Evaluation of gene-based therapies for human Usher syndrome and other hereditary retinal disorders.

To date, more than 270 different genes responsible for hereditary retinal disorders (HRDs) have been identified, and several others have been mapped. For the majority of the patients, there is no clinically approved remedy to cure or stop the vision loss. The pathomechanism underlying retinal disorders as well as preclinical testing of therapies in progress have often proven difficult in research since rodent animal models don´t show clear signs of progressive retinal degeneration or don´t develop an ocular phenotype representing the human patient ocular condition.

 

Our team develops and characterizes large animal models and patient-specific cellular models to decipher the pathomechanisms and evaluate the potential of gene-based therapies, especially for the Usher syndrome. Our therapy strategies include gene augmentation using adeno-associated viruses (AAV), genome editing, exon skipping and translational read-through of nonsense mutations by small molecules, translational read-through drugs (TRIDs) such as Ataluren.

Specific projects are:

1. Preclinical validation of AAV-mediated USH1C gene therapy in the USH1C pig model.
2. Delineate molecular differences in patient-specific cellular models.
3. Determine the impact of gene-based therapies in mitigating the disease phenotypes in cellular models.
4. Establish a screening platform to identify novel drugs targeting disease causing nonsense variants.

 

Preclinical validation of AAV-mediated USH1C gene therapy in the USH1C pig model.

Landmark clinical trials involving adult Leber congenital amaurosis (LCA) 2 patients reported some improvement of visual function and no serious complications by AAV (adeno-associated virus)-based gene addition therapy. This led to the approval of the first AAV-based drug (LUXTURNA^TM) in this subgroup of LCA patients. However, for most other retinal diseases, including Usher syndrome there are currently no effective cures available.

Our expression and localisation studies of USH1C/harmonin in the human retinae showed that the USH1C gene is frequently alternatively spliced and quantitative RNA-seq analysis revealed harmonin a1 as the most abundant transcript of USH1C in the human retina. Bulk mRNA-seq, Western blots as well as immunofluorescence and immunoelectron microscopy confirmed abundant expression and localization of harmonin in Müller glia cells and retinal neurons identifying both cell types as targets for gene therapy in USH1C patients. Our preliminary data in USH1C pigs indicate that subretinal injected AAVs encoding for harmonin a1 can revert the pathologic retinal phenotype in the animals.

 

 

Delineate molecular differences in patient-specific cellular models.

Patient-derived cellular models, such as fibroblasts, patient-derived retinal organoids provide the opportunity for testing personalized treatments in vitro. We use patient-derived cell lines, e.g., USH2A, USH1B (MYO7a) and USH2C (ADGRV1) patient fibroblasts and investigate whether the mutations result in cellular and or molecular changes in fibroblasts, such as changes in ciliogenesis, ciliary length and activation of signaling pathways when compared to control cells.

The ability to model human inherited retinal disease (IRD) through patient stem cell has created unprecedented opportunities in the field of research on photoreceptor/retinal development, degeneration and preclinical drug testing. We generated induced pluripotent stem cells (iPSC) obtained from patients and healthy controls and differentiate them into photoreceptor precursors, retinal pigment epithelium (RPE) and retinal organoids allowing the modelling of rare and complex retinal diseases in a human context in order to analyse the pathomechanism underlying the diseases.

 

 

Determine the impact of gene-based therapies in mitigating the disease phenotypes in cellular models.

Upon identification of cellular and molecular differences in our patient-specific cellular models (fibroblasts and IPSC-derived organoids) we evaluate gene-based therapeutic interventions. Our group is focusing on i) gene addition using recombinant adeno-associated virus (AAV) ii) genome editing iii) exon skipping using antisense-oligonucleotides and iv) the pharmaco-genetic read-through therapy using translational read-through inducing drugs (TRIDs). Latter compounds target in-frame nonsense mutations accounting for ~11% of all mutations causing hereditary disorders

 

Establish a screening platform to identify novel drugs targeting disease causing nonsense variants.

Pathogenic nonsense variants introduce a premature termination codon in the coding sequence of genes. The underlying molecular basis for disorders caused by nonsense mutations can be the increased decay of the mRNA by nonsense-mediated mRNA decay (NMD) or premature translation termination at the site of the nonsense mutation, or combined effects of both pathways, all resulting in the lack of full-length protein expression. Pharmacological compounds, so called TRIDs target premature termination codons by inducing the translational read-through of nonsense mutations and/or interfering with NMD and thereby restore functional protein expression. Such TRIDs include e.g., Ataluren (Translarna, PTC124) which has conditional marketing authorization to treat Duchenne muscular dystrophy caused by nonsense mutations. However, application of currently identified TRIDs is not sufficient to fully restore normal protein levels. We hypothesize that the identification of novel compounds inducing read-through of nonsense mutations would increase the chance of finding the “right” drug for a particular disease caused by a nonsense mutation (= “wrong” stop).

For most of our projects, recent reviews are available which are indicated in the list of publications. Our projects are embedded in international collaborations as well as in scientific networks at JGU Mainz.