Chemically modified guide RNAs facilitate genome editing in primary human cells 

A recent work from the Porteus lab (Stanford, California) presents the use of chemically altered gRNAs to improve low efficiency genome editing rates in primary human T cells and CD34+ hemopoietic stem cells. Modified sugar-phosphate backbone renders the gRNA more stable in cells leading to enhanced gene engineering, including more efficient non-homologous end joining and homology directed repair rates. These gRNAs have a particular advantage over unmodified gRNAs when the system is delivered as a ribonucleoprotein particle (RNP, i.e. Cas9 protein and gRNA) or as pure RNAs (Cas9 mRNA and gRNA), advantages over plasmid DNA expressed Cas9 are less clear. The authors claim there is somewhat better on/off target ratio with modified gRNAs, but we have to be cautious with this: the authors modified the terminal bases on the gRNA, perhaps functionally truncating them. Truncated gRNAs have significantly higher specificity according to the paper of from the K. Joung lab. Nevertheless, this system could be highly advantageous when working with primary cells, where genome editing rates are relatively low.

If you are interested in the paper, you can find it here.