Monday, September 28, 2015

Move over, Cas9: Cpf1 may be your new #CRISPR competition.

This past weekend at the Pilgrimage music festival in Franklin, TN I had the pleasure of seeing Weezer rock out, then I walked a few hundred yards to another stage to watch Wilco do the same.  These bands each had their own stage, but in the CRISPR world, Cas9 is a superstar that now has to share the stage with a newcomer:  Cpf1.  (Yeah, I know that's a goofy setup but it really was a good festival and it was on my mind.  Now on to the science.)

Last Friday Feng Zhang’s group published a paper in Cell that immediately grabbed a lot of attention, and rightly so.  They reveal that the Cpf1 class of CRISPR effector proteins may be an attractive alternative to Cas9.   Although Cpf1 has many similarities to Cas9, it has some significant differences that are very interesting – and could lead to improved efficiencies for some types of gene editing.  

Cpf1 Is a Single RNA-Guided Endonuclease of a Class 2 CRISPR-Cas System.  Zetsche et al., 2015, Cell 163, 1–13October 22, 2015  (Avail. online Sept. 25 2015).

Here is my summary…First, they reviewed some background on CRISPR systems in bacteria to cover the basis of the study.   There are two major classes of CRISPR systems based mainly on the proteins involved; the cleavage effectors of class 1 are complexes of multiple proteins, while the class 2 effectors are single proteins like Cas9.  Within class 2 there are two subtypes of systems: those with Cas9, and another that has Cpf1.   Cpf1 means “CRISPR from Prevotella and Francisella 1”.  Some bacterial species carry both Cas9-CRISPR and Cpf1-CRISPR loci in their genomes.   Like Cas9, Cpf1 has RuvC-like DNA cleavage domains but it lacks some of the other domain and neighbor-gene features of Cas9, so it’s clearly distinct in its evolution.    It’s a largish protein of ~1300 amino acids, similar in size to Cas9.

They picked the Cpf1-CRISPR gene system of Francisella novicida strain U112 to study first since there were clear homologies of Cpf1-CRISPR spacer sequences to various prophage in this species – further suggesting that Cpf1 is important in bacterial immunity and so it’s well adapted to slice and dice target DNAs.   (Sidebar: what’s F. novicida? A pretty rare human pathogen, originally isolated from the Great Salt Lake in Utah.  It’s related to the better known bug F. tularensis which is one of the most infectious pathogens known.)     

By transferring the F. novicida Cpf1-CRISPR gene locus into E. coli they quickly established that it prefers a  “TTN” PAM motif that is located 5’ to its protospacer target – not 3’, as per Cas9.  So right away it’s distinct in having a PAM that isn’t G-rich and is on the opposite side of the protospacer. 

Like Cas9, Cpf1 binds a crRNA that carries the protospacer sequence for base-pairing the target.  But for me the biggest surprise in the paper is that unlike Cas9, Cpf1 does not require a separate tracrRNA – in fact, there’s no sign of a tracrRNA gene at the Cpf1-CRISPR locus.   Thus, Cpf1 merely needs a cRNA that is about 43 bases long –of which 24 nt is protospacer and 19 nt is the constitutive direct repeat sequence.   This is very different than Cas9 – even by fusing the crRNA and tracrRNA, the single RNA that Cas9 needs is still ~100 nt long.

Furthermore, the Cpf1 crRNA does not have the long stemloop structure that is typical of RNAseIII-mediated processing to cut it out of its primary transcript.   It has a much shorter stemloop that is required for Cpf1 activity, however.   But surprisingly, Cpf1 itself is apparently directly responsible for cleaving the 43-base cRNAs apart from the primary transcript in the first place! This isn’t conclusively proven yet, but is pretty likely based on their experiments.   

Next, two more surprises comes from the cleavage sites on the target DNA.  The cut sites are staggered by about 5 bases.  This should create “sticky overhangs” that might be exploitable to enable gene editing via NHEJ-mediated-ligation of DNA fragments with matching ends.   And, the cut sites are in the 3’ end of the protospacer, distal to the 5’ end where the PAM is.    The cut positions usually follow the 18th base on the protospacer strand and the 23rd base on the complementary strand (the one that pairs to the crRNA).

They tested if they could inactivate the DNA cleavage domains via homologous mutations in codons known to do this in Cas9.  However, the resulting Cpf1 mutants don’t have “nickase” activity – they can’t cut either strand.   So it’s not clear that Cpf1 nickases can be made and in fact the authors suggest that the cleavage might require some sort of dimerization.  I can’t visualize how that would work yet but I’m sure it will be figured out in the near future…

Base substitution experiments then showed that, as per Cas9 CRISPRs, there is a “seed” region close to the PAM in which single base substitutions completely prevent cleavage activity.   Therefore, unlike the Cas9 CRISPR target the cleavage sites and the seed region do not overlap.  This immediately suggests a potential improvement over Cas9 in mammalian HDR-mediated repair efficiency.    This is because any initial cleavage events that might lead to “simple” NHEJ indels might still be substrates for cleavage  - and thus allowing additional chances for HDR-mediated edting to occur.  With Cas9, an indel mutation will almost always disrupt the target seeds and then it’s game over for HDR.     

Finally, they did the important work to screen various Cpf1 proteins from different bacterial species to see if any would actually work in mammalian cells.  This is because that despite codon optimization and attachment of nuclear localization signals, most of these bacterial proteins just don’t work right when you put them inside human or mouse cells.  Therefore they tested 16 different Cpf1 proteins.  Of these, for seven proteins they could identify PAM signatures using their E. coli assay.  They all had similar T-rich PAMs. 

Of these seven proteins, only two worked well in human HEK293 cells – AbCpf1, from an Acidaminococcus, and LbCpf1, which is from a Lachnospiraceae; interestingly these are apparently both anaerobic bacteria sometimes found in mammalian intestines.  Anyway these can both generate indels at specific targets in human cells at a rate similar to Cas9 – typically, 10-20% Surveyor assay numbers were observed, when they tested HEK cells following simple transfections.

Bottom line: Cpf1 may be the real deal as a serious competitor for Cas9.  Is Cas9 suddenly obsolete?  Hardly.  First, we don't yet know if Cpf1 is as specific as Cas9 – though there is every reason to think it may be.   So the off-target effects need to be carefully measured. Second, we don’t know how widespread the targeting efficiencies will be across sites (although the initial tests seem very promising).  Third, although Cpf1 may be better than Cas9 for mediating insertions of DNA, it’s not yet been shown if that is true.   However it may have some nice advantages over Cas9, not the least of which is that its guide RNA is only 43 bases long.  It will thus be feasible to purchase directly synthesized guide RNAs for Cpf1, perhaps with chemical modifications to enhance stability.

Probably, Cpf1 and Cas9 will both be in the spotlight for a long while to come.  Look for more Cpf1 papers to start coming out very soon and for Cpf1 plasmids to appear in Addgene.  Happy CRISPRing, everyone.