For the first time, researchers have treated an animal model of a genetic disorder using a viral vector to deliver genome-editing components in which the disease- causing mutation has been corrected. Delivery of the vector to newborn mice improved their survival while treatment of adult animals, unexpectedly, made them worse, according to a new study by investigators from the Perelman School of Medicine at the University of Pennsylvania The team published their findings this week in Nature Biotechnology.
“Correcting a disease-causing mutation following birth in this animal model brings us one step closer to realizing the potential of personalized medicine,” said senior author James Wilson, MD, PhD, a professor of Medicine and director of the Orphan Disease Center at Penn. “Nevertheless, my 35-year career in gene therapy has taught me how difficult translating mouse studies to successful human treatments can be. From this study, we are now adjusting the gene-editing system in the next phases of our investigation to address the unforeseen complications seen in adult animals.” Wilson is also director of the Penn Gene Therapy Program.
The Wilson lab focused on liver as a target for gene editing since they had solved the problem of gene delivery in this organ in previous work using traditional gene therapy using vectors based on adeno-associated virus (AAV). However, gene replacement therapy with AAV is not ideal for treating genetic diseases of the liver that manifest as newborns since the non-integrating genome is lost as developing liver cells proliferate.
Because of this Wilson, co-first author Lili Wang, PhD, a research associate professor of Pathology and Laboratory Medicine, and collaborators, thought that the newborn liver might be an ideal organ for AAV-mediated gene correction using CRISPR-Cas9, an RNA-guided genome-editing technology that uses the bacteria protein Cas9. With CRISPR-Cas9 the corrected mutation will persist as the vector genome is lost.
This hypothesis was tested in a mouse model of a rare metabolic urea-cycle disorder caused by a deficiency in an enzyme called ornithine transcarbamylase (OTC). The urea cycle is a series of six liver enzymes that help rid the body of ammonia, a breakdown product of protein metabolism. When one of these enzymes is missing or deficient, ammonia accumulates in the blood and travels to the brain, causing a multitude of problems, including brain damage and death.
OTC deficiency is the most common of the urea-cycle disorders, occurring in one out of every 40,000 births. A mutated OTC gene can cause an enzyme that is shorter than normal, the wrong shape, or may not be produced at all. The genetic mutation responsible for OTC occurs on the X chromosome, so women are typically carriers, while their sons with the mutated gene suffer the disease.