Sorry CRISPR, but there's a new genomic editor in town — and this one's better than you. It's a new approach to site-specific gene targeting that will allow scientists to safely replace disease-causing genes with functional copies. And they've already used it to relieve the effects of hemophilia in mice.
One of the most important biotechnological breakthroughs of the last several years is a genomic editing approach called CRISPR/Cas9. It allows researchers to replace a faulty, or mutated, version of a gene with a working copy. The technique is relatively easy to use, it can be used to modify multiple genes, and it features quick turn-around times. Currently, scientists are using it to create specific strains of lab animals, but in future it could be used for gene drives and to treat genetic diseases in humans. More controversially, it could conceivably be used for human enhancement and to confer entirely new characteristics altogether.
CRISPR, however, is not without its drawbacks. It requires the co-delivery of an enzyme called an endonuclease to snip the recipient's DNA at specific locations, and it relies on the co-insertion of genetic "on" switches called promoters to activate the new gene's expression. These lead to an increased chance of adverse effects, including cancer and various DNA abnormalities.
Now, Stanford researcher Mark Kay has found a new way to go about genome editing that avoids these problems. The new technique is considered safer and longer-lasting than other methods. Kay and his colleagues demonstrated the new technique by enabling mice with hemophilia to produce a missing blood clotting factor. From the Stanford release:
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.
The integration of the clotting factor gene was key to the successful experiment; other clinical trials involving hemophilia have relied 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.
Amazing, we're inching closer to the day when this technology will finally be made available to people suffering from virtually any kind of genetic disorder.
Read the entire study at Nature: "Promoterless gene targeting without nucleases ameliorates haemophilia B in mice".