In 1999, gene therapy – once touted as one of biomedicine’s most promising fields – suffered a major and highly publicized setback. Jesse Gelsinger, an 18-year-old participating in a clinical trial testing treatment for a rare immune disorder, suffered a fatal immune reaction to the virus used to deliver the gene. The field has been slow to recover from this early tragedy. But now clinical trials of gene therapy are taking place worldwide, mainly for cancer and diseases linked to a single known gene.
Gene therapy has enormous potential to treat disease – if scientists can find safe delivery technologies. In the therapy, a copy of a gene is delivered to a specific tissue to correct an abnormality – for example, the lungs in cystic fibrosis. The gene is often delivered with viruses, which have evolved the ability to deliver their genes into human cells. The danger is that the virus can trigger a vigorous immune response in the patient, as happened with Gelsinger.
James Wilson, a scientist at the University of Pennsylvania, who headed the trial in which Gelsinger died, has spent the last seven years searching for safer and more effective ways to deliver therapeutic genes. He’s taking advantage of the natural diversity of viruses to find candidates that have evolved the optimal characteristics for gene therapy. He tells Technology Review about some promising options.
Technology Review: Why do we need better delivery viruses, commonly known as vectors?
James Wilson: We recognized five or six years ago that the technology we had was wholly inadequate for what we wanted to achieve in the clinic. Back in the ’80s we used the vectors that were available off the shelf, not vectors that were perfected for use in gene therapy. But what we found was that they were not sufficiently efficient and/or caused an inflammatory or immune response. For many applications, that meant these vectors were either toxic or ineffective.
TR: What are the most important qualities in a delivery vector?
JW: First of all, when injected in vivo [into a live animal or person] they must target cells with high efficiency. Second, they must be easy to manufacture. And third, these vectors must evade the immune system.
TR: Do you have any vectors that possess these qualities?
JW: We initially worked a lot with adenoviruses [the virus behind the common cold], which were first brought into humans to treat cystic fibrosis. But they were found to be very immunogenic, meaning they caused severe inflammation.
For the last five years, we have studied adenovirus-associated viral (AAV) vectors. They seem to not cause inflammation – they evade immune detection. These vectors also seem to be very stable. For example, we injected monkeys with an AAV vector eight years ago – and gene expression has gone down only about three-fold since then. [This long-term expression is significant because gene expression of many types of vectors disappears over time.]