Given that I’m a huge fan of Jennifer Doudna’s team’s work on CRISPR/cas9, I’m surprised I haven’t written about it here before. If you really want to read about CRISPR at length, I really recommend Doudna’s book, A Crack in Creation — but if you just want a quick primer, that I can do!
CRISPR stands for “clustered regularly interspaced short palindromic repeats” (you can see why CRISPR has caught on as the acronym). CRISPR sequences are found natively in the bacteria and archaea, and it’s actually thought to be something like a bacterial version of an immune system. These sections of DNA actually contain DNA sequences from viruses that have previously attacked the cell, and they prevent some phage viruses from doing damage to the cell. cas9 is a “CRISPR-associated system”, and hangs out next to the CRISPR sequences. Together, the two can be used to edit the genetic sequence of any species with high specificity.
How does this work? cas9 acts as a pair of “molecular scissors”, cutting DNA at a point specified by a guide RNA which binds to a target sequence, right where the DNA needs to be edited. This system was used originally by bacteria and archaea to snip out bits of phage DNA to keep for their immune response (though remember that when I say immune response, it’s nothing like as complex as the human immune response — this is a metaphor).
Anyway, once cas9 has cut the DNA, the cell’s own repair systems will fix the break — using DNA you’ve introduced. This is really useful to fix genetic defects that originate in short sequences or even single bases, or to add a new gene.
However, CRISPR/cas-9 isn’t yet a miracle cure. It’s not successful 100% of the time, and sometimes the breaks in DNA are fixed wrong. This can introduce dangerous mutations, and if the wrong piece of DNA is cut, prevent production of a protein you actually need. In addition, there’s no way right now to get it working in a living person, so the cure for various genetic diseases is still a way away.
Still, it’s a powerful tool for research and there’s a lot of potential. I predict there’ll be a Nobel for this technique in the end, and that it or a related technique will be the answer to our gene editing prayers.