Beyond CRISPR: What’s current and upcoming in genome editing (Covid 19)

Genome editing means CRISPR to most people. Yet methods using zinc-finger nucleases, transcription activator-like effector nucleases (TALENs), and meganucleases have their own unique strengths. All of these techniques rely on cellular DNA–repair mechanisms. Options that don’t—base editing, epigenetic editing, and site-specific recombinases—offer further advantages.

How did “genome editing” become a household phrase so quickly? This question, posed by Jerel Davis of the investment firm Versant Ventures, opened a gene-editing panel at the 2019 Life Science Innovation Northwest (LSINW) conference in Seattle, Washington. “Genome editing is a juxtaposition of two discoveries,” explained panelist Philip Gregory from the gene and cell therapy company Bluebird Bio: Nucleases can make double-stranded DNA breaks (DSBs) at specific sequences, and DSBs activate repairs that can change DNA.

DSB repair has two mechanisms. Nonhomologous end joining (NHEJ) links ends together, often creating insertions and deletions (indels) in the process. In genome editing, this can be used to knock out gene function. Homology-directed repair (HDR) fixes DSBs using DNA with a similar sequence. Providing cells with external homologous donor DNA introduces edits via HDR.

Many genome-editing systems work by activating DSB repair at specific sites using engineered zinc-finger nucleases (ZFNs), transcription activator-like effector-based nucleases (TALENs), or meganucleases (1). Currently, the dominant genome-editing method is CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats-CRISPR-associated protein 9) (2). How do researchers choose among these systems?

“The primary consideration is the end product,” says Jon Hennebold, Oregon Health & Science
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