Scientists Create First Healthy Mice with Two Fathers

A group of stem cell researchers has successfully engineered embryonic stem cells to create a bi-paternal mouse, a mouse with two male parents, that survived to adulthood. Their findings, published in the journal Cell Stem Cell detail how targeting specific genes involved in reproduction enabled the team to overcome significant obstacles in unisexual reproduction in mammals.

Previous efforts to create bi-paternal mice had been unsuccessful, as the embryos would only develop to a certain stage before ceasing growth. In this study, led by Corresponding Author Wei Li from the Chinese Academy of Sciences (CAS) in Beijing, the researchers concentrated on imprinting genes, which play a critical role in regulating gene expression. “This work will help to address several limitations in stem cell and regenerative medicine research,” said Li.

The unique characteristics of imprinting genes have led scientists to believe that they are a fundamental barrier to unisexual reproduction in mammals. Even when constructing bi-maternal or bi-paternal embryos artificially, they fail to develop properly, and they stall at some point during development due to these genes.”

Qi Zhou, Study Co-Corresponding Author, Chinese Academy of Sciences

Earlier attempts to produce a bi-paternal mouse involved using ovarian organoids to derive oocytes from male pluripotent stem cells, which were then fertilized with sperm from another male.

However, when homologous chromosomes - those that separate during meiosis to form oocytes and sperm - originated from the same sex, imprinting abnormalities occurred, resulting in severe developmental defects.

In this study, the team individually modified 20 key imprinting genes using various techniques, such as frameshift mutations, gene deletions, and edits to regulatory regions. These modifications not only enabled the creation of bi-paternal animals that occasionally survived to adulthood but also produced stem cells with enhanced pluripotency stability.

These findings provide strong evidence that imprinting abnormalities are the main barrier to mammalian unisexual reproduction. This approach can significantly improve the developmental outcomes of embryonic stem cells and cloned animals, paving a promising path for the advancement of regenerative medicine.”

Guan-Zheng Luo, Study Co-Corresponding Author, Sun Yat-sen University

Despite these advancements, the researchers acknowledge several limitations in their work. Only 11.8% of viable embryos developed to birth, and not all of the pups that were born survived to adulthood due to developmental defects. Among those that did reach adulthood, many exhibited altered growth, shortened lifespans, and sterility, although they showed improved cloning efficiency.

Further modifications to the imprinting genes could potentially facilitate the generation of healthy bi-paternal mice capable of producing viable gametes and lead to new therapeutic strategies for imprinting-related diseases.”

Zhi-Kun Li, Study Co-Corresponding Author, Chinese Academy of Sciences

The team plans to continue investigating how imprinting gene modifications can enhance the developmental potential of embryos. They also aim to adapt their experimental methods for use in larger animals, such as monkeys.

However, this will require significant time and effort, as the combinations of imprinting genes in monkeys differ substantially from those in mice. Whether this technology will eventually be applied to address human diseases remains uncertain.

Current ethical guidelines from the International Society for Stem Cell Research prohibit heritable genome editing for reproductive purposes and the use of human stem cell-derived gametes for reproduction, as these practices are currently considered unsafe.