Two differentially spliced forms of topoisomerase IIalpha and beta mRNAs are conserved between birds and humans

Studying vertebrate topoisomerase 2 function using a conditional knockdown system in DT40 cells

DT40 is a B-cell lymphoma-derived avian cell line widely used to study cell autonomous gene function because of the high rates with which DNA constructs are homologously recombined into its genome. Here, we demonstrate that the power of the DT40 system can be extended yet further through the use of RNA interference as an alternative to gene targeting. We have generated and characterized stable DT40 transfectants in which both topo 2 genes have been in situ tagged using gene targeting, and from which the mRNA of both topoisomerase 2 isoforms can be conditionally depleted through the tetracycline-induced expression of short hairpin RNAs. The cell cycle phenotype of topo 2-depleted DT40 cells has been compared with that previously reported for other vertebrate cells depleted either of topo 2α through gene targeting, or depleted of both isoforms simultaneously by transient RNAi. In addition, the DT40 knockdown system has been used to explore whether excess catenation arising through topo 2 depletion is sufficient to trigger the G2 catenation (or decatenation) checkpoint, proposed to exist in differentiated vertebrate cells.

Common transcriptional signature of tumor-infiltrating mononuclear inflammatory cells and peripheral blood mononuclear cells in hepatocellular carcinoma patients

Hepatocellular carcinoma (HCC) is frequently associated with infiltrating mononuclear inflammatory cells. We performed laser capture microdissection of HCC-infiltrating and noncancerous liver-infiltrating mononuclear inflammatory cells in patients with chronic hepatitis C (CH-C) and examined gene expression profiles. HCC-infiltrating mononuclear inflammatory cells had an expression profile distinct from noncancerous liver-infiltrating mononuclear inflammatory cells; they differed with regard to genes involved in biological processes, such as antigen presentation, ubiquitin-proteasomal proteolysis, and responses to hypoxia and oxidative stress. Immunohistochemical analysis and gene expression databases suggested that the up-regulated genes involved macrophages and Th1 and Th2 CD4 cells. We next examined the gene expression profile of peripheral blood mononuclear cells (PBMC) obtained from CH-C patients with or without HCC. The expression profiles of PBMCs from patients with HCC differed significantly from those of patients without HCC (P < 0.0005). Many of the up-regulated genes in HCC-infiltrating mononuclear inflammatory cells were also differentially expressed by PBMCs of HCC patients. Analysis of the commonly up-regulated or down-regulated genes in HCC-infiltrating mononuclear inflammatory cells and PBMCs of HCC patients showed networks of nucleophosmin, SMAD3, and proliferating cell nuclear antigen that are involved with redox status, the cell cycle, and the proteasome system, along with immunologic genes, suggesting regulation of anticancer immunity. Thus, exploring the gene expression profile of PBMCs may be a surrogate approach for the assessment of local HCC-infiltrating mononuclear inflammatory cells.

mAMSA resistant human topoisomerase IIbeta mutation G465D has reduced ATP hydrolysis activity

Type II Human DNA Topoisomerases (topos II) play an essential role in DNA replication and transcription and are important targets for cancer chemotherapeutic drugs. Topoisomerase II causes transient double-strand breaks in DNA, forming a gate through which another double helix is passed, and acts as a DNA dependent ATPase. Mutations in topoII have been linked to atypical multi-drug resistance. Both human Topoisomerase II isoforms, α and β, are targeted by amsacrine. We have used a forced molecular evolution approach to identify mutations conferring resistance to acridines. Here we report mutation βG465D, which was selected with mAMSA and DACA and is cross-resistant to etoposide, ellipticine and doxorubicin. Resistance to mAMSA appears to decrease over time indicating a previously unreported resistance mechanism. G465D lies within the B′ domain in the region that contacts the cleaved gate helix. There is a 3-fold decrease in ATP affinity and ATP hydrolysis and an altered requirement for magnesium in decatenation assays. The decatenation rate is decreased for the mutated G465D protein. And we report for the first time the use of fluorescence anisotropy with intact human topoisomerase II.

Rapid exchange of mammalian topoisomerase II alpha at kinetochores and chromosome arms in mitosis

A stable cell line (GT2-LPk) derived from LLC-Pk was created in which endogenous DNA topoisomerase II alpha (topoII alpha) protein was downregulated and replaced by the expression of topoII alpha fused with enhanced green fluorescent protein (EGFP-topoII alpha). The EGFP-topoII alpha faithfully mimicked the distribution of the endogenous protein in both interphase and mitosis. In early stages of mitosis, EGFP-topoII alpha accumulated at kinetochores and in axial lines extending along the chromosome arms. During anaphase, EGFP-topoII alpha diminished at kinetochores and increased in the cytoplasm with a portion accumulating into large circular foci that were mobile and appeared to fuse with the reforming nuclei. These cytoplasmic foci appearing at anaphase were coincident with precursor organelles of the reforming nucleolus called nucleolus-derived foci (NDF). Photobleaching of EGFP-topoII alpha associated with kinetochores and chromosome arms showed that the majority of the protein rapidly exchanges (t1/2 of 16 s). Catalytic activity of topoII alpha was essential for rapid dynamics, as ICRF-187, an inhibitor of topoII alpha, blocked recovery after photobleaching. Although some topoII alpha may be stably associated with chromosomes, these studies indicate that the majority undergoes rapid dynamic exchange. Rapid mobility of topoII alpha in chromosomes may be essential to resolve strain imparted during chromosome condensation and segregation.

DNA topoisomerase II is essential for preimplantation mouse development

Topoisomerase II (topo II) is an essential enzyme that alters DNA topology. This activity is important for a variety of chromosome functions including DNA replication, transcription, recombination, and chromosome condensation and segregation. Previously we localized topo II in mouse gametes and preimplantation embryos using isoform-specific antibodies demonstrating the presence of the enzyme in oocytes and embryos, but not sperm. To probe functions of topo II during preimplantation development, we treated mouse zygotes with 100 nM teniposide, and assessed embryo morphology and DNA replication. Teniposide blocked cleavage in 69% embryos; the remainder cleaved once but had abnormal nuclei. Teniposide-treated embryos were devoid of topo II immunofluorescence. Teniposide also prevented DNA replication, implicating topo II in this process. Embryos treated with a 2 hr pulse of teniposide recovered and developed to the blastocyst stage, indicating 100 nM teniposide did not induce apoptosis. To more specifically analyze topo IIalpha function, we treated zygotes with topo IIalpha-targeted antisense oligodeoxynucleotides. Most zygotes arrested at the 2-cell stage while controls developed into blastocysts indicating topo IIalpha is essential for preimplantation development. The absence of topo IIalpha, but not beta immunofluorescence in antisense-treated embryos confirms the specificity and impact of the treatment. In addition, topo IIalpha is newly synthesized at the 2-cell stage. These results establish an essential function for topo II in mouse preimplantation embryonic development.

DNA topoisomerase II distribution in mouse preimplantation embryos

DNA topoisomerase II (topo II) is an essential enzyme that mediates a variety of chromosome activities including DNA replication, transcription, recombination, and chromosome condensation and segregation. Isoform-specific anti-topo II antibodies were used to determine the distribution of topo II alpha and beta in mouse gametes and embryos. Immunoblot analysis with two anti-topo IIalpha antibodies revealed that a 170 kDa topo IIalpha band was present in ovary and testis. Mature sperm exhibited an 89 kDa band only, which may be a degradation product of topo IIalpha. Immunoblots probed with a monoclonal antibody that recognizes both isoforms, showed bands at 170 and 180 kDa, which correspond to topo IIalpha and beta, respectively. An additional 100 kDa band was also present in ovary and testis. Mature sperm did not exhibit staining with this antibody. We also localized topo II in mouse gametes and embryos up to the blastocyst stage using immunofluorescence microscopy. While both isoforms were found in nuclei and nucleoli of germinal vesicle oocytes, topo IIalpha localized to metaphase chromosomes during meiosis, and only to nucleoli during embryonic interphase. Topo IIbeta was absent from chromosomes of metaphase II oocytes, but localized to embryonic interphase nuclei. Both full-length isoforms were absent from sperm, indicating topo II is stored maternally. These results identify topo II as an important component of mouse oocyte and embryonic chromatin, and suggest its involvement in oocyte maturation and preimplantation embryonic development. The different immunofluorescent staining patterns indicate topo IIalpha and beta may serve different roles during the embryonic cell cycle.