Endoreplication and its role in tumor development and progression / 中国肿瘤临床

Most diploid cells proliferate by proceeding through the canonical G1 (DNA pre-synthesis), S (DNA synthesis), G2 (DNA post-synthesis), and M(mitosis) phases of the cell cycle. However, there is another type of cell cycle that occurs frequently in both plants and animals, known as endoreplication. Endoreplication consists of alternating periods of G and S phases without cytokinesis, which results in polyploidy. It is indispensable for normal development, organ formation, and wound healing in humans. In recent years, con-siderable attention has been paid to delineating the connections of endoreplication with tumorigenesis and tumor progression. Here, we review the role of endoreplication in normal human development and discuss its possible role in tumor development and the un-derlying molecular mechanisms.

Cell cycle and DNA content of mitotic cells in brain ganglia of Drosophila larvae.

The programmes of replication of hetero- and euchromatin regions, mitotic cell cycle and the DNA content in metaphases in brain ganglia from late third instar larvae of Drosophila melanogaster (wild type and a tumour bearing mutant, 1(2)gl, strain) and of Drosophila nasuta were examined by autoradiography of [3H]thymidine labelled (continuous or pulse) cells and by cytophotometry, respectively. Brain ganglia labelled continuously with [3H]thymidine for 24 h in vitro showed a significantly high proportion of cells with incorporation of radioactivity restricted to heterochromatin only. Pulse labelling of brain ganglia from larvae of Drosophila melanogaster and Drosophila nasuta followed by chase for different time intervals showed that (i) the frequency of labelled metaphases was more than 50% within 15 to 30 min of chase and remained higher than 50% in nearly all the chase samples till 24 h, (ii) euchromatin labelled metaphases appeared with a low frequency within 1 to 4 h chase period but the heterochromatin labelled metaphases continued to be more common in the later chase samples also, (iii) single chromatid labelled second cycle metaphases were seen within 1 to 4 h after the pulse, but their frequency did not increase in the later samples. Cytophotometry of feulgen-DNA and Hoechst 33258 stained metaphases in late third instar larval brain ganglia revealed a greater variation in the DNA content of individual metaphases, although the means were close to the expected 4 C content. It appears that in relation to the known asymmetric cell divisions of neuroblast and other neural cells, the mitotically active cells in brain ganglia comprise a heterogenous population with widely varying lengths of the different phases of cell cycle; some of them may not cycle regularly and may possibly have a discontinuous S-phase.

In situ study of chorion gene amplification in ovarian follicle cells of Drosophila nasuta.

The temporal and spatial pattern of replication of chorion gene clusters in follicle cells during oogenesis in Drosophila melanogaster and Drosophila nasuta was examined by [3H] thymidine autoradiography and by in situ hybridization with chorion gene probes. When pulse labelled with [3H] thymidine, the follicle cells from stage 10-12 ovarian follicles of both Drosophila melanogaster and, Drosophila nasuta often showed intense labelling at only one or two sites per nucleus. In situ hybridization of chorion gene probes derived from Drosophila melanogaster with follicle cell nuclei of Drosophila melanogaster and Drosophila nasuta revealed these discrete [3H] thymidine labelled sites to correspond to the two amplifying chorion gene clusters. It appears, therefore, that in spite of evolutionary divergence, the organization and programme of selective amplification of chorion genes in ovarian follicle cells have remained generally similar in these two species. The endoreplicated and amplified copies of each chorion gene cluster remain closely associated but the two clusters occupy separate sites in follicle cell nucleus.