New 'prime' genome editing offers promise

An improved genome editing method could potentially correct 89 percent of known genetic defects causing disease, US scientists say.

In the study published in Nature, the team from the Broad Institute of MIT and Harvard University in Cambridge, Massachusetts performed more than 175 different edits in human and mouse cells using their new genome editing approach called prime editing. Among others, the experiments demonstrated corrections of the mutations behind sickle cell anaemia and Tay-Sachs disease, neither of which could be fixed by previous genome-editing systems.

'Prime editors offer more targeting flexibility and greater editing precision,' said Professor David Liu, lead author of the study.

Since the development of CRISPR/Cas9 for genome editing, the field has been subject to constant improvements. Although CRISPR-Cas9 greatly improved scientists' ability to modify gene sequences, its limitations include poor efficiency and off-target effects. Furthermore, it has been observed that making double-strand breaks in the DNA with Cas9 enzyme, while relying on the natural DNA repair mechanisms to reconnect them, leads to the introduction of uncontrolled DNA insertions and deletions, which can potentially result in dangerous mutations.

Professor Liu's prime editing describes a modified version of this machinery which only cuts a single DNA strand of the double helix and includes a reverse transcriptase enzyme attached to Cas9. This enables the controlled introduction of new DNA from an RNA template included in the prime editor, resulting in only one product, rather than a mix of unpredictable changes. The precise character of the editing method also reduces the off-target effects seen with CRISPR-Cas9.

'The specificity of the changes that this latest tool is capable of could also make it easier for researchers to develop models of disease in the laboratory, or to study the function of specific genes,' said Professor Liu.

However, the authors pointed out that prime editing will need to be tested further to fully understand its effects and robustness, especially when used in animal models. Moreover, despite being able to correct a large number of genetic anomalies resulting in disease, prime editing cannot fix genetic errors consisting of too many copies of a gene or complete deletions of genes.

The team estimates that prime editing could fix errors in the 89 percent of the 75,000 variants associated with disease in the ClinVar database developed by NIH.

'Prime editing may not be able to make the very big DNA insertions or deletions that CRISPR–Cas9 is capable of - so it's unlikely to completely replace the well-established editing tool,' Professor Erik Sontheimer at the University of Massachusetts Medical School in Worcester, who was not involved in the study, told Nature. 'Different flavours of genome-editing platforms are still going to be needed for different types of edits.'