Scientists genetically engineer mice with two male parents

By Stephen Beech

Mice with two male parents that lived to adulthood have been created by stem cell scientists.

But the genetically engineered rodents were unable to reproduce.

Chinese researchers used embryonic stem cell engineering to create bi-paternal mice - ones with two male parents.

Their research, published in the journal Cell Stem Cell, targeted a particular set of genes involved in reproduction allowed the team to overcome previously insurmountable challenges in unisexual reproduction in mammals.

Scientists have attempted to create bi-paternal mice before, but the embryos developed only to a certain point and then stopped growing.

The Chinese team focused on targeting imprinting genes, which regulate gene expression in several ways.

Study leader Professor Wei Li, of the Chinese Academy of Sciences (CAS) in Beijing, said: “This work will help to address a number of limitations in stem cell and regenerative medicine research."

Co-corresponding author Dr Qi Zhou, also of CAS, said: “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.”

Earlier attempts to make a bi-paternal mouse used ovarian organoids to derive oocytes from male pluripotent stem cells.

Those oocytes were then fertilized with sperm from another male.

However, when the homologous chromosomes - the chromosomes that divide during meiosis to create oocytes and sperm - originated from the same sex, imprinting abnormalities arose, leading to "severe" developmental defects.

For the new study, the Chinese team modified 20 key imprinting genes individually using a number of different techniques, including frameshift mutations, gene deletions, and regulatory region edits.

They found that not only did the edits allow the creation of bi-paternal animals that sometimes lived to adulthood, but they also led to stem cells with more stable pluripotency.

Co-corresponding author Dr Guan-Zheng Luo, of Sun Yat-sen University in Guangzhou, China said: “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.”

But the Chinese researchers noted several limitations that their work still needs to address.

Only 11.8% of the viable embryos were capable of developing until birth, and not all the pups that were born lived to adulthood due to developmental defects.

Most of those that did live to adulthood had altered growth and a shortened lifespan.

And the mice that lived to adulthood were sterile, although they did exhibit increased cloning efficiency.

Co-corresponding author Dr Zhi-Kun, Li of CAS, added: “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."

The Chinese team plan to continue studying how modifying imprinting genes may lead to embryos with higher developmental potential.

They also aim to extend the experimental approaches developed in mice to larger animals, including monkeys.

But they say that will require "considerable" time and effort because the imprinting gene combinations in monkeys differ significantly from those in mice.

Whether stem cell technology will ultimately be applied towards solving human disease remains unclear.

The International Society for Stem Cell Research's ethical guidelines does not allow heritable genome editing for reproductive purposes nor the use of human stem cell-derived gametes for reproduction because they are deemed as currently unsafe.