Development of an Usher Gene Therapy

SUMMARY Mutations in the USH2A gene are the cause of the Usher syndrome type II (USH2) that is characterized by retinitis pigmentosa (RP) and hearing impairments. USH2 is the leading genetic cause of combined blindness and deafness. It affects about 400,000 subjects worldwide. There is no cure or therapy for USH2A- associated photoreceptor degeneration. The most common USH2A mutations are located in in-frame exon 13. Skipping of exon 13 has been investigated as therapy, even though a clinical observation suggested that exon 13 skipping may be pathogenic. Gene repair using the CRISPR/Cas9 machinery represents another approach to correct exon 13 mutations. Although highly efficient ex vivo after enrichment for corrected cells, it is not known whether the in situ homology-directed repair of possibly 25% of cells will suffice. A gene transfer vector controls its efficacy by its transduction rate. It addresses all pathogenic mutations within the entirety of the USH2A gene (rt470 mutations, NHLBI/NIH Exome Sequencing Project). The USH2A coding cDNA is 15.6kb in length. It cannot be accommodated by conventional adeno-associated virus (AAV) and lentivirus vectors, whose payloads are limited to 5kb and 10kb, respectively. We have developed a fully deleted adenoviral vector (GreGT) that avoids contaminations with helper viruses and replication competent adenoviruses (RCA). With a payload of up to 33kb, we hypothesize that it will efficiently deliver a full-length USH2A coding cDNA to its photoreceptor targets. We expect that these studies will establish the GreGT vector as platform for the treatment of other defects afflicting large genes. Our application builds upon a collaboration between UD Staerz (Greffex, Inc.), who as an experienced immunologist and molecular biologist has guided the development of different vector systems, and Jun Yang (John A Moran Eye Center, University of Utah), who has developed a unique expertise in the area of retinal degeneration.