A team of scientists in China has succeeded in creating offspring of apparently healthy mice from two mice of the same sex (females). The researchers also generated offspring of two male mice, but all those young died shortly after birth.
The results have been published in the journal Cell Stem Cell and show that although we are getting closer and closer to two men or two women having their own biological children, the new technique still faces serious obstacles should it be attempted in humans.
“It is never too much to emphasize the risks and the importance of security, before any human experiment is involved,” says Wei Li at the Chinese Academy of Sciences in Beijing. “But we believe that our work takes it closer,” he adds.
The biggest obstacle to creating babies from parents of the same sex is a phenomenon called genetic imprinting. In mammals, certain genes are deactivated in the sperm genome, by adding epigenetic markers to the DNA. These markers do not change the underlying DNA sequence, but ensure that the gene is not expressed. Different genes are deactivated in the ovules.
Unfortunately, the imprint means that if in some way the genomes of two females, or two males, are combined in an ovule and it begins to develop, the resulting embryo will die. But in 2004, a team in Japan managed to create the first mammal with two mothers. They achieved this by removing a DNA fragment in one of the genomes to mimic the effect of the imprint, but from 500 attempts only two mice survived to adulthood.
On this occasion, Li’s team has greatly improved the success rate by removing three bits of DNA to better mimic the imprint. Of 200 attempts to create a mouse with two mothers, scientists succeeded 27 times. These mice grew normally, while those with less deletions were abnormally small.
In parallel, Li and his colleagues also created 12 mice of male parents (from a total of 500 attempts). However, they had to remove 7 DNA fragments and none of the offspring survived until adulthood.
Although surprising, this is not the first time that they have had mice of male parents. In 2010, a team used a genetic trick to make male mouse cells grow inside female mice. Females with ovaries derived from male cells mated with normal males. For obvious ethical reasons this technique could not be applied in humans.
The risks of applying them in humans
On the other hand, Li mice reveal more about which genes are crucial for normal development. However, it is not clear if the results apply to other mammals such as humans; The team will try trying to create monkeys with two mothers. And before we can think about trying to create human babies with same-sex parents, we would need a way to mimic the imprints without resorting to genetic modifications. Because eliminations would have detrimental effects on later generations.
This could be achieved with the editing of CRISPR genes; and several groups, including Li’s, are trying to add or remove epigenetic markers without changing the underlying DNA sequence. “We tried but we still don’t know the answer,” says the researcher.
However, even if it worked, there are still questions about how safe the method is. “Defective imprints do lead to human diseases,” says New Scientist Azim Surani of the University of Cambridge, who discovered the phenomenon of the imprint in 1984. Any manipulation could have serious consequences, he warns.
The same case of Li mice can serve as an example. Although mice seem healthy, having removed only three DNA fragments means they may have more subtle problems that can only be noticed over time.