The LET-RBE spectra for cell killing for cultured mammalian cells exposed to accelerated heavy ions were investigated to design a spread-out Bragg peak beam for cancer therapy at HIMAC, National Institute of Radiological Sciences, Chiba, prior to clinical trials. Cells that originated from a human salivary gland tumor (HSG cells) as well as V79 and T1 cells were exposed to (3)He-, (12)C- and (20)Ne-ion beams with an LET ranging from approximately 20-600 keV/micrometer under both aerobic and hypoxic conditions. Cell survival curves were fitted by equations from the linear-quadratic model and the target model to obtain survival parameters. RBE, OER, alpha and D(0) were analyzed as a function of LET. The RBE increased with LET, reaching a maximum at around 200 keV/micrometer, then decreased with a further increase in LET. Clear splits of the LET-RBE or -OER spectra were found among ion species and/or cell lines. At a given LET, the RBE value for (3)He ions was higher than that for the other ions. The position of the maximum RBE shifts to higher LET values for heavier ions. The OER value was 3 for X rays but started to decrease at an LET of around 50 keV/micrometer, passed below 2 at around 100 keV/micrometer, and then reached a minimum above 300 keV/micrometer, but the values remained greater than 1. The OER was significantly lower for (3)He ions than the others.

Bistable epigenetic switches are fundamental for cell fate determination in unicellular and multicellular organisms. Regulatory proteins associated with bistable switches are often present in low numbers and subject to molecular noise. It is becoming clear that noise in gene expression can influence cell fate. Although the origins and consequences of noise have been studied, the stochastic and transient nature of RNA errors during transcription has not been considered in the origin or modeling of noise nor has the capacity for such transient errors in information transfer to generate heritable phenotypic change been discussed. We used a classic bistable memory module to monitor and capture transient RNA errors: the lac operon of Escherichia coli comprises an autocatalytic positive feedback loop producing a heritable all-or-none epigenetic switch that is sensitive to molecular noise. Using single-cell analysis, we show that the frequency of epigenetic switching from one expression state to the other is increased when the fidelity of RNA transcription is decreased due to error-prone RNA polymerases or to the absence of auxiliary RNA fidelity factors GreA and GreB (functional analogues of eukaryotic TFIIS). Therefore, transcription infidelity contributes to molecular noise and can effect heritable phenotypic change in genetically identical cells in the same environment. Whereas DNA errors allow genetic space to be explored, RNA errors may allow epigenetic or expression space to be sampled. Thus, RNA infidelity should also be considered in the heritable origin of altered or aberrant cell behaviour.

The biological effects of radiation originate principally in damages to DNA. DNA damages by X rays as well as heavy ions are induced by a combination of direct and indirect actions. The contribution of indirect action in cell killing can be estimated from the maximum degree of protection by dimethylsulfoxide (DMSO), which suppresses indirect action without affecting direct action. Exponentially growing Chinese hamster V79 cells were exposed to high-LET radiations of 20 to 2106 keV/mum in the presence or absence of DMSO and their survival was determined using a colony formation assay. The contribution of indirect action to cell killing decreased with increasing LET. However, the contribution did not reach zero even at very high LETs and was estimated to be 32% at an LET of 2106 keV/mum. Therefore, even though the radiochemically estimated G value of OH radicals was nearly zero at an LET of 1000 keV/mum, indirect action by OH radicals contributed to a substantial fraction of the biological effects of high-LET radiations. The RBE determined at a survival level of 10% increased with LET, reaching a maximum value of 2.88 at 200 keV/mum, and decreased thereafter. When the RBE was estimated separately for direct action (RBE(D)) and indirect action (RBE(I)); both exhibited an LET dependence similar to that of the RBE, peaking at 200 keV/mum. However, the peak value was much higher for RBE(D) (5.99) than RBE(I) (1.89). Thus direct action contributes more to the high RBE of high-LET radiations than indirect action does.

In eukaryotes, initiation of DNA replication is a strictly controlled process, so that chromosomal DNA is precisely duplicated once per cell cycle. Recent studies using different systems show that a number of proteins are involved in the initiation of DNA replication, a process that is largely conserved from yeast to human (1, 2). Current models indicate that initiation consists of two steps. In the first step, the origin recognition complex (ORC), 1 Cdc6, Cdt1, and Mcm2-7 proteins sequentially assemble on the replication origins to form the prereplicative complex, from late mitosis to early G 1 phase. The ORC, a complex of six proteins (Orc1-6), binds replication origins (3, 4) and recruits Cdc6. Cdc6 in turn coordinates with Cdt1 to load the Mcm2-7 complex (5-10), a presumed replicative helicase (11, 12), on the chromatin template. In the second step, Cdc7/Dbf4 kinase and S phase cyclin-dependent kinases (S-Cdks) activate the prereplicative complex and trigger DNA replication by loading Cdc45 onto each origin with programmed timing (13-16). Cdc45 facilitates assembly of the replication machinery by recruiting replication protein A and DNA polymerases (17, 18).Eukaryotes contain multiple parallel pathways to ensure that the prereplicative complex is not re-assembled until the segregation of chromosomes in mitosis. Cdc6 is either degraded via the ubiquitin-dependent pathway (19) or exported out of the nucleus (20). Phosphorylation of Mcm2-7 complex by cdc2 kinase initiates dissociation from chromatin during S phase (21). Cdt1 is regulated by protein expression as well as interactions with geminin to ensure that it is active only in the G 1 phase (22,23).Mcm10 (Dna43) was originally discovered in Saccharomyces cerevisiae while screening to identify other mcm mutants (24). Previous studies performed in S. cerevisiae suggest that Mcm10 has multiple roles in DNA replication. The mcm10 mutant is defective in initiation of DNA replication at the non-permissive temperature (24) and causes stalling of replication forks when the replication machinery passes through origins that do not fire (24). In addition, Mcm10 mediates the loading of the Mcm2-7 complex onto replication origins (25), and interacts genetically with Cdc45, DNA polymerase ␦ and ⑀, which are required for the elongation steps of DNA replication (26). Therefore, it appears that Mcm10 is involved in both origin activation and elongation, although the mechanisms by which the protein interacts with multiple replication factors at each step remain to be elucidated.Mcm10 homologs have additionally been discovered in Schizosaccharomyces pombe and Caenorhabditis elegans (25,27). Recently, we identified Drosophila and human homologs of Mcm10 and demonstrated that human Mcm10 interacts with the mammalian Orc2 and Mcm2-7 complex (28). We also confirmed that human Mcm10 binds chromatin during S phase and dissociates in G 2 phase (28), whereas yeast Mcm10 remains bound to chromatin throughout the cell cycle (25,26). To clarify the mechanism of regulation of Mcm10 funct...

Chromosomal double-strand breaks (DSBs) in mammalian cells are usually repaired through either of two pathways: end-joining (EJ) or homologous recombination (HR). To clarify the relative contribution of each pathway and the ensuing genetic changes, we developed a system to trace the fate of DSBs that occur in an endogenous single-copy human gene. Lymphoblastoid cell lines TSCE5 and TSCER2 are heterozygous (+/-) or compound heterozygous (-/-), respectively, for the thymidine kinase gene (TK), and we introduced an I-SceI endonuclease site into the gene. EJ for a DSB at the I-SceI site results in TK-deficient mutants in TSCE5 cells, while HR between the alleles produces TK-proficient revertants in TSCER2 cells. We found that almost all DSBs were repaired by EJ and that HR rarely contributes to the repair in this system. EJ contributed to the repair of DSBs 270 times more frequently than HR. Molecular analysis of the TK gene showed that EJ mainly causes small deletions limited to the TK gene. Seventy percent of the small deletion mutants analyzed showed 100- to 4,000-bp deletions with a 0- to 6-bp homology at the joint. Another 30%, however, were accompanied by complicated DNA rearrangements, presumably the result of sister-chromatid fusion. HR, on the other hand, always resulted in non-crossing-over gene conversion without any loss of genetic information. Thus, although HR is important to the maintenance of genomic stability in DNA containing DSBs, almost all chromosomal DSBs in human cells are repaired by EJ.

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Heavy-ion radiation accounts for the major component of absorbed cosmic radiation and is thus regarded as a significant risk during long-term manned space missions. To evaluate the genetic damage induced by heavy particle radiation, gpt delta transgenic mice were exposed to carbon particle irradiation and the induced mutations were compared with those induced by reference radiations, i.e., X-rays and gamma-rays. In the transgenic mouse model, deletions and point mutations were individually identified as Spi(-) and gpt mutations, respectively. Two days after 10 Gy of whole-body irradiation, the mutant frequencies (MFs) of Spi(-) and gpt were determined. Carbon particle irradiation significantly increased Spi(-) MF in the liver, spleen, and kidney but not in the testis, suggesting an organ-specific induction of mutations by heavy-ion irradiation. In the liver, the potency of inducing Spi(-) mutation was highest for carbon particles (3.3-fold increase) followed by X-rays (2.1-fold increase) and gamma-rays (1.3-fold increase), while the potency of inducing gpt mutations was highest for gamma-rays (3.3-fold increase) followed by X-rays (2.1-fold increase) and carbon particles (1.6-fold increase). DNA sequence analysis revealed that carbon particles induced deletions that were mainly more than 1,000 base pairs in size, whereas gamma-rays induced deletions of less than 100 base pairs and base substitutions. X-rays induced various-sized deletions and base substitutions. These results suggest that heavy-ion beam irradiation is effective at inducing deletions via DNA double-strand breaks but less effective than X-ray and gamma-ray irradiation at producing oxidative DNA damage by free radicals.

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