Researchers from the City of Hope improve the CRISPR-Cas9 system through novel sequence changes to the trans-activating RNA (tracrRNA).
The improved tool could help to fast track new therapies for HIV, sickle cell disease and other immune conditions. The results were published in Scientific Reports on November 6.
CRISPR/Cas9 systems are a powerful genome-editing tool which requires CRISPR RNA (crRNA) with an interchangeable 20 nucleotide complementary sequence to a target DNA site, and a trans-activating crRNA (tracrRNA) scaffold recognized by a catalytically active Cas9 protein.
TracrRNA is derived from Streptococcus pyogenes bacteria and is a part of the components used to guide the genetic scissors (Cas9) to the right gene sequence. The system uses guide RNA (gRNA) to facilitate target site activation and new gene insertion.
The research team identifies specific modifications to the gRNA that significantly enhance the Cas9 ribonucleoprotein complex (RNP) activity.
This is the first time that scientists have systematically gone through gRNA sequences to change it and improve the technology.
Tristan Scott, PhD, who is a lead author of the study and a staff research scientist at City of Hope’s Center for Gene Therapy said that their CRISPR-Cas9 design may be the difference between trying to cut a ribeye steak with a butter knife versus slicing it with a steak knife, Other scientists have also tried to improve the CRISPR cutting through chemical modifications, but that’s a quite expensive process and is like diamond-coating a blade.
Instead, we have designed a better pair of scissors you can buy them at any convenience store.
First, they screened U-modified tracrRNAs and identified nucleotide substitutions that improved Cas9 RNP knockdown of HIV reporter cell lines and observed this enhanced targeting of the long terminal repeat of HIV.
The modified tracrRNAs improved the knockout activity of an essential HIV co-receptor, C-C chemokine receptor type 5 (CCR5).
CCR5 is a current target in clinical trials seeking to re-engineer a person’s immune system to be resistant to HIV. Furthermore, the modified tracrRNA improved target accuracy and subsequently increased the inactivation of CCR5.
Improved targeting and improved gene insertion were also observed at the HBB gene and the BCL11A site.
The researchers hope that these results could lead to more “clean” gene editing in cell and mouse model experiments. More pronounced results could quicken the new therapies from the laboratory to patients’ bedsides. Scott said that his team is at the beginning of this long scientific process.