RESEARCH HIGHLIGHTS STEM CELLS Fatherless stem cells Two independent research groups derive cells do not sur- haploid mouse embryonic stem cells and vive in culture and propose their use for genetic screens. postulated that this The two copies of the mammalian autoso- might be because mal genome, one inherited from the father the cells mistakenly and the other from the mother, are important inactivate their sin- for normal life, but they complicate genetic gle X chromosome. experiments. To study a recessive mutation “We had a very implicated in disease, for instance, one must nice theory before Haploid mouse embryonic stem cells. Chromosome spread of a haploid mouse change both copies of the gene sequence. If we actually did the embryonic stem cell line and chimeric mice derived in part from the same one had haploid cells in hand, it would make experiment,” Wutz cell line. Reproduced from Nature; image courtesy of Anton Wutz. life in the laboratory much easier. laughs. This is what drove Ulrich Elling and Josef What the researchers instead saw, inde- practical implications: it means that the cells Penninger at the Institute of Molecular pendently of Penninger and colleagues, was can be mutagenized in the haploid state and Biotechnology of the Austrian Academy of that many outgrowths from parthenogenetic then turned into homozygous diploids for Sciences, Vienna, to revisit early attempts at mouse embryos in fact had a sizeable fraction phenotypic analysis. This has the advan- deriving such cells. It had long been thought of cells with only a single copy of the genome. tage that the cells studied are more akin to that the haploid state is incompatible with Haploidy, in other words, is not as lethal to normal mammalian cells, but the system is stable mammalian cells; surviving cells were mammalian cells as it had previously been still an artificial one, as Penninger and Wutz typically seen to turn diploid. But, especially thought. The gentle conditions (so-called 2i both emphasize (cells could aquire genetic because Tijn Brummelkamp and colleagues conditions, in which two signaling pathways changes during diploidization, and pater- had reported almost-haploid human tumor are inhibited) that Wutz and colleagues used nally imprinted genes will not be expressed); cells, Elling and Penninger thought it was to derive the embryonic stem cells may have experiments must therefore be designed © 2012 Nature America, Inc. All rights reserved. America, Inc. © 2012 Nature worth looking into this problem again. contributed to the high percentage of haploid accordingly. Penninger and colleagues, “If you look at some of [Nobel Prize cells in their cultures (Leeb & Wutz, 2011). for instance, separate the effects of specific laureate] Martin Evans’ old papers,” says Both the Vienna and the Cambridge mutations from potential background effects Penninger, “they had haploid cells in there; groups could mutagenize the haploid cells, by generating mutant alleles that are revers- npg the problem was they could not maintain the former with retroviral vectors and the lat- ible with Cre recombinase. them. Basically we did again what Martin ter with transposon-based vectors, and indi- Undoubtedly there are genome-wide did 28 years ago but we had a FACS sorter.” cate that the cells will be useful for forward resources of mutant haploid mouse embry- The researchers used standard protocols to genetic screens. Both groups also note that, onic stem cells on the horizon. In parallel derive mouse embryos parthenogenetically although the cells stay haploid for a few pas- with the efforts of the mouse gene-targeting (without fertilization), grew embryonic stem sages in culture, it is necessary to sort out the consortia, and with cell collections for selec- cells from these parthenotes and then used diploid cells that accumulate. What is more, tion-free screening of homozygous mutant FACS to sort out the small remaining frac- differentiation results in ‘diploidization’ of the mouse embryonic stem cells (Horie et al., tion of haploid cells. They found that, with cells. This happens both in vitro, if the cells 2011), this will provide exciting possibilities periodic sorting, they could maintain hap- are differentiated in embryoid bodies, and in for the cellular study of mammalian gene loid mouse embryonic stem cell lines (Elling vivo, if the cells are used to generate chimeric function. et al., 2011). mice. Critically, once they have diploidized, Natalie de Souza Anton Wutz and Martin Leeb at the the cells are apparently broadly functional Wellcome Trust Center for Stem Cell because they are capable of contributing to RESEARCH PAPERS Elling, U. et al. Forward and reverse genetics through Research, University of Cambridge, came many tissue types in the mouse. derivation of haploid mouse embryonic stem cells. Cell at the problem from a different angle; they Although haploid cell biology is itself Stem Cell 9, 563–574 (2011). thought that haploid mouse cells might be a likely to be a rich research seam, the most Horie, K. et al. A homozygous mutant embryonic stem useful system in which to study the way cells immediate use of these cells will be in unbi- cell bank applicable for phenotype-driven genetic screening. Nat. Methods 8, 1071–1077 (2011). count their X chromosomes. They were also ased screens and other more targeted stud- Leeb, M. & Wutz, A. Derivation of haploid embryonic influenced, explains Wutz, by the prevailing ies of gene function. In this context, the ten- stem cells from mouse embryos. Nature 479, 131–134 notion that normal haploid mammalian dency of the cells to diploidize has important (2011). NATURE METHODS | VOL.9 NO.2 | FEBRUARY 2012 | 121.