New type of Usher Syndrome discovered: USH IV
The team of the Hearing & Genes Expert Centre of Radboudumc lately made a discovery: Usher Syndrome includes four different clinical types. The researchers, with Hedwig Velde as principal author, recently published their study and findings in the leading Human Genetics. With the identification of minor faults (mutations) in the ARSG gene and the description of a new clinical picture, they confirm the discovery of a fourth type of Usher Syndrome.
This really is an important discovery, which gives more clarity about a number of patients with atypical Usher complaints without a genetic diagnosis. In the meantime, following the identification of ARSG as Usher gene, globally fifteen people have still been diagnosed, now that they all appear to have mutations in the ARSG gene. As it has been demonstrated that these patients have a common pattern of symptoms, this is no longer an atypical picture, but it makes up a new clinical type.
A patient with an atypical clinical picture
Very rarely a patients visits the outpatients’ clinic showing symptoms that correspond with the clinical picture of Usher Syndrome (loss of hearing combined with retinitis pigmentosa), but which picture deviates from the familiar Usher types. This is called an atypical clinical picture. In some cases no generic cause is found in the Usher genes that are known so far. Consequentially, these patients are unfortunately sent home again without having been diagnosed (and without any clarity).
Hedwig Velde is researcher and doctoral candidate at the ENT section Hearing & Genes of the Radboudumc. She is studying patients who suffer from loss of hearing but who have not been genetically diagnosed. With her research team she confirmed a new Usher Syndrome type, which is caused by minor faults in the ARSG gene.
A publication from the year 2018 written by a group of scientists in Israel described the discovery of the ARSG gene with Arylsulfatase G as a protein that might be involved with Usher Syndrome. The researchers from Israel described five persons from three families who all had the same minor fault in the ARSG gene. Such a publication may give other researchers ideas for their studies.
Studying the DNA of several people within one family sharing the same symptoms is a big help for scientists. Hedwig Velde: “There is a big chance that all patients within the family share the same genetic cause. When outside the family that has been studied another patient is found with the same atypical clinical picture and a minor fault in the same gene, this may confirm the relation between the gene and the clinical picture. Of course, the chance of coming across this patient is really small. Usher Syndrome is very rare.”
Until the national Expert Centre in the Radboud UMC saw a patient with this atypical clinical picture of Usher Syndrome and Hedwig Velde and other researchers in the Radboud UMC continued the study that was started by the team in Israel in 2018.
New type now confirmed
With the publication of Hedwig Velde c.s. the researchers confirmed this new type. The researchers found minor faults in the same gene (the ARSG gene), which creates the codes for the Arylsulfatase G protein. This protein is involved in the degradation of another protein and the idea is that malfunctioning of Arylsulfatase G will lead to an adverse accumulation of the protein that normally should be destroyed. With this study the research team also demonstrated that the minor faults in the ARSG gene that have been found really result in a non-functioning protein.
The clinical picture of the type does not fit in with the already known Usher types I, II and III. Apart from a later starting age of both the loss of hearing and the retinitis pigmentosa, the ophthalmic defects are more centrally located. This means that the vision problems with these patients rather occur in the central part of the field of vision as opposed to the other Usher types, which usually show problems in the outer part of the field of vision (the periphery). As the clinical picture is consistent with all USH IV patients, researchers of the Radboud UMC are of the opinion that this is not atypical Usher, but a new clinical type: Usher Syndrome type 4.
Hedwig: “By publishing these findings, we as researchers hope to start up a discussion in the scientific world. Various studies may together lead to the confirmation that the findings are correct or, in some cases, rebut these findings. In case of USH IV it is the accumulation of evidence in several publications that enables us to confirm that this clinical type really is a new Usher type.”
By now, globally several patients have been diagnosed for this Usher Syndrome type and for minor faults in the same ARSG gene. Previously, these patients used to be categorised in the group ‘diagnosis unknown with atypical Usher symptoms’.
The course of Usher Syndrome type IV
Both the loss of hearing and the complaints related to retinitis pigmentosa start at a later age with people suffering from USH IV. Patients started to develop complaints concerning hardness of hearing between the ages of 20 and 40 and the retinitis pigmentosa between the ages of 40 and 60. Based on the audiograms of USH IV patients, the research team has been able to calculate that the loss of hearing starts about the age of 17.
The course and the progressiveness are not necessarily milder than with the other Usher types. “We still have little insight into the course of USH IV, because only fifteen patients have been described and we therefore have to base our findings on this small group.”
Genetic tests or not?
With this discovery the researchers of the Radboudumc have managed to fit in yet another piece of the ‘Usher puzzle’.” Thanks to this, a part of the patients with an unknown diagnosis will eventually be given clarity and this is really important to this group of patients.
Unfortunately, there still are people for whom the Usher-related symptoms cannot be confirmed by a diagnosis. This makes genetic tests so important!
The physicians indicate that, of course, the choice is still to be made by the patient. One patient attaches a lot of value to a confirmation by means of genetic tests, while another does not.
Hedwig: “There are various reasons to have genetic tests done or not. An advantage of a genetic diagnosis is that with this the development of a disorder can better be predicted and that this may help the patient to adapt to the situation. Imagine that you are hard of hearing at a young age and that there is a small chance of becoming visually impaired. However, if you know that you will be visually impaired, then you had better concentrate on the kind of care that will help you both early and later in life. For instance, in this case learning sign language will not be a long-term solution for your loss of hearing, but good hearing aids may make a substantial contribution.”
Genetic tests will also help the scientific world to get further. For example, as scientific research allows for comparing the DNA of various patients, new genetic causes can be discovered. Besides, this offers a possibility for meticulously mapping out the relation between a minor fault in a gene and the corresponding complaints.
Hedwig: “Because of this, future patients can be better informed about their diagnoses. On the other hand, it is also important for any future genetic treatments to know the exact underlying deviations in the DNA.”
Usher Syndrome: 4 types and 11 genes involved
In 2022, type IV and the ARSG gene will be added to the list of Usher types and genes involved in the development of Usher Syndrome. So at this moment, Usher Syndrome distinguishes 4 types with 11 different genes involved. [Ed.: This evidence is not entirely conclusive for USH1J (CIB2) yet]
For all these genes scientific evidence has been provided that minor faults (mutations) in these genes will result in Usher Syndrome.
The Knowledge Portal of the Usher Syndrome Foundation provides a complete overview of the genes with the names of the ‘protein involved’.
Here you can read the publication of the article by Hedwig Velde c.s. in Human Genetics.