Gene editing is a very compelling concept for physicians. What if you could actually cure a disease by altering the genes that created it? Then your patients wouldn't need drugs and other therapies, which often involve high costs and dangerous side effects. This revolutionary approach could either remove the disease or reduce it to a nonthreatening level.
Slowly but surely, researchers are trying to bring the concept of gene editing closer to clinical reality. Still, no one is saying that this therapy would be commercially available any time soon. Use of gene editing on humans is just beginning to enter clinical trials. At this point, research is focusing only on a small number of diseases that affect relatively small populations.
In gene editing, "the idea is not to treat the disease but to physically change the DNA in a way that cures the disease," says Fyodor Urnov, PhD, a genetic biologist and senior scientist at Sangamo BioSciences, a California company that owns the rights to a form of gene-editing technology called "zinc-finger nucleases."
More than 3000 diseases have been linked to mutations in individual genes, but researchers are starting with diseases that are most likely to yield positive results. These include HIV and diseases that involve a defect in only one gene, such as hemophilia, sickle cell disease, and beta thalassemia. Meanwhile, "there are many diseases that we are not looking at, such as heart disease, because they have contributions from multiple genes," Dr Urnov says.
How Gene Editing Works
Gene editing—more properly called "genome editing"—involves removing and adding specific bits of DNA in a patient's genome. The process is a lot like cutting and pasting words, which is why the process is called "editing." Sangamo's zinc-finger nucleases are engineered from natural enzymes and introduced into the blood, or into the brain or other organs. They can also be used outside the body on stem cells or T cells, which are then introduced into the body.
The zinc finger is able to locate a particular set of defective genes, make a break in the DNA strands there, and introduce new bits of DNA to take their place. "The beauty of this is that we can rely on a natural process to repair the break," Dr Urnov says. "We let Mother Nature do its work."
Dr Urnov says this process has become much more than just a concept. It has been shown to work on mice and other animals in research labs and is now beginning to be used in clinical trials. Sangamo is already in mid-stage clinical trials for HIV and is hoping to get approval for trials on beta thalassemia, sickle cell disease, and hemophilia.
"Gene editing is a reality," he says. "We can edit the genome of human cells so that they make a new therapeutic protein, or we can knock out a gene in order to have a therapeutic effect."
The next steps in developing gene editing are clear, Dr Urnov says. "We have done a lot of work on mice models, and now we have to translate that to the human setting." At this point, neither Dr Urnov nor anyone else can say when gene editing would be ready for normal clinical use on patients.
New Competition From CRISPR
The zinc-finger method was developed almost 20 years ago, but Dr Urnov says research on it only began to hit its stride in 2005, when it was first used to correct a mutant gene in human cells. In the next 3 years, zinc fingers were successfully used to add a whole gene to a specific place in the DNA and then to remove a specific gene, he says.
Sangamo has garnered millions of dollars from investors, industry partners, and grant funding agencies to research gene editing and translate the potential of the technology to the clinic. Company representatives say that Sangamo expects to end the year with at least $200 million in cash and investments, not to mention its partnerships with other biotech companies, such as Biogen and Shire.
Recently, however, interest in gene editing has begun to include a rival method to zinc fingers, called "clustered regularly interspaced short palindromic repeats" (CRISPR). Developed just 3 years ago, CRISPR has been hailed as a gene-editing wunderkind by the
New York Times,
[1] the
Wall Street Journal,
[2] and the
New Yorker.
[3]
Despite an ongoing battle over the patent for CRISPR, the technique is beginning to attract substantial investments. In November 2014, Intellia Therapeutics announced
[4] a $15 million funding round led by Novartis and Atlas Venture to develop CRISPR.
Many genetic researchers, such as Gang Bao, PhD, at the Georgia Institute of Technology, have switched from zinc fingers to CRISPR, according to a 2014 article
[5] in
MIT Technology Review. Using RNA molecules rather than zinc fingers, CRISPR "has quickly spread through biology laboratories" because "it is so precise and cheap to use," according to a November 2015 article
[6] in
MIT Technology Review.
For all of its merits, however, CRISPR isn't as accurate as zinc fingers in locating specific DNA strands, Dr Urnov and many others contend. "For this reason, it will be difficult to develop as a therapeutic technology," Dr Urnov says.
Nonetheless, there's now a company called CRISPR Therapeutics, based in Cambridge, Massachusetts, whose stated mission
[7] is "to develop transformative gene-based medicines for patients with serious diseases."
Possible Use Against HIV
Sangamo's first substantial research into human applications for gene editing focused on HIV. The goal is to mimic the CCR5-delta 32 mutation, a very rare natural gene mutation that allows T cells to resist infection by HIV. CCR5-delta 32 came to light when doctors observed that some sexual partners and needle-sharers of AIDS patients didn't contract the virus. Then a patient in Berlin, Germany, was in effect cured of the HIV infection after receiving a bone marrow transplant from a donor who had the mutation.
"The goal is to modify HIV patients' immune system so that their own cells can destroy the virus," Dr Urnov says. This would mean that they would no longer have to be on individual "cocktails" of antiretroviral drugs for the rest of their lives.
Sangamo is currently active in three clinical trials involving gene therapy for HIV—two using T cells and one using stem cells. Dr Urnov says he's cautiously optimistic about the trial results so far. In one trial, "nine subjects have been able to stay off their meds for an extended period of time," he says. "The longest period of time has been more than a year and a half."
Dr Urnov says Sangamo would have to complete larger clinical trials before the HIV approach could gain approval by the US Food and Drug Administration (FDA), and the trials could be quite costly. If the results of the current trials are promising, the company would look for a
partner to share the expenses for further development of this therapy, he says.
Researchers are also using CRISPR to mimic the CCR5-delta 32 mutation for HIV. In research
[8] published in the
Proceedings of the National Academy of Sciences last year, a team of hematologists engineered a particular white blood cell to be HIV-resistant after altering the genome of induced pluripotent stem cells.
Targeting Some Simple Diseases
Meanwhile, Dr Urnov says Sangamo is focusing on other diseases that are thought to be the best fit for the gene-editing approach, such as sickle cell disease, beta thalassemia, and hemophilia. These diseases were chosen because they're monogenic—meaning they're caused by a genetic defect in a single gene. Developing the therapy would be relatively straightforward because "there's an unambiguous correlation between a mistake in the gene and the disease," Dr Urnov says.
Sangamo has partnered with Biogen to fund research and develop a gene-editing therapeutic to treat beta thalassemia and sickle cell disease. The two companies plan to file investigational new drug (IND) applications for clinical trials for beta thalassemia in the first half of 2016 and for sickle cell disease in the second half of 2016, Sangamo representatives say.
Both diseases are incurable and require ongoing therapy to keep the patient alive. Beta thalassemia is a blood disorder that reduces the production of hemoglobin, requiring lifetime blood transfusions. Sickle cell disease causes red blood cells to become misshapen and break down, decreasing the amount of oxygen in the blood. Patients with sickle cell disease need to have blood transfusions, iron chelation therapy, and other treatments and medications.
Gene editing for both diseases involves the clever idea of switching from the adult hemoglobin gene, which has been malfunctioning, to the fetal hemoglobin gene, which is in good shape. So it's a switch that would, in effect, cure the patient of the disease. Why are there two genes? The fetal hemoglobin gene is used in the womb when the fetus is taking oxygen from the mother's bloodstream. At birth, it's turned off and the adult gene is turned on. Already, in preclinical research in mice, it has been shown that gene editing with zinc fingers can turn the fetal gene back on, Dr Urnov says.
Meanwhile, CRISPR is also being used to edit sickle cell genes in research led by Dr Bao at Georgia Tech. In the 2014 article in
MIT Technology Review, he said that even if gene editing could not remove all the sickle cells, it would still have the desired effect. "Even if we can replace 50%, a patient will feel much better," he said. "If we replace 70%, the patient will be cured."
Developing a Therapy for Hemophilia B
Dr Urnov is enthusiastic about using zinc fingers on hemophilia, another monogenic disease. The research is focusing on hemophilia B, which is caused by a defect in the gene for clotting factor IX (FIX).
In a study
[9] published in
Nature in 2011, laboratory mice were virtually cured of hemophilia by using the zinc-finger technology. Scientists at Sangamo and at the Center for Cellular and Molecular Therapeutics at the Children's Hospital of Philadelphia spliced a new FIX gene into the gene sequence of a damaged FIX gene. This technique raised the level of the clotting protein only marginally, to about 5% of normal levels, but this was enough to have a dramatic impact on the mice's condition.
Subsequently, a Sangamo researcher led a similar study on 15 monkeys injected with zinc-finger nucleases and normal versions of FIX. Afterward, the monkeys' livers began producing much higher levels of FIX, and protein levels in the blood reached as much as 10% of normal levels. Dr Urnov adds that researchers have improved results using this technique by targeting the albumin gene in the liver.
Sangamo plans to file an IND application by the end of the year to begin the first clinical trial for its hemophilia B treatment. The company used to collaborate with Shire on research for hemophilia and Huntington disease, but in September, Sangamo announced
[10] that although the companies will keep their collaboration, Shire will focus its research on Huntington disease and Sangamo will focus on hemophilia.
Dr Urnov says Sangamo is also looking into the use of gene editing for lysosomal storage disorders, such as Hurler and Hunter syndromes. These diseases involve defects of the lysosomes, which act as recycling sites in cells, breaking down unwanted material into simple products for the cell to use to build new materials, and currently there are no cures. Company officials say Sangamo plans to file an IND application with the FDA for Hurler syndrome by the end of 2015 and for Hunter syndrome in the first half of 2016.
Ethical Considerations
Dr Urnov and most other researchers are exclusively using gene editing on somatic cells—cells that aren't involved in the reproductive process and thus will disappear with the death of their host. But there has been some talk about using gene editing for the germline—cells that will be passed on to succeeding generations. Many scientists aren't comfortable with this sort of research, because no one knows what sort of side effects gene editing might produce. If they entered the germline, they might be passed down to future generations.
In March, Dr Urnov and other Sangamo representatives wrote an opinion piece
[11] in
Nature calling for a moratorium on editing the germline. Also that month, some scientists involved in the CRISPR technology made basically the same plea.
[12] These scientists will further discuss the issue in a meeting in December. Although the United States doesn't directly ban it, the National Institutes of Health won't fund human embryo research.
Dr Urnov is concerned that if the scientific community doesn't act against embryo research, authorities might move to ban all kinds of gene editing, including research on somatic cells that could be potentially life-changing for patients with some of these genetic diseases.
Researchers do agree on one thing, however: Gene editing shows great potential for clinicians, but it could take many years to develop.
"Although still in its infancy, genome editing presents tantalizing opportunities for tackling a number of diseases that are beyond the reach of previous therapies," according to a review of the approach in
Nature Medicine[13] earlier this year. "The technology will require a number of iterations to systematically optimize its efficacy, safety and specificity."
Nonetheless, Dr Urnov is optimistic that at least some breakthroughs will occur within the next decade. "There is no question—none whatsoever—that over the next decade the clinical landscape will change, and genome editing will play a major role in this," he said in a July lecture
[14] at the University of California, Berkeley. "We will think clinically on how to manage genetic and other diseases in a fundamentally new light."
http://www.medscape.com/viewarticle/856498
