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UK Duo Generates Haploid Mouse Stem Cells

By a GenomeWeb staff reporter

NEW YORK (GenomeWeb News) – A pair of researchers from the Wellcome Trust Centre for Stem Cell Research at the University of Cambridge has developed the first haploid mammalian embryonic stem cells by nabbing stem cells from mouse embryos created using unfertilized mouse eggs.

As they reported online today in Nature, Martin Leeb and Anton Wutz were able to sort through mouse embryonic stem cells derived from mouse embryos made from unfertilized eggs to find those containing one copy of each chromosome rather than two. They subsequently found that the embryonic stem cell lines made from these haploid cells appear to maintain their developmental potential, though they typically diploidize to form chimeric embryos. From their findings so far, the researchers are optimistic that such cell lines may become a tool for future genetic screening studies.

"Any genetic change we introduce to the single set of chromosomes will have an easy-to-determine effect," Wutz, the study's corresponding author, said in a statement. "This will be useful for exploring in a systematic way the signaling mechanisms within [a] cell and how networks of genes regulate development."

The availability of a haploid zebrafish system has made it possible to do more simple genetic screens in that vertebrate model organism, Leeb and Wutz explained, since each gene is present just once in the genome. But researchers have been less successful in their previous efforts to coax chromosome copies away from mice, a mammalian model animal.

For the current study, the duo used strontium chloride to activate unfertilized mouse eggs, generating mouse embryos containing individual copies of each chromosome. From there, they plucked blastocyst cells from the embryos, using them to generate dozens of embryonic stem cell lines.

After expanding these lines and tossing out those that they found to be diploid by flow cytometry, the researchers were left with six embryonic stem cell lines in which at least 10 percent of cells were haploid. They then enriched for these haploid cells and expanded the cultures again to get haploid embryonic mouse stem cell lines.

By using this approach in several mouse genetic backgrounds, the team generated 25 haploid stem cell lines. Through a series of experiments, Leeb and Wutz showed that these lines had similar colony morphology, genetic integrity, and copy number variation profiles as typical mouse embryonic stem cell lines. The researchers also identified pluripotency markers and embryonic stem cell-like expression patterns in the haploid lines.

Meanwhile, their developmental experiments using these haploid embryonic stem cells suggest that they frequently form chimeric embryos that continue to develop after this diploidization.

"We have observed rapid diploidization when haploid [embryonic stem] cells differentiate," the study authors wrote. "The resulting diploid parthenogenetic cells can contribute to development."

Through a pilot screen for genes involved in mismatch repair, the researchers showed that the haploid embryonic stem cells also hold promise for helping to find mutations affecting autosomal genes, hinting that they may pave the way for a better understanding of mammalian genetics in general.

"This technique will help scientists overcome some of the significant barriers that have so far made studying the functions of genes so difficult," Wellcome Trust molecular and physiological sciences head Michael Dunn, who was not involved in the study, said in a statement. "This is often the first step towards understanding why mutations lead to disease and, ultimately, to developing new drugs treatments."

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