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Chimeric mice with rat chromosomes
University of Yamanashi
Colossal Biosciences, a now-defunct company, may be on the verge of revival. Scientists successfully transplanted rat chromosomes, preserved for over a year, into living mouse cells, paving the way for potential reconstruction of mammoth chromosomes within living cells. They crafted a complete mouse embedded with rat chromosomes in specific cells.
“Once we refine the technology, we will begin testing on elephant cells,” stated Teruhiko Wakayama from Yamanashi University in Japan. “If we can introduce elephant chromosomes into mouse embryonic stem cells, we would be eager to proceed with mammoth chromosomes as well.”
<p>The immediate objective of this research is to explore gene activity from extinct animals in contemporary species, potentially revealing insights beyond mere gene sequence analysis. This study also holds implications for conservation and de-extinction initiatives. For instance, in 2004, we preserved tissue from the Hawaiian pouri, an extinct bird. The unique characteristics of biological systems suggest that chromosome transfer is crucial for reviving this species.</p>
<p>An animal's genome is organized into structures known as chromosomes. During cell division, these extensive DNA strands tightly coil into the classic cylindrical shapes depicted in textbooks. These "condensed chromosomes" can be visually identified in living cells by employing dyes that bind to the surrounding proteins, enabling non-invasive observation.</p>
<p>Wakayama's method involves extracting a cell's nucleus and injecting it into an egg cell, initiating chromosome condensation—a process akin to nuclear transfer techniques utilized in cloning. This technique was first applied by Wakayama for cloning a mouse shortly after the birth of Dolly the sheep.</p>
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<p>Upon injection of the nucleus, the egg is treated with enzymes to assist in chromosome separation. A single chromosome is then retrieved using a minute hollow needle and introduced into another egg. As this egg develops into an embryo, the chromosomes subsequently integrate within all embryonic stem cells.</p>
<p>After honing this technique with mouse chromosomes, Wakayama extended the research to rats genetically modified for green fluorescence. Blood cells extracted from the tail of one such rat, preserved for over a year, successfully generated mouse embryonic stem cells containing additional chromosomes from the genetically altered rat.</p>
<p>These cells were then introduced into standard mouse embryos and implanted into female mice, resulting in chimeric animals with rat chromosomes in some of their cells. These mice appear normal but fluoresce green under UV light, indicative of the original rat cells.</p>
<p>The research team aims to create mice with an additional rat chromosome in every cell, but this objective remains unachieved. Currently, the technique appears effective only for chromosome 9 in rats, as attempts to add other chromosomes hinder embryo development. "We are actively exploring various methods to enhance the success rate," noted Wakayama.</p>
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<p class="ArticleImageCaption__Title">This baby mouse has cells with rat chromosomes that glow green.</p>
<p class="ArticleImageCaption__Credit">University of Yamanashi</p>
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<p>The observed interference in embryonic development may stem from activity elsewhere in the rat chromosomes. If this is the case, researchers might need to deactivate genes on supplementary chromosomes, similar to how one X chromosome is inactivated in female mammalian cells. However, Wakayama remains optimistic that this won't be necessary.</p>
<p>The team has acquired samples of frozen elephant tissue from the zoo for experimental purposes. They are collaborating with a team that <a href="https://doi.org/10.1038/s41598-019-40546-1">successfully extracted and studied cell nuclei</a> and are considering using mammoth chromosomes from a 28,000-year-old specimen named Yuka.</p>
<p>Cloning efforts to revive mammoths from these cells are unlikely to succeed due to extensive DNA damage. Nonetheless, Professor Wakayama believes recovery of individual chromosomes for study in living cells is within reach.</p>
<p>“Even a single successful transfer is a significant milestone,” remarked <a href="https://reviverestore.org/about-us/#Ben_Novak">Ben Novak</a>, of Revive & Restore, a conservation organization. "This research could have pivotal implications for passerine birds," a diverse group that encompasses over half of all bird species, where body tissue cells such as skin and muscle no longer carry complete chromosomal information. Reproductive cells, however, retain an extra chromosome essential for survival.</p>
<p>The Hawaiian puli is a passerine bird with only sterile male tissue frozen, necessitating the introduction of two chromosomes from a closely related species—one chromosome from reproductive cells and a W chromosome specific to female birds. "Though this would lead to partial hybrids, it could facilitate the revival of the species," Novak explained.</p>
<p>Wakayama's research is not unprecedented; in 2022, a Japanese team managed to <a href="https://doi.org/10.1016/j.ajhg.2021.12.015">create a rat with an additional human chromosome 21</a> to study Down syndrome. However, this method involves significant genetic modification, making it less applicable for conservation efforts.</p>
<p>The prevalence of extra reproductive chromosomes might be more common than currently recognized, according to Novak, suggesting much of the tissue being preserved by biobanks may be incomplete.</p>
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