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takyon writes:

The Scientist reports that University of Tokyo researchers have created a CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats using CRISPR associated protein 9) enzyme for gene editing that only works when activated by blue light. Photoactivatable CRISPR-Cas9 offers greater precision and control of gene editing:

Recently, University of Tokyo chemist Moritoshi Sato and his colleagues developed pairs of photoswitching proteins called Magnets, which use electrostatic interactions to come together when activated by light. The team has also used photoactivatable technology to develop a light-activated CRISPR-based transcription system to target specific genes for expression. Now, Sato's group has taken this one step further, using its Magnet proteins to create a photoactivatable Cas9 nuclease (paCas9) for light-controlled genome editing.

"The existing Cas9 does not allow to modify genome of a small subset of cells in tissue, such as neurons in the brain," Sato told The Scientist in an e-mail. "Additionally, the existing Cas9 often suffers from off-target effects due to its uncontrollable nuclease activity.... We have been interested in the development of a powerful tool that enables spatial and temporal control of genome editing."

The researchers created paCas9 by first splitting the Cas9 protein into two inactive fragments. They then coupled each fragment with one Magnet protein of a pair. When irradiated with blue light, the Magnets come together, bringing with them the split Cas9 fragments, which then merge to reconstitute the nuclease's RNA-guided activity. Importantly, the process is reversible: when the light is turned off, the paCas9 nuclease splits again, and nuclease activity is halted. "Such an on/off-switching property of paCas9 is the most important breakthrough previously unattainable," Sato said.

From the abstract:

We describe an engineered photoactivatable Cas9 (paCas9) that enables optogenetic control of CRISPR-Cas9 genome editing in human cells. paCas9 consists of split Cas9 fragments and photoinducible dimerization domains named Magnets. In response to blue light irradiation, paCas9 expressed in human embryonic kidney 293T cells induces targeted genome sequence modifications through both nonhomologous end joining and homology-directed repair pathways. Genome editing activity can be switched off simply by extinguishing the light. We also demonstrate activation of paCas9 in spatial patterns determined by the sites of irradiation. Optogenetic control of targeted genome editing should facilitate improved understanding of complex gene networks and could prove useful in biomedical applications.

Original Submission