Mice can only be repeatedly cloned for so long
Xinhua/Zhou Qi/Imago/Alamy
A clone is supposed to be a perfect genetic replica, but a remarkable 20-year study has revealed that this isn’t entirely true. The research shows that clones accumulate numerous additional mutations, and these can reach dangerous levels if clones are repeatedly cloned. These findings have significant implications for the agricultural use of cloning, conservation of endangered species, attempts to resurrect extinct species, and even the potential application of cloning technology in humans.
The primary question is why clones have so many more mutations. One possibility is that adult body cells being cloned gather more mutations than egg or sperm cells. Teruhiko Wakayama from Yamanashi University in Japan suggests that the cloning process itself might be a contributing factor. He states, “Unfortunately, while clones were initially believed to be identical to the original, it is now evident that this is not the case, indicating potential issues with their use. We need to prove that mutations from cloning are not problematic.”
The idea of cloning mammals was once thought impossible. As body cells grow and specialize, they acquire chemical tags that govern gene activity. For example, skin cell DNA is “programmed” to create skin cells. However, the birth of Dolly the sheep in July 1996 demonstrated that transferring the nucleus of an adult cell into an empty egg could reset its genome, allowing development. Shortly thereafter, Wakayama produced Cumulina, the first cloned mouse, in October 1997.
To evaluate his team’s mouse-cloning technique, Wakayama began cloning clones in 2005. “Just as copying a painting results in lower image quality, I wanted to see how clones compared to the original,” he explains.
In 2013, Wakayama and his colleagues revealed that they had successfully cloned clones for 25 consecutive generations, producing over 500 mice from the original donor. “The cloned mice in our experiments showed no physical abnormalities in any generation, lived as long as normal mice, and were healthy,” Wakayama reports.
This level of success hasn’t been achieved with other species. Cloned dogs still face a high rate of health issues, and no one has successfully cloned a primate from an adult cell. While Wakayama believed repeated cloning in mice could go on indefinitely, the success rate dwindled until, by the 58th generation, no clones survived.
To uncover the cause, the team sequenced the genomes of 10 mice from different generations. They found an average of over 70 mutations per clone generation, three times more than in naturally reproducing mice. Large-scale mutations began appearing in the cloned mice after the 27th generation, eventually leading to the loss of an entire X chromosome.
One explanation could be that animals have evolved mechanisms to protect sperm and egg cells from mutations and eliminate harmful mutations during reproduction, leaving adult body cells with more mutations. For example, a recent study showed mutations accumulate eight times faster in blood cells than in sperm. Therefore, if the adult cells being cloned already have more mutations, the clones will as well.
Wakayama believes the nuclear transfer process might be responsible for some additional mutations. “It’s not surprising that the nucleus—essentially the DNA—could be damaged by the physical shock,” he says. “I think developing a gentler nuclear transfer method could reduce the mutation rate in cloned embryos, although I don’t have a clear solution yet.”
Shoukhrat Mitalipov from Oregon Health & Science University is skeptical. He argues, “Any observed increase in mutation rates in clones likely reflects the genomic state of the donor cells, rather than a consistent effect of the nuclear transfer process itself.”
While human cloning is prohibited in many countries, researchers like Mitalipov are investigating the use of nuclear transfer to create matching tissues or organs for medical treatments and generate sperm and egg cells to address infertility. Wakayama’s findings highlight the importance of careful donor cell selection and screening, according to Mitalipov. “Ideally, donor cell populations should be assessed for harmful variants. When necessary, gene-editing techniques could be used to fix known deleterious mutations.”
However, if the cloning process itself is causing mutations, this wouldn’t be sufficient. To clarify, these results don’t indicate that cloning techniques are too hazardous to use—the mutation rate per generation is still relatively low, and cells can be screened post-cloning for dangerous mutations—but they do suggest there are more potential issues than previously thought. An already challenging technology just became even more complex.
