Inherited retinal diseases (IRDs) are caused by one or more mutations in the DNA. Depending on the case, genetic mutations can cause the lack of an important protein or the production of a non-functional or even harmful protein in retinal cells. This leads to the onset of deficits in visual function and, in the most severe cases, to the death of photoreceptor cells, with consequent loss of vision.
Unlike multifactorial diseases, which are determined by environmental, behavioral and genetic factors, inherited diseases depend solely, or at least predominantly, on the presence of a “”defect”” in the DNA. Since parents pass on half of their DNA to their children, diseases with a genetic basis can be transmitted from one generation to another and for this reason they are defined as “”inherited””.
The inheritance pattern of genetic diseases changes depending on whether they are based on recessive mutations – which cause the disease only if they are inherited from both parents – or dominant mutations – which cause the disease even if they are inherited from only one of the two parents. X-linked diseases are caused by mutations present on the X chromosome, and if such mutations are recessive the disease arises mainly in males. Mitochondrial diseases are caused by mutations present in the DNA contained in the mitochondria, small cellular organelles that produce energy; since mitochondria are inherited from the mother, mitochondrial diseases are typically transmitted maternally.
Taken individually, inherited retinal diseases are usually rare diseases, but on the whole, they represent an important cause of low vision and legal blindness in children, teen-agers and adults of working age. The chances of treatment for these pathologies are closely linked to the possibility of correcting the responsible genetic defect, an option that was unimaginable until recently but which today is fortunately becoming a real therapeutic option.
Among the various therapeutic techniques which are being developed, gene therapy was the first to obtain approval for a drug, and namely for the treatment of Leber’s congenital amaurosis and retinitis pigmentosa caused by the homozygous mutation of the RPE65 gene. The drug employs a viral vector which carries a corrected version of the RPE65 gene into the cells of the patients’ retina, in this way being able to compensate for the genetic defect and reduce visual deficits resulting from the disease. If the treatment is given early, significant therapeutic results are obtained.
The development of new technologies in medical genetics is not easy to achieve, however there are now thousands of researches underway that are helping to significantly speed up the advancement of new therapies. The possibility of having access to an approved therapy or, as an alternative, to a promising new therapy which is still under clinical trial, is subject to the exact knowledge of the genetic defect that causes the disease. To date, in fact, more than 300 genes have been identified whose mutations can give rise to over 50 different inherited retinal diseases. While it is easy to understand that different diseases are caused by mutations of different genes or by different mutations of the same gene, it may not be obvious that the same genetic disease can be caused by mutations of different genes or by different mutations of the same gene. Furthermore, since different diseases can present very similar clinical pictures, the only way to know which mutation is responsible for an inherited retinal disease is to perform a genetic test.
Genetic tests allow to identify and precisely characterize the genetic mutation(s) present in a patient suffering from an inherited disease and represent an essential step in order to guide the patient towards a possible therapeutic path, be it an approved therapy or, where possible, an advanced clinical trial.
Genetic tests are performed on samples of saliva taken by rubbing a cotton swab on the inner surface of the cheek, a completely painless and extremely fast procedure (less than a minute). The test results must be analyzed by a geneticist expert in the field, who will be able to explain the meaning of any mutations found, both in terms of evolution of the pathology and in terms of therapeutic possibilities.