Faults in three different genes may lead to Usher Syndrome 2. USH2a, ADGRV1 (= USH2c) and WHRN (= USH2d). The USH2d gene can, apart from mutations leading to Usher Syndrome, also have mutations that cause non-syndromic loss of hearing. People suffering from non-syndromic loss of hearing are hard of hearing, but no retinitis pigmentosa (RP) is involved. The USH2a gene can also have mutations that are responsible for non-syndromic RP. These patients do develop retinitis pigmentosa but they do not experience a significant loss of hearing.
More information about these genes, the proteins involved and the various mutations is to be found in ‘Usher Syndrome and DNA diagnostics’.
Which developments, innovations and studies are going on for patients suffering from Usher Syndrome type 2? In this article we will discuss the current developments with respect to therapy.
Various types of mutations within the USH2a gene.
Over 500 different mutations have been described for the USH2A gene. These mutations can largely be divided into differents types, being nonsense mutations, splicing mutations, deletions and insertions and missense mutations.
- Nonsense mutations:
Every cell of our body contains a kind of “protein machine”. This machine reads the blueprint of the genes and uses this information to produce proteins. When this machine comes across a so-called “nonsense” mutation in a gene, the production of the protein that is coded by this specific gene stops. This results in a shortened, non-functional or even absent protein. About 35% of all mutations (changes) in the USH2A gene are “nonsense” mutations.
- Splicing mutations:
In case of a fault in the pre-mRNA splicing, a part of the gene is not included in the mRNA (exon skipping) or a wrong part is included in the mRNA (pseudo-exon). Both options also lead to a stop signal and therefore we classify them with the nonsense mutations.
A deletion means that a small piece of genetic code has disappeared. This results in a shortened, non-functional or absent protein during the protein production.
An insertion means that an extra piece of genetic code has been inserted into a gene. This leads to a disruption of the protein production and, consequently, a non-functional or absent protein.
- Missense mutations:
Proteins consist of a chain of amino acids. During the protein production process, a “missense” mutation in the genetic code changes an amino acid into another amino acid. In this case the protein is produced, but it looses its function because of this one fault.
In the article ‘From DNA to protein’ you can read more about the proteins and the different mutations.
Start Ataluren for test persons suffering from USH type 2a
The medicine Ataluren (TranslarnaTM) binds itself to the “protein machine” and weakens the recognition of the stop signal and can “overwrite” this. This leads to the production of normal proteins over the full length. It is a powder that is drunk solved in water three times a day. In safety studies involving more than a thousand patients (> 2 years; with another disorder than Usher Syndrome) only minimum side effects were observed, such as diarrhoea of nausea during the first week of use. By now, Ataluren has been approved in Europe as a medicine for treating Duchenne muscular dystrophy (a serious form of muscular dystrophy) as a consequence of “nonsense” mutations in the DMD gene.
In the laboratories of Kerstin Nagel-Wolfrum (Johannes Gutenberg Universität, Mainz, Germany) and Mariya Moosajee (UCL, London, UK) it was demonstrated that in cultivated cells of a patient with “nonsense” mutations in USH2A, about 20 to 25% of the normal quantity of USH2a protein is produced again after administration. This might be sufficient to effectively slow down the progression of the disease. The laboratory in London now wants to start testing this medicine with patients suffering from retinitis pigmentosa caused by “nonsense” mutations in USH2a and USH1C (also see ‘Development of medicines for Usher Syndrome type 1’)
For application of this medicine to patients suffering from Usher a clinical testing environment is needed and any possible outcomes must be known. In order to map this out, the Moorfields Eye Institute in London selected 57 patients with USH2A for a natural development study. This group of patients was equally divided based on the various types of mutations.
This study demonstrated, among other things, that measuring the eyesight is not the best indicator for analysing the effects of a treatment, as with most Usher Syndrome patients the eyesight remains quite stable for many years. By means of an OCT scan (Optical Coherence Tomography), which can determine the number of intact light-sensitive photoreceptors, an annual decrease of photoreceptors of 7% has been observed. This decrease is larger with younger patients (below 30 years old) than it is with older patients. This argues in favour of particularly including this group of young patients in natural development studies and clinical trials.
People suffering from Usher Syndrome experience a narrowing down of the field of vision within which sharp observation is possible. It is like looking through a tunnel (= tunnel vision). By means of funduscopy the retina can be made visible and a ring-shaped structure can be observed in the perimeter of the field of vision. This examination also makes accumulation of waste products produced by the degeneration of the retinal pigment epithelium (RPE) visible. The natural development study in London demonstrated that the diameter of the ring-shaped structure in the retina of Usher Syndrome type 2a patients annually decreases by an average of 11%. Therefore this is a good parameter for determining whether a therapeutic treatment has the desired result.
A cross-over test is started with the medicine Ataluren in the Moorfields Eye Hospital. A cross-over study is a study in which test persons are all administered the medicine for a certain period and a placebo for a certain period. It is expected that it will take a total period of two years to guarantee absolute safety and to be able to determine whether the deterioration of the vision is inhibited or stabilised.
AON therapy (“genetic patch”) for USH2a exon 13 mutations
Erwin van Wijk of the Radboudumc in Nijmegen, the Netherlands, studies the therapeutic effect of antisense oligonucleotides (AON) for the future treatment of Usher Syndrome.AONs can be regarded as a “genetic patch” that covers the area of the mutation and makes it invisible. In this way, the cause of Usher Syndrome is removed and, hopefully, the deterioration of the eyesight (and possible the hearing) of this group of patients is stopped.
Zebrafishes turned out to be perfect models for studying the vision problems caused by mutations in Usher genes. Therefore, with the help of the CRISPR/Cas9 system a mutation (= small deletion) was made in exon 13 of the USH2A gene of the zebrafish. The result of this was that USH2A protein was no longer produced in these zebra fishes and that the eyesight of these fishes deteriorated.
By injecting “genetic patches” working against the USH2A exon 13 of the zebrafish, this exon was effectively skipped, making the part of the USH2A gene being translated into protein a bit smaller. Zebrafish larva that were given this treatment showed, apart from a partly recovered production of the USH2A protein, a significant improvement of eyesight. The next step will be to translate these findings from the zebrafish model to human beings. The molecule (= genetic patch) optimised for treating people is called QR-421a. The analysis of mouse eyes injected with QR-421a showed that the molecule indeed goes to the photoreceptors. Subsequently, it was demonstrated that QR-421a does not have any toxic properties.
Start of clinical trial for AON treatment for USH2a
ProQR Therapeutics from Leiden, the Netherlands, treated the first patient with mutations in exon 13 of the USH2Agene with QR-421a in the phase 1/2 STELLAR trial on 11 March 2019. This STELLAR trial is based on the above-described study of Erwin van Wijk.
The primary objectives of this clinical trial are to test the safety of QR-421a and prove the effectiveness of the treatment.
The QR-421a medicine will be administered by means of an injection in the eyeball, a so-called intravitreal injection. Additional reading on this trial can be found in: “ProQR will be start with first trials Usher syndrome 2A’
Unravel the disease mechanism of USH2a-related deterioration of the retina by means of stem cell technology
The lab of Anai Gonzalez Cordero of University College in London cultivates retinal organoids using cells coming from a USH2a patient. Retinal organoids, also called “eyes in a culture dish”, are made from stem cells and are used to get better insight into Usher Syndrome and to test the effects of various medicines. In connection with this, skin cells of a patient are re-programmed into stem cells and then developed further into specific body tissues, in this case a retina.
Do you want to know more about cultivating a retina from skin cells? Watch this video “Seeing with your skin”
Medicine study for reduction of ER stress
The above-mentioned “protein machine” in our cells is found in the endoplasmatic reticulum (ER), an organel in our cells, built up with a system made of tubes and channels. The ER plays an important part in the production of new proteins and the creation of protein complexes. The research group of Monte Westerfield (University of Oregon, Eugene, USA) studies the effect of CDH23 (= USH1D), USH1C and MYO7A (= USH1B) in the zebra fish model. It seems that various mutations in these genes prevent the production and transport of the protein complex, consisting of the three proteins mentioned, into and out of the ER. As a result of this, this incomplete protein complex accumulates in the ER, which leads to ER stress. Prolonged ER stress leads to the photosensory and mechanosensory cells dying slowly. It was also demonstrated that bright/intense light accelerates this process. Monte Westerfield, University of van Oregon USA, advises to wear sunglasses in environments with bright/intense light to reduce the ER stress as much as possible.
They want to test medicines focused on reducing the ER stress in a zebra fish model in follow-up studies, with the objective to slow down the deterioration of hearing and eyesight. Presently, these medicines are already used for treating Parkinson’s disease and Alzheimer’s disease. If this treatment works, this could be applied for, or in combination with, any gene-therapeutic treatment.
Repair of the c.2299delG mutation in USH2a with the help of CRISPR/Cas9
Carla Fuster Garcia from Valencia, Spain, tries to repair faults/mutations in the USH2a gene with the help of the CRISPR/Cas9 system. Here she concentrates on the most frequently occurring mutation in the USH2a gene: c.2299delG.
The CRISPR/Cas9 system is based on an important part of the bacterial defence mechanism against viral infections. The scientists Jennifer Doudna (University of California, USA) and Emmanuelle Charpentier (Max Planck Instituut in Berlin, Germany) discovered in the laboratory that they could modify this defence mechanism themselves. What’s more, they could very precisely cut and paste in the DNA at the desired place. In this way, pieces of DNA can be easily, quickly, cheaply and extremely precisely be processed in all types of cells. CRISPR/Cas9 can enable or disable genes and adjust these in plants, animals and even people.
Fuster Garcia applied this system to cultivated skin cells of a USH2a patient with the aim to repair the c.2299delG mutation.
With these cell models she demonstrated that the mutation (albeit not really efficiently yet) can be repaired without being confronted with the adverse side-effects in the cell. The next step will be to increase the efficiency of the repair and to test this method in animal models or in a retina cultivated by making use of stem cells.
“Know your genes”
In order to be eligible for a natural development study and future clinical trials, it is really important to know the genetic cause of the disorder. Apart from this, the therapies that are currently under development are gene-specific or even mutation-specific. Therefore it is crucial to undergo a genetic test. Read more about this in ‘Usher Syndrome and DNA diagnostics’
Gene therapy for Usher Syndrome type 2
Research into unravelling and a treatment for Usher Syndrome costs a lot of money. As Usher Syndrome is a rare disease, the governments makes little money available to stimulate research. The mission of the Usher Syndrome Foundation is: “A treatment for Usher Syndrome in 2025!” Help us and donate for further scientific research, giving all people suffering from Usher Syndrome a realistic prospect of treatment.
#stopUSH and make our dream come true!
Also read ‘‘Who knows USHIE?’ and read how USHIE you can help to collect a million euros for scientific research
This series was established thanks to:
Ivonne Bressers, Cindy Boer en Willem Quite (Ushersyndroom Foundation),
Ronald Pennings, Erwin van Wijk, Erik de Vrieze en Bas Hartel (Radboudumc),
Lisé Nijman (English translations)