Researchers gene-edit mouse egg cells, suggest potential use in humans for preventing mitochondrial disease

Scientists have suggested a potential alternative to controversial mitochondrial replacement, so called ‘3-person IVF’, for preventing inherited genetic disorders by using genome-editing technology to prevent specific mitochondrial DNA from being passed from mothers to offspring in mice. The study, published in the journal Cell, also reported the technique was used to correct human mitochondrial DNA inserted into mouse egg cells. Scientists have recently called for a moratorium on using genome-editing techniques in human germ cells.

The following researchers gave their take on the study.


Dr. Paul Knoepfler, Associate Professor, Cell Biology & Human Anatomy, School of Medicine, University of California, Davis (webpage):

Expertise: stem cells and regenerative medicine; epigenomics of cancer and stem cells with a particular interest in pediatric tumors.

“I am of two minds regarding the new paper from Juan Carlos Izpisua Belmonte’s group published in Cell using gene editing to reverse mutations associated with mitochondrial disease. On the one hand, the paper is technically solid and presents a convincing case that mitochondrial disease-associated mutations can be successfully addressed via gene editing technology in mice and in vitro in cells in the lab. On the other side, however, this paper raises concerns as it argues that such gene editing technology could in the future be used clinically as an approach to prevent mitochondrial disease in humans. A find this motion concerning in light of outstanding scientific and ethical questions regarding human germline genetic modification.

“As to the paper itself, the work demonstrates a gene editing induced shift in so-called “heteroplasmy”, the degree to which mitochondrial DNA contains different kinds of mutations. In principle, such a shift could lead to a more “normal” mitochondrial condition with fewer mutations. The main concern with gene editing work from a technical perspective is the possibility of off-target effects, meaning that the gene editing machinery could go to the wrong place in the genome and make a deleterious change there. The authors only briefly mention that they did not observe significant off-target effects in this study and that is reassuring, but they do not show much data in that regard. I would have found it helpful if they provided additional data on how they addressed potential off-target effects. The authors used two approaches in this study: (1) work in mouse cells and in actual mice, and (2) studies in a hybrid system where they fused mouse oocytes (eggs) with human somatic cells bearing mitochondrial mutations. Data from both models supported the conclusions.

“In theory, if the authors wanted to argue a stronger case for eventual human therapeutic use for this technology, they might have also included another model system and data from that in the form of actual human oocytes. Of course, gene editing of human oocytes bearing mitochondrial DNA mutations would have potentially been more controversial. Even so, the argument in this paper for eventual clinical use in humans for this technology would as the authors point out involve human germline genetic modification and creation of human beings with altered mitochondrial genetic makeup, which is very controversial at this time. Given the recent and growing calls for a moratorium on germline human gene editing and gene modification (including one from me on my blog via the ABCD plan I proposed), I expect this paper to generate a lot of discussion given its tone promoting translational extensions of the work into humans. The idea that a gene editing approach to mitochondrial disease in humans could be superior to the so-called “3-person IVF” or “mitochondrial transfer” approach to mitochondrial disease just recently approved in the UK but not permitted in the US is somewhat provocative as well. At the same time, I believe that this kind of study from a technical standpoint is exactly what we need to learn more about the outcomes of gene editing including the extent of off-target effects.”


Dr. April Pyle, Associate Professor, Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles (webpage)

Expertise: basic and translational human pluripotent stem cell biology; stem cell based therapeutic approaches for patients with muscular dystrophy.

“The authors present exciting data in which they reduced mitochondrial genomes in mouse egg and one cell stage embryos using mitochondria-targeted nucleases. The authors propose that this approach may be applied and developed to prevent the transgenerational transmission of human mitochondrial diseases. Although this is a nice proof of concept study in mice it is not clear if this approach will be immediately translatable to humans.

“The authors stated, “We expect that this method will reduce the percentage of mutated mitochondrial DNA below the threshold for triggering mitochondrial diseases in humans.” However, additional studies in mice would be required to overcome existing issues, including technical barriers that may prove difficult in humans as well as concerns regarding stability and off-target effects (the potential for an unintended location in the genome being edited) using genome editing approaches.

“In mice, embryos with mitochondria levels below a specific threshold develop normally during the pre-implantation stages, but subsequently fail to implant in the uterus or fail to develop. Thus the thresholds required for safe development of this technology in humans would need to be carefully evaluated.

“There may be additional technical hurdles in translating this approach to humans. Open questions remain as to the percentage reduction of mutated DNA that would be needed in order to see a meaningful clinical gain, and how reducing the mitochondrial genome could affect key cellular processes and proper development needs to be evaluated.

“These results are exciting and could have broad reaching implications for mitochondrial diseases. However, as with any new technology development it is imperative that research is followed using international guidelines. As such the International Society for Stem Cell Research has asked for a “moratorium on attempts at clinical application of nuclear genome editing of the human germ line to enable more extensive scientific analysis of the potential risks of genome editing and broader public discussion of the societal and ethical implications.”

“Mitochondrial replacement therapy does not entail direct modification to the nuclear genome and is already undergoing a rigorous evaluation in many countries including the UK and United States. However there is no question that this will require continuous oversight from larger scientific and ethical communities to ensure that the technology is safely and effectively translated to the patients in need, without halting the progress of this exciting research. ”


Dr. Debra Mathews, Assistant Professor, Pediatrics, Assistant Director for Science Programs, Berman Institute of Bioethics, Johns Hopkins University (webpage):

Expertise: bioetchics and policy issues related emerging biotechnologies.

“The investigators note that their technique avoids the problem of requiring biological materials from three people, as do the now permitted mitochondrial transfer techniques. While this is true and may solve a significant ethical issue for some, the questions of safety and of germline modification remain, as do concerns about manipulating the embryo, in that particular application of their technique. In each of these cases we can ask: Can we do it technically? Is it safe? And ought we do it? The first two are questions for science and medicine, the last is a question for each society considering introducing the technique into medical care and reproduction. Some applications of the technique (e.g., the treatment of oocytes) may be more acceptable than others (e.g., the treatment of embryos) — or not — but only public discussion and debate will bring this out.”


Dr. Jennifer Kuzma, Goodnight-NCGSK Foundation Distinguished Professor, School of Public and International Affairs, Co-Director, Genetic Engineering & Society Program, North Carolina State University (webpage):

Expertise: governance systems for emerging technologies, particularly genetic engineering for environmental, agricultural, health and industrial applications.

“This paper takes the approach of knocking out mitochondria carrying disease by using genome editing, so from what I understand, the intention of the researchers is not to add new genes that are integrated into the genome. Regardless, I think that the precautious approach that the authors of the calls for moratoria make in recent Nature and Science pieces should apply to this case.

“What is a “starting point” for human genome editing is debatable. I believe the decisions to fund and conduct such research puts us past the starting point, and now, the soon-to-be collaboration of this research team with IVF clinics brings us past the “starting point” and into the applied research and development phases. However, there will likely be a long process in testing for safety and efficacy and for the ability to successfully eliminate these diseases in animal models. For example, although these genome editing techniques are meant to be specific, there are safety issues with off-target cutting rates and potential toxicity or other unintended effects that need to be addressed. If successful, there will be an even longer phase for human trials. Regardless, I think the study is a very significant step to germline genomic editing in humans with the intention of continuing down the path of development and use.

“As important are broader questions about who should have access to this technology (will it be available to all, or only those who can afford it?); where should users of the technology, doctors, and patients draw the line about what is “disease treatment” and what is an “undesirable trait”; who has the right to make those decisions; and how do we structure open and transparent processes for safety review, public dialogue, and ethical considerations. Another very important ethical issue that should be considered is the autonomy of parents to ensure survivability and reduce disease burdens in their children.

“We are at an opportune moment where we should begin to consider what kind of society we want and how technologies should or should not have a place in that future.   The pace of technology development is exceeding our abilities to reflect on these questions and respond with trusted governance systems. We are lacking capacity and infrastructure in our policy systems for public dialogue about emerging technologies governance in the face of biotechnology, nanotechnology, neurotechnology, robotics, big data science, and the convergence of these.

“Specifically, we need leadership from trusted, independent bodies to shepherd societal dialogues about the use of genome editing technology. Participants should include those from diverse disciplinary groups (not just natural scientists, but also social and behavioral scientists, medical practitioners, lawyers, ethicists, risk analysts, etc.), affected stakeholders (such as patient groups, and health care workers), and interested citizens. We should openly consider questions about differing viewpoints on how far the technology should proceed, who funds it, under what conditions it should be allowed, what processes are needed for ensuring safe & responsible use, what the downstream impacts on human populations and societies could be, and how to make informed-consent processes rigorous. These are not decisions to rush, but to carefully reflect upon before the trials in human embryos commence.

“In the interim, perhaps more experiments should continue to demonstrate disease treatment in animal models and continue to study the safety of the process in animal models, as we begin to structure and conduct a public discourse on the societal implications of genome editing in human beings.”


Dr Matthew Porteus, Associate Professor of Pediatrics, Division of Stem Cell Transplantation, School of Medicine, Stanford University (webpage)

Expertise: hematopoetic stem cell transplantation and using homologous recombination as a precise method of genome modification for therapeutic and research purposes.

“Prior work has shown that one can engineer nucleases to target the mitochondrial DNA.  This work represents an extension of that work in that it applies the approach to a specific application.  In most respects this is a proof of concept paper because they use engineered nucleases (enzymes that cut DNA) to target mitochondrial DNA in mouse embryos and specific mitochondrial mutations.  Mitochondria with differing DNA can be present in the same cell, a state known as heteroplasmy. That you can specifically create breaks in the mitochondria carrying a mutation to cause a “heteroplasmy shift” is very interesting.  Additionally, the group demonstrates that they can target human mitochondria which is residing in a mouse zygote is quite interesting and compelling. I think it is important that they achieved this using standard restriction endonucleases and the TALEN system.  In principle it should also work with the more recently discovered CRISPR/Cas9 gene editing system.

“The mitochondrial genome is maternally inherited and gets passed down from generation to generation.  The function of mitochondria has tremendous effects on human health, the importance of which is still under appreciated. It is not just severe mitochondrial diseases but the function of mitochondria probably affects all aspects of human physiology including such aspects as aging and regeneration.  Because mitochondrial DNA is passed from one generation to another, I think it should be considered much as we consider editing the nuclear DNA. That being said, I think basic technological studies such as these are important for us to understand what is possible and what are the limitations.  To fully understand the process, we need to move beyond mice.”



Selective Elimination of Mitochondrial Mutations in the Germline by Genome Editing‘ by Reddy et al, published in Cell on Thursday, April 23, 2015.


Declared interests (see GENeS register of interests policy):

No other interests declared

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