Preclinical Development and Safety of Laterally Spreading Dual AAV.SPR-MYO7A Vectors for the Treatment of Usher Syndrome Type 1B
Usher syndrome type 1B (USH1B), caused by mutations in MYO7A, is the most severe form of Usher syndrome and accounts for more than 50% of USH1 cases. Affected patients are born profoundly deaf with vestibular dysfunction and develop progressive retinitis pigmentosa, for which no approved treatment exists. The MYO7A coding sequence exceeds the packaging capacity of adeno-associated virus (AAV), the gold standard vector for retinal gene therapy. To overcome this, we developed dual AAV hybrid and overlap platforms to reconstitute full-length MYO7A expression in photoreceptors and retinal pigment epithelium. Modification of cDNA split points, and reduction of non-coding sequence yielded optimized vector pairs that produced higher full-length MYO7A levels with substantially reduced truncated protein. Packaging in the laterally spreading AAV.SPR capsid enabled supraphysiological MYO7A expression in cynomolgus macaques at macular and distal retinal sites without requiring foveal detachment, an important safety consideration for USH1B patients with remaining central vision. A six-month GLP toxicology study confirmed that ATSN-301, an AAV.SPR-based dual vector reconstituting MYO7A driven by the smCBA promoter, was well tolerated at all doses, with a No Observed Adverse Effect Level established at 1.2E11 vg/eye, supporting its clinical advancement for USH1B.