Development of AAV-mediated mini-gene therapy for Usher syndrome type 1F, a combined deafness and blindness

Deafness and blindness are two of the most common and most devastating neurological disorders.  Seldom fatal, they separate patients from the world they live in and the people they love, for a lifetime.  In many cases, combined deafness and blindness result from a single gene defect, usually with a mutant gene copy inherited from both parents.  Usher syndrome is a hereditary deafness and blindness caused by mutation of any of nine genes. One gene, PCDH15, causes Usher syndrome type 1F, manifesting as profound congenital deafness and progressive blindness. There is no treatment.  Usher 1F occurs especially in the Ashkenazi Jewish population; there are ~3000 patients in the United States.

Following the striking success of Luxturna therapy for RPE65 blindness, gene addition for Usher 1F is an attractive approach. However the PCDH15 coding sequence, at 5.8 kb, is too large for AAV.  PCDH15 is a chain-like protein of 11 links, with binding domains at N- and C-termini.  We developed a number of mini-PCDH15 constructs that retain the binding domains but lack 3-8 links, and that consequently fit in AAV vectors.  Many mini-PCDH15 variants show normal binding in vitro. Since the deafness phenotype is more severe than blindness in mouse models, we assay function by testing cochlear receptor cells in vitro and hearing in vivo. AAVs encoding mini-PCDH15s have been assayed for their ability to preserve hearing in a Pcdh15-deficient mouse model that normally becomes completely deaf.   Some (but not all) of the minis, when injected into the cochleas of neonatal Pcdh15-deficient mice, produce substantial rescue of hearing. Because mechanical stress on PCDH15 is greater in the ear than in the eye, we believe that constructs that successfully rescue hearing will also rescue vision. In future work, we will test these constructs in the eye to assess rescue of the blindness phenotype.