Nickase NmCas9 unsilences paternal Ube3a in a mouse model of Angelman syndrome without causing AAV vector integration.

Angelman syndrome (AS) is a severe neurodevelopmental disorder caused by loss of maternal UBE3A. In neurons, the paternal (pat)UBE3A allele is silenced by a long non-coding antisense transcript called Ube3a-ATS. Previous genome-editing approaches used active nucleases to unsilence patUbe3a by disrupting Ube3a-ATS. However, these methods create DNA double-strand breaks (DSBs) and promote integration of adeno-associated virus (AAV) vector genomes, both of which raise potential safety concerns. Here, we found that a nickase Neisseria meningitidis Cas9 variant (nNmCas9-D15A) disrupted Ube3a-ATS transcription when targeted to the non-template strand and unsilenced patUbe3a in cultured mouse neurons without generating DSBs or causing AAV integration. Intracerebroventricular delivery of AAV9-nNmCas9-D15A in AS model mice potently and durably reduced Ube3a-ATS and elevated Ube3a throughout the cerebral cortex and hippocampus for at least 6 months. Further, this vector restored UBE3A expression in ~ 87% of cortical neurons, which compares favorably to previously reported efficiencies with active Cas9, dead Cas9, and zinc finger nuclease vectors. These results demonstrate that nNmCas9 is a highly effective and potentially safer genome editor for the treatment of AS.