On the same day in 2018, two publications first reported on Cas13d.Cas13d was found in a genomic data search for class 2 CRISPR-Cas systems which target RNA by Arbor Biotechnologies, Cambridge researchers lead by David Cheng and David Scott and published in Molecular Cell. Cas13d was also pulled out from analyzing genomic sequence data by a team lead by Patrick Hsu at the Salk Institute for Biological Studies and published in Cell. Their sequence came from Ruminococcus flavefaciens XPD2002 and is also known as CasRx.
Compared with other known RNA targeting effectors, Cas13a, Cas13b and Cas13c, Cas13d is 20% smaller. The CRISPR-Cas13d system has a WYL domain which modulates the RNase activity of the Cas13d target and collateral RNase activity. There are no target-flanking sequences needed to target RNA. Cas13d orthologs from Eubacterium siraeum and Ruminococcus sp. are active.
Hsu’s group used CasRx to manipulate alternative splicing in neurons from a patient with frontotemporal dementia, that has a mutation in the MAPT gene which codes for tau protein. The MAPT mutation favors a splicing pattern that produces more of the longer of two alternative isoforms. Expression of dCasRx, a dead version that does not have nuclease activity, in patient neurons brought the ratio between the long and short form of the MAPT transcript closer to that of control cells.