Limited Lifespan of Cloned Mice
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Cloning involves creating genetically identical copies, yet extensive research over the last 20 years reveals unexpected complexities. Clones often accumulate additional mutations, and if the cloning process is repeated, these mutations can reach lethal levels. This discovery presents important implications for cloning in agriculture, conservation, and even medical applications involving humans.
The core issue lies in the numerous mutations within clones. Adult somatic cells may accumulate more mutations than gametes (egg or sperm cells). Researchers such as Teruhiko Wakayama from the University of Yamanashi in Japan suggest that the cloning process may also contribute to these mutations. “While we once believed clones were identical to their originals, the accumulated mutations present significant challenges,” Wakayama states. “Our goal is to confirm that these mutations do not lead to complications.”
Historically, cloning mammals was deemed implausible because cellular differentiation adds various chemical tags that regulate gene activity. The successful birth of Dolly the sheep in July 1996 demonstrated that transferring the nucleus of an adult cell into an empty egg could effectively reprogram the genome, enabling cell growth. Shortly after, in October 1997, Wakayama created the first cloned mouse, Kumulina.
To evaluate the efficacy of his team’s cloning technique, Wakayama initiated cloning experiments in 2005. “Similar to how a reproduced painting loses detail, we aimed to assess the quality of the clones against the original,” he explains.
By 2013, Wakayama’s team had successfully generated over 500 mice from a single donor across 25 cloning generations, claiming, “Each cloned mouse exhibited no physical anomalies and maintained normal lifespan and health.” However, this level of success has not been replicated in other species. Cloned dogs continue to face health complications, and no primate has been cloned using adult cells to date. Initially, Wakayama believed repeated cloning in mice could extend indefinitely, yet by the 58th generation, not one clone survived.
To uncover the reasons behind this decline, the research team sequenced the genomes of ten different mice from various generations. They found an average of over 70 mutations per clonal generation, three times higher than in the naturally bred control group. Notably, after the 27th generation, significant mutations began to accumulate, even leading to the loss of the entire X chromosome.
This issue may stem from evolutionary mechanisms that protect gametes from mutations while allowing adult somatic cells to accrue more mutations. Recent studies suggest mutations accumulate eight times faster in blood cells compared to sperm. Thus, if the original cloned adult cell harbored numerous mutations, so too would the resulting clones.
Wakayama also posits that the nuclear transfer process may induce additional mutations. “It’s plausible that physical shock during nuclear transfer can damage the DNA,” he remarks. “If we can devise gentler nuclear transfer techniques, we might lower the mutation rate in cloned embryos—but we’re still seeking solutions.”
Shukrat Mitalipov, a professor at Oregon Health and Science University, remains skeptical. “The mutation rate evident in cloned subjects probably reflects the genomic nature of donor cells rather than being an inherent consequence of nuclear transfer,” he states.
While human cloning is prohibited in many regions, researchers like Mitalipov are exploring nuclear transfer’s potential for generating tissues and organs that are compatible for treatments, as well as for creating sperm and egg cells for infertility therapies. Wakayama’s findings highlight the necessity of thorough donor cell screening to prevent deleterious mutations. “Evaluating donor cell populations for harmful mutations is vital; if needed, gene editing could correct identified issues.”
Nevertheless, if the cloning process itself is responsible for inducing mutations, it presents additional challenges. Nonetheless, these findings do not signal that cloning techniques entail insurmountable risks. The mutation rate per generation remains relatively low, and safety screenings can be conducted post-cloning. However, they underscore the complexities inherent in cloning technology.
Source: www.newscientist.com
