Researchers have created mouse embryonic stem cells that contain only one set of chromosomes, rather than two, according to a study publishing today (September 7) in Nature. The new cells could be used to do more rapid analysis of gene networks involved in mammalian development.
“This opens the opportunity to do genetics in mammalian systems on a more facile basis than has been possible previously,” said Allan Bradley, a stem cell geneticist at the Wellcome Trust Sanger Institute, who was not involved in the study. “In mammalian systems you’re dealing with a diploid genome, so when you’re now dealing with a haploid genome it significantly improves the ability to do experiments.”
Introducing random mutations into the genome and observing how the mutated cells function has dramatically improved our understanding of gene networks in bacteria and yeast, said Anton Wutz, lead author of the paper and a stem cell scientist at the Wellcome Trust for Stem Cell Research at University of Cambridge. But unlike bacteria, mice and other mammals have two sets of chromosomes in all their cells (except gametes), so randomly knocking out a gene on one chromosome still leaves its counterpart functional, making it difficult to tease apart a gene’s function. Knocking out both copies of the gene requires targeted point mutations—a more difficult process compared to random mutagenesis.
“If you take our haploid embryonic stem cells and you introduce a mutation in a gene, you can immediately assess what the loss of gene function does to the cell,” Wutz said.
To make the haploid stem cells, Wutz and his colleague and Martin Leeb stimulated unfertilized eggs to begin dividing in vitro. While many cells simply converted to diploidy by not dividing into two cells at the final stage in cell division, a small percentage remained haploid as they divided into a multicellular embryo. From those cells, they then derived haploid blastocysts, extracted stem cells from the inner part of the embryo and grew them in culture.
As a demonstration of how the technique could be used, the team transfected the haploid cell lines with a transposon and a plasmid to induce mutation. They then put the cells in a toxic broth that normally kills cells with functioning mismatch repair genes. After a while, several colonies became resistant to the chemicals, and subsequent analysis revealed two new mutations that were responsible.
The team also injected the haploid cells back into mouse embryos and produced chimeric animals that express the haploid cell lines in their fur and other parts of their bodies. Mouse embryos could be used to study how gene networks work during mammalian development, Wutz said.
Cells still had a tendency to revert back to their natural state of having two sets of chromosomes, and as the cell lines are passaged, the number of diploid cells tends to grow, Wutz said. But that can easily be dealt with by sorting cells by staining their chromosomes and then counting the number in each cell, he added.
Wutz, A., and Leeb, M. “Derivation of haploid embryonic stem cells from mouse embryos,” Nature, doi:10.1038/nature10448, 2011.