Researchers for the first time have successfully fused two chromosomes in mice and shown that the new karyotype can be transmitted to offspring. The researchers used a strategy that involved haploid embryonic stem cells and gene editing, illustrating the feasibility of large-scale engineering of DNA in mammals.

The work has been published in Science in an article entitled “A sustainable mouse karyotype created by programmed chromosome fusion.”

To date, chromosome-level engineering has only been accomplished in yeast. The ability to manipulate DNA on a large scale in higher eukaryotes like mammals has remained a challenging goal. Success in this area could provide important insights into how rearrangements of chromosomes influence evolution.

“Over longer time scales…karyotype changes caused by chromosome rearrangements are common,” the researchers wrote in their article. “Rodents have 3.2 to 3.5 rearrangements per million years, whereas primates have 1.6.”

Such seemingly small changes have big impacts. Human chromosome 2 was formed by the fusion of two chromosomes that remain separate in gorillas, for example. On the level of an individual, chromosomal fusions or translocations can result in conditions such as infertility and diseases like childhood leukemia.

Mouse with New Karyotype
An adult mouse that carries the new karyotype consisting of chromosomes 4 and 5 fused together. [Qiang Wang]

The laboratory mouse has a standard karyotype of 40 chromosomes. This consists of 19 pairs of autosomes plus the X and Y chromosomes. Engineering large-scale changes to this set has been technically challenging.

According to co-first author Li-Bin Wang, researcher with the Chinese Academy of Sciences and the Beijing Institute for Stem Cell and Regenerative Medicine, the difficulty is that the process requires deriving stem cells from unfertilized mouse embryos, and the cells only contain one set of chromosomes. “Genomic imprinting is frequently lost [in these haploid embryonic stem cells], meaning the information about which genes should be active disappears…limiting their pluripotency and [potential for] genetic engineering,” Wang said.

“We recently discovered that by deleting three imprinted regions, we could establish a stable sperm-like imprinting pattern [in these cells],” the authors explained in their article. “Because they have yeast-like haploidy and passage-persistent pluripotency, we used these cells in this study to test the feasibility of chromosome engineering in mammals.”

The researchers fused the two largest mouse chromosomes (chromosomes 1 and 2) and two medium-sized chromosomes (chromosomes 4 and 5). Karyotypes carrying fused chromosomes 1 and 2 displayed arrested mitosis, polyploidization, and embryonic mortality. On the other hand, the smaller fused chromosome composed of chromosomes 4 and 5 could be passed on to homozygous offspring.

“Some engineered mice showed abnormal behavior and postnatal overgrowth, whereas others exhibited decreased fecundity, suggesting that although the change of genetic information was limited, fusion of animal chromosomes could have profound phenotypic effects,” the researchers wrote in their article.

Capn11, which is located on a rearranged chromosome, might have contributed to the phenotypes,” they continued. The researchers also point to chromosome segregation errors as potential contributors.

The weakened fertility also demonstrated the importance of accumulating chromosomal rearrangements to establishing reproductive isolation—a key evolutionary sign of the emergence of a new species. “With a lower birth rate, homozygous Chr4+5 mice were derived by mating heterozygous parents, suggesting that one fusion was insufficient for reproductive isolation in mice,” they wrote.