There are many different retinal diseases simply because many different steps of the visual process can be affected. However, they all have one thing in common: correction of the relevant defective gene currently provides the only possibility of permanent cure. Prof. Dr. Mathias Seeliger and his group of researchers at the Institute of Ophthalmic Research at the University Hospital in Tübingen is specifically focused on the development of gene therapies for the treatment of neurodegenerative eye diseases. The techniques, developed in cooperation with colleagues from the University of Munich, have been lastingly successful in mice. A gene therapy for the treatment of this kind of retinal disease is now close to clinical testing.
Prof. Dr. Mathias Seeliger has been specifically working on congenital retinal diseases for more than 15 years. Seeliger is an engineer, medical doctor and head of the ocular neurodegeneration research group at the Institute of Ophthalmic Research at the University of Tübingen. Amongst other things, Seeliger’s work involves congenital retinal diseases. Patients with congenital retinal diseases exhibit modifications in the genes that are responsible for the proper function of the retinal cells. Most of these modifications are point mutations which, depending on where they occur in the DNA strand, may lead to the complete termination of protein synthesis, a lower than normal expression of the relevant protein in the cell or the production of a faulty 'disturbing' protein. All this affects retinal vision and results in a number of serious eye diseases.
In healthy visual processes, light stimuli are registered by the sensory cells of the retina – the rods and cones; photons are absorbed by the photoreceptive pigment within the photoreceptor cell; the shape change of the pigment leads to a biochemical signaling cascade, which is converted into an electrical signal. This signal is passed on to the next level, i.e. the bipolar cells. Amacrine and ganglion cells, retinal interneuron cells which convert the signals into a universal language that is understood by most neurons, are at the end of the retinal light processing chain. “The visual information is converted into a number of action potentials within a specific period of time that is lower or higher than normal,” says Seeliger. “Errors can occur at any point in this chain of reactions; and these errors may affect a patient’s vision and even lead to total blindness. That said, defective general body functions like vitamin A deficiency, diabetes mellitus or vascular disease may also affect vision.”
The University Eye Hospital in Tübingen offers a special consultation for people with congenital retinal diseases, one of very few in Germany. When a patient comes here, the doctors initially establish which eye disease he or she has, i.e. which step of the complex visual process is impaired. Numerous diagnostic methods are used to identify the disease, including ophthalmological examination, electrophysiological and imaging methods. “Nowadays, we also use genetic tests. But around ten years ago, it was still extremely difficult to pinpoint the exact retinal disease,” says Seeliger. Purely clinical diagnoses are especially difficult in patients with progressive stages of disease. Depending on the disease, it is now possible to identify the disease-causing genetic modification in a large percentage of patients.
There are mouse models for many genetic eye diseases. The animals have exactly the same genetic defect as the patients who come to the clinic. These models are used to study neurodegenerative eye diseases on the molecular level as well as to develop therapies. For around ten years now, the scientists from Tübingen have been successfully using gene therapy approaches aimed at correcting the errors arising from defective genes. Seeliger and his colleagues benefit from another specialty area in Tübingen, i.e. the development of diagnostic methods and devices. They have also adapted most diagnostic devices used for human patients for use in mice, which facilitates comparison of the findings with human data and transfer of the results to humans.
Adeno-associated viruses (AAV) have been identified as the most efficient tools for transferring healthy genes into the retina. AAV bind particularly well to the retinal photoreceptors. “The viral envelope only contains the viral proteins that are necessary for gene transfer,” says Seeliger. “The viral particles bind to the cells and release the content – i.e. a corrected gene with the previously missing information.” A specific promoter ensures that the information is only transcribed in the retinal cells for which it is intended. Successful gene therapy improves eye function, potentially permanently. Researchers can later use specific histological markers to identify the correctly synthesized protein, thereby confirming the success of the therapy. The method works quite well in mice for certain eye diseases and the objective is now to transfer the method to humans.
The investigators decided to test the application of gene therapy for a human eye disease on which Seeliger’s group has been working with a group from Munich for the past 15 years: achromatopsia, an inherited condition caused by mutations in any of several genes. Achromatopsia is characterized by the loss of the retinal channels that convert biochemical signals into electrical ones. No electrical signal can be generated, cone vision is impaired. “This condition is extremely distressing for those affected. As the centre of the macula only consists of cones, and a high cone density also exists close to the centre, the visual acuity of individuals with this congenital form of disorder is reduced to about 1/10th of normal levels. Nystagmus, i.e. an uncontrolled oscillatory movement of the eyes, is another disturbing symptom associated with the disorder, as are the inability to see clearly in bright light and the lack of colour discrimination. That said, it is nothing wrong with the cone cell itself, the disorder is only due to the inability of the channels to convert biochemical signals into electrical activity,” says Seeliger.
However, gene therapy leads to problems when the treated cell is unable to regulate the amount of protein produced. Too much protein might in the worst case lead to the death of the cells. The channels of achromatopsia patients consist of two subunits (α and β) encoded by different genes. Since the genetic defect only affects one of the two subunits, the number of channels produced can be controlled by the other, unaffected, subunit. An overdose can therefore be excluded. This is most likely one of the reasons why gene therapy of achromatopsia in mice has led to permanent cure.
The transfer of the method to humans would be the next logical step. Gene therapies are subject to the same approval regulations as drugs for treating headaches or heart problems. Extensive dossiers must be submitted to the regulatory authorities. For example, toxicological safety must be proven and the drug must be manufactured in certified production plants under GMP conditions. All this costs several million euros – an obstacle that often stops such projects in their tracks, unless an investor with a financial interest in the therapy can be found. As achromatopsia and many other congenital retinal diseases are very rare, it goes without saying that drug producers tend to have little interest in developing treatments for these diseases. “This is a particular issue for the therapy of specific genetic defects. A gene therapy cannot be broadly applied to several diseases as it is tailored to a specific gene defect and only effective in this particular case. A similar sum of money will be required for another specific gene therapy, and so on,” explains Seeliger.
However, in this case the scientists were fortunate to find a private sponsor. The project is being supported by a foundation for a total period of five years and is now into its 3rd year. Originally designed for the mouse model, the vectors have now been replaced by those containing the human gene. Since the human promoter also works in mice, the scientists are able to assure the quality of the vector batches produced in the French city of Nantes. “So far, everything has gone to plan, and we are expecting the first clinical batch to arrive within the next few days,” says Seeliger. “We will then test it in the mouse model and we are confident that it will be approved for use in human patients.” At present, the team is selecting people from a large number of eligible patients from Germany and abroad whose disease is most suited to achieving the results the study is aiming to achieve. This would be the first gene therapy in Germany for treating a particular eye disease. Everyone is hoping that the investigators will be able to attract sponsors for the remaining clinical trial phases.
Further information:Prof. Dr. Mathias SeeligerSchleichstr. 4/372076 TübingenTel. +49 (0)7071-29 80718E-mail: firstname.lastname@example.org