Groundbreaking genetic breakthrough in reproductive science

In a major scientific advancement, scientists in China have successfully bred mice with two biological fathers, achieving a milestone in reproductive biology. The study, published in Cell Stem Cell on January 28, demonstrates that these bipaternal mice can survive into adulthood—a significant improvement over previous attempts at such genetic engineering.

This is not the first instance of bipaternal reproduction in mice. In 2023, a Japanese research team accomplished a similar feat using a different methodology. However, the Chinese team’s approach offers new insights into the role of imprinting genes—genes whose activity depends on whether they are inherited from the mother or father. Dysfunctions in these genes are linked to serious human disorders, including Angelman syndrome.

“I admire the work—I think it’s an important approach,” said Keith Latham, a professor of animal science and reproductive biology at Michigan State University, who was not involved in the study. “This is another significant step forward in understanding the biology of imprinting.”

Genetic modifications enable bipaternal reproduction

The Chinese research team pioneered a novel method to overcome the genetic barriers that typically prevent offspring from having two male biological parents. Their approach involved 20 precise genetic modifications to alter the function of imprinting genes.

In the experiment, scientists began by removing DNA from an immature egg cell (oocyte) of a female mouse. They then introduced sperm into the egg to generate specialized embryonic stem cells, which were later combined with sperm from another male mouse. The resulting fertilized egg developed into a mouse pup carrying genetic material from two fathers.

This technique contrasts with the 2023 Japanese study, where scientists converted adult male skin cells into stem cells capable of developing into egg cells. By ensuring these cells retained two X chromosomes—the typical female chromosomal structure—the team fertilized them with sperm, producing a limited number of viable offspring.

Addressing imprinting gene challenges

A key challenge in bipaternal reproduction is genomic imprinting, a biological process where offspring inherit two copies of a gene—one from each parent—but only one remains active. If both copies are activated or silenced, severe developmental abnormalities can occur.

“Our approach directly targets imprinted genes, which have long been suspected to play a central role in bi-paternal reproductive barriers,” explained Zhi-kun Li, an associate professor at the Chinese Academy of Sciences in Beijing and co-lead author of the study.

Initially, attempts to produce bipaternal mice resulted in embryos that either failed to develop or died shortly after birth due to abnormalities such as umbilical hernias and enlarged internal organs. However, through incremental genetic modifications, scientists successfully addressed these issues.

• With 18 genetic tweaks, mice required assistance in infancy but reached adulthood.
• With 19 tweaks, placental growth issues persisted, but postnatal development improved.
• The final, 20th modification resolved placental complications, leading to healthier offspring.

Implications for future research and human applications

Despite the breakthrough, bipaternal mice in the study exhibited shorter lifespans and infertility, raising questions about the long-term viability of this approach. By contrast, the Japanese team’s bipaternal mice survived to adulthood and retained fertility.

The Chinese team aims to refine their gene-editing approach to produce healthier bipaternal animals. “Our next steps include refining the gene-editing approach to produce healthier bi-paternal animals,” Li stated, suggesting that further modifications may resolve lingering health concerns.

Experts believe this research could have wide-ranging applications beyond reproductive science.

• Understanding imprinting disorders: The study could aid in developing treatments for genetic imprinting disorders in humans, potentially through gene-editing or drug-based interventions.
• Agricultural advancements: The findings may help breeders cultivate desirable traits in livestock through genetic manipulation.
• Ethical and technological considerations: While promising, the technology faces significant hurdles before human applications can be considered. “We need to fully understand the risks and benefits before attempting to apply this in humans,” Latham cautioned.

Dr. Kotaro Sasaki, an associate professor at the University of Pennsylvania School of Veterinary Medicine, echoed these concerns, noting that “some technological hurdles and ethical quandaries all hamper clinical application in humans shortly.”

A step toward the future of reproductive biology

The successful creation of bipaternal mice marks a significant advancement in genetic engineering, shedding light on fundamental biological processes and potential future applications in medicine and agriculture. While much work remains before such techniques could be considered for human reproduction, this study underscores the rapid progress being made in reproductive genetics.