Introducing precise modifications into genes for repair is one of the greatest hopes in medicine. A promising gene editing tool is CRISPR-Cas9, CRISPR which stands for Clustered regularly interspersed short palindromic repeats, and CRISPR-associated protein 9. As the name suggests CRISPR-CAS9 is a two-part system that can copy and cut specific DNA sequences. This microbial adaptive immune system participates in immunity against viruses.
There are several ways to deliver the CRISPR-CAS9 system into a target cell. One way is using an Adenovirus carrying CRISPR-CAS9 in its genome. The virus docks to a receptor on the cell membrane and is endocytosed via a clatherin mediated pathway. In the low pH environment of the endosome, the viral capsid ruptures releasing the viral DNA, which can now enter the nucleus. Inside the nucleus, the genome is unwound by the host topoisomerases and separated by DNA helicases.
The host RNA polymerase transcribes the CRISPR-Cas9 sequence into mRNA and RNA that is then chaperoned into the cytoplasm. The host ribosome translates the mRNA into Cas9 enzymatic protein. Cas9 then associates with the CRISPR RNA to form a functional unit.
One of the functions of the CRISPR-Cas9 system is deletions of specific DNA sequences. The CRISPR RNA determines the target specificity and a guide RNA determines where Cas9 cleaves the DNA.
After CRISPR-CAS9 finds its target sequence and produces a double strand break, the host DNA repairs the double stranded break using DNA polymerases to insert random nucleotides until all strands are joined. This mechanism ultimately creates a shift in the reading frame for the target allele producing a loss of function essentially deleting the gene.
Another function of the CRISPS-Cas9 system is addition of specific DNA sequences. A gene of interest, is introduced along with the CRISPR-CAS9 system. After Cas9 produces a double stranded break, the host DNA repair mechanism repairs the break via homologous-directed repair where 3’ to 5’ exonuclease activity takes place producing “sticky-ends”. The ends of the gene of interest have nucleotide sequences that are complimentary to one of the sticky-ends. DNA polymerases then use the gene of interests as a template to repair the DNA break.