Gene Therapy Targeting PCSK9

Figure 1. Shown is the principle of using the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system for genome editing. The programmable guide RNA (gRNA) complementary binds to a specific DNA sequence termed “protospacer”. The DNA–gRNA complex recruits a nuclease such as Cas9. The nuclease has to interact with a specific DNA sequence next to the protospacer termed “protospacer-adjacent motif” (PAM) in order to work. If these requirements are fulfilled, the nuclease introduces a double strand break in the DNA. Endogenous DNA repair mechanisms follow. These repair mechanisms are error-prone and lead to random insertions or deletions, which, in turn, render the gene and the encoded protein dysfunctional. In a further development, the catalytic domain of Cas9 has been deactivated and fused to a base editor, which does not introduce double strand breaks, but precise single-nucleotide changes.

Figure 2. Genome editing of PCSK9 using a base editor that introduces a single-nucleotide change at the splice site of exon 1 and intron 1. This results in retention of intron 1 in the final mRNA, which contains an in-frame stop codon, leading to premature termination of protein synthesis.