Abstract
Here we provide additional confirmatory data and clarifying discussion, including sequencing data showing extensive regions with heterozygosity and in some cases greater than 2 alleles in CRISPR treated mice throughout the genome, which are all progeny of inbred mice purchased from a commercial vendor (JAX). Furthermore, the heterozygous mutations were mostly within 7-10bp adjacent to NGG or NGA nucleotide sequences, the preferred Protospacer Adjacent Motif (PAM) for the SpCas9. The multiple alleles in these cases cannot simply be explained by parental inheritance. The summary statements in our Correspondence reflect observations of a secondary outcome following successful achievement of the primary outcome using CRISPR to treat blindness in Pde6b/rd1 mice. As the scientific community considers the role of WGS in off-target analysis, future in vivo studies are needed where the design and primary outcome focuses on CRISPR off-targeting. We agree that a range of WGS controls are needed that include parents, different gRNAs, different versions of Cas9, and different in vivo protocols. We look forward to the publication of such studies. Combined, these results will be essential to fully understand off-targeting and can be used to create better algorithms for off-target prediction. Overall, we are optimistic that some form of CRISPR therapy will be successfully engineered to treat blindness.