There are a number of human diseases that are caused by mutations in single genes. Hemophilia B occurs when point mutations cause disruption of the function of blood clotting factor IX. This results in excessive bleeding and blood loss because of impaired blood clotting due to low levels of factor IX. For years, scientists have tried to treat diseases that are caused by mutations in single genes with a process known as gene therapy. Gene therapy is a treatment that aims to insert a cloned gene into the genome of a cell to restore a functional gene product. In the past, gene therapy has had many setbacks. Insertion of a cloned gene into the genome of a cell often occurred at random sites. This had the potential of insertion into and disruption of normal genes in the genome and could lead to additional mutagenesis and eventual carcinogenesis. In addition, gene therapy often inserted cloned genes into locations of the genome where they would not have the normal regulatory elements in place such as promoters and enhancers. Genome editing is a new technique that attempts to insert cloned genes into a native position in the genome and replace the mutant gene. This prevents random insertion and also preserves the normally active genetic machinery of the endogenous regulatory elements. Researchers lead by Dr. Katherine High, from the Children’s Hospital of Philadelphia and the University of Pennsylvania School of Medicine, have used the technique of genome editing to insert a functional copy of the clotting factor IX gene into mice that have hemophilia B. Their results were recently published online in the journal Nature. The method of genome editing that the researchers used involves using enzymes called zinc finger nucleases to cut, or splice, the genome at specifically directed sites. The researchers used a strain of mice that had the mutant human hemophilia B gene cloned into their genome. With help of geneticists from Sangamo BioSciences, the researchers developed zinc finger nucleases that were able to splice out the mutant human factor IX gene in the genome of the mice and insert a normal functional copy. The researchers did this by constructing a viral vector that specifically targeted the liver of the mice, which is where factor IX is produced. This type of gene therapy, using genome editing, caused the serum levels of factor IX to rise to 3-7% of the typical amount of the deficient clotting factor. This resulted in a significantly fasting clotting time and decreased the severity of hemophilia B, effectively producing a functional cure in the mice. When the livers of the mice were resected and allowed to regenerate, the new liver tissue was shown to also be able to produce functional factor IX. This suggests that the genome editing technique that was used was able to stably integrate the functional factor IX gene into the genome and that this functional gene was passed onto the regenerated liver tissue. The authors wrote, “[zinc finger nuclease] -driven gene correction can be achieved in vivo, raising the possibility of genome editing as a viable strategy for the treatment of genetic disease” and that “clinical translation of these results will require optimization of correction efficiency and a thorough analysis of off-target effects in the human genome”. Future research should focus on targeting the functional gene templates to the correct cell type, limiting the risk that zinc finger nucleases may splice wrong areas of the genome, and constructing suitable viral vectors that are able to carry and deliver the functional gene.
Hojun Li et al. “In vivo genome editing restores haemostasis in a mouse model of haemophilia” Nature published online 26 June 2011