What is the current progress of genome editing?

Genome Editing Progress

The biotechnology company Recombinetics got media attention for using TALENs to breed polled (hornless) cows—which saves farmers the trouble of dehorning them. The project started in 2012,  the company continues to work on making the editing more efficient.

Given its popularity and availability, CRISPR dominates genome-editing predictions. CRISPR-based systems will continue to improve incrementally, Carlson says. Researchers regularly publish about improved gRNAs with higher efficiency or specificity. Multiple Cas-type enzymes have been discovered or engineered with different PAMs or activities. For example, Cas13 targets RNA and is the foundation of RNA base editing. This method and Liu’s DNA base editing are licensed to Beam Therapeutics, whose co founders include Liu and Feng Zhang, who developed CRISPR for mammalian cells.

CRISPR methodological improvements include treating cells with small molecules during editing to nudge DSB repair away from NHEJ and toward HDR. Controllable systems switch on Cas9 using light or small molecules, limiting its activity in order to reduce off-target effects. Researchers are scouring the microbial world for new Cas-type enzymes and entirely new genome-editing systems. “We’re still identifying new molecules with editing capacity and we don’t fully understand the editing tools we have,” Hennebold says. “We still have a lot to learn.”

The practice of using CRISPR to correct disease-causing mutations is growing: Editas Medicine and Allergan announced human in vivo CRISPR-therapy trials for an inherited blindness. A potential hurdle to therapeutic CRISPR is the possibility of human immune responses to its bacterial components. For instance, a majority of tested blood samples showed existing immune responses to Cas9, which is commonly taken from Staphylococcus or Streptococcus bacteria .

The genome-editing wish list includes better methods for multiplexing—editing more than one gene at a time. For example, multiplexing would speed developments in T-cell–based immunotherapy, which works for many patients but requires altering multiple genes. And plant scientists often want to create “stacks” of linked genes that are inherited together as a package for resistance to disease, pests, and other agricultural threats. Multiplexing would accelerate creating these products.

In principle, multiplexing is simple with CRISPR, requiring only the introduction of a single Cas enzyme and of gRNAs and template DNAs for each targeted gene. Gunawardane has tried CRISPR multiplexing to tag multiple genes in the same cell and says it’s achievable, but in practice, gets increasingly complicated with each added gene. Systems using SSRs, ZFNs, or meganucleases may offer advantages such as smaller components that allow easier introduction.

Ask scientists about genome-editing challenges and they mention delivery of components into cells. They say to watch for transient systems that deliver editing enzymes as proteins instead of their genes so that the proteins are degraded after acting instead of being continuously expressed. Limiting activity in this manner could reduce off-target effects. Gao notes that DNA-independent delivery of genome-editing systems could alleviate concerns about genetically modified organisms (GMOs). “Proteins can’t integrate into the genome,” she says, “so if no foreign DNA is delivered at all, the resulting plants should be considered non-GMO.”

CRISPR is already very powerful, and so many people are working on it and other genome-editing systems that they’ll inevitably continue to improve, Gao says. “Scientists like to make new tools and new technology,” she says, “so we’re really seeing progress every day. Now, we say we can edit any target in principle, but in five years it will be true.”

References: 

• X. S. Liu et al., Cell 172, 979–992 (2018), doi: 10.1016/j.cell.2018.01.012.
• L. Villiger et al., Nat. Med. 24, 1519–1525 (2018), doi: 10.1038/s41591-018-0209-1.
• A. Pickar-Oliver, C. A. Gersbach, Nat. Rev. Mol. Cell Biol. 20, 490–507 (2019), doi: 10.1038/s41580-019-0131-5.
• C. T. Charlesworth et al., Nat. Med. 25, 249–254 (2019), doi: 10.1038/s41591-018-0326-x.