Mouse chromosomes get supersized but find their limits.

Abstract

© The Author(s) 2022. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/ licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com Humans diverged from primates when an ancestral chromosomal fusion—the event when two chromosomes join together to form one—gave us 23 instead of 24 sets of chromosomes. In muntjac deer, small deer native to South and Southeast Asia, chromosome fusions occurred so often that Indian muntjacs have only 3 chromosomes, whereas Chinese muntjacs have 23 (1). Fusions matter not only during the evolution of species but can also cause diseases such as cancer or Down’s syndrome. While fusions occur often in nature, engineering events like these on purpose have been difficult to do. The field of synthetic genomics attempts feats like this, along with building new designer chromosomes for applications in medicine, agriculture and industrial processing. Completely synthetic genomes have been built for bacteria (2), as well as for yeast (2). Additionally, it was shown that all 16 yeast chromosomes can be fused into one single chromosome 12 megabases long (3). Besides these achievements, it remained an open question what the actual size limit of a single chromosome would be, for example, whether the 100–200 megabase mammalian chromosomes could be fused and whether changes like these would persist through multiple generations. Answers could be used to model speciation and human diseases, as well as biologically ‘contain’ engineered organisms from natural populations. In a groundbreaking new study (4), researchers from the Chinese Academy of Sciences have generated the largest designed fusion chromosomes so far reported in mice as their research model. To technically achieve this, they used Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) to make targeted DNA breaks. These breaks would induce recombination—a natural repair process of the cell—thereby fusing the two largest mouse chromosomes 1 and 2 into a single one, in two different orientations, followed by fusing medium size chromosomes 4 and 5. The longest of these chromosomes was 377 megabases long and functional. In addition, they accomplished all this engineering in haploid mouse embryonic stem cells (i.e. cells with only one set of chromosomes), showing the potential to make mice easier to engineer using haploid cells in a Petri dish. Although the authors could generate heterozygous embryos with the largest fused chromosomes, one fused orientation was lethal to the developing embryo, while embryos of the other orientation grew to adulthood. Yet, the resultant mice could not breed homozygous offspring. Surprisingly, the 308 megabase medium-sized fused chromosome mice could mate and propagate the chromosomes through five mouse generations. The developmental difficulties with the long chromosomes stemmed from issues during cell division. Chromosomes have relatively balanced amounts of DNA on their left and right arms, as chromosomes are split by the attachment point for cell division called a centromere. Due to limitations of the technique, the engineered chromosomes that were generated were extremely unbalanced, with the short arm having little DNA. Due to this, the authors found that there was a limit to the size difference one arm could have relative to the other. Having one arm too big caused the fused chromosomes to lag behind during cell division, and this led to extra unwanted copies of the chromosome in daughter cells. They also found that the chromosomes distributed poorly to sperm cells, causing a reproductive fitness disadvantage. In essence, these results suggest that population bottlenecks are necessary to fixate chromosome fusion events during speciation. It is now clear that by using CRISPR we may program chromosome fusions in mammals to model diseases like cancer or to understand speciation events like in muntjacs. But future studies will have to offer more precision by balancing the two arms. If so, would it be possible even to build chromosomes out to the gigabase scale as found in some species? Besides addressing technical limitations, the field of synthetic genomics will also need to lead ethical conversations to allay societies’ possible concerns about these techniques. As the ultimate goal of the field is to use these chromosomes to program cell factories for manufacturing future foods, chemicals and medicines, we should only use these techniques if they are well understood, safe and publicly accepted.