Delivering the DNA and CRISPR Cancer Screens
The gene therapy field revives, once again; Using CRISPR to probe tumors. Day Two from the CRL World Congress meeting.
James Wilson is a self-described refugee in the up-and-down field of gene therapy. As one of the early pioneers who tested the notion of replacing or adding a gene to correct a faulty, disease-causing one, Wilson, the director of the Gene Therapy Program at University of Pennsylvania’s (UPenn) Perelman School of Medicine, lived through the enormous media hype that drove the field during the 1980s and 1990s and the crushing defeat that followed in 1999, when an 18-year-old man became the first person to die during a gene therapy trial.
The adenoviral vector delivering the DNA was blamed for the fatality and safety concerns shelved programs all across the country. The field was revived again in 2012 when European regulators approved the world’s first gene therapy, a drug to treat a rare genetic defect that causes recurring acute pancreatitis. But the Dutch biotech uniQure withdrew Glybera five years later because for various reasons, including the whopping US$1 million price per dose, the drug was only used on one person.
But recent successes are, once again, stirring excitement in the field, noted Wilson during a keynote address Sept. 27 at the Charles River World Congress on Animal Models in Drug Discovery and Development. “We are now in a new phase of commercial development that is generating some of the excitement [felt] decades ago,” says Wilson.
Earlier this year, the US Food and Drug Administration approved a gene therapy drug for a rare form of leukemia. The cancer immunotherapy drug, developed by scientists from UPenn in partnership with Novartis, was the first gene therapy to be approved in the US, but it's in the realm of orphan diseases where the most of the action is occurring..
Wilson talked a lot about the contributions of large animal species—who he gamely listed as credits at the end of his talk!—for helping to de-risk the technical challenges of choosing the right adeno-associated viral (AAV) vectors to chauffeur genetic material, and for contributing to major breakthroughs in the treatment of rare diseases, including a genetic form of blindness, a collection of around 50 rare liver diseases known as lysosomal storage disorders (LSDs) and the blood disorder, hemophilia.
Wilson says his lab and others have made considerable progress in determining optimal delivery systems for some of these experimental gene therapy drugs. Many rare diseases impact the central nervous system, but finding drugs to treat them are made even more difficult by the blood-brain barrier, which by necessity must shield the body from foreign material that could be harmful but which also, consequently, refuses most drugs entry.
Wilson says they have found a way around this obstacle by testing different delivery systems that deliver the drug directly into the brain or into the cerebrospinal fluid. Using an animal model of an LSD characterized by abnormally high levels of an enzyme called IDUA, they managed to restore normal levels of IDUA by injecting an AAV9 vector containing a normal genetic sequence of the enzyme. The gene therapy also effectively treated brain storage lesions triggered by the LSD. The drug is now entering early clinical trials.
RNA Guideposts for Cancer
Most oncologists agree that we are living in a kind of cancer treatment renaissance led, of course, by the growing number of immunotherapy drugs. But there is still so much that we don’t understand about how and why some tumors respond to treatment while others do not. Keynote speaker David Sabatini, a biology professor at MIT and a member of the Whitehead Institute, says CRISPR-Cas9 screening of cancer cells can help us probe tumors in unprecedented detail, and perhaps tell us which genes of interest are important for the life-span of the tumor, which ones are detrimental and which ones are inconsequential. The goal is to identify new targets for personalized drug therapies.
To analyze these tumors, his lab first grows millions of immortalized cells and then uses CRISPR to knock out one gene in each cell. During this process, each cell is marked with a genetic “barcode” that helps to determine which gene has been knocked out. The cells are then grown in barcoded cells in a dish to see which ones survive and which ones die, which offer clues for which genes are essential for cancer cell growth. Because many of the genes in cancer cells are essential for healthy cells, sniffing out genes that represent a cancer’s weak link is a tricky process. One thing Sabatini has learned from doing using these CRISPR screens is that tumors, like people can’t escape their roots. “The cancer cell remembers where it came from,” he says. “Whatever those genes were when transcription happened, those genes turn out to matter.