The revolutionary genome editing method, CRISPR-Cas9, broadens horizons immeasurably in the health field. Discovered four years ago, it makes it possible to rapidly, efficiently and cheaply modify the DNA of plants, animals and humans to correct genetic anomalies. However, changing the genetic code of human embryos to create custom babies would present ethical problems. Christine Pourcel, researcher at the Institute for Integrative Biology of the Cell (I2BC) at the University of Paris-Saclay contributed to the discovery.
MedicalExpo e-mag: Did CRISPR-Cas9 grow out of bacteria’s natural defenses against viruses?
Christine Pourcel: Yes. It’s based on a natural system that resembles vaccination. When a bacterium is first infected by a foreign aggressor, it stores a bit of the attacker’s genetic information in its chromosome. If the same aggressor returns, the bacterium is prepared to counterattack to destroy it.
ME e-mag: How does it work?
Christine Pourcel: The CRISPR structure is the genetic element where the virus-related information is stored, much as in an archive. Then there’s a series of genes which produce proteins called Cas—CRISPR-associated—because they are always associated with the CRISPR element. Among others, these might be nucleases, capable of cutting the DNA. Cas9 is one of the nucleases of the CRISPR-Cas system. This is a rather exceptional protein because it has multiple functions usually carried out by three of four different proteins. The big discovery was to use this single protein with multiple properties, including the ability to cut DNA. To cut DNA in a precise location, we use a characteristic of the CRISPR-Cas system—to put a bit of homologous RNA right on the targeted spot.
Here’s the methodology used by scientists: We determine the target on the DNA. We choose a segment and copy it as RNA. We have an outside company synthesize this bit of RNA. It’s easy and doesn’t cost much because it’s not very long. Next, we inject this RNA and the Cas9 protein into the target cell. Together, they attach to the section of chromosome matching the RNA and make the cut. This triggers a repair mechanism.
ME e-mag: Is it also possible to use the cut to insert a new piece of DNA?
Christine Pourcel: Yes. We can put something else in the cell to replace the cut part. This involves a recombination mechanism which will replace the abnormal area with what we supplied, i.e., a bit of functioning DNA. For example, we can use this to repair a genetic defect. The technique has been tested on numerous animals—flies, worms, fish, mice and others—and on diverse cell cultures. It seems to work every time.
ME e-mag: What are the possible applications?
Christine Pourcel: Setting aside human applications for the moment, there is an enormous number of possibilities for improving plants or the productivity of livestock. For example, it’s already been tried on farmed fish, pigs and cows. The Chinese have used it to modify rice to improve its ability to grow in dry areas and to resist insects. It’s actually a new way to make GMOs that is faster, simpler and more effective.
They might even be better than current GMOs. Today, the process leaves something in the plant’s chromosome, that makes it resistant to an antibiotic, for example. This new method leaves nearly no trace of the modification. It’s as if we achieved the result via natural selection.
It’s actually a new way to make GMOs that is faster, simpler and more effective.
ME e-mag: Have there been studies on the method’s effects?
Christine Pourcel: It’s exactly same as for today’s GMOs. Will the modified Chinese rice create agricultural or human health problems a few years from now? For the moment, we don’t know.
Another animal application with potential consequences would be the elimination of a pest species, like the malaria mosquito. It’s theoretically possible. We don’t know the consequences, but it would mean human intervention in natural selection. We already do this, in fact, since our simple presence eliminates certain animal species. But using the genetic methods we’ve been talking about would accelerate the process. It also poses an ethical problem.
ME e-mag: Let’s talk about genetically-modified humans.
Christine Pourcel: The first application, the one that creates the most fear, is the modification of the human embryo. The method already has been used in China with apes, and is theoretically possible with people. The Chinese fixed a genetic defect in ape embryos and re-implanted them in a female. Two babies were born with the modification and in good health. The Chinese also have modified human embryonic cells which had been fertilized in vitro. But they didn’t re-implant them. They demonstrated that they could target and modify a human mutation. Thus, if we were to re-implant such embryos in a woman, we could produce a baby with a modified gene. At that point, the baby would have a new mutation in its genetic heritage which would be transmitted to its descendants. For the moment, this is forbidden, but someone might do it some day. It’s like cloning. At this point we haven’t heard of anyone doing it with a human.
This is very different from genetic therapy applied to a human after birth, where we wouldn’t target reproductive cells, but a defective organ to try to correct a mutation. Gene therapy has been studied for decades, but hasn’t always worked very well. Now, we’ll be able to do much more.
ME e-mag: Will we be able to “correct” hereditary illnesses?
Christine Pourcel: Yes, diseases like myopathy or cystic fibrosis. It all depends on the defect. With certain well-identified defects that are not too serious, it will be possible to replace a portion of the sick gene with a healthy one. Even if we don’t achieve 100% success, we can still do something. For example, with cystic fibrosis, if we give patients who can’t breathe 30% of their respiratory capacity, it would be an enormous improvement. In general, for every disease whose genetic origin is known, we can theoretically treat patients by intervening in the defective gene, even if we can’t cure them completely.
ME e-mag: How about cancer?
Christine Pourcel: Yes. I’ve been talking about DNA mutations, but we also can control the gene’s expression with this system. That is, instead of modifying the gene itself, we would limit what it produces. For example, we know that there are proteins that cause tumors. We might try to stop the expression of such a cancer protein. We would target the tumor. The CRISPR-Cas9 system would “turn off” or stop the function of the gene responsible for the tumor. There are many other applications, notably in cardiovascular diseases.
The first application, the one that creates the most fear, is the modification of the human embryo.
ME e-mag: Are errors possible?
Christine Pourcel: Yes. The protein might make the cut in the wrong place and create another problem. This is a potential danger that has already been seen in traditional gene therapy. For example, some children treated for an immune deficiency syndrome wound up with another illness. These are possible risks, but each time we do a risk-benefit analysis.
ME e-mag: Why didn’t this discovery receive the Nobel prize this year?
Christine Pourcel: There are several reasons. One is the pitched patent battle being waged by two American laboratories, each of which claims to be the inventor. One is Jennifer Doudna of Berkeley, a researcher to whom the invention has been attributed, along with Emmanuelle Charpentier, a French woman. The other is Feng Zhang of MIT’s Broad Institute, near Boston. His lab was doing exactly the same thing. He published after the two women, who are well-known, but applied for the patent before them. There’s an enormous amount of money at stake.