The advantage to editing genes in embryos is that you have a good chance of having the edit adopted by all cells across the organism, something that is not true when the editing is done postnatally.
So one way scientists figure out what a gene does is by disabling it in an embryo and seeing what effect its removal has.
If you disable or “knock out” a particular gene in the embryos of mice, for instance, and they don't develop eyes, odds are that gene was involved in making eyes. (Yes, those experiments have been done.)
Up until recently, mice were the most advanced organisms routinely used in gene knockout studies. Scientists would disable a gene in the rodents, see what happens, and extrapolate those results to people. Knocking out genes in mice has been useful in studying diseases like cancer, obesity and diabetes, says the National Human Genome Research Institute in this fact sheet.
But of course, people aren’t mice. Numerous examples exist of the eradication of equivalent genes in both species that result in different effects for each.
A recent study on human development published in the journal Nature describes an experiment by a team of scientists in London who used the gene-editing tool CRISPR-Cas9 to disable a gene called Oct4 in viable human embryos, an experiment that had previously been performed on mice. Oct4 was chosen because it is known to be critical to early development.
As expected, losing Oct4 was catastrophic for human embryo development. What was less expected is that the effects were even more severe than had previously been seen in the embryos of mice.
While none of the main types of cells present at the earliest stages of human embryos managed to develop properly, absent Oct4, that was not true of the mice embryos that lacked the gene. In those, the cells that go on to become the placenta survived. The conclusion: Oct 4 is instrumental in development of the placenta in humans, but not in mice.
The experiment helped uncover the specific function of the eradicated gene. Knock-out experiments like this one done in human embryos could one day lead to new treatments for infertility that may not have been discovered by relying only on mouse studies.
Disabling a Human Gene
The Crick Institute study started out with 54 fertilized human eggs. The researchers microinjected 37 of these embryos with the best gene-editing tool available, Cas9, to target the Oct4 gene for destruction. The researchers also microinjected 17 embryos with untargeted Cas9 as a control, to make sure the effects were not due to the injection procedure itself.
At the earliest stages of development, human embryos with and without a working Oct4 gene behaved similarly. But later on, only 7 of the 37, or 19 percent, of the embryos without the gene went on to develop into a blastocyst, in which cells have grown and divided to number roughly 200. Meanwhile, 8 of the 17, or 47 percent, of the embryos that still contained the gene survived to become blastocysts.
The other noteworthy result: Of the human embryos that made it to the blastocyst stage, absent Oct4, neither the blastocyst outer layer nor its inner mass developed properly. The outer cells usually go on to become the placenta, while the inner cells develop into a person. In mice, removal of Oct4 did not damage these proto-placenta cells.
The conclusion: Oct4 is not critical to the development of the placenta in mice, but it is in humans.
Ethics of Human Gene Editing
Scientists created a stir recently after they successfully edited DNA in a human embryo in order to correct a disease-causing mutation, the first time this has been done. The researchers used embryos that were viable but created expressly for use in the experiment. Most previous work on human embryos had been done on embryos that were incapable of developing into fetuses.
In the Oct4 experiment, the embryos were donated by couples who had conceived via in vitro fertilization and would have otherwise been discarded, as surplus embryos are often created in IVF to increase the odds of successfully conceiving a child. Given the severity of the loss of the Oct4 gene, they never would have developed into human beings, anyway.
As gene-editing techniques progress, the ethics of editing embryonic DNA will come more into play. Most scientists argue that at this time, no one should proceed if the embryo will go on to fully develop and be born. This proscription even includes gene editing that might cure a life-threatening genetic disease. Why? The arguments include the continuing, high failure rate when even the best gene-editing techniques are applied, and the dangers of edits occurring at off-target locations in the genome. Both could result in birth defects or unintended consequences for the human germline — the genetic material that is heritable from one generation to the next.
These views are consistent with a report from an international committee convened by the U.S. National Academy of Sciences and the National Academy of Medicine in Washington, D.C., which argues that the technology for human genome editing is not yet sophisticated enough to allow the editing of genes that can be passed on to offspring. When the technology is ready, the NAS says, it should only be allowed under very specific circumstances, such as a case of both parents who have a genetic disease and want to conceive a child. (Here is a pro vs con discussion of human genome editing from Science.)
But the fact is, like it or not, scientists now have the ability to edit the genes in a human fertilized egg. As capabilities increase, so will the moral stakes. The questions could soon be : When is it okay to fix broken genes in order to prevent life-threatening genetic diseases? And when is it not?
As CRISPR-Cas9 pioneer Jennifer Doudna has counseled, the ethical debates need to be kept current with the rapidly progressing technology.
Dr. Barry Starr is a scientist in the Department of Genetics at Stanford University. He runs the Stanford at The Tech program and the Understanding Genetics website with The Tech Museum of Innovation in San Jose, California.