Debugging Our DNA: From Gene Therapy to CRISPR
STAT recently released a phenomenal three-part video series on biomedicine in collaboration with Retro Report. The latter is known for their bite-sized documentaries that follow up on old stories and explore how they figure into today’s news. In this series they recount the hopes and aspirations associated with sequencing the human genome and precision medicine, and retrace the often tortuous history of biotechnologies past and present, from gene therapy to burgeoning gene editing tools like CRISPR.
In Part 2, we’re reminded of the discomfiting story of Jesse Gelsinger, whose death set the field of gene therapy back more than a decade. Gene therapy is one of our earliest attempts to correct for disorders arising from our DNA, with the first clinical trial conducted in May 1989 by the National Institutes of Health. At bottom, gene therapy tries to reverse the course of genetic disorders and other diseases, usually by inserting a working gene variant into the body to compensate for a faulty one.
Huntington’s disease, for example, is a rare neurodegenerative disorder caused by a mutation in the huntingtin gene on chromosome 4. (Naming genes by what goes wrong when they mutate is a popular convention in genetics.) Humans have two copies of this gene, one from each parent. If just one copy has the mutation that gives rise to the disease, the individual will inherit the disorder. We call this an autosomal dominant disorder, which according to the rules of inheritance, means that any child of an affected parent has a 50% chance of suffering the condition. Cystic fibrosis, on the other hand, is an autosomal recessive disorder, requiring two copies of the mutated gene to express the disease.
What gene therapy aims to do is supply the cells with a normal or non-mutated copy of the gene(s) responsible for these and other disorders. This was the goal behind Jesse Gelsinger’s treatment in 1999. The 18 year-old Gelsinger suffered from OTCD (ornithine transcarbamylase deficiency), a metabolic condition that impairs the body’s ability to process nitrogen in the blood. As with the others mentioned above, OTCD has a genetic explanation: it’s caused by a mutation in the OTC gene located on the X chromosome. Its inheritance is X-linked recessive, meaning men more commonly suffer from the condition than do women.
In a clinical trial overseen by James Wilson, founder of University of Pennsylvania’s former Institute for Human Gene Therapy, scientists used a viral vector to deliver an unmutated copy of the OTC gene into Gelsinger’s blood. The antigenic presence triggered a massive immune response, sparking a deadly high fever and lethal levels of blood clotting. Gelsinger did not survive.
Wilson and his research cohort quickly found themselves on the wrong end of political stilettos. The FDA, NIH, a Senate subcommittee, and his own university launched investigations and held hearings to probe the ethical, legal and scientific implications of gene therapy. Wilson, in addition to being sued by Gelsinger’s family and the Justice Department, was slapped with a five-year ban from human clinical trials. Around the same time, two others died in Europe from therapeutic DNA delivery, almost shuttering the field for good.
In the case of Gelsinger, his immune response and subsequent death were the result not of the insertion of the functioning allele but of the delivery mechanism — the viral vector. James Wilson’s team used what are called adenoviruses, specialized viruses that take up the functional DNA before being injected into the host. What we think happened is that Gelsinger had already been exposed to the particular adeno-associated virus used in the study. When the particles made it to his liver, his previously acquired antibodies induced the body-wide bout of inflammation that overwhelmed his immune system.
We still use viruses to deliver DNA in gene therapeutic trials. But Wilson and others have made great strides in the time since Gelsinger’s death, discovering a number of viral candidates far better suited for genetic treatments. The ones in use today are not only more efficient and provoke little to no immune response, they can also be tailored to specific types of human tissue, from muscle tissue to the liver and even the brain. Subjacent to a 2013 article by Carl Zimmer are a number of examples of disorders currently being treated by gene therapy interventions, including hemophilia, muscular dystrophy, blindness, and even nicotine addiction. To date, over 2,500 gene therapy treatments have been approved for clinical trials.
Gelsinger’s story underlines the at times tragic cost of clinical trials in exchange for future cures. As you’ll see in the videos below, many of the participants in these trials volunteer knowing that their decision could ultimately prevent others from suffering as they have. There are never guarantees in these arrangements, but the possibility of successful treatment of a debilitating disease often outweighs the risk for many patients.
Retro Report’s recap of the fall and rise of gene therapy helps set the stage for CRISPR, a genetic engineering tool that’s become a remarkably powerful way to edit DNA in any species. One key distinction to keep in mind when it comes to gene therapy and its successor, is that, unlike CRISPR, gene therapy doesn’t actually repair broken genes; it simply provides a working variant alongside the defective one. That is, the mutated one remains in place. With CRISPR, we are actually changing the DNA in living cells.
Because of this, it’s always been unclear how long gene therapy treatments, if successful, will last. In the case of one woman in the documentary who was treated for ADA deficiency at a young age, we learn that the therapeutic benefits subsided over time, and that she’s had to continually receive treatment over the course of her life. This is important because a successful gene therapy trial does not mean that the patients were cured or that the benefits they received from the treatment are persistent.
With CRISPR and other gene editing kits (there are several flavors), the mutated code is snipped out and replaced with healthy DNA. Rather than rely on viruses to deliver the DNA, CRISPR makes use of Cas9, an enzyme that can cut DNA directly. When the enzyme’s guide RNA finds a match within a selected strip of DNA, it deletes the matching portion, disabling it. A 3D illustration does wonders for understanding how this system works: Genome Editing with CRISPR-Cas9 by the McGovern Institute at MIT is still my favorite.
While CRISPR is currently being put to extraordinary use by humans, we didn’t invent this technology. We discovered it by studying bacteria, which have been using it as a defense against viruses and other foreign elements for hundreds of millions of years. Scientists have simply co-opted this natural mechanism by changing the guide RNA to match the specific DNA sequence we want to cut in an organism’s genome.
Where it gets interesting is what happens after the molecular scissors excise the target DNA. Normally, the cell’s repair mechanisms kick into gear and attempt to reconstitute the missing section of DNA. This process can introduce mutations that often end up disabling the gene altogether. Sometimes, knocking out the defective DNA is all that’s needed and we can stop there. In other cases we might want to replace the mutant gene with a healthy copy, effectively editing the DNA sequence. This is the step that’s proved a bit trickier in practice.
When Chinese researchers attempted this in human embryos, CRISPR managed to cut DNA in only a fraction of the 71 embryos that survived, out of a total of 86, and only in a fraction of those 71 did cells manage to take up the new DNA. In other words, 28/71 were successfully spliced, but only a few of the 28 ended up with the intended DNA. (Note that the Chinese team used non-viable zygotes only — i.e., the zygotes were fertilized by two sperm, so they had an extra set of chromosomes.) The ability of cells to take up a desired sequence of DNA without introducing off-target effects is where the rubber meets the road right now in genetic engineering.
CRISPR and other high-profile biotech have undoubted potential and are amazingly cool. But we still have a long way to go before we’ll be rewriting the DNA underlying SNP-related diseases like cystic fibrosis, sickle-cell anemia, Tay-Sachs disease, or breast cancer, or curing neurological disorders such as Alzheimer’s and Parkinson’s, to say nothing of enhancing or regulating traits controlled by multiple genes (see polygenic trait). The time required to refine our understanding of gene-based treatment, moreover, will provide us the necessary opportunity to assess these technologies at the level of ethics and define the boundaries of permissible application and misuse.
The hope is that one day these more precise molecular techniques now at our disposal will eliminate common genetic diseases entirely. But comparing today’s headlines with those of yesterday suggest that, in some ways, we’ve been here before. The under-delivered promises made alongside advancements in gene therapy and the $2.7 billion Human Genome Project should remind us of the untold complexity that awaits us as we delve ever deeper into the labyrinthine realm of biomedicine.
Enjoy STAT’s series The Code below.
External link: ‘The Code’: roots of today’s most promising genetic technologies
Further reading:
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