Hidden rAAV Breakpoints Detected Using Single-Molecule, Modified Base Sequencing
The AAV genome is a single stranded DNA molecule packaged in an icosahedral protein capsid. Vector genomes of plus and minus polarities are packaged and complementary genomic strands hybridize when lysed in vitro. Standard sequencing library methods cause loss of information from individual genomes when mismatches and gaps are repaired. To retain original molecular information, modified bases are used during the repair step which allows pre-existing DNA to be distinguished from DNA added during library preparation. Modified bases introduced during repair are identified using the Sequel II system and used to detect HIDdEN DNA breakpoints (HIDEN-Seq). The most frequent breakpoints in an AAV vector subject to high strand breakage during packaging were linked to adjacent secondary structure, prompting changes in nearby sequences to reduce breakage. This use of modified bases for localizing DNA breaks enables better vector design, resulting in higher quality gene therapy vectors. The same approach can be used in other systems where knowledge of pre-existing sequence and structure is important.