- How does nonsense suppression therapy work?
- Is it available to patients now?
- Is it safe?
Nonsense suppression therapy is a drug-based treatment targeting conditions caused by nonsense mutations. A nonsense mutation introduces an abnormal “stop” signal into a gene that prematurely halts protein production, resulting in a shortened protein that does not function properly. There are a number of drugs, for example ataluren (Translarna™), which influences the protein-making machinery within the cell to “ignore” these abnormal “stop” signals and produce normal full-length functioning protein.
Experiments using animal or cell models suggest that these drugs work for aniridia, choroideremia, retinitis pigmentosa, Usher syndrome, and Bardet–Biedl syndrome.[2-9] A molecular therapy that safely targets nonsense mutations, which contribute to over a third of genetic eye disease, could therefore treat a substantial proportion of patients in a disease- and gene-independent manner, making this approach both practical and economical.
How does nonsense suppression therapy work?
This form of treatment is based on a class of antibiotics called aminoglycosides, in which gentamicin is a common example. These drugs bind to the protein-making machinery of bacteria and interfere with their ability to read the instructions to make a protein, resulting in a jumbled non-functional protein being formed. This leads to the bacteria dying because it cannot function properly without the correct proteins.
These antibiotics were found to also bind to human protein-making machinery but much less efficiently. Hence, in the presence of a nonsense mutation, because of the unnatural position of the mutation, the drug weakens the recognition of the abnormal stop signal, and allows the correct or closely matched protein to be formed. This can produce up to 25% of normal functioning protein, which has been shown to be able to prevent disease progression in several genetic eye diseases.
Is it available to patients now?
Ataluren (Translarna™) is the only drug that is currently been used in patients as a nonsense suppression therapy. It is approved for the treatment of Duchenne Muscular Dystrophy in the European Union, as clinical benefit was noted in the 6-minute walk test in patients when taken orally as a powder dissolved in water. It was recently tested in a clinical trial (STAR; NCT 02647359) for aniridia, but the main outcome of the trial was not achieved, which was maximum Reading Speed.
There are a number of other drugs that have been shown to work well for choroideremia, retinitis pigmentosa, Usher syndrome, and Bardet–Biedl syndrome in the laboratory setting including designer aminoglycosides and amlexanox.[11-15] These drugs are still in the very early stages of development. It is hoped that these drugs will be reformulated so that treatment can be applied directly to the eye, either as eye drops or injections, similar to gene therapy.[4,16] Further clinical trials with well-designed outcome measures based on careful natural history studies are required, so that a response to treatment (if there is one) can be measured effectively.
Is it safe?
Ataluren is well-tolerated as an oral treatment. There have been no major adverse side effects. The most common adverse reactions reported were vomiting, diarrhoea, nausea, headache, upper abdominal pain, and flatulence, all occurring in ≥5% of all ataluren-treated patients, who are all affected by Duchenne Muscular Dystrophy.
1) Correct delivery approach
Ataluren is currently administered orally. This may not reach the eye in high enough concentrations and it is also debatable whether there may be off-target effects if it is treating the whole body. Our entire genetic code (genome) is composed of approximately 3 billion molecules of DNA, and we have 3-4 million changes in our genome that make us individuals. However, 10,000 of these changes (mutations) can lead to disease, and hence, some nonsense mutations may be silently residing in other genes. An oral drug may interfere with these leading to unwanted effects. It would be beneficial to reformulate these drugs so we can just treat the tissues of the eye and have a local effect.
2) Unknown long term side effects and dosage
We do not know how long we would expect to see a treatment response, and thus how frequently we need to dose patients. This needs to be conducted in higher animal model systems (animal models that closely resemble the condition of interest) to establish the optimum local delivery approaches, correct dosages, safety and efficacy of treatment.
- Jauregui R, Park KS, Tanaka AJ, et al. Spectrum of Disease Severity and Phenotype in Choroideremia Carriers. Am J Ophthalmol. Jun 7 2019;doi:10.1016/j.ajo.2019.06.002
- Moosajee M, Gregory-Evans K, Ellis CD, Seabra MC, Gregory-Evans CY. Translational bypass of nonsense mutations in zebrafish rep1, pax2.1 and lamb1 highlights a viable therapeutic option for untreatable genetic eye disease. Hum Mol Genet. Dec 15 2008;17(24):3987-4000. doi:10.1093/hmg/ddn302
- Wang X, Gregory-Evans K, Wasan KM, Sivak O, Shan X, Gregory-Evans CY. Efficacy of Postnatal In Vivo Nonsense Suppression Therapy in a Pax6 Mouse Model of Aniridia. Mol Ther Nucleic Acids. Jun 16 2017;7:417-428. doi:10.1016/j.omtn.2017.05.002
- Gregory-Evans CY, Wang X, Wasan KM, Zhao J, Metcalfe AL, Gregory-Evans K. Postnatal manipulation of Pax6 dosage reverses congenital tissue malformation defects. J Clin Invest. Jan 2014;124(1):111-6. doi:10.1172/jci70462
- Moosajee M, Tracey-White D, Smart M, et al. Functional rescue of REP1 following treatment with PTC124 and novel derivative PTC-414 in human choroideremia fibroblasts and the nonsense-mediated zebrafish model. Hum Mol Genet. Aug 15 2016;25(16):3416-3431. doi:10.1093/hmg/ddw184
- Ramsden CM, Nommiste B, A RL, et al. Rescue of the MERTK phagocytic defect in a human iPSC disease model using translational read-through inducing drugs. Sci Rep. Mar 3 2017;7(1):51. doi:10.1038/s41598-017-00142-7
- Schwarz N, Carr AJ, Lane A, et al. Translational read-through of the RP2 Arg120stop mutation in patient iPSC-derived retinal pigment epithelium cells. Hum Mol Genet. Feb 15 2015;24(4):972-86. doi:10.1093/hmg/ddu509
- Goldmann T, Overlack N, Wolfrum U, Nagel-Wolfrum K. PTC124-mediated translational readthrough of a nonsense mutation causing Usher syndrome type 1C. Hum Gene Ther. May 2011;22(5):537-47. doi:10.1089/hum.2010.067
- Forsythe E, Kenny J, Bacchelli C, Beales PL. Managing Bardet–Biedl Syndrome—Now and in the Future. Mini Review. Frontiers in Pediatrics. 2018-February-13 2018;6(23)doi:10.3389/fped.2018.00023
- Moosajee M, Ramsden SC, Black GC, Seabra MC, Webster AR. Clinical utility gene card for: choroideremia. Eur J Hum Genet. Apr 2014;22(4)doi:10.1038/ejhg.2013.183
- Richardson R, Smart M, Tracey-White D, Webster AR, Moosajee M. Mechanism and evidence of nonsense suppression therapy for genetic eye disorders. Exp Eye Res. Feb 2017;155:24-37. doi:10.1016/j.exer.2017.01.001
- Gonzalez-Hilarion S, Beghyn T, Jia J, et al. Rescue of nonsense mutations by amlexanox in human cells. Orphanet J Rare Dis. Aug 31 2012;7:58. doi:10.1186/1750-1172-7-58
- Goldmann T, Rebibo-Sabbah A, Overlack N, et al. Beneficial read-through of a USH1C nonsense mutation by designed aminoglycoside NB30 in the retina. Invest Ophthalmol Vis Sci. Dec 2010;51(12):6671-80. doi:10.1167/iovs.10-5741
- Goldmann T, Overlack N, Möller F, et al. A comparative evaluation of NB30, NB54 and PTC124 in translational read-through efficacy for treatment of an USH1C nonsense mutation. EMBO Mol Med. Nov 2012;4(11):1186-99. doi:10.1002/emmm.201201438
- Sarkar H, Mitsios A, Smart M, et al. Nonsense-mediated mRNA decay efficiency varies in choroideremia providing a target to boost small molecule therapeutics. Hum Mol Genet. Jun 1 2019;28(11):1865-1871. doi:10.1093/hmg/ddz028
- Way CM, Lima Cunha D, Moosajee M. Translational readthrough inducing drugs for the treatment of inherited retinal dystrophies. Expert Review of Ophthalmology. 2020/05/03 2020;15(3):169-182. doi:10.1080/17469899.2020.1762489
- McDonald CM, Campbell C, Torricelli RE, et al. Ataluren in patients with nonsense mutation Duchenne muscular dystrophy (ACT DMD): a multicentre, randomised, double-blind, placebo-controlled, phase 3 trial. The Lancet. 2017;390(10101):1489-1498. doi:10.1016/S0140-6736(17)31611-2