"Previous work using CRISPR in mouse embryos and other kinds of human cell had already demonstrated that editing chromosomes can cause large, unwanted effects. But it was important to demonstrate the work in human embryos as well, says Urnov, because different cell types might respond to genome editing differently."
It says of one study "Of 18 genome-edited embryos, about 22% contained unwanted changes". Such a failure rate probably isn't such a big deal in some applications. In embryos, especially human embryos, it's definitely a big deal.
The worst case scenario would be a viable but badly diseased embryo.
I wonder if they can tell which of these outcomes they got from the experiment.
Some quick reasons:
- A 25 base-pair RNA can probably kinda-sort-sometimes bind/recognize sequences as short as around 5 basepairs. It may not be the majority of incidences, but it can happen.
- There are different Cas9 proteins. They do many things, with some are more effective at certain activities and some eliciting a stronger host response. There's a balancing act.
- The human genome is somewhere around 1000 times larger than a bacteria like E. coli.
- We have seen them! Tons of them! Scientists will customize and tweak proteins and RNAs to match a given organism. It's obviously harder in humans...