Nearly 4,000 years after mammoths disappeared from the face of the Earth, the U.S. company Colossal is attempting to bring them back to life using ancient DNA preserved in permafrost. While they have not yet been able to bring them back to life, they have just witnessed the birth of the first mammoth mouse in their laboratory, or as they have named it, the Colossal Woolly Mouse.
This is an ordinary mouse that has been simultaneously modified with seven genes and now displays the fur of a mammoth. In other words, it has the color, texture, and thickness reminiscent of woolly mammoth phenotypes, which also provided the conditions to adapt to extreme cold. "This marks a pivotal moment in our de-extinction mission," said Ben Lamm, co-founder and CEO of Colossal Biosciences.
Colossal's mammal team explored 121 mammoth and elephant genomes to identify significant genes affecting hair and other cold adaptation traits. They then focused on a set of genes in which mammoths had evolved compared to their Asian elephant counterparts. Finally, they refined the list to use ten genes related to hair length, thickness, texture, and color, as well as lipid metabolism, that were compatible with mouse expression.
"By designing multiple cold-tolerant mammoth evolutionary pathways in a living species, we have demonstrated our ability to recreate complex genetic combinations that nature took millions of years to create. This success brings us one step closer to our goal of bringing back the woolly mammoth," noted Lamm.
The team edited the mouse genome using a simplified strategy that combined three editing technologies: RNP-mediated knockout, multiplexed precision genomic editing, and precision homology-directed repair (HDR). They then made eight simultaneous edits, some with efficiencies of up to 100%, to modify seven genes.
Inactivating the Mc1r gene changes the dark color of the hair to a yellowish or reddish tone, similar to red-haired people and animals, or as mammoths had. Inactivating the Fgf5 gene causes the hair to grow up to three times longer than normal. Inactivating the Fam83g, Fzd6, Tgm3, Astn2, Krt25, Tgfa, and Krt27 genes alters the growth pattern, causing it to curl, form curls, and become thicker, resembling mammoth hair. The final appearance of the edited mouse is that of a woolly mouse, with thick, long, curly, red hair, likely much more prepared to withstand low temperatures than unmodified wild mice.
"Although the mice have a striking golden coat, they are otherwise healthy, indicating that the method used is not harmful," noted Denis Headon, group leader and principal investigator at the Roslin Institute, University of Edinburgh, in statements to the Science Media Centre.
They also edited the Fabp2 gene, involved in lipid metabolism, which is thought to contribute to sufficient body fat storage to insulate from the cold and sustain during long winters. Currently, the woolly mice with this latter gene do not yet accumulate more weight than their unmodified siblings, suggesting that other genetic modifications will be necessary to acquire this characteristic.
"This is a significant step towards validating our approach to resurrecting lost extinction traits with the aim of restoring them," said Dr. Beth Shapiro, Chief Scientific Officer at Colossal.
The implications of this advancement go beyond the laboratory. The Colossal Woolly Mouse is not only the first living animal designed to express multiple cold-adapted traits using mammoth genetic orthologs, but it also serves as a living model for studying cold climate adaptations in mammals. Future analyses will enhance our understanding of how multiple genes work together to manifest physical traits.
"We are demonstrating that we can rationally design and build complex genetic adaptations, with profound implications for the future of de-extinction and multi-gene engineering," noted George Church, Professor of Genetics at the Wyss Institute, Harvard Medical School, and co-founder of Colossal.
According to Lluís Montoliu, researcher at the National Center for Biotechnology (CNB-CSIC), this is a historic genetic achievement, as he pointed out in statements to the Science Media Centre. "There are researchers who leave no stone unturned. They are capable of carrying out and completing the most fanciful and extravagant ideas we can imagine. Ideas that the rest of us mortals dismiss as impossible or unfeasible. For these researchers, nothing is impossible. Their conviction of having the answer to problems, their perseverance, and stubbornness often earn them successes that the scientific community applauds with a mix of surprise and bewilderment."
Less enthusiastic is Louise Johnson, an evolutionary biologist at the University of Reading in the United Kingdom: "Seeing these mice is a bit like looking into the past, but with a very selective telescope. It is interesting work, but the idea that we can bring something back from extinction is a false hope. What has been done here is not trivial, but of the ten different mutations introduced in the mice, only a few cause the mouse gene to approach a known mammoth gene. In theory, mice like these could be produced simply by crossing mice with rare fur."
"But a mammoth is much more than an elephant with a fur coat," noted Tori Herridge, Associate Professor at the Faculty of Biosciences, University of Sheffield. Therefore, how close Colossal is to seeing mammoths roaming around. Using DNA obtained from various frozen and relatively well-preserved mammoth carcasses in the Siberian tundra, Colossal has been able to obtain a high-quality mammoth genome to compare it with that of the Asian elephant, the closest living evolutionary relative to the woolly mammoth. There are about 500,000 changes between the two genomes that Church and his colleagues aim to incorporate, one by one, using Asian elephant cells in culture as starting material. "This will take them some time, and it is only the first of the technical challenges they will need to solve," noted Lluís Montoliu.
"They will need to reconstruct mammoth embryos using Asian elephant egg cells and nuclei of edited cells through nuclear transfer (cloning) and gestate them, likely in some extrauterine system yet to be invented, improving existing systems that allow for gestation and growth outside the maternal uterus in lambs and premature babies. All of this will take quite some time, unpredictable to calculate. And that's why they need intermediate successes to justify their project and allow them to progress, demonstrating, with much simpler experiments, that it is possible to edit an animal's genome to incorporate selected traits," Montoliu concluded.