AAV vector-derived elements integrate into Cas9-generated double-strand breaks and disrupt gene transcription.

We previously developed an adeno-associated virus (AAV) Cas9 gene therapy for Angelman syndrome that integrated into the genome and prematurely terminated Ube3a-ATS. Here, we assessed the performance of 3 additional AAV vectors containing S. aureus Cas9 in vitro and in vivo, and 25 vectors containing N. meningitidis Cas9 in vitro, all targeting single sites within Ube3a-ATS. We found that none of these single-target gRNA vectors were as effective as multi-target gRNA vectors at reducing Ube3a-ATS expression in neurons. We also developed an anchored multiplex PCR sequencing method and analysis pipeline to quantify the relative frequency of all possible editing events at target sites, including AAV integration and unresolved double-strand breaks. We found that integration of AAV was the most frequent editing event (67%-89% of all edits) at three different single target sites, surpassing insertions and deletions (indels). None of the most frequently observed indels were capable of blocking transcription when incorporated into a Ube3a-ATS minigene reporter, whereas two vector derived elements-the poly(A) and reverse promoter-reduced downstream transcription by up to 50%. Our findings suggest that the probability that a gene trapping AAV integration event occurs is influenced by which vector-derived element(s) are integrated and by the number of target sites.