Development of a Gene Therapy for Usher Syndrome Type 1B (USH1B)

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Usher syndrome (USH) comprises a group of genetic diseases inherited in an autosomal recessive pattern that are characterized by hearing loss, vestibular dysfunction and retinitis pigmentosa (RP). Clinically, USH is classified into three principal groups (USH1-3). USH1B is caused by mutations in the gene MYO7A encoding an unconventional myosin motor protein. MYO7A is expressed in rod and cone photoreceptors (PR) and in retinal pigment epithelium (RPE). So far, there is no treatment for USH1B patients. While gene supplementation approaches are principally applicable to USH1B, the MYO7A coding sequence (6.6 kb) exceeds the cargo capacity of recombinant adeno-associated virus (rAAV) vectors (about 4.7 kb) that are used in most current gene therapy studies. To overcome this problem, we utilize in the present proposal a novel dual rAAV vector approach (REVeRT) in which MYO7A is recombined in Myo7a-deficient mice (Myo7aretKO) by trans-splicing of two mRNAs that are transcribed from two separate rAAVs. As a second strategy, missing Myo7a function will be compensated by transcriptional activation of the Myo7b gene encoding a motor protein that is homologous to Myo7a but is not expressed in the adult retina. To achieve in vivo gene activation catalytically inactive dCas9 fused to transcriptional activators (dCas9-VPR) will be used. Biochemical, histological and functional assays will be used to monitor therapeutic outcome of the different gene therapy approaches. Finally, rescue of MYO7A as well as transactivation of MYO7B will be examined in MYO7A-deficient retinal organoids generated in the project SP3 of this consortium.