The published report confirms previous rumors and speculation that such experiments had been conducted, and confirmed the position that these experiments would indeed be conducted regardless of any moratorium on the work. Previously, the CRISPR/Cas9 system had only been demonstrated in adult human cells and with animal embryos.
In their paper, the team led by Junjiu Huang who is a gene function researcher at Sun Yat-sen University used only non-viable embryos which cannot be implanted and result in a live birth. Their publication reports that the method was successfully able to modify the gene responsible for β-thalassaemia, a potentially fatal blood disorder. However, the researchers also report that their results show that there remain serious obstacles to using the method in medical applications anytime in the near future.
In this procedure, embryos are injected with the enzyme complex CRISPR/Cas9, which binds and splices DNA at specific locations along the double helix. The CRISPR/Cas9 enzyme complex can be programmed to locate a specific problematic gene of interest which is then replaced.
In this experiment the Chinese scientists tested to see if the technique could replace a gene in a single-cell fertilized human embryo. Then, in theory, all cells produced as the embryo developed would then have the repaired gene.
However, the Chinese team reports that of the injected 86 embryos only 71 survived, and of these 59 were genetically tested. Only 28 of those embryos were successfully spliced by CRISPR/Cas9 and of these only a fraction of the embryos contained the correctly modified DNA sequence, specifically only 14 samples perfectly matched the donor.
The researchers conclude that “off-target effect of CRISPR/ Cas9 should be investigated thoroughly before any clinical application” of the technique and that “CRISPR/Cas9 has off-target effect in human tripronuclear embryos” therefore “our work highlights the pressing need to further improve the fidelity and specificity of the CRISPR/Cas9 platform” and that such improvements would be “a prerequisite for any clinical applications of CRSIPR/Cas9-mediated editing.”
This publication was widely reported in the mainstream media to be the first germline modification of humans, however as reported only non-viable embryos were used. So arguably this is not a true real world germline modification. It is of course an important experiment that shows such modification is indeed technically possible. However, this was largely suspected already based on existing demonstrated work with animal embryos.
The results are at this time negative for clinical use since the experiment suggests that the CRISPR/Cas9 method is not specific enough in its action and also it was in the end relatively ineffective at performing the desired modification producing only 14/71 correctly modified embryos.
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