As previously mentioned, at 6.6 kb,
31 the
MYO7A coding sequence is relatively large. Its size precludes the use of standard adeno-associated viruses (AAVs) as vectors for USH1B-targeted genetic augmentation therapy due to their relatively small cargo capacity of about 5 kb.
38–40 Workshop participants heard from representatives of two groups of researchers working to develop dual AAV vectors, a technology based on the inherent ability of two AAV genomes to undergo concatemerization and homologous recombination. With a dual AAV vector, the
MYO7A coding sequence is divided into two pieces, and each piece is carried by one AAV. Via a region of complementary sequence shared between these two pieces, they are joined back together through homologous recombination to allow expression of full-length
MYO7A in retinal cells and, ultimately, generation of a full-length protein.
41–43 Alberto Auricchio, MD, from the Telethon Institute of Genetics and Medicine (TIGEM) in Naples, Italy, presented research that led to development of the hybrid dual AAV vector serotype 8 (AAV.8.
hMYO7A), which has been shown to result in reconstitution of full-length myosin VIIA to therapeutic levels in the USH1B shaker1 mouse model.
41 He reviewed work to date from the UshTher initiative in Europe, including toxicity and Good Manufacturing Practice lot manufacture and release testing, a dose–response study in shaker1 mice, and nonclinical studies in NHPs. He presented a timeline toward submitting the Investigational Medicine Product Dossier to regulators and moving to a clinical trial as early as 2022. Dr. Boye provided an overview of research from the USH1B genetic therapy development program at Atsena Therapeutics that is also investigating dual AAV vectors for use in this capacity. Her team has optimized hybrid vectors by altering split points between the front and back half vectors, as well as codon modifying the front half vector sequence to prevent production of truncated protein. They also have optimized overlap dual AAV vectors, a platform that relies on slightly different methods of recombination, to efficiently produce full-length
MYO7A. She reported Atsena Therapeutics is currently conducting testing in mouse and NHP models to determine an optimal candidate vector and, from there, will move into studies to enable an investigational new drug application and a clinical trial. Hybrid dual AAV vector systems developed by the UshTher initiative and Atsena also have been shown to have potential to treat the vestibular dysfunction in USH1B when tested in the shaker1 mouse model. A possible caveat to this research: the dual AAV vector–based genetic augmentation therapy systems Dr. Auricchio's organization is researching contain
MYO7A transcript variant 1, which, as Dr. Williams pointed out, is not as common as variant 2 in the human retina.
16,44 Dr. Boye's organization is employing a version that encompasses properties of transcript variants 1 and 2. Both have shown the ability to correct melanosome migration in the shaker1 mouse. However, the significance of the different isoforms on retinal function in primates is unknown.