This isn’t a surprising fact, as CRISPR is a system that has been, possibly independently, developed among a vast swath of single celled life. There are different versions of such systems all over the place and we continue to find more.
A New Type
We’ve previously discussed a rather new type of CRISPR formerly referred to as C2c2. Since then, it has been re-classified as Cas13 and officially labeled as a Type VI (Type 6) system, different from all the others. Even as a newly recognized class, it already has both an a and b variant and more is being learned about it every day.
This particular system is unique because it focuses on cutting RNA sequences, instead of DNA sequences. Such an aim means that it is used against very different types of viruses that may attack bacteria and that require a different approach.
Another unique facet of the Cas13a variant specifically is how it causes the CRISPR complex to act in a somewhat berzerker fashion. Once a target single-stranded RNA has been identified and the Cas protein binds to it, it begins to rapidly cut up the RNA at multiple points along it, specifically focusing on cutting at the nucleotides A and U.
Except it doesn’t stop. After cutting the offending viral RNA to pieces, it continues to target and destroy other RNA throughout the cell. This could, if left unchecked, kill the cell outright.
Speculation and Findings
Famed scientist and publicizer of the uses of CRISPR, Jennifer Doudna, and her colleagues at UC Berkeley suggested that this was exactly what this Cas13a system was meant to do. It was purposefully supposed to act as a self-destruct system to remove any infected cells from the population while protecting the rest.
In the same paper that held such an argument that they published in October/September of last year, they reported on their discovery that Cas13a has a dual functionality. In addition to its ability to induce cleavage of single-stranded RNA, it is also capable of detecting those viral RNAs in the first place with a frightening precision. An incredibly small amount of RNA from deadly viruses like Zika can be detected by Cas13a, opening up options there for its use as an infection detector system.
But there’s far more that can be done than just that. In another paper published by Doudna and her fellow researchers on May 4th, they discussed their findings on 10 new Cas13a protein varieties obtained from across the bacterial phylum. Their functions are what groups them together, but they are inherently structured quite differently.
The researchers found that there are two distinct groups these proteins fall into, as mentioned earlier, with one focusing on cutting uracil and the other on cutting adenine. The subfamilies have 7 proteins in the former group and 4 in the latter, if you include the original Cas13a protein.
They use completely separate crRNAs in order to identify their particular targets. A bioinformatics model was also unable to predict these separate groupings, implying that the evolution of this system and process must be more detailed than our models are yet able to predict.
The tests and research found that these two groups can be multiplexed together and used simultaneously for multiple types of RNA cleavage at once. This yet again increases the possibilities for future usage.
A New Family To Discover
For now, Doudna and her colleagues wish to continue filling out the Cas13a family with any other varieties they can discover and then get to work on first utilizing them for RNA detection technologies. Precise viral detection at low quantities of viral RNA can have a number of medical uses and, beyond that, this CRISPR family may be able to be used to defend against the viruses directly, if they can be made to stop at just destroying the viral RNA itself.
As always, there’s still more work to be done.
Photo CCs: Zika-virus-3D from Wikimedia Commons