Improvements in gene editing tools and the steady advance in the knowledge of how the human genome works have opened up the possibility of performing modification-editing processes in the DNA sequence of the early human embryo genome for different purposes, both therapeutic (removal of the mutations responsible for hereditary genetic diseases) and enhancement (endowing individuals with specific characteristics).
Many scientists hold different positions on this matter. Herein, we summarize some of the positions for and against this technique, published in a recent article.
Current status of embryo gene editing
The clinical application of genome editing technologies to embryos obtained by in vitro fertilization (IVF) is still premature due to significant unresolved safety and efficacy issues that raise ethical concerns.
Germline gene editing has been proposed as a strategy for targeted correction of mutations in the germline that could potentially replace embryo selection.
However, both technical and ethical considerations make germline gene editing unacceptable for clinical application at present.
CRISPR, the gene editing tool
The CRISPR-Cas9 gene editing technique has been a major breakthrough in the possibilities of precisely modifying certain DNA regions, at levels that were unimaginable only relatively recently.
CRISPR induces double-stranded DNA breaks (DSBs) that can be repaired by two mechanisms. The first and most common mechanism is called “non-homologous end joining” (NHEJ), whereby the cell repairs a break in its DNA by joining the free ends of the DNA. This pathway is the more “sloppy” of the two, and often results in the random addition or removal of nucleotides around the site of the DNA break, causing insertions or deletions in the genetic code. In genome engineering, this allows scientists to stop a gene from working (similar to removing a page from the middle of an instruction manual).
Less frequently, a second mechanism called “homology-directed repair” (HDR) can be employed to correct the DSB using the embryo’s own repair machinery, allowing an altered gene to be replaced with a healthy copy.
Early embryo gene editing
Genetic modifications made at the earliest stage of embryonic development, particularly the zygote stage, will affect all the cells of the future organism. However, early embryos also appear to be more vulnerable to DNA damage at this stage, presumably due to a deficiency in the DNA repair mechanism present before the activation of the embryonic genome. This observation alone makes a compelling argument against the clinical use of CRISPR-Cas9 in embryo gene editing.
Most of the scientific evidence suggests that interventions in the germline have the potential to negatively impact embryonic development.
It appears that the editing efficacy increases when microinjection of the CRISPR components is performed at fertilization (zygote) or in the early stages of embryonic development. Additionally, optimizing the stability of CRISPR reagents can reduce mosaicism, whereby cell populations with distinct genetic compositions are obtained.
After generating the DSB, HDR uses an uncut homologous DNA sequence or, alternatively, a synthetic counterpart that acts as a template harbouring the correct sequence for restoration of the damaged copy. It seems that human embryos prefer to use the endogenous template over the synthetic one, through a mechanism that is largely not understood.
Most embryos still resolve generated DSBs by mutagenic NHEJ rather than HDR, which is required for the correction of most mutations associated with human disease.
Furthermore, NHEJ induces additional mutations (small insertions and deletions known as “indels”) rather than correcting the existing ones. This predominant form of repair is therefore suboptimal for gene editing, except when the intention is to disrupt gene function completely.
Because HDR occurs infrequently (<10%) in cells of early human embryos, the success of editing is low. This is especially problematic because IVF provides only a limited number of embryos. Moreover, its low efficacy requires the use of many embryos, with the ethical consequences that this entails, since they are discarded in any event.
Arguments in favour of embryo gene editing
Theoretically, the application of genome editing to IVF-derived embryos could have some advantages, such as the removal of a mutation during the preimplantation phase, which could avoid the development of a genetic disease phenotype.
In fact, if the treatment is performed on the embryo before the 4-cell stage, when the major wave of embryonic gene expression begins, it is likely that the affected gene to be corrected would not be transcribed until after the normal genetic sequence has been restored, allowing its subsequent repair. Genome editing in the preimplantation stage also has the advantage that the number of cells that need to be modified is very small. For example, at the zygote stage, changes induced by the reagents used to edit the genome can be virtually guaranteed by microinjection into the single cell. For the later preimplantation stages, microinjection may be impractical, but until around day 3 after fertilization, all the blastomeres (cells that make up the embryo) have part of their surface in contact with the external environment. This gives them the possibility that the changes induced by the genome editing components added to the medium could perform their function using various methods of transfection (introduction of foreign genetic material into eukaryotic cells using plasmids or other tools). Thus, any alterations to the genome could be passed to subsequent generations, potentially remaining in the human gene pool forever, a point that generates great uncertainty about its possible consequences.
For other authors, however, these permanent changes in the genome offer the possibility of permanently removing certain mutations responsible for serious hereditary diseases, avoiding their appearance in the offspring.
Objections to embryo gene editing
If, instead of editing the early embryo by modifying the genome of all its cells, this editing is limited to somatic cells, where the changes are transmitted only to the individual, the risk associated with the hereditary transmission of the changes introduced would be avoided, in the event that they have errors that compromise the health of the next generations and human evolution.
Furthermore, future generations affected would not be able to consent to interventions that could have a material impact on their health and well-being.
In 2017, the National Academies of Sciences and National Academy of Medicine reviewed the ethical, legal, and scientific concerns about genome editing. They concluded that genome editing in the early embryo that involves its hereditary transmission is unacceptable at the present time. The safety of genomic editing can also be challenged by the existence of pleiotropy, which consists of the production by a single gene of two or more unrelated effects, with unpredictable consequences.
In this scenario, disruption of a gene could produce unforeseen and unwanted effects if that gene contributes to more than one function.
Treatment or enhancement? The crossroads of germline gene editing
A survey conducted by the Royal Society in the United Kingdom reported that 83% of participants supported germline genome editing to treat incurable diseases, although many drew a line at this editing being done for enhancement rather than therapeutic ends. For example, 60% were opposed to heritable gene editing to improve intelligence.
Conclusion
If genome editing is ever to be applied to human embryos, safety concerns will have to be addressed. There is evidence that the cells of early preimplantation embryos set in motion genetic repair mechanisms to reverse DSBs induced by CRISPR/Cas9 technology applied in gene editing processes, causing additional unresolved damage. This has potentially serious consequences for the subsequent evolution of the genetically altered embryo, although some researchers claim to have induced DSBs in human embryos in vitro in a research setting, with promising prospects.
The most compelling argument for avoiding germline therapy in the United States is that it will eventually be used as a feature-enhancing intervention on tested embryos, perhaps sooner than many anticipate, when safe and effective editing methods are developed. The unpredictable consequences of modification of the human genome, which remains in the offspring, could alter the human condition itself in a direction difficult to foresee. Even when gene editing techniques have improved their accuracy, safety and efficacy, close monitoring is required to ensure that their application is for therapeutic purposes and not for enhancement.
Before that happens, the ban on germline human genome editing must remain, given its current lack of safety, the still-unknown process of interaction of genes and their functionality, and the abundant evidence on the associated serious risks to the health and well-being of offspring and future generations.
Julio Tudela
Bioethics Observatory – Institute of Life Sciences
Catholic University of Valencia
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