Despite calls for a moratorium on CRISPR gene-editing in human germline cells, Chinese scientists have used the system on nonviable preimplantation human zygotes. [istock/knorre]
Despite calls for a moratorium on CRISPR gene-editing in human germline cells, Chinese scientists have used the system on nonviable preimplantation human zygotes. [istock/knorre]

Blurring an ethical line, scientists at Sun Yat-sen University in Guangzhou have used the CRISPR/Cas9 gene-editing system not on human adult cells or animal embryos, as has been done on many occasions, but on human embryos. To date, scientists have been reluctant to edit human germline cells for fear of passing the revisions—and unknown consequences of those revisions—down to future generations. The Chinese scientists, however, advanced where others had hesitated, having taken the precaution of working with human embryos that they had deemed nonviable.

If the scientists expected to escape controversy, they were mistaken. Concerns and protests have been rising since the scientists’ work appeared April 18 in the journal Protein & Cell. For example, in Nature, George Daly, a stem-cell biologist at Harvard Medical School, was quoted as follows: “I believe this is the first report of CRISPR/Cas9 applied to human pre-implantation embryos and as such the study is a landmark, as well as a cautionary tale. Their study should be a stern warning to any practitioner who thinks the technology is ready for testing to eradicate disease genes.”

Dr. Daly’s comment highlighted that the study by the Chinese group indicated that the CRISPR/Cas9 system did not perform very well at all in human embryos. In many cases, CRISPR/Cas9 failed to make the desired change—the replacement of the HBB gene, mutations of which have been implicated in the disease β-thalassemia. Also, CRISPR/Cas9 appeared to introduce various off-target changes.

The details appeared in an article entitled, “CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes.” It noted that out of 86 human embryos that were subjected to genetic manipulation, 71 survived. Of the surviving embryos, 54 were genetically tested. Just 28 of these 54 embryos were successfully spliced. An even smaller number turned out to contain the desired genetic material. Also, off-target effects were seen.

“The efficiency of homologous recombination directed repair (HDR) of HBB was low, and the edited embryos were mosaic,” the authors reported in their paper. “Off-target cleavage was also apparent in these 3PN zygotes as revealed by the T7E1 assay and whole-exome sequencing. Furthermore, the endogenous delta-globin gene (HBD), which is homologous to HBB, competed with exogenous donor oligos to act as the repair template, leading to untoward mutations.”

Other criticisms of the work were not limited to its technical shortcomings, but also introduced “slippery slope” concerns. On the NPR website, an article quoted a spokesperson from a watchdog group, the Center for Genetics and Society.

“This paper demonstrates the enormous safety risks that any such attempt would entail, and underlines the urgency of working to forestall other such efforts,” wrote spokesperson Marcy Darnovsky. “The social dangers of creating genetically modified human beings cannot be overstated.”

News coverage in Nature reminded readers that in recent months, many scientists had publicized their concerns that CRISPR/Cas9 was proving so powerful that temptations to edit human germline cells could become hard to resist. For example, last March a group of prominent scientists issued a warning in the pages of Science. The paper—“A prudent path forward for genomic engineering and germline modification”—essentially called for a moratorium on gene-editing in germline cells.

A moratorium was also proposed by the International Society for Stem Cell Research (ISSCR). In response to the Chinese study, the group renewed its warnings against human clinical germline genome editing.

“Over the past few decades, research involving the modification of the DNA sequence in a cell has allowed scientists to investigate disease and develop new medical treatments,” said ISSCR president Rudolf Jaenisch, Ph.D. “However, it is too soon to apply these technologies to the human germ line, the inherited DNA, in a clinical setting, and any research involving the use of human embryos and reproductive cells should be undertaken with care and in accordance with strict ethical guidelines.”

For their part, the Chinese scientists, who were led by gene-function researcher Junjiu Huang, Ph.D., emphasized that their study underscored the need to more comprehensively understand the mechanisms of CRISPR/Cas9-mediated genome editing in human cells: “[We] support the notion that clinical applications of the CRISPR/Cas9 system may be premature at this stage.”

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