The biological functions of nuclear topoisomerase I (Top1) have been difficult to study because knocking out TOP1 is lethal in metazoans. To reveal the functions of human Top1, we have generated stable Top1 small interfering RNA (siRNA) cell lines from colon and breast carcinomas (HCT116-siTop1 and MCF-7-siTop1, respectively). In those clones, Top1 is reduced ∼5-fold and Top2α compensates for Top1 deficiency. A prominent feature of the siTop1 cells is genomic instability, with chromosomal aberrations and histone γ-H2AX foci associated with replication defects. siTop1 cells also show rDNA and nucleolar alterations and increased nuclear volume. Genome-wide transcription profiling revealed 55 genes with consistent changes in siTop1 cells. Among them, asparagine synthetase (ASNS) expression was reduced in siTop1 cells and in cells with transient Top1 down-regulation. Conversely, Top1 complementation increased ASNS, indicating a causal link between Top1 and ASNS expression. Correspondingly, pharmacologic profiling showed l-asparaginase hypersensitivity in the siTop1 cells. Resistance to camptothecin, indenoisoquinoline, aphidicolin, hydroxyurea, and staurosporine and hypersensitivity to etoposide and actinomycin D show that Top1, in addition to being the target of camptothecins, also regulates DNA replication, rDNA stability, and apoptosis. Overall, our studies show the pleiotropic nature of human Top1 activities. In addition to its classic DNA nicking-closing functions, Top1 plays critical nonclassic roles in genomic stability, gene-specific transcription, and response to various anticancer agents. The reported cell lines and approaches described in this article provide new tools to perform detailed functional analyses related to Top1 function. [Cancer Res 2007;67(18):8752–61]

The Fukushima Daiichi Nuclear Power Plant (FNPP) accident released large amounts of radioactive substances into the environment. In order to provide basic information for biokinetics of radionuclides and for dose assessment of internal exposure brought by the FNPP accident, we determined the activity concentration of radionuclides in the organs of 79 cattle within a 20-km radius around the FNPP. In all the specimens examined, deposition of Cesium-134 (¹³⁴Cs, half-life: 2.065 y) and ¹³⁷Cs (30.07 y) was observed. Furthermore, organ-specific deposition of radionuclides with relatively short half-lives was detected, such as silver-110m (^110mAg, 249.8 d) in the liver and tellurium-129m (^129mTe, 33.6 d) in the kidney. Regression analysis showed a linear correlation between the radiocesium activity concentration in whole peripheral blood (PB) and that in each organ. The resulting slopes were organ dependent with the maximum value of 21.3 being obtained for skeletal muscles (R² = 0.83, standard error (SE) = 0.76). Thus, the activity concentration of ¹³⁴ Cs and ¹³⁷Cs in an organ can be estimated from that in PB. The level of radioactive cesium in the organs of fetus and infants were 1.19-fold (R² = 0.62, SE = 0.12), and 1.51-fold (R² = 0.70, SE = 0.09) higher than that of the corresponding maternal organ, respectively. Furthermore, radiocesium activity concentration in organs was found to be dependent on the feeding conditions and the geographic location of the cattle. This study is the first to reveal the detailed systemic distribution of radionuclides in cattle attributed to the FNPP accident.

One difficulty in analyzing the damage response is that the effect of damage itself and that of cellular response are hard to distinguish in irradiated cells. In mouse zygotes, damage can be introduced by irradiated sperm, while damage response can be studied in the unirradiated maternal pronucleus. We have analyzed the p53-dependent damage responses in irradiated-sperm mouse zygotes and found that a p53-responsive reporter was efficiently activated in the female pronucleus. [³H]thymidine labeling experiments indicated that irradiated-sperm zygotes were devoid of G₁/S arrest, but pronuclear DNA synthesis was suppressed equally in male and female pronuclei. p53^−/− zygotes lacked this suppression, which was corrected by microinjection of glutathione S-transferase-p53 fusion protein. In contrast, p21^−/− zygotes exhibited the same level of suppression upon fertilization by irradiated sperm. About a half of the 6-Gy-irradiated-sperm zygotes managed to synthesize a full DNA content by prolonging S phase, while the other half failed to do so. Regardless of the DNA content, all the zygotes cleaved to become two-cell-stage embryos. These results revealed the presence of p53-dependent pronuclear cross talk and a novel function of p53 in the S-phase DNA damage checkpoint of mouse zygotes.

Fractionated radiotherapy (RT) is widely used in cancer therapy for its advantages in the preservation of normal tissues. However, repopulation of su