From DNA ‘Scissors’ to a ‘Pencil’ … Precise Gene Analysis in Human Embryos With Fewer Off-Target Effects

A gene-editing technique that works like a “pencil,” rewriting DNA one letter at a time instead of cutting it with “scissors,” has been applied to research on early human embryo development. The findings point to the possibility of precisely analyzing gene function while reducing side effects compared with existing techniques.

The National Biotechnology Policy Research Center reported the findings in its “BioINwatch,” published on the 17th, citing a Nature News & Views article published on the 9th (local time).

Research on early human development is important for understanding the basic principles of infertility and assisted reproduction, genetic diseases, and regenerative medicine. However, much of the knowledge about early mammalian development comes from studies of mouse embryos, limiting its application because of differences between mice and humans.

Conventional CRISPR-Cas9 cuts both strands of DNA and then relies on the cell’s repair mechanisms. This process can cause large DNA deletions, chromosomal structural abnormalities, and mosaicism, in which cells with different editing outcomes are mixed, making precise interpretation difficult.

Researchers therefore applied base editing to human embryos, changing specific DNA bases without cutting the DNA. The adenine base editor (ABE) used in this study changes adenine (A) to guanine (G) to introduce a desired single-base substitution.

The Bower team precisely edited the NANOG gene and established that it is essential for epiblast formation during early human development. Human embryonic stem cells with disrupted NANOG lost the molecular characteristics of pluripotency and could not maintain their stem cell state.

When the same method was applied to human embryos, blastocysts could still form without NANOG function, but epiblast formation was impaired. The researchers suggested that, unlike in mice, humans may have species-specific molecular mechanisms that compensate for NANOG function in primitive endoderm formation.

The Jerabek team analyzed the efficiency and safety of adenine base editing in human embryos, targeting the PCSK9, HBG1 and HBG2 genes. Direct delivery of the base editor to fertilized eggs in protein form enabled consistent editing and introduced the desired variants into both alleles of the target genes without disrupting blastocyst development.

By contrast, when the base editor was delivered as mRNA, many embryos arrested at an early developmental stage. The researchers said that the delivery method, as well as the editor itself, is an important variable.

The study points to the possibility of directly investigating which genes are essential for normal human embryo development and how human development differs from that of other animals. It is expected to help expand foundational knowledge for infertility treatment, assisted reproduction and regenerative medicine.

However, chromosomal abnormalities, off-target editing and mosaicism were observed in some embryos even when protein-form base editing was used. Human embryo base editing still requires further technical improvements in precision and delivery efficiency before it can be used clinically.

In particular, if an edited human embryo leads to a birth, the edit would constitute heritable germline editing, with the variant passed on to the next generation. The researchers emphasized that, at present, the most appropriate use of human embryo base editing is not clinical gene correction but a basic research tool for elucidating the principles of early human development within an appropriate ethical framework.