[Corrected 10/13/16, 1:00 p.m. See below.] Another academic group has published results of animal experiments that use the gene editing tool CRISPR-Cas9 to treat sickle cell disease, adding to the hope that one day, the devastating genetic condition and others like it could be fixed by changing a patient’s DNA.
The latest research, published today in Science Translational Medicine, described how cells from sickle cell patients, altered with CRISPR-Cas9 to carry a normal blood-making gene, could produce healthy blood cells when transplanted into mice. The work combined the efforts of blood-disease researchers and clinicians at UCSF Benioff Children’s Hospital Oakland, the Berkeley, CA-based Innovative Genomics Initiative, which was cofounded by CRISPR-Cas9 pioneer Jennifer Doudna, and Dana Carroll of the University of Utah. [Edited to clarify the names and affiliations of the research leaders.]
The newly published work and other gene-editing approaches to sickle cell disease are years away from being tested in humans, so it’s wise to temper enthusiasm. But sickle cell disease, also known as sickle cell anemia, has no cure except for risky bone marrow transplants that are only available for the lucky few who can find a matching donor. Doctors are getting better at managing care of chronic patients, in part with a generic drug called hydroxyurea, but the costs to patients’ quality of life and to the healthcare system remain high.
“Even with optimal care, it’s likely to shorten anybody’s life span by decades,” says David Martin, a pediatric hematologist, coauthor of the new study, and chair of the Center for Genetics at Children’s Hospital Oakland Research Institute.
The disease affects roughly 100,000 Americans, mostly of African descent. It causes their red blood cells, normally round, to curl into sticky crescents and clog blood vessels. The worst complications are strokes, deadly lung conditions, bouts of excruciating pain, anemia, and organ failure. The disease is more prevalent, with worse outcomes, in areas like sub-Saharan Africa where malaria is endemic.
Researchers know which gene mutation causes sickle cell. People who have inherited the mutation from both parents have the full-blown disease. Those with only one copy of the mutation have sickle cell “trait” and typically don’t have symptoms of the disease, although the connection between genetics and disease is not perfectly drawn.
Fixing that gene—it’s called beta-globin, and it’s on chromosome 11—makes sickle cell a tempting target for gene therapy, which is why even incremental clinical news from groups like Bluebird Bio (NASDAQ: BLUE) attract scrutiny.
Like gene therapy, gene editing might fix the genetic flaw, but with more precision. Two industry collaborators, Sangamo Biosciences (NASDAQ: SGMO) and Biogen (NASDAQ: BIIB), are using the gene editing system known as zinc fingers to develop a sickle cell treatment, but it has yet to reach clinical trials.
Meanwhile, several academic groups are working with CRISPR-Cas9. Researchers at St. Jude Children’s Research Hospital in Memphis, TN, and Dana-Farber Cancer Institute in Boston are using it to make a genetic snip that coaxes the fetal version of beta-globin, which typically goes silent after birth, to stay on and make healthy red blood cells. (Normally, the fetal gene is switched off after birth and the adult version kicks in; mutations in the adult gene cause sickle cell disease, as well as another blood disease called beta thalassemia.)
The work published today takes a different tack: Fix the mutant version of adult beta-globin. One set of their experiments used stem cells from a sickle cell patient. They repaired the faulty beta-globin gene and put the stem cells into lab mice that wouldn’t reject the human cells.
Four months later, the researchers inspected the mice. They estimated that 2 to 5 percent of their stem cells had been successfully edited—“relatively high efficiency,” according to Martin—and were producing normal blood cells. That might not seem promising. But experience with bone marrow transplants has shown researchers that, because of the peculiarities of sickle cell biology, a low level of replacement might be enough to alter the course of the disease.
“If you have 10 percent normal bone marrow you’ll be cured, but we think the necessary percentage is actually lower,” says Martin. His colleague Mark Walters, also a co-author, has seen patients recover with as low as two percent of their bone marrow cells functioning normally.
That would be good news, because one lesson from the new study is that the stem cells that make our blood are difficult to edit without losing their “stemness,” or ability to regenerate. On top of that, the researchers are using CRISPR not to knock out a gene, but to cut out the bad version and insert a new version—a much tougher task. “Replacement would certainly be the best if it can be accomplished,” Martin says, but he acknowledges that boosting fetal beta-globin is attractive because it only requires a knockout. Martin says he and his colleagues are working on that, too, but it “isn’t ready for primetime.”
One caveat of the replacement work is related to sample size. Because stem cells from sickle cell patients are hard to obtain, many of the cells the researchers used were drawn from healthy donors. With the healthy cells, the researchers deployed the same CRISPR-Cas9 tools to insert the sickle cell mutation, a sort of mirror image of the procedure that fixed the mutation in sickle stem cells. What worked going in one direction should work going in the other direction, Martin says, but the researchers will need to reproduce the experiment in larger batches of sickle stem cells drawn from patients to feel more comfortable.
Now they need to raise money for more mouse studies. Another task in the next set of experiments is to watch for long-term safety problems. CRISPR-Cas9 uses molecular scissors guided by a tiny piece of code to match up and snip the right sequence of DNA. But a snip in the wrong place could theoretically lead to an unwanted mutation and to cancer. Some working with CRISPR feel that such “off target effects,” if rare, would likely be checked by the body’s own safeguards. There have been no red flags so far, but the Children’s-IGI-Utah group will watch what happens in their mice over a longer period of time. Jacob Corn, coauthor of the paper and scientific director of the Innovative Genomics Initiative, told me last year that the safety risks of gene editing “should keep everyone in the hematopoietic field up at night.”
Because sickle cell is for some people a manageable disease, the safety of a genetic fix will be a big question with regulators. It’s one thing to green light a drug that might cause cancer when it’s treating cancer patients with no other recourse; it’s another when patients could have decades of life remaining.
“It’s something we’re grappling with,” says Martin. “How far do you need to go to demonstrate safety?”
[The original version of this story said that Children’s Hospital Oakland led the research team, which is not the case..]
Image of sickled red blood cells courtesy of Scooter DMU via Creative Commons.