Prime Editing: Genome Editing for Rare Genetic Diseases Without Double-Strand Breaks or Donor DNA
An article published in Nature (Anzalone et al., 2019) reports the development of a genome editing experimental approach that mediates all possible base-to-base conversions, “indels,” and combinations in human genome without the need of double-strand breaks (DSBs) or donor DNA (dDNA) templates. Prime editing, the novel method of genome editing, exploits a longer-than-usual single guide RNA (gRNA), known as prime editing gRNA (pegRNA), and a fusion protein consisting of Cas9 H840A nickase fused to an engineered reverse transcriptase (RT) enzyme. Described as “search-and-replace” base-editing technology, prime editing supplies the desired genetic construct in an extension to the gRNA, which is then converted to DNA using the RT enzyme. The new approach eliminates the need for co-delivery of a corrective DNA template, performs all possible nucleotide substitutions (including those for a sizeable proportion of genetic disorders), resolves frameshifts induced by indels and confers fewer off-target edits when compared with conventional CRISPR-Cas devices. Prime editing is an exciting new complement to existing CRISPR editing systems and may even be an improvement in many cases. However, prime editing introduces new challenges. Overcoming these obstacles and applying prime editing in vivo, will give rise to new genome editing therapies for rare genetic diseases.