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Improved base-editors advance CRISPR based genome editing

17 February 2020
Appeared in BioNews 1035

Refined base editing techniques that appear to reduce off-target effects may improve the accuracy of future gene therapies, according to new research.

In two papers recently published in Nature Biotechnology, scientists from Harvard University, the Broad Institute of MIT, and the Howard Hughes Medical Institute, reported how they improved the action of CRISPR/Cas9 base editing by engineering enzymes that can precisely target DNA bases without introducing as many unwanted errors.

In the first paper, scientists created novel cytosine base editors that reduced off-target edits by 10 to 100 fold. The second paper describes the way scientists designed a new generation of enzymes that are capable of targeting a larger section of pathogenic mutations.

Professor David Liu, senior author of the two papers said: 'Since the era of human genome editing is in its fragile beginnings, it's important that we do everything we can to minimise the risk of any adverse effects when we start to introduce these gene editors into people.'

Base editing, since its introduction in 2016 (see BioNews 848), has allowed researchers to change one of the four individual DNA bases adenine (A), cytosine (C), thymine (T) and guanine (G) into another, rather than cutting through the DNA to add, remove or replace sections.

However, last year scientists identified that the enzymes used to change C to T not only acted on the desired target but also on other locations in the genome, introducing off-target changes. These random changes were concerning as the technique could potentially cause harm if used for gene therapies in people.

These off-target changes appeared throughout the genome, and the only way to discover them was by sequencing the whole, edited genome, which is difficult, slow and expensive to carry out.

Professor Liu and his team developed ways to discover the off-target changes in bacteria and humans without the need to sequence the whole genome. They used methods to screen various enzymes in search of those that would be the best base editing enzymes. The result of their research was the discovery of a collection of enzymes that can change DNA base C to T without causing as many off-target mutations.

Many heritable diseases, including sickle cell anaemia, are caused by a single DNA base pair change. The researchers have shown that their novel enzymes are able to base edit the mutation that causes sickle cell anaemia, which has previously been difficult to access with the standard CRISPR/Cas9 approach.

SOURCES & REFERENCES
Building better base editors
Harvard University |  10 February 2020
Continuous evolution of SpCas9 variants compatible with non-G PAMs
Nature Biotechnology |  10 February 2020
Evaluation and minimization of Cas9-independent off-target DNA editing by cytosine base editors
Nature Biotechnology |  10 February 2020
Super-precise CRISPR tool enhanced by enzyme engineering
Nature |  10 February 2020
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