Mice with two dads have been created using CRISPR

Mice with two fathers have been born—and have survived to adulthood—following a complex set of experiments by a team in China. 

Zhi-Kun Li at the Chinese Academy of Sciences in Beijing and his colleagues used CRISPR to create the mice, using a novel approach to target genes that normally need to be inherited from both male and female parents. They hope to use the same approach to create primates with two dads. 

Humans are off limits for now, but the work does help us better understand a strange biological phenomenon known as imprinting, which causes certain genes to be expressed differently depending on which parent they came from. For these genes, animals inherit part of a “dose” from each parent, and the two must work in harmony to create a healthy embryo. Without both doses, gene expression can go awry, and the resulting embryos can end up with abnormalities.

This is what researchers have found in previous attempts to create mice with two dads. In the 1980s, scientists in the UK tried injecting the DNA-containing nucleus of a sperm cell into a fertilized egg cell. The resulting embryos had DNA from two males (as well as a small amount of DNA from a female, in the cytoplasm of the egg).

But when these embryos were transferred to the uteruses of surrogate mouse mothers, none of them resulted in a healthy birth, seemingly because imprinted genes from both paternal and maternal genomes are needed for development. 

Li and his colleagues took a different approach. The team used gene editing to knock out imprinted genes altogether.

Around 200 of a mouse’s genes are imprinted, but Li’s team focused on 20 that are known to be important for the development of the embryo.

In an attempt to create healthy mice with DNA from two male “dads,” the team undertook a complicated set of experiments. To start, the team cultured cells with sperm DNA to collect stem cells in the lab. Then they used CRISPR to disrupt the 20 imprinted genes they were targeting.

These gene-edited cells were then injected, along with other sperm cells, into egg cells that had had their own nuclei removed. The result was embryonic cells with DNA from two male mice. These cells were then injected into a type of “embryo shell” used in research, which provides the cells required to make a placenta. The resulting embryos were transferred to the uteruses of female mice.

It worked—to some degree. Some of the embryos developed into live pups, and they even survived to adulthood. The findings were published in the journal Cell Stem Cell.

“It’s exciting,” says Kotaro Sasaki, a developmental biologist at the University of Pennsylvania, who was not involved in the work. Not only have Li and his team been able to avoid a set of imprinting defects, but their approach is the second way scientists have found to create mice using DNA from two males.

The finding builds on research by Katsuhiko Hayashi, now at Osaka University in Japan, and his colleagues. A couple of years ago, that team presented evidence that they had found a way to take cells from the tails of adult male mice and turn them into immature egg cells. These could be fertilized with sperm to create bi-paternal embryos. The mice born from those embryos can reach adulthood and have their own offspring, Hayashi has said.

Li’s team’s more complicated approach was less successful. Only a small fraction of the mice survived, for a start. The team transferred 164 gene-edited embryos, but only seven live pups were born. And those that were born weren’t entirely normal, either. They grew to be bigger than untreated mice, and their organs appeared enlarged. They didn’t live as long as normal mice, and they were infertile.

It would be unethical to do such risky research with human cells and embryos. “Editing 20 imprinted genes in humans would not be acceptable, and producing individuals who could not be healthy or viable is simply not an option,” says Li.

“There are numerous issues,” says Sasaki. For a start, a lot of the technical lab procedures the team used have not been established for human cells. But even if we had those, this approach would be dangerous—knocking out human genes could have untold health consequences. 

“There’s lots and lots of hurdles,” he says. “Human applications [are] still quite far.”

Despite that, the work might shed a little more light on the mysterious phenomenon of imprinting. Previous research has shown that mice with two moms appear smaller, and live longer than expected, while the current study shows that mice with two dads are overgrown and die more quickly. Perhaps paternal imprinted genes support growth and maternal ones limit it, and animals need both to reach a healthy size, says Sasaki.

This is what might happen if the US withdraws from the WHO

On January 20, his first day in office, US president Donald Trump signed an executive order to withdraw the US from the World Health Organization. “Ooh, that’s a big one,” he said as he was handed the document.

The US is the biggest donor to the WHO, and the loss of this income is likely to have a significant impact on the organization, which develops international health guidelines, investigates disease outbreaks, and acts as an information-sharing hub for member states.

But the US will also lose out. “It’s a very tragic and sad event that could only hurt the United States in the long run,” says William Moss, an epidemiologist at Johns Hopkins Bloomberg School of Public Health in Baltimore.

A little unfair?

Trump appears to take issue with the amount the US donates to the WHO. He points out that it makes a much bigger contribution than China, a country with a population four times that of the US. “It seems a little unfair to me,” he said as he prepared to sign the executive order.

It is true that the US is far and away the biggest financial supporter of the WHO. The US contributed $1.28 billion over the two-year period covering 2022 and 2023. By comparison, the second-largest donor, Germany, contributed $856 million in the same period. The US currently contributes 14.5% of the WHO’s total budget.

But it’s not as though the WHO sends a billion-dollar bill to the US. All member states are required to pay membership dues, which are calculated as a percentage of a country’s gross domestic product. For the US, this figure comes to $130 million. China pays $87.6 million. But the vast majority of the US’s contributions to the WHO are made on a voluntary basis—in recent years, the donations have been part of multibillion-dollar spending on global health by the US government. (Separately, the Bill and Melinda Gates Foundation contributed $830 million over 2022 and 2023.)

There’s a possibility that other member nations will increase their donations to help cover the shortfall left by the US’s withdrawal. But it is not clear who will step up—or what implications changing the structure of donations will have.

Martin McKee, a professor of European public health at the London School of Hygiene and Tropical Medicine, thinks it is unlikely that European members will increase their contributions by much. The Gulf states, China, India, Brazil, and South Africa, on the other hand, may be more likely to pay more. But again, it isn’t clear how this will pan out, or whether any of these countries will expect greater influence over global health policy decisions as a result of increasing their donations.

Deep impacts

WHO funds are spent on a range of global health projects—programs to eradicate polio, rapidly respond to health emergencies, improve access to vaccines and medicines, develop pandemic prevention strategies, and more. The loss of US funding is likely to have a significant impact on at least some of these programs.

It is not clear which programs will lose funding, or when they will be affected. The US is required to give 12 months’ notice to withdraw its membership, but voluntary contributions might stop before that time is up. 

For the last few years, WHO member states have been negotiating a pandemic agreement designed to improve collaboration on preparing for future pandemics. The agreement is set to be finalized in 2025. But these discussions will be disrupted by the US withdrawal, says McKee. It will “create confusion about how effective any agreement will be and what it will look like,” he says.

The agreement itself won’t make as big an impact without the US as a signatory, either, says Moss, who is also a member of a WHO vaccine advisory committee. The US would not be held to information-sharing standards that other countries could benefit from, and it might not be privy to important health information from other member nations. The global community might also lose out on the US’s resources and expertise. “Having a major country like the United States not be a part of that really undermines the value of any pandemic agreement,” he says.

McKee thinks that the loss of funding will also affect efforts to eradicate polio, and to control outbreaks of mpox in the Democratic Republic of Congo, Uganda, and Burundi, which continue to report hundreds of cases per week. The virus “has the potential to spread, including to the US,” he points out.

“Diseases don’t stick to national boundaries, hence this decision is not only concerning for the US, but in fact for every country in the world,” says Pauline Scheelbeek at the London School of Hygiene and Tropical Medicine. “With the US no longer reporting to the WHO nor funding part of this process, the evidence on which public health interventions and solutions should be based is incomplete.”

Moss is concerned about the potential for outbreaks of vaccine-preventable diseases. Robert F. Kennedy Jr., Trump’s pick to lead the Department of Health and Human Services, is a prominent antivaccine advocate, and Moss worries about potential changes to vaccination-based health policies in the US. That, combined with a weakening of the WHO’s ability to control outbreaks, could be a “double whammy,” he says: “We’re setting ourselves up for large measles disease outbreaks in the United States.”

At the same time, the US is up against another growing threat to public health: the circulation of bird flu on poultry and dairy farms. The US has seen outbreaks of the H5N1 virus on poultry farms in all states, and the virus has been detected in 928 dairy herds across 16 states, according to the US Centers for Disease Control and Prevention. There have been 67 reported human cases in the US, and one person has died. While we don’t yet have evidence that the virus can spread between people, the US and other countries are already preparing for potential outbreaks.

But this preparation relies on a thorough and clear understanding of what is happening on the ground. The WHO provides an important role in information sharing—countries report early signs of outbreaks to the agency, which then shares the information with its members. This kind of information not only allows countries to develop strategies to limit the spread of disease but can also allow them to share genetic sequences of viruses and develop vaccines. Member nations need to know what’s happening in the US, and the US needs to know what’s happening globally. “Both of those channels of communication would be hindered by this,” says Moss.

As if all of that weren’t enough, the US also stands to suffer in terms of its reputation as a leader in global public health. “By saying to the world ‘We don’t care about your health,’ it sends a message that is likely to reflect badly on it,” says McKee. “It’s a classic lose-lose situation.”

“It’s going to hurt global health,” says Moss. “It’s going to come back to bite us.”

Update: this article was amended to include commentary from Pauline Scheelbeek.