A lot of attention has been paid lately to the idea of genome editing. This technique allows researchers to precisely modify an animal's DNA to replace one version of a gene with another, or to add a working copy for a mutated gene. An approach called CRISPR/Cas9 in particular has garnered interest with its ease of use, ability to modify multiple genes, and relatively quick turnaround time when making specific strains of laboratory animals like mice for study.
Now pediatrician and geneticist Mark Kay, MD, PhD, has published in Nature a new way to conduct genome editing that could give CRISPR a run for its money because it could be both safer and longer-lasting than other methods. As described in our press release:
The approach differs from that of other hailed techniques because it doesn’t require the co-delivery of an enzyme called an endonuclease to clip the recipient’s DNA at specific locations. It also doesn’t rely on the co-insertion of genetic “on” switches called promoters to activate the new gene’s expression.
Inclusion of endonucleases and promoters run the risk of a gamut of adverse effects in the recipient, from cancers if the promoter turns on the wrong gene in the genome to an unwanted immune response geared toward the foreign proteins. The researchers in Kay's lab, including postdoctoral scholar and study lead author Adi Barzel, PhD, found a way around their use, and showed that it worked to enable mice with hemophilia to produce a missing blood clotting factor:
The technique devised by the researchers uses neither nucleases to cut the DNA nor a promoter to drive expression of the clotting factor gene. Instead, the researchers hitch the expression of the new gene to that of a highly expressed gene in the liver called albumin. The albumin gene makes the albumin protein, which is the most abundant protein in blood. It helps to regulate blood volume and to allow molecules that don’t easily dissolve in water to be transported in the blood.
The researchers used a modified version of a virus commonly used in gene therapy called adeno-associated virus, or AAV. In the modified version, called a viral vector, all viral genes are removed and only the therapeutic genes remain. They also relied on a biological phenomenon known as homologous recombination to insert the clotting factor gene near the albumin gene. By using a special DNA linker between the genes, the researchers were able to ensure that the clotting factor protein was made hand-in-hand with the highly expressed albumin protein.
As Kay, who is also a member of the Stanford Cancer Institute, the Stanford Child Health Research Institute and Stanford Bio X, explained, the integration of the clotting factor gene is key to the successful treatment (other clinical trials involving gene therapy for hemophilia rely on the expression of a free floating, unintegrated gene in the nucleus):
The real issue with AAV is that it’s unclear how long gene expression will last when the gene is not integrated into the genome. Infants and children, who would benefit most from treatment, are still growing, and an unintegrated gene could lose its effectiveness because it’s not copied from cell to cell. Furthermore, it’s not possible to re-administer the treatment because patients develop an immune response to AAV. But with integration we could get lifelong expression without fear of cancers or other DNA damage.
Previously: Gene “editing” could correct a host of genetic disorders, Policing the editor: Stanford scientists devise way to monitor CRISPR effectiveness and Both a doctor and a patient: Stanford physician talks about his hemophilia