Two years ago, in spring 2015, Liang et al. published the first report of gene-editing in human embryos using CRISPR/Cas9 (mentioned previously here, here and here). At the time no high-profile journal was willing to take on the risk of publishing what was perceived to be a controversial study. Liang et al. were trying to correct mutations in the human beta-globin gene – mutations in this gene can lead to a group of diseases called beta thalassaemias, including sickle cell anaemia – in human embryos that had been fertilised by two sperm cells (and could therefore never develop). In fact, the take-home message from their study was that using the techniques available to them at the time led to a host of unwanted side effects, including the creation of mutations at other sites in the embryo genome and the “correction” of the beta-globin gene with a similar gene called delta-globin.
Last month, a different group (Ma et al. – four first authors and five corresponding authors!) published more work on human embryo CRISPR/Cas9 gene-editing, this time in Nature. Like Liang et al. this paper also tried to tackle a monogenic disease, a disease that is caused by a well-defined mutation in a single gene, called hypertrophic cardiomyopathy. The affected gene is MYBPC3 and when mutated (denoted as DeltaGAGT in the figure below) this leads to a thickening of the heart muscle, which in turn can cause heart failure. The authors used donor sperm with the MYBPC3 mutation together with healthy oocytes to perform their experiments. In the first approach the eggs were fertilised by the sperm and only subsequently, during S phase, were the guide RNA, Cas9 protein and a piece of non-mutated donor DNA injected. The guide RNA was designed to specifically recognise the mutant version of MYBPC3, which recruits the Cas9 protein to make a cut in the DNA, and then the donor DNA would serve as a template to repair the sperm’s mutated gene. Ma et al. observed that this technique worked but often generated so-called mosaic embryos, which contained a mixture of healthy and mutated cells. This incomplete gene correction happened because during S phase both the maternal and paternal chromosomes duplicate and therefore the CRISPR/Cas9 system would have to correct two mutated MYBPC3 genes before the first cell division.
- Schematic depicting CRISPR/Cas9 stage at zygote stage (top) versus together with sperm (bottom) – copied directly from Ma et al, 2017
In a second approach, Ma et al. wanted to overcome this mosaicism by injecting the CRISPR components together with the sperm during the M phase of the oocyte. Now only one copy of mutant MYBPC3 had to be corrected and this succeeded in producing completely healthy embryos. Ma et al. also checked to make sure that these embryos did not carry any unwanted, off-target mutations.
Last but not least, Ma et al. provided evidence that often the human zygote used the healthy maternal gene to provide a template for the repair of the mutated paternal gene, instead of the injected DNA template. This is significant because in most cell types the DNA double-strand breaks caused by Cas9 are usually repaired in an imprecise manner (called non-homologous end joining) and lead to further mutations. Ma et al. therefore argued that “human gametes and embryos employ a different DNA damage response system”.
This finding could be of huge importance, both to the basic understanding of human embryonic development as well as to potential therapeutic CRISPR/Cas9 applications. However, four days after the Nature paper was published online, several prominent scientists posted a riposte on the pre-print server bioRxiv. Egli et al. criticised the first paper quite heavily by raising theoretical objections/concerns; they couldn’t have tried to replicate the experiments in such a short time frame. [Note that this pre-print was, of course, not peer-reviewed, although the authors have confirmed that they were trying to get their work published in Nature as well.]
Among other more technical issues to do with the way in which healthy and mutant genes were detected, Egli et al. pointed out that after fertilisation the maternal and paternal chromosomes remain physically separated (indicated by the arrows in the figure below) until just before the first cell division. Therefore, Egli et al. argued, it is highly unlikely that the healthy maternal MYBPC3 gene could serve as a template for the repair of the mutant paternal gene. This strikes me as a strong argument, not being at all familiar with early human development. Overall, Egli et al. suggested that Ma et al. were simply not detecting the mutant gene in their embryos but not providing good enough evidence of a corrected gene. The scientific debate will, no doubt, continue and I think having bioRxiv as such a rapid place for the exchange of ideas can drive scientific discourse.
- Pictures of a human zygote (fertilised egg/oocyte) and its very early development – copied directly from Egli et al, 2017
Since this is a digest it should also contain some other relevant CRISPR/Cas9-related news. One of the post docs I met at Cold Spring Harbor Laboratory in 2014, Serif Senturk, published a paper early this year in which the authors show how they can switch CRISPR on or off in living cells. They did this by fusing the Cas9 protein to another, destabilising protein domain, which caused the attached Cas9 to get degraded. However, when a “shield molecule” was added to the cells, the destabilising domain was no longer active and the Cas9 could accumulate. This innovation counteracts the problem of off-target effects, which are often due to the long duration that Cas9 is active for. Pretty neat system, I think.
Schematic depicting Cas9 fused to a destabilising domain – copied directly from Senturk et al, 2017
Egli D, Zuccaro M, Kosicki M, Church G, Bradley A, Jasin M (2017) Inter-homologue repair in fertilized human eggs?
Liang P, Xu Y, Zhang X, Ding C, Huang R, Zhang Z, Lv J, Xie X, Chen Y, Li Y, Sun Y, Bai Y, Songyang Z, Ma W, Zhou C, Huang J (2015) CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes. Protein Cell: 1-10
Ma H, Marti-Gutierrez N, Park SW, Wu J, Lee Y, Suzuki K, Koski A, Ji D, Hayama T, Ahmed R, Darby H, Van Dyken C, Li Y, Kang E, Park AR, Kim D, Kim ST, Gong J, Gu Y, Xu X et al. (2017) Correction of a pathogenic gene mutation in human embryos. Nature 548: 413-419
Senturk S, Shirole NH, Nowak DG, Corbo V, Pal D, Vaughan A, Tuveson DA, Trotman LC, Kinney JB, Sordella R (2017) Rapid and tunable method to temporally control gene editing based on conditional Cas9 stabilization. Nature Communications 8: 14370