Researchers merge mouse chromosomes in Scientific First

For the first time, researchers fused two chromosomes together in vitro, resulting in live mice with new karyotypes. The new technique, detailed in a study published today (August 25) in Science, may help study chromosomal evolution and may also aid research into the adverse health effects of chromosomal fusions in humans, experts say.

“[The researchers] now have this beautiful toolbox. . . they can do a lot of really smart CRISPR engineering,” says Harmit Malik, an evolutionary biologist at the Fred Hutch Research Institute in Seattle, who was not involved in the study. The scientist “It’s a tour de force. . . many issues that we thought were not possible to address in a genetically treatable way are now completely genetically treatable.

See: “How chaos in chromosomes helps spread cancer”

Most species have a fixed number of chromosomes, the tightly coiled thread-like structures that organize and separate a cell’s DNA during cell division and division. Over the evolutionary history of many organisms, chromosomes may have separated and fused, causing profound physiological and behavioral changes. These changes may even be an important driver of speciation, but scientists lack direct evidence for this hypothesis, as they have only been able to observe this phenomenon in nature.

Chromosomal fusions are also common in cancer and have been linked to health problems including infertility, aneuploidy and childhood diseases. Thus, researchers have long sought the ability to precisely manipulate chromosomes in model organisms, particularly mammals, in hopes of studying fusions from a medical and evolutionary perspective.

See: “Simplified creation of artificial chromosomes”

To fuse chromosomes in mice, the researchers used a technology they first developed in yeast: Briefly, they injected friendly modified haploid mouse embryonic stem cells (hESCs) with a CRISPR-Cas9 system that targets and eliminates the genes responsible for the production of telomeres and centromeres. on two specific chromosomes. As a result, the targeted chromosomes compressed.

In all, the researchers created three different fusion lines. For two of them, the researchers fused the two longest mouse chromosomes (Chr1 and Chr2), but in one fusion the second chromosome was flipped (Chr2+1), as opposed to the right side (Chr1+ 2) – and for some reason in the latter part of chromosome 1 split off to join chromosome 17, so the merged chromosome was a bit smaller. They also ligated chromosomes four and five (Chr4+5) to create a separate set of haESCs.

All three haESC lineages could divide and grow, although the researchers had difficulty maintaining haploidy in Chr2+1 cells. The cells tended to become polypoid, which the researchers realized was because the chromosomes didn’t separate properly in some of these cells, so some would end up extra after mitosis. This signaled to the researchers that there might be a limit to the size of chromosomes in mice before sorting errors begin to occur.

They then attempted to create mice with these new haploid karyotypes by injecting a chromosome-fused hESC into a wild-type oocyte. But there was a problem: the cells were imprinting, meaning the maternal genes were epigenetically silenced after fertilization. Imprinting can cause embryos to grow too quickly and sap the mother’s resources. So the researchers knocked out three fingerprint genes in hESCs and solved the problem. Malik says that now that researchers have overcome the imprinting phenomenon, “the world is their oyster when it comes to genetic engineering.”

Mice with fused chromosomes (4 and 5)

Qiang Wang

John Postlethwait, an evolutionary biologist at the University of Oregon who was not involved in the work, wrote in an email to The scientist that it seems to him that “it is possible to grow mammalian haploid cells in culture, hit their genomes with Crisprs, then inject the haploid cell into an unfertilized egg and raise a baby mouse. It’s incredible.

Mice with fused chromosomes showed some adverse consequences from their genetic changes. Ch2+1 mice did not grow at all, while Chr1+2 mice reached adulthood but were sterile. Chr1+2 mice also grew significantly faster than normal mice and “showed a high level of anxiety in an open-field anxiety test,” writes study co-author Libin Wang, synthetic biologist at the Chinese Academy of Sciences, in an email to The scientist. The team attributes these phenotypic effects to changes in Capn11 gene expression, a gene that codes for a protein thought to be important for spermatogenesis, which was lower in Ch1+2 mice than in normal mice. “Although the modification of genetic information has been limited, the fusion of animal chromosomes could have profound phenotypic effects,” Wang writes.

Chr4+5 mice, on the other hand, could produce offspring and mate with normal mice. But when they did, their litters were small, indicating they may have suffered from abnormal chromosome sorting during meiosis, Wang says.

Malik says his own interest is meiosis, specifically how certain chromosome fusions give some eggs a competitive advantage over others. He says scientists could now study this phenomenon in a controlled way and identify the fusions most likely to transfer to the next generation of mice. He also says that this toolkit will also be useful for cancer researchers, who can now “suddenly model” the “rather dramatic rearrangement[s] of the genome” that occur in cancer cells.

Postlethwait writes that the study “is a bold attempt to reengineer vertebrate chromosomes” and that it could also help test various theories about chromosome evolution. Postlethwait has worked with fish from Antarctica, which have many chromosomes presumably fused together, so he says he’s particularly interested in researching chromosome fusions in fish. He says the new study will allow researchers to “understand the evolutionary questions [about] the limits and selective pressures that cause . . . that chromosome fusions occur.

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