Applications of the genome editing system CRISPR are appearing at a furious pace, and gathering momentum toward therapeutic use in human cells. Indeed, Chinese scientists recently began a human clinical trial using CRISPR-edited cells to fight lung cancer, and U.S. clinical trials are slated to begin in 2017. But leading up to this exciting milestone, researchers performed some editing on the CRISPR system itself.
Strategies used to modulate Cas9 activity. (Computational and Structural Biotechnology Journal - Programmable Genome Editing Tools and their Regulation for Efficient Genome Engineering)
(a) Group II intron (GII)-based switch,
(b) separating Cas9 into two peptides, termed split-Cas9,
(c) Tetracycline-inducible and reversible expression system, and
(d) ligand-dependent dimerization of split-Cas9.
(e) Light-dependent dimerization of split-Cas9, termed photoactivatable Cas9 (paCas9),
(f) intein-Cas9, which are activated by splicing of a ligand-dependent intein,
(g) and unstable destabilizing domain-Cas9 (DD-Cas9) fusions, which are degraded unless provided with the ligand, Shield.
Abbreviations:
CAG = cytomegalovirus early enhancer/chicken β-actin promoter;
Cas9 = clustered regularly interspaced short palindromic repeat (CRISPR)-associated protein 9;
Cas9′ = partial Cas9;
dCas9 = dead Cas9;
FKBP = FK506 binding protein;
FRB = FKBP-rapamycin binding;
IPTG = isopropyl β-D-1-thiogalactopyranoside;
KRAB = Krüppel-associated box; MN = meganuclease;
mRNA = messenger RNA;
rtTA = reverse tetracycline-controlled transcriptional activator;
sgRNA = single-guide RNA; TRE = tetracycline response element;
T7 = T7 RNA polymerase promoter;
4-HT = 4-hydroxytamoxifen;
DD = destabilizing domain;
nMag = negative Magnet;
pMag = positive Magnet;
sgRNA = single-guide ribonucleic acid.
Scientists quickly began tweaking CRISPR in the right places, and now innovative molecular features are making it work even better and for more cell types. The rapid emergence of CRISPR applications means that clinical trials related to HIV, cancer, sickle cell disease, and other diseases are on the horizon.
Today CRISPR is a cutting-edge tool for many more researchers, and more suitable for future therapeutic use than other gene-modulating methods. “Back when RNA interference [RNAi] hit, it went into hyperdrive,” says Mark Behlke, senior vice president and chief scientific officer at Integrated DNA Technologies (IDT), which supplies RNAi and CRISPR reagents. “But now CRISPR makes that look like a child’s game—it’s just mind-blowing.”
CRISPR reagents get a makeover
Genome editing reagents are being developed and employed at a rapid rate. In order to increase specificity and avoid or reduce toxicity issues due to off-target activities strategies are now being developed to provide temporal control over the DNA-cutting activities of genome editing tools. There are a variety of novel approaches that have been employed so far but there is tremendous potential that is offered by nucleic acid-based regulatory switches that could be incorporated into the expression vectors of genome editing reagents. The development of programmable genome editing tools along with the ability of controlling the temporal and spatial expression of such editing reagents promises to be a very active and challenging research area.
Faster, cheaper, and easier to use than gene editing methods such as TALENs (transcription activator-like effector nucleases) or zinc-finger nucleases, CRISPR was quickly seized upon by researchers in many fields. For example, cancer researchers transformed cell lines with plasmids containing DNA that encoded CRISPR guide RNA (gRNA) and Cas9 (CRISPR-associated protein 9) to create different cancer cell lines for study.
But Matt Porteus, a physician and associate professor of pediatrics at Stanford University School of Medicine, had a different initial experience with CRISPR. “Everyone was saying that CRISPR would solve all the problems of the world, but when we tried to use CRISPR DNA plasmids in cells that we thought were important for therapeutic applications, like hematopoietic cell lines or other primary human cell types, the system didn’t work at all,” he says. So the Porteus lab developed a different delivery method for CRISPR/Cas9 editing in human primary cells, one that doesn’t require DNA plasmids.
Variations of this method exist that introduce CRISPR/Cas9 reagents into cells in the form of ribonucleoproteins (RNPs).
“The researcher combines these reagents [gRNA and Cas9 protein] and allows them to form a complex for five to ten minutes” to create RNPs, says Jon Chesnut, senior director of synthetic biology R and D at Thermo Fisher Scientific. “The CRISPR RNPs can then be delivered to the cell by lipid nanoparticles or electroporation.”
One of the keys to improving CRISPR in primary cells, as well as other cell types, is the recent enhancement of reagents. IDT developed chemically modified gRNAs that are resistant to nuclease degradation inside cells. The company also manufactures gRNAs in the form of two shorter RNAs (as in the original bacterial system) that form a complex, instead of a single, longer gRNA. MilliporeSigma also plans to offer two-part synthetic gRNAs as “SygRNAs.”
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Reposted via Next Big Future
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