Blm-deficient Embryonic Stem Cells Research Articles

A Recessive Genetic Screen for Components of the RNA Interference Pathway Performed in Mouse Embryonic Stem Cells.

Several key components of the RNA interference (RNAi) pathway were identified in genetic screens performed in non-mammalian model organisms. To identify components of the mammalian RNAi pathway, we developed a recessive genetic screen in mouse embryonic stem (ES) cells. Recessive genetic screens are feasible in ES cells that are Bloom-syndrome protein deficient (Blm-deficient). We constructed a reporter cell line in Blm-deficient ES cells to isolate RNAi mutants using a simple drug selection scheme. This chapter describes how we used retroviral gene-traps to mutagenize the reporter cell line and select for RNAi mutants. Putative RNAi mutants were confirmed using a separate functional assay. The location of the gene-trap was then identified using molecular techniques such as splinkerette PCR. Our screening strategy successfully isolated several mutant clones of Argonaute 2, a vital component of the RNAi pathway.

Mannosidase 2, alpha 1 Deficiency Is Associated with Ricin Resistance in Embryonic Stem (ES) Cells

Host gene products required for mediating the action of toxins are potential targets for reversing or controlling their pathogenic impact following exposure. To identify such targets libraries of insertional gene-trap mutations generated with a PiggyBac transposon in Blm-deficient embryonic stem cells were exposed to the plant toxin, ricin. Resistant clones were isolated and genetically characterised and one was found to be a homozygous mutant of the mannosidase 2, alpha 1 (Man2α1) locus with a matching defect in the homologous allele. The causality of the molecular lesion was confirmed by removal of the transposon following expression of PB-transposase. Comparative glycomic and lectin binding analysis of the Man2α1 (−/−) ricin resistant cells revealed an increase in the levels of hybrid glycan structures and a reduction in terminal β-galactose moieties, potential target receptors for ricin. Furthermore, naïve ES cells treated with inhibitors of the N-linked glycosylation pathway at the mannosidase 2, alpha 1 step exhibited either full or partial resistance to ricin. Therefore, we conclusively identified mannosidase 2, alpha 1 deficiency to be associated with ricin resistance.

A piggyBac transposon-based genome-wide library of insertionally mutated Blm-deficient murine ES cells.

Cultured mouse or human embryonic stem (ES) cells provide access to all of the genes required to elaborate the fundamental components and physiological systems of a mammalian cell. Chemical or insertional mutagenesis of Blm-deficient mouse ES cells can be used to generate genome-wide libraries of homozygous mutant ES cells, which are the substrates for conducting phenotype-driven loss-of-function genetic screens. However, the existing insertional mutation libraries are limited by incomplete genomic coverage. In this study, we have explored the use of piggyBac (PB) transposon-mediated mutagenesis to extend the genomic coverage of mutation libraries in Blm-deficient ES cells. A library composed of 14,000 individual gene-trap clones was generated and a recessive genetic screen conducted to identify cells with defects in DNA mismatch repair (MMR) genes. Independent mutations in all known genes of the pathway Msh2, Msh6, Pms2, and Mlh1 were recovered in these screens. The genomic coverage in this library confirms its utility as a new genetic resource for conducting recessive genetic screens in mammalian cells.

A recessive genetic screen for host factors required for retroviral infection in a library of insertionally mutated Blm-deficient embryonic stem cells

A recessive genetic screen of an insertionally mutated Blm-/- ES cell library identifies host factors required for retroviral infection, and confirms that mCat-1 is the ecotropic murine leukaemia virus receptor in ES cells.

Mismatch repair genes identified using genetic screens in Blm-deficient embryonic stem cells.

Phenotype-driven recessive genetic screens in diploid organisms require a strategy to render the mutation homozygous. Although homozygous mutant mice can be generated by breeding, a reliable method to make homozygous mutations in cultured cells has not been available, limiting recessive screens in culture. Cultured embryonic stem (ES) cells provide access to all of the genes required to elaborate the fundamental components and physiological systems of a mammalian cell. Here we have exploited the high rate of mitotic recombination in Bloom's syndrome protein (Blm)-deficient ES cells to generate a genome-wide library of homozygous mutant cells from heterozygous mutations induced with a revertible gene trap retrovirus. We have screened this library for cells with defects in DNA mismatch repair (MMR), a system that detects and repairs base-base mismatches. We demonstrate the recovery of cells with homozygous mutations in known and novel MMR genes. We identified Dnmt1(ref. 5) as a novel MMR gene and confirmed that Dnmt1-deficient ES cells exhibit micro-satellite instability, providing a mechanistic explanation for the role of Dnmt1 in cancer. The combination of insertional mutagenesis in Blm-deficient ES cells establishes a new approach for phenotype-based recessive genetic screens in ES cells.

Eme1 is involved in DNA damage processing and maintenance of genomic stability in mammalian cells.

Yeast and human Eme1 protein, in complex with Mus81, constitute an endonuclease that cleaves branched DNA structures, especially those arising during stalled DNA replication. We identified mouse Eme1, and show that it interacts with Mus81 to form a complex that preferentially cleaves 3'-flap structures and replication forks rather than Holliday junctions in vitro. We demonstrate that Eme1-/- embryonic stem (ES) cells are hypersensitive to the DNA cross-linking agents mitomycin C and cisplatin, but only mildly sensitive to ionizing radiation, UV radiation and hydroxyurea treatment. Mammalian Eme1 is not required for the resolution of DNA intermediates that arise during homologous recombination processes such as gene targeting, gene conversion and sister chromatid exchange (SCE). Unlike Blm-deficient ES cells, increased SCE was seen only following induced DNA damage in Eme1-deficient cells. Most importantly, Eme1 deficiency led to spontaneous genomic instability. These results reveal that mammalian Eme1 plays a key role in DNA repair and the maintenance of genome integrity.