsychiatric abnormalities which severely impact upon the sufferer’s quality of life.
On a more positive note, HD does lend itself quite well to RNAi research, being an inherited neurological disease whose origins are purely genetic, as opposed to sporadic Alzheimer’s disease, whose aetiology is rooted in both genetic and environmental factors . However, early studies investigating the in vivo applications of therapeutic RNAi for HD and AD alike have indeed gathered what seems to be promising results. In one study, a significant reduction in the expression of a pathogenic Huntingtin allele in the striatum was observed after rAAV-delivered RNAi in a HD mouse model , concurrent with a significant attenuation of the pathological and behavioural abnormalities which are commonly associated with HD.
With respect to AD, there are still strong genetic targets for therapeutic RNAi, despite genetics themselves only making up half of the clinical picture. For instance, a known genetic risk factor for the development of AD is a mutation in the genes encoding for amyloid precursor protein. As amyloid precursor protein (APP) is not an essential gene, using RNAi to silence APP (and indirectly Aβ production) is a valid method by which to reduce plaque formation . Indeed, suppressing the expression of APP in transgenic mice has been shown to cause a decline in Aβ production. In one study, by co-inoculating a mutated-APP containing virus vector designed to elicit the rapid accumulation of Aβ, with an RNAi herpes simplex virus vector, the authors demonstrated a successful in vivo inhibition of Aβ accumulation .
Promising, yes, but perhaps not translatable. The anatomical and functional differences between the nervous systems of rodents and humans really are vast, and it would be a huge and irresponsible leap to suggest that these findings are sufficient grounds to upscale to human clinical trials. We were fortunate enough to talk with Professor James Uney about these issues, a Professor of Molecular Neuroscience at Bristol University whose research, amongst other interests, involves the investigation of the coding and non-coding genes that regulate neuronal function.
“Whilst transgenic mice models are good to model the expression of specific gene mutations, it’s problematic when upscaling to humans, as our brains are just so much bigger, and our genomes more complex.
Another issue is how do you sustain the effects of the treatment, and getting the molecules to the target cells without any off-target effects?
At present, we are unable to target specific neuronal populations, and it’s likely that CRISPR/CAS9 techniques will supercede RNA in this way.
When we become able to do this, we’ll see a huge change in the way diseases are treated.
Further, it may be that for these therapies to be effective, they have to be injected directly into the brain, in order to optimise the clinical benefit of this therapy.
Economically, its much cheaper to inject into the spine, and there have been some promising results from a recent clinical trial doing this research.
However, it’ll be a long time before we see these methods in the clinic, what with the time it takes to advance between clinical trials”.
The trial to which Professor Uney refers is the IONIS-HTT trial, which is testing the effect of an antisense drug designed to reduce the production of all forms of the Huntingtin protein (HTT). So far, significant reductions the mutant Huntingtin protein have been achieved, and this is certainly an area of research to keep your eye on for new and exciting developments. You can read more about the trial here: http://ir.ionispharma.com/news-releases/news-release-details/ionis-htt-rx-rg6042-top-line-data-demonstrate-significant.
It’s clear that we're making steady progress, and the phase 2 results of this trial will be able to give us some much needed and important information about how these methods work in human populations. Given the time it takes to advance between these phases, it might unfortunately be some time before we can read all about it, and how they have surmounted the issues which preclude RNAi gaining more momentum. If at the end of it all, it transpires that RNAi must be administered cortically in order to reap meaningful results, then this would be an understandably unattractive means of treatment for many people. However, if it proves to be truly effective, and enables those with degenerative diseases to take an extra step toward taking getting back the reins on their lives, then this might be a small price to pay. There can be no question that the economic burden imposed upon sufferers of neurodegenerative disease is extraordinary, and we appear tantalisingly close to the point where we may be able to bring the destructive trajectory of neurodegenerative disease to a standstill.
By Matthew Price
Interested in writing for us? Email us at firstname.lastname@example.org and we'll get back to you as soon as possible. We look forward to hearing from you!
1. Fire, A., Xu, S., Montgomery, M.K., Kostas, S.A., Driver, S.E. and Mello, C.C., 1998. Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. nature, 391(6669), p.806.
2. Dykxhoorn, D.M., Novina, C.D. and Sharp, P.A., 2003. Killing the messenger: short RNAs that silence gene expression. Nature reviews Molecular cell biology, 4(6), p.457.
3. Raoul, C., Barker, S.D. and Aebischer, P., 2006. Viral-based modelling and correction of neurodegenerative diseases by RNA interference. Gene therapy, 13(6), p.487.
4. Gonzalez-Alegre, P., 2007. Therapeutic RNA interference for neurodegenerative diseases: From promise to progress. Pharmacology & therapeutics, 114(1), pp.34-55.
5. Davidson, B.L. and Paulson, H.L., 2004. Molecular medicine for the brain: silencing of disease genes with RNA interference. The Lancet Neurology, 3(3), pp.145-149.
6. Mandel, R.J., Manfredsson, F.P., Foust, K.D., Rising, A., Reimsnider, S., Nash, K. and Burger, C., 2006. Recombinant adeno-associated viral vectors as therapeutic agents to treat neurological disorders. Molecular Therapy, 13(3), pp.463-483.
7. Miller, V.M., Gouvion, C.M., Davidson, B.L. and Paulson, H.L., 2004. Targeting Alzheimer’s disease genes with RNA interference: an efficient strategy for silencing mutant alleles. Nucleic acids research, 32(2), pp.661-668.
8. Harper, S.Q., Staber, P.D., He, X., Eliason, S.L., Martins, I.H., Mao, Q., Yang, L., Kotin, R.M., Paulson, H.L. and Davidson, B.L., 2005. RNA interference improves motor and neuropathological abnormalities in a Huntington's disease mouse model. Proceedings of the National Academy of Sciences, 102(16), pp.5820-5825.
9. Emilien, G., Maloteaux, J.M., Beyreuther, K. and Masters, C.L., 2000. Alzheimer disease: mouse models pave the way for therapeutic opportunities. Archives of neurology, 57(2), pp.176-181.
10. Hong, C.S., Goins, W.F., Goss, J.R., Burton, E.A. and Glorioso, J.C., 2006. Herpes simplex virus RNAi and neprilysin gene transfer vectors reduce accumulation of Alzheimer's disease-related amyloid-β peptide in vivo. Gene therapy, 13(14), p.1068.