You know you’ve struck marketing gold when a brand becomes a so-called “proprietary eponym.” Need to blow your nose? Grab a Kleenex. Track some sand from the beach onto your floor? Hoover it up.
In biology, Crispr is the proprietary eponym of the moment. The gene-editing technique is so inexpensive and easy to use that, in just four years, it’s become a ubiquitous tool in labs across the world. And soon, it could jump from bench-top workhorse to human therapeutic. In late October, a Chinese team deleted a gene out of a lung cancer patient’s lymphocytes and then injected the edited cells back into his bloodstream, and more cancer-related trials are planned next year in both the US and China.
But the jury’s still out on whether Crispr will be as transformative as a medical therapy as it has been as lab tool. Plenty of gene-editing techniques have been attempted as therapies, but few have made significant impacts—especially when it comes to diseases as complex as cancer. A better place to start testing gene therapies is with inherited blood disorders, like sickle cell anemia and beta thalassemia.
These diseases are a good comparison point because they’re relatively easy to treat. Both arise from mutations to a single gene, which in this case result in malfunctioning red blood cells that starve the body’s organs of oxygen. And while it’s tricky to edit cells in a body while they’re in a body, it’s much easier with blood diseases: You just take blood cells out, treat them, and put them back—better known as a bone-marrow transplant.
Researchers have thrown a number of gene-editing techniques at these diseases, hoping one might become the standard of care for the more than 100,000 people in the US who suffer from them. But if you ask experts in the field, the smart money’s on Crispr. “The Crispr field is moving at such a lightning-speed rate,” says Stuart Orkin, a hematologist-oncologist at Boston Children's Hospital. “Many of the issues that people raise as potential problems are being solved—and they’re being solved at a faster rate than other techniques.”
A potential competitor?
Early this month, researchers reported harnessing Crispr to edit bone marrow stem cells from humans with sickle cell. Then they grafted them into mice to see how long the edited cells survived. Stem cells in the bone marrow give rise to all cells in the blood, including red blood cells; so presumably editing them would mean the correct gene would be incorporated into the red blood cells they create.
After four months, edited cells remained in the mouse’s bone marrow, making up about 6 percent of the total population. That was a three-fold improvement on a similar study from Berkeley scientists, who less than a month earlier reported finding only 2 percent edited cells in the marrow of mice after the same amount of time elapsed.
Meanwhile, in late October, over on the East coast, a team from Yale and Carnegie Mellon revealed results of a new, alternative gene-editing technique—one that doesn’t require a transplant. They managed to find 7 percent of bone marrow cells to be edited after five months in mice with the human mutation for beta thalassemia, simply by injecting them with synthetic DNA-like polymers (usefully called PNAs) via IV.
At first glance, that might seem like a more viable gene therapy strategy. To begin with, the technique doesn’t involve cutting the genome, which can lead to errors. Instead, a nanoparticle ferries the PNA into cells along with a snippet of DNA to correct a mutation. The PNA binds to a matching section of DNA and appears as a “pothole” that needs fixing, says Peter Glazer, chair of Yale’s Department of Therapeutic Radiology. The cell’s repair machinery then uses that template DNA to replace the divot.
With Crispr, in comparison, an enzyme called Cas9 cuts a targeted sequence of DNA out of the genetic code, leaving the repair machinery to fill the gap using a template DNA segment that scientists supply. Since Cas9 is a fairly active enzyme, there are concerns it could make cuts elsewhere in the genome, as it persists in cells after editing the beta globulin gene. Further, in both the Stanford and Berkeley studies, often when a cut was made the DNA template wasn’t used to guide the patch. That incorrect fix might stop red blood cells from forming sickle shapes, but it could render them dysfunctional—effectively trading sickle cell for beta thalassemia.
But editing alone is not enough. It’s important that the correct cells are modified. Scientists raised concerns that the PNAs were not editing stem cells, but rather cells that are farther along the path to becoming full-fledged blood cells. That could mean any therapeutic effect would be temporary, and that a human version of this therapy might require regular IV treatments. With Crispr, since cells are being brought outside the body and treated in the lab, it’s easier to ensure that it’s the actual stem cells that are being edited. And if a Crispr team can get a higher fraction of edited stem cells to persist in bone marrow, a one-time treatment could permanently alleviate a blood disorder.
Crispr’s trump card
According to Matthew Porteus, the pediatrician who led the Stanford sickle cell study, most scientists agree that there should be at least 10 percent modified cells persisting in bone marrow to have a clinical benefit. And the improvement in his study following so soon after the Berkeley team’s proof-of-principle suggests that the bar could be cleared in short order. “Both of our groups have shown the blueprint,” says Porteus. “And it should be easy for the next groups to adopt our recipes.”
A huge advantage of the technique is what led to its mass adoption in the lab—the gene-editing systems are simple and easy to make. PNAs, on the other hand, involve complex chemistry reminiscent of zinc finger nucleases (ZFNs), which less than a decade ago were the gold standard in gene-editing. Zinc fingers are pairs of proteins that each target a sequence of three DNA bases to bind to specific parts of the genome and break off a segment of DNA. While there are ZFNs that are as effective as Crispr at editing genes, building a pair of zinc fingers takes months. “To make a really good pair of ZFNs takes a lot of time,” says Donald Kohn of UCLA’s Broad Stem Cell Research Center. “Any lab can make 20 Crisprs tomorrow.”
That disparity means that when a problem arises for Crispr to solve, many groups around the world can easily take a crack at it. Meanwhile, the Yale/Carnegie Mellon team is essentially the only one refining the PNA technique. But that doesn’t mean they should abandon their efforts. “From a patient perspective, we need to have alternative approaches that people are developing,” says Porteus. “Because in a few years, we might stumble upon a fatal flaw in the Crispr technology that we can't solve.”
But until we hit that deal breaker, a future where people suffering from genetic blood disorders will soon have their pesky mutations Crispr’d out of their DNA for good is coming into clearer view.
