A team of scientists at the Chinese Academy of Sciences have made a groundbreaking achievement in genetic engineering. They have successfully used gene editing in embryonic stem cells to create mice with two male parents. These bi-paternal mice are the first to reach adulthood. The team, led by researchers Zhi-kun Li, Wei Li, and Qi Zhou, published their results in Cell Stem Cell on January 28, 2025. This scientific achievement sheds light on the main difficulties of mammalian unisexual reproduction and opens doors for advancements in regenerative medicine.
Genomic imprinting is a process where only one copy of a gene is expressed; the other is silenced due to modifying chemical tags added to the DNA sequence during germ cell (egg or sperm cell) formation. Usually, diploid genes (genes with two sets of chromosomes) have a substitute if one set is defective. However, when only one copy can be expressed, the negative effects of mutations are unavoidable for that gene. Therefore, even alleles which are typically recessive can be expressed if the dominant allele is silenced. The genes expressed from an imprinted area differ depending on the sex of the parent, which is why the presence of both maternal and paternal genes is essential for development. This is the difficulty many scientists face when creating bi-maternal or bi-paternal embryos; for example, when egg cells derived from male pluripotent stem cells are fertilised, developmental defects occur due to both chromosomes originating from the same sex.
Using CRISPR – a technique to cut and add DNA at precise locations on a strand – they introduced changes at twenty different positions on imprinting genes to establish a stable pattern. These changes include frameshift mutations (inserting or deleting basic building blocks of DNA), gene deletions and regulatory edits. Although there are over 200 imprinting genes in mice, they targeted only twenty that are important in embryonic development. The gene-edited cells were inserted into egg cells without any genetic material.
In previous research, bi-maternal mice were successfully created. They were small in size but survived to adulthood, while bi-paternal pups resulted in excessive organ growth, which led to their death. The hypothesis was that paternal genes promote growth while maternal genes restrict it.
Despite the relatively low survival rate of 11.8% for embryos until birth, this was a huge improvement from previous studies, where a smaller number of genes were edited, resulting in abnormalities such as the absence of placenta development. The surviving adults still showed overgrowth, had shorter lifespans, and were sterile, though they otherwise appeared normal, including developing normal sexual characteristics.
“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,” said co-author Zhi-Kun Li, a developmental biologist at CAS.
Imprinting disorders cause neurodevelopmental impairment, affecting growth and metabolism and are present from birth. Examples include Angelman syndrome, which causes intellectual disabilities and impaired speech.
The team will continue to study ways to improve embryos’ developmental barriers, aiming to experiment with this approach on other animals, including primates. However, we still need to understand a lot more before we even consider finding a safe and ethical way to edit the genome in human stem cells for unisexual reproduction.
