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The CRISPR-Cas9 system is famous for its ability to snip threads of DNA at precise locations like a pair of razor-sharp scissors, giving researchers unprecedented gene-editing capabilities. But for the Cas9 protein to accurately cut and paste genes, it first has to locate the exact spots in the DNA that it needs to cut.
For Cas9 to find a specific location on the genome, scientists must first equip it with a snippet of “guide RNA,” a small piece of RNA whose bases are complementary to the DNA sequence of interest. The bulky protein first unzips the double-stranded DNA and scans through until it finds the sequence that matches the guide RNA, and then latches on.
To harness CRISPR’s gene-targeting ability, the researchers took a deactivated Cas9 protein — a variant of Cas9 that can find a specific location on DNA, but doesn’t cut it — and tethered it to transistors made of graphene. When the CRISPR complex finds the spot on the DNA that it is targeting, it binds to it and triggers a change in the electrical conductance of the graphene, which, in turn, changes the electrical characteristics of the transistor. These changes can be detected with a hand-held device developed by the team’s industrial collaborators.
Aran conceived of CRISPR-Chip while a postdoctoral scholar at UC Berkeley and developed it as an assistant professor at KGI. (UC Berkeley photo by Stephen McNally)
Graphene, built of a single atomic layer of carbon, is so electrically sensitive that it can detect a DNA sequence “hit” in a full-genome sample without PCR amplification.
“Graphene’s super-sensitivity enabled us to detect the DNA searching activities of CRISPR,” Aran said. “CRISPR brought the selectivity, graphene transistors brought the sensitivity and, together, we were able to do this PCR-free or amplification-free detection.”
Aran hopes to soon “multiplex” the device, allowing doctors to plug in multiple guide RNAs at once to simultaneously detect a number of genetic mutations in minutes.
“Imagine a page with a lot of search boxes, in our case transistors, and you have your guide RNA information in these search boxes, and each of these transistors will do the search and report the result electronically,” Aran said.
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