For the first time, scientists edited a gene in fertilized human eggs critical to early development. The experiments helped the researchers learn about fundamental human biology in a way they could not through research on mice.

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Researchers led by Kathy Niakan, a developmental biologist at the Francis Crick Institute in London, disabled a gene that codes for a protein called OCT4, known to be active in stem cells that can develop into all the cell types found in the body, reports Gretchen Vogel for Science. Turning the gene off ensured that cells from fertilized human eggs failed to form placental cells, yolk sac cells or even cells that would typically become a fetus.
Disabling the same gene in mouse embryos gives different results: Those embryos became balls of mostly placental cells. The findings suggest that the gene controls the fate of several cell lineages and plays a slightly different role in humans than in mice.
"This is opening up the possibility of using a really powerful, precise genetics tool to understand gene function," Niakan tells Rob Stein of NPR. "We would have never gained this insight had we not really studied the function of this gene in human embryos."
The researchers reported their results yesterday in Nature.
Even given that insight, the work is more of a proof of principle—a demonstration of the power and utility of the CRISPER-Cas9 genome editing technique, Vogel reports. The technology is sort of like a pair of molecular scissors that allow researchers to cut specific chucks of DNA from the genome and even replace the code with their own instructions.
Already scientists have wielded the tool to make a range of important advances and discoveries, such as engineering custom lab animals and testing potential cancer treatments. Recent years have also seen several forays into the territory of human genome editing. In August, U.S-based scientists wielded CRISPR to correct a mutation that causes a deadly heart condition. (Other scientists have since questioned those recent claims, reports Ewen Callaway for Nature.)
However, for every step taken down this road, editorials and opinion pieces urge caution.
"The concerns are that we would be opening the door to fertility clinics vying to offer gene-editing to make future children taller or stronger or whatever they wanted to market," Marcy Darnovsky, head of a genetics watchdog group called the Center for Genetics and Society, tells NPR. "That could put us into a situation where some children were perceived to be biologically superior to other children."
Niakan and her colleagues' work, however, is a far cry from that situation. The researchers had to apply to the Human Fertilisation and Embryology Authority, an institution set up in the United Kingdom that rigorously reviews proposed embryo-editing research, Vogel reports for Science.
"Day 5 embryo" (left) shows the embryo on the fifth day of development and "Day 5 edited embryo" shows an edited embryo without OCT4. It does not form a proper blastocyst, showing that OCT4 is needed for blastocyst development (Dr Kathy Niakan/Nature)
In classic genetic studies, researchers routinely disable genes to figure out how they function, reports Ricki Lewis for the Genetic Literacy Project. But CRISPR allows them to do the same with greater precision and accuracy.
The new research used fertilized cells donated after people had undergone in vitro fertilization treatments. "When it comes to illuminating early human development, there’s nothing that measures up to using the real thing: human cells and tissues," writes Lewis.
The researchers disabled the gene that codes for OCT4 very early in development. In more 80 percent of the 41 fertilized ovums tested, the growing and dividing cells failed to form a hollow sphere of about 200 cells, called a blastocyst. Many in vitro fertilization efforts also fail at this stage, so it is a critical juncture for researchers to understand.
"By understanding the key genes that are involved in the development of the blastocyst, this can really inform our understanding of this important, critical window of human development," Niakan tells NPR.
In an editorial, Nature itself praised the research as an example of how human genome editing research should be done:
The particular requirements of studies will differ, but a strong framework for assessing them as early as possible seems the best way to ensure that they meet the highest standards. Regulators, funders, scientists and editors need to continue working together to define the details of the path forward for germline genome editing, so that the valuable resources and tools now at our disposal are used with good judgment.
Future experiments could use CRISPR to investigate the role of other genes. And experts will watch closely to monitor the ethics of that work.