Asymmetric Parental genome engineering by Cas9 during mouse meiotic exit

Toru Suzuki*, Maki Asami* & Anthony C.F. Perry

*co-first author

Scientific Reports 4, Article number:7621 (2014)

Mammalian genomes can be edited by injecting pronuclear embryos with Cas9 cRNA and guide RNA (gRNA) but it is unknown whether editing can also occur during the onset of embryonic development, prior to pronuclear embryogenesis. We here report Cas9-mediated editing during sperm-induced meiotic exit and the initiation of development. Injection of unfertilized, mouse metaphase II (mII) oocytes with Cas9 cRNA, gRNA and sperm enabled efficient editing of transgenic and native alleles. Pre-loading oocytes with Cas9 increased sensitivity to gRNA ~100-fold. Paternal allelic editing occurred as an early event: single embryo genome analysis revealed editing within 3 h of sperm injection, coinciding with sperm chromatin decondensation during the gamete-to-embryo transition but prior to pronucleus formation. Maternal alleles underwent editing after the first round of DNA replication, resulting in mosaicism. Asymmetric editing of maternal and paternal alleles suggests a novel strategy for discriminatory targeting of parental genomes.

Schematic depicting a model for Cas9-mediated editing following injection of mII oocytes (mII). Limitted editing of maternal alleles during the gamete-to-embryo transition is inherent to the system, whereas limitted editing of paternal alleles in zygotes is because available targets have already been removed. Pb2, second polar body; pn, pronucleus.