Harnessing the full power of CRISPR-mediated genome editing

In the 30 years since the establishment of the HFEA, science in general has advanced exponentially. From the first derivation of human embryonic stem cells (ESCs) in 1998 (Thomson et al, 1998), the publication of the first human genome sequence in the early 2000s (International Human Genome Sequencing Consortium, 2004), to today’s cutting-edge techniques in microscopy and gene expression profiling, we have been able to move from an era of descriptive, observational studies of the human embryo to uncovering the molecular underpinnings of early human development. With this in mind, it is almost impossible to imagine what can be achieved in the next three decades.

In the shorter term, perhaps the next 10 years, we are certain to have leveraged the suite of advanced culture and analysis techniques to gain a better mechanistic understanding of the early specialisation events that cells undergo during the first 14 days of human embryogenesis. Due to errors in such events, as many as 75% of fertilised eggs fail to develop beyond these early stages. Thus, continued exploration of the changes occurring in the first two weeks of life will help us to understand why some of these failures occur and how they may be avoided during assisted reproduction.

Central to this goal will be to harness the full power of CRISPR-mediated genome editing, a technique to accurately and precisely “cut and paste” sections of DNA to make desired changes. This method has already been shown to work in human embryos, to study the critical function of a gene in development (Fogarty et al, 2017). In the near future, improved techniques will allow for the translation of long-standing “transgenics” methods into the human embryo. Transgenic model organisms have enabled numerous insights into diverse aspects of biology. For instance, an organism can be modified to carry a section of DNA encoding a component that will cause all cells of a particular type to glow or fluoresce. With advanced live imaging methods, the light from these cells can then be tracked in space and time to gain insight into the role and fate of these cells in a living organism or embryo. For instance, we could ask: when do placental- and embryo-fated cells diverge in their function? With the pace of development of CRISPR-based techniques, we are not so far away from achieving such unprecedented degree of detail in our studies of the human embryo.

However, it is important to bear in mind that with each advance in our ability to study and manipulate the human embryo comes a whole host of ethical considerations. For instance, even now when the technology for extended culture of human embryos is in its infancy, there is debate around the relative benefit of relaxing or removing the “14-day rule”. Indeed, in recently published guidelines, the International Society for Stem Cell Research (ISSCR) announced a major change such that it could permit, under certain strict criteria, studies on human embryos grown in a laboratory for more than 2 weeks (Lovell-Badge et al, 2021). From a purely scientific standpoint, the benefits of such experiments, once possible, are clear. Many crucial developmental processes take place after the first two weeks of development, including the emergence of germ cell progenitors, which will go on to form either eggs or sperm. Understanding the pathways that lead to successful germ cell formation could be hugely beneficial in assisted reproduction, especially by pinpointing where these pathways are most likely to go wrong. For instance, it could enable restoration of function in otherwise sub-optimal germ cells from patients dealing with infertility. Alternatively, the same pathway could be commandeered to drive germ cell formation from a patient’s own cells. Feasibly, within the next 30 years, such technology could enable same sex couples to have genetically related children by deriving eggs from a male partner or sperm from a female. Importantly, society will have to decide whether such advances are suitably beneficial to outweigh concerns around extended human embryo culture.

Another area of research that we predict will accelerate in the next decade is the use of stem cell-derived embryo models. This encompasses a variety of structures generated from hESCs that model, to varying degrees of accuracy, different stages of human embryo development. As these models continue to advance, with increased complexity and similarity to the “true” human embryo, the question arises – how do we delineate a “true” gamete-derived embryo from a stem cell-derived model? And how does the 14-day rule, or any other regulation, apply to these structures? Given their origin, should scientists have free reign to study stem-cell derived embryo models, likely providing an easier path to insights from later stages of development and manipulations to the developmental process? Or, is resemblance to an embryo sufficient to justify assigning such models the same status of a “true” embryo? And at what point would this become the case?

Looking back on the last 30 years, and thinking about the next, it is an undeniably exciting time to be involved in human embryo research. But one must always take care to remember the special considerations owed to such research and the invaluable contributions made by those patients willing to donate.

Rebecca Lea

Rebecca Lea was a PhD student and postdoctoral researcher in the lab of Professor Niakan. Her work focussed on the role of transcriptional regulators in early human embryo development and human embryonic stem cells.

Kathy Niakan

Kathy Niakan is Mary Marshall and Arthur Walton Professor of the Physiology of Reproduction, Chair of the Cambridge Reproduction Strategic Research Initiative and Director of the Centre for Trophoblast Research at the University of Cambridge. She is an Honorary Group Leader at the Babraham Institute and an Affiliate Researcher of the Cambridge Stem Cell Institute. Kathy is also a Group Leader at the Francis Crick Institute. Her laboratory investigates the molecular mechanisms that direct cell fate in human embryos. She has a daughter born following infertility treatment.

International Human Genome Sequencing, C. (2004). Finishing the euchromatic sequence of the human genome. Nature, 431, 931-45.

Thomson, J. A., Itskovitz-Eldor, J., Shapiro, S. S., Waknitz, M. A., Swiergiel, J. J., Marshall, V. S. & Jones, J. M. (1998). Embryonic stem cell lines derived from human blastocysts. Science, 282, 1145-7.

Fogarty, N. M. E., Mccarthy, A., Snijders, K. E., Powell, B. E., Kubikova, N., Blakeley, P., Lea, R. A., Elder, K., Wamaitha, S. E., Kim, D., et al. (2017). Genome editing reveals a role for OCT4 in human embryogenesis. Nature, 550, 67-73.

Lovell-Badge, R., Anthony, E., Barker, R. A., Bubela, T., Brivanlou, A. H., Carpenter, M., Charo, R. A., Clark, A., Clayton, E., Cong, Y., et al. (2021). ISSCR Guidelines for Stem Cell Research and Clinical Translation: The 2021 update. Stem Cell Reports, 16, 1398-1408.