Gene editing on and off-target

Gene editing is certainly the future of medicine. And CRISPR/Cas9 has accelerated that revolution. But there are two sides to gene editing with CRISPR. There are the edits that you want - on-target - and those you don't want - off-target.

There are many challenges to gene editing. Off-target edit prediction, quantification, and reduction are key parts of a successful gene editing.

On-target editing

CRISPR has been revolutionary because it makes on target editing much easier than it had been before. But it's really simple when you have a simple task. And version 1 of CRISPR is best at breaking things. This breaking is called non-homologous end joining (NHEJ). It's quite powerful at breaking things in the genome and this can be put to great therapeutic use. In my time at CRISPR Therapeutics, we built an entire clinical program that essentially breaks a piece of DNA that is involved in turning of fetal hemoglobin once you're born (blue γ line in figure below). When you break this you turn back on fetal hemoglobin and for people with Sickle Cell Disease and β-Thalassemia this should in theory bring back a second form hemoglobin that doesn't have issues like sickling and can in turn relieve the symptoms of the disease. It turns out that seems to be working for over 20 patients to date and will hopefully be approved for treatment by the FDA and other regulatory agencies in the near future.

The full NEJM paper

Off-target editing

But CRISPR can edit outside of its intended editing on-target region. Assessing how much, if any, of this off-target editing happens is a large part of the work when doing gene editing with CRISPR. The FAQ below answers some common questions about off-target experiments and links to the paper published by my team at CRISPR Therapeutics that evaluates several off-target laboratory methods.

FAQ on off-target editing