Mechanistic insights into the survival curve of HeLa cells with a short shoulder and their S phase-specific sensitivity†.

HeLa cells are a cell line with two unique cellular features: a short-shouldered survival curve and two peaks of radioresistance during the cell cycle phase, while their underlying mechanisms remain unclear. We herein proposed that these radiobiological features are due to a common mechanism by which radiation suppresses homologous recombination repair (HRR) in a dose-dependent manner. This radio-suppression of HRR is mediated by an intra-S checkpoint and reduces survivals of cells in S phase, especially early S phase, resulting in both short shoulder and radioresistance with two peaks in the cell cycle. This new explanation may not be limited to HeLa cells since a similar close association of these features is also observed in other type of cells.

Prediction of late adverse events in pelvic cancer patients receiving definitive radiotherapy using radiation-induced gamma-H2AX foci assay.

Radiation can induce DNA double-stranded breaks, which are typically detected by the fluorescence of phosphorylated histone H2AX. In this study, we examined the usefulness of the dynamics of radiation-induced gamma-H2AX foci of peripheral blood lymphocytes (PBLs), as a marker of DNA repair ability, in predicting late adverse events from radiotherapy. A total of 46 patients with cervical, vaginal and anal canal cancers treated with radical radiotherapy between 2014 and 2019 were included in this analysis. Concurrent chemotherapy was administered in 36 cases (78.3%). Peripheral blood was obtained before treatment, and then irradiated ex vivo with 1 Gy X-ray. The ratio of radiation-induced gamma-H2AX foci in PBLs measured at 30 min and at 4 h was defined as the foci decay ratio (FDR). With a median follow-up of 54 months, 9 patients (19.6%) were observed to have late genitourinary or gastrointestinal (GU/GI) toxicity. The FDR ranged from 0.51 to 0.74 (median 0.59), with a significantly higher incidence of Grade 1 or higher late adverse events in the FDR ≥ 0.59 group. In multivariate analysis, FDR ≥ 0.59 and hypertension also emerged as significant factors associated with the development of late toxicities. Overall, our results suggest that measurement of radiation-induced gamma-H2AX foci in PBLs may predict the risk of late GU/GI toxicities from chemoradiotherapy, which can enable tailoring the radiation dose to minimize adverse effects.

DOSE AND DOSE-RATE DEPENDENCE OF DSB-TYPE MUTANTS INDUCED BY X-RAYS OR TRITIUM BETA-RAYS: AN APPROACH USING A HYPERSENSITIVE SYSTEM.

To evaluate biological effects triggered by low levels of radiation, we established a uniquely sensitive experimental system to detect somatic mutations. By using the system, we found that mutant frequencies induced by X-rays were statistically significant at doses over 0.15 Gy, and a linear dose relationship with the mutant frequency was observed at doses over 0.15 Gy. The mutation spectra analysis revealed that mutation events generated by X-ray doses below 0.1 Gy were similar to those observed in unirradiated controls. In addition, a significant inflection point for both, the mutant frequency and the mutation spectra, was found at dose-rates around 11 mGy/day when cells were cultured in medium containing tritiated water. Because induced radiation-type events presented a clear dose/dose-rate dependency above the critical dose or the inflection point, these observations suggest that mutation events generated by radiation could change at a threshold dose-rate or a critical dose.

NBS1 I171V variant underlies individual differences in chromosomal radiosensitivity within human populations.

Genetic information is protected against a variety of genotoxins including ionizing radiation (IR) through the DNA double-strand break (DSB) repair machinery. Genome-wide association studies and clinical sequencing of cancer patients have suggested that a number of variants in the DNA DSB repair genes might underlie individual differences in chromosomal radiosensitivity within human populations. However, the number of established variants that directly affect radiosensitivity is still limited. In this study, we performed whole-exome sequencing of 29 Japanese ovarian cancer patients and detected the NBS1 I171V variant, which is estimated to exist at a rate of approximately 0.15% in healthy human populations, in one patient. To clarify whether this variant indeed contributes to chromosomal radiosensitivity, we generated NBS1 I171V variant homozygous knock-in HCT116 cells and mice using the CRISPR/Cas9 system. Radiation-induced micronucleus formation and chromosomal aberration frequency were significantly increased in both HCT116 cells and mouse embryonic fibroblasts (MEFs) with knock-in of the NBS1 I171V variant compared with the levels in wild-type cells. These results suggested that the NBS1 I171V variant might be a genetic factor underlying individual differences in chromosomal radiosensitivity.

Health effects triggered by tritium: how do we get public understanding based on scientifically supported evidence?

The Commission for 'Corresponding to Radiation Disaster of the Japanese Radiation Research Society' formulated a description of potential health effects triggered by tritium. This was in response to the issue of discharging water containing tritium filtered by the Advanced Liquid Processing System (ALPS), generated and stored in Fukushima Daiichi Nuclear Power Station after the accident. In this review article, the contents of the description, originally provided in Japanese, which gives clear and detailed explanation about potential health effects triggered by tritium based on reliable scientific evidence in an understandable way for the public, were summarized. Then, additional information about biochemical or environmental behavior of organically bound tritium (OBT) were summarized in order to help scientists who communicate with general public.

Induction of somatic mutations by low concentrations of tritiated water (HTO): evidence for the possible existence of a dose-rate threshold.

Tritium is a low energy beta emitter and is discharged into the aquatic environment primarily in the form of tritiated water (HTO) from nuclear power plants or from nuclear fuel reprocessing plants. Although the biological effects of HTO exposures at significant doses or dose rates have been extensively studied, there are few reports concerning the biological effects of HTO exposures at very low dose rates. In the present study using a hyper-sensitive assay system, we investigated the dose rate effect of HTO on the induction of mutations. Confluent cell populations were exposed to HTO for a total dose of 0.2 Gy at dose rates between 4.9 mGy/day and 192 mGy/day by incubating cells in medium containing HTO. HTO-induced mutant frequencies and mutation spectra were then investigated. A significant inflection point for both the mutant frequency and mutation spectra was found between 11 mGy/day and 21.6 mGy/day. Mutation spectra analysis revealed that a mechanistic change in the nature of the mutation events occurred around 11 mGy/day. The present observations and published experimental results from oral administrations of HTO to mice suggest that a threshold dose-rate for HTO exposures might exist between 11 mGy/day and 21.6 mGy/day where the nature of the mutation events induced by HTO becomes similar to those seen in spontaneous events.

Mitotic cells can repair DNA double-strand breaks via a homology-directed pathway.

The choice of repair pathways of DNA double-strand breaks (DSBs) is dependent upon the cell cycle phases. While homologous recombination repair (HRR) is active between the S and G2 phases, its involvement in mitotic DSB repair has not been examined in detail. In the present study, we developed a new reporter assay system to detect homology-directed repair (HDR), a major pathway used for HRR, in combination with an inducible DSB-generation system. As expected, the maximal HDR activity was observed in the late S phase, along with minimal activity in the G1 phase and at the G1/S boundary. Surprisingly, significant HDR activity was observed in M phase, and the repair efficiency was similar to that observed in late S phase. HDR was also confirmed in metaphase cells collected with continuous colcemid exposure. ChIP assays revealed the recruitment of RAD51 to the vicinity of DSBs in M phase. In addition, the ChIP assay for gamma-H2AX and phosphorylated DNA-PKcs indicated that a part of M-phase cells with DSBs could proceed into the next G1 phase. These results provide evidence showing that a portion of mitotic cell DSBs are undoubtedly repaired through action of the HDR repair pathway.

Exploration of genetic basis underlying individual differences in radiosensitivity within human populations using genome editing technology.

DNA double-strand breaks (DSBs) induced by ionizing radiation (IR) are the initial and critical step in major alteration of genetic information and cell death. To prevent deleterious effects, DNA repair systems recognize and re-join DNA DSBs in human cells. It has been suggested that there are individual differences in radiosensitivity within human populations, and that variations in DNA repair genes might contribute to this heterogeneity. Because confounding factors, including age, gender, smoking, and diverse genetic backgrounds within human populations, also influence the cellular radiosensitivity, to accurately measure the effect of candidate genetic variations on radiosensitivity, it is necessary to use human cultured cells with a uniform genetic background. However, a reverse genetics approach in human cultured cells is difficult because of their low level of homologous recombination. Engineered endonucleases used in genome editing technology, however, can enable the local activation of DNA repair pathways at the human genome target site to efficiently introduce genetic variations of interest into human cultured cells. Recently, we used this technology to demonstrate that heterozygous mutations of the ATM gene, which is responsible for a hyper-radiosensitive genetic disorder, ataxia-telangiectasia, increased the number of chromosomal aberrations after IR. Thus, understanding the heterozygous mutations of radiosensitive disorders should shed light on the genetic basis underlying individual differences in radiosensitivity within human populations.

Induction of somatic mutations by low-dose X-rays: the challenge in recognizing radiation-induced events.

It is difficult to distinguish radiation-induced events from spontaneous events during induction of stochastic effects, especially in the case of low-dose or low-dose-rate exposures. By using a hypersensitive system for detecting somatic mutations at the HPRT1 locus, we investigated the frequency and spectrum of mutations induced by low-dose X-rays. The mutant frequencies induced by doses of >0.15 Gy were statistically significant when compared with the spontaneous frequency, and a clear dose dependency was also observed for mutant frequencies at doses of >0.15 Gy. In contrast, mutant frequencies at doses of <0.1 Gy occurred at non-significant levels. The mutation spectrum in HPRT-deficient mutants revealed that the type of mutations induced by low-dose exposures was similar to that seen in spontaneous mutants. An apparent change in mutation type was observed for mutants induced by doses of >0.2 Gy. Our observations suggest that there could be a critical dose for mutation induction at between 0.1 Gy and 0.2 Gy, where mutagenic events are induced by multiple DNA double-strand breaks (DSBs). These observations also suggest that low-dose radiation delivered at doses of <0.1 Gy may not result in DSB-induced mutations but may enhance spontaneous mutagenesis events.

Evaluation of ATM heterozygous mutations underlying individual differences in radiosensitivity using genome editing in human cultured cells.

Ionizing radiation (IR) induces DNA double-strand breaks (DSBs), which are an initial step towards chromosomal aberrations and cell death. It has been suggested that there are individual differences in radiosensitivity within human populations, and that the variations in DNA repair genes might determine this heterogeneity. However, it is difficult to quantify the effect of genetic variants on the individual differences in radiosensitivity, since confounding factors such as smoking and the diverse genetic backgrounds within human populations affect radiosensitivity. To precisely quantify the effect of a genetic variation on radiosensitivity, we here used the CRISPR-ObLiGaRe (Obligate Ligation-Gated Recombination) method combined with the CRISPR/Cas9 system and a nonhomologous end joining (NHEJ)-mediated knock-in technique in human cultured cells with a uniform genetic background. We generated ATM heterozygous knock-out (ATM +/-) cell clones as a carrier model of a radiation-hypersensitive autosomal-recessive disorder, ataxia-telangiectasia (A-T). Cytokinesis-blocked micronucleus assay and chromosome aberration assay showed that the radiosensitivity of ATM +/- cell clones was significantly higher than that of ATM +/+ cells, suggesting that ATM gene variants are indeed involved in determining individual radiosensitivity. Importantly, the differences in radiosensitivity among the same genotype clones were small, unlike the individual differences in fibroblasts derived from A-T-affected family members.

Mutations in the FHA-domain of ectopically expressed NBS1 lead to radiosensitization and to no increase in somatic mutation rates via a partial suppression of homologous recombination.

Ionizing radiation induces DNA double-strand breaks (DSBs). Mammalian cells repair DSBs through multiple pathways, and the repair pathway that is utilized may affect cellular radiation sensitivity. In this study, we examined effects on cellular radiosensitivity resulting from functional alterations in homologous recombination (HR). HR was inhibited by overexpression of the forkhead-associated (FHA) domain-mutated NBS1 (G27D/R28D: FHA-2D) protein in HeLa cells or in hamster cells carrying a human X-chromosome. Cells expressing FHA-2D presented partially (but significantly) HR-deficient phenotypes, which were assayed by the reduction of gene conversion frequencies measured with a reporter assay, a decrease in radiation-induced Mre11 foci formation, and hypersensitivity to camptothecin treatments. Interestingly, ectopic expression of FHA-2D did not increase the frequency of radiation-induced somatic mutations at the HPRT locus, suggesting that a partial reduction of HR efficiency has only a slight effect on genomic stability. The expression of FHA-2D rendered the exponentially growing cell population slightly (but significantly) more sensitive to ionizing radiation. This radiosensitization effect due to the expression of FHA-2D was enhanced when the cells were irradiated with split doses delivered at 24-h intervals. Furthermore, enhancement of radiation sensitivity by split dose irradiation was not seen in contact-inhibited G0/G1 populations, even though the cells expressed FHA-2D. These results suggest that the FHA domain of NBS1 might be an effective molecular target that can be used to induce radiosensitization using low molecular weight chemicals, and that partial inhibition of HR might improve the effectiveness of cancer radiotherapy.

Histone chaperone FACT regulates homologous recombination by chromatin remodeling through interaction with RNF20.

The E3 ubiquitin ligase RNF20 regulates chromatin structure through ubiquitylation of histone H2B, so that early homologous recombination repair (HRR) proteins can access the DNA in eukaryotes during repair. However, it remains unresolved how RNF20 itself approaches the DNA in the presence of chromatin structure. Here, we identified the histone chaperone FACT as a key protein in the early steps of HRR. Depletion of SUPT16H, a component of FACT, caused pronounced defects in accumulations of repair proteins and, consequently, decreased HRR activity. This led to enhanced sensitivity to ionizing radiation (IR) and mitomycin-C in a fashion similar to RNF20-deficient cells, indicating that SUPT16H is essential for RNF20-mediated pathway. Indeed, SUPT16H directly bound to RNF20 in vivo, and mutation at the RING-finger domain in RNF20 abolished its interaction and accumulation, as well as that of RAD51 and BRCA1, at sites of DNA double-strand breaks (DSBs), whereas the localization of SUPT16H remained intact. Interestingly, PAF1, which has been implicated in transcription as a mediator of FACT and RNF20 association, was dispensable for DNA-damage-induced interaction of RNF20 with SUPT16H. Furthermore, depletion of SUPT16H caused pronounced defects in RNF20-mediated H2B ubiquitylation and thereby, impaired accumulation of the chromatin remodeling factor SNF2h. Consistent with this observation, the defective phenotypes of SUPT16H were effectively counteracted by enforced nucleosome relaxation. Taken together, our results indicate a primary role of FACT in RNF20 recruitment and the resulting chromatin remodeling for initiation of HRR.

Effects of depletion of dihydropyrimidine dehydrogenase on focus formation and RPA phosphorylation.

Gimeracil, an inhibitor of dihydropyrimidine dehydrogenase (DPYD), partially inhibits homologous recombination (HR) repair and has a radiosensitizing effect as well as enhanced sensitivity to Camptothecin (CPT). DPYD is the target protein for radiosensitization by Gimeracil. We investigated the mechanisms of sensitization of radiation and CPT by DPYD inhibition using DLD-1 cells treated with siRNA for DPYD. We investigated the focus formation of various kinds of proteins involved in HR and examined the phosphorylation of RPA by irradiation using Western blot analysis. DPYD depletion by siRNA significantly restrained the formation of radiation-induced foci of Rad51 and RPA, whereas it increased the number of foci of NBS1. The numbers of colocalization of NBS1 and RPA foci in DPYD-depleted cells after radiation were significantly smaller than in the control cells. These results suggest that DPYD depletion is attributable to decreased single-stranded DNA generated by the Mre11/Rad50/NBS1 complex-dependent resection of DNA double-strand break ends. The phosphorylation of RPA by irradiation was partially suppressed in DPYD-depleted cells, suggesting that DPYD depletion may partially inhibit DNA repair with HR by suppressing phosphorylation of RPA. DPYD depletion showed a radiosensitizing effect as well as enhanced sensitivity to CPT. The radiosensitizing effect of DPYD depletion plus CPT was the additive effect of DPYD depletion and CPT. DPYD depletion did not have a cell-killing effect, suggesting that DPYD depletion may not be so toxic. Considering these results, the combination of CPT and drugs that inhibit DPYD may prove useful for radiotherapy as a method of radiosensitization.

Yield of single- and double-strand breaks and nucleobase lesions in fully hydrated plasmid DNA films irradiated with high-LET charged particles.

We measured the yield and spectrum of strand breaks and nucleobase lesions produced in fully hydrated plasmid DNA films to determine the linear energy transfer (LET) dependence of DNA damage induced by ion-beam irradiation in relation to the change in the atomic number of ions. The yield of isolated damage was revealed as a decrease in prompt SSBs with increasing LET of He(2+), C(5+,6+) and Ne(8+,10+) ions. On the other hand, the yields of prompt DSBs increased with increasing ion LET. SSBs were additionally induced in ion-irradiated DNA film by treatment with two kinds of base excision repair proteins (glycosylases), Nth and Fpg, indicating that base lesions are produced in the hydrated DNA film. This result shows that nucleobase lesions are produced via both chemical reactions with diffusible water radicals, such as OH radicals, and direct energy deposition onto DNA and the hydrated water layer. Nth-sensitive sites deduced to be pyrimidine lesions, such as 5,6-dihydrothymine (DHT), showed a relatively larger yield than Fpg-sensitive sites deduced to be purine lesions, such as 7,8-dihydro-8-oxo-2'deoxyguanine (8-oxoGua), for all ion exposures tested. The yield of SSBs or DSBs observed by enzyme treatment decreased noticeably with increasing LET for all tested ions. These results indicated that higher-LET ions preferentially produce a complex type of damage that might compromise the activities of the glycosylases used in this study. These findings are biologically important since, under cell mimicking conditions, persistent DNA damage occurs in part due to direct energy deposition on the DNA or hydrated water shell that is specifically induced by dense ionization in the track.

NBS1 recruits RAD18 via a RAD6-like domain and regulates Pol η-dependent translesion DNA synthesis.

Translesion DNA synthesis, a process orchestrated by monoubiquitinated PCNA, is critical for DNA damage tolerance. While the ubiquitin-conjugating enzyme RAD6 and ubiquitin ligase RAD18 are known to monoubiquitinate PCNA, how they are regulated by DNA damage is not fully understood. We show that NBS1 (mutated in Nijmegen breakage syndrome) binds to RAD18 after UV irradiation and mediates the recruitment of RAD18 to sites of DNA damage. Disruption of NBS1 abolished RAD18-dependent PCNA ubiquitination and Polη focus formation, leading to elevated UV sensitivity and mutation. Unexpectedly, the RAD18-interacting domain of NBS1, which was mapped to its C terminus, shares structural and functional similarity with the RAD18-interacting domain of RAD6. These domains of NBS1 and RAD6 allow the two proteins to interact with RAD18 homodimers simultaneously and are crucial for Polη-dependent UV tolerance. Thus, in addition to chromosomal break repair, NBS1 plays a key role in translesion DNA synthesis.

Gimeracil, an inhibitor of dihydropyrimidine dehydrogenase, inhibits the early step in homologous recombination.

Gimeracil (5-chloro-2, 4-dihydroxypyridine) is an inhibitor of dihydropyrimidine dehydrogenase (DPYD), which degrades pyrimidine including 5-fluorouracil in the blood. Gimeracil was originally added to an oral fluoropyrimidine derivative S-1 to yield prolonged 5-fluorouracil concentrations in serum and tumor tissues. We have already reported that gimeracil had radiosensitizing effects by partially inhibiting homologous recombination (HR) in the repair of DNA double strand breaks. We investigated the mechanisms of gimeracil radiosensitization. Comet assay and radiation-induced focus formation of various kinds of proteins involved in HR was carried out. siRNA for DPYD were transfected to HeLa cells to investigate the target protein for radiosensitization with gimeracil. SCneo assay was carried out to examine whether DPYD depletion by siRNA inhibited HR repair of DNA double strand breaks. Tail moments in neutral comet assay increased in gimeracil-treated cells. Gimeracil restrained the formation of foci of Rad51 and replication protein A (RPA), whereas it increased the number of foci of Nbs1, Mre11, Rad50, and FancD2. When HeLa cells were transfected with the DPYD siRNA before irradiation, the cells became more radiosensitive. The degree of radiosensitization by transfection of DPYD siRNA was similar to that of gimeracil. Gimeracil did not sensitize DPYD-depleted cells. Depletion of DPYD by siRNA significantly reduced the frequency of neopositive clones in SCneo assay. Gimeracil partially inhibits the early step in HR. It was found that DPYD is the target protein for radiosensitization by gimeracil. The inhibitors of DPYD, such as gimeracil, could enhance the efficacy of radiotherapy through partial suppression of HR-mediated DNA repair.

Regulation of homologous recombination by RNF20-dependent H2B ubiquitination.

The E3 ubiquitin ligase RNF20 regulates chromatin structure by monoubiquitinating histone H2B in transcription. Here, we show that RNF20 is localized to double-stranded DNA breaks (DSBs) independently of H2AX and is required for the DSB-induced H2B ubiquitination. In addition, RNF20 is required for the methylation of H3K4 at DSBs and the recruitment of the chromatin-remodeling factor SNF2h. Depletion of RNF20, depletion of SNF2h, or expression of the H2B mutant lacking the ubiquitination site (K120R) compromises resection of DNA ends and recruitment of RAD51 and BRCA1. Consequently, cells lacking RNF20 or SNF2h and cells expressing H2B K120R exhibit pronounced defects in homologous recombination repair (HRR) and enhanced sensitivity to radiation. Finally, the function of RNF20 in HRR can be partially bypassed by forced chromatin relaxation. Thus, the RNF20-mediated H2B ubiquitination at DSBs plays a critical role in HRR through chromatin remodeling.

Gimeracil sensitizes cells to radiation via inhibition of homologous recombination.

BACKGROUND AND PURPOSE: 5-Chloro-2,4-dihydroxypyridine (Gimeracil) is a component of an oral fluoropyrimidine derivative S-1. Gimeracil is originally added to S-1 to yield prolonged 5-FU concentrations in tumor tissues by inhibiting dihydropyrimidine dehydrogenase, which degrades 5-FU. We found that Gimeracil by itself had the radiosensitizing effect. METHODS AND MATERIALS: We used various cell lines deficient in non-homologous end-joining (NHEJ) or homologous recombination (HR) as well as DLD-1 and HeLa in clonogenic assay. gamma-H2AX focus formation and SCneo assay was performed to examine the effects of Gimeracil on DNA double strand break (DSB) repair mechanisms. RESULTS: Results of gamma-H2AX focus assay indicated that Gimeracil inhibited DNA DSB repair. It did not sensitize cells deficient in HR but sensitized those deficient in NHEJ. In SCneo assay, Gimeracil reduced the frequency of neo-positive clones. Additionally, it sensitized the cells in S-phase more than in G0/G1. CONCLUSIONS: Gimeracil inhibits HR. Because HR plays key roles in the repair of DSBH caused by radiotherapy, Gimeracil may enhance the efficacy of radiotherapy through the suppression of HR-mediated DNA repair pathways.

Homologous recombination but not nucleotide excision repair plays a pivotal role in tolerance of DNA-protein cross-links in mammalian cells.

DNA-protein cross-links (DPCs) are unique among DNA lesions in their unusually bulky nature. The steric hindrance imposed by cross-linked proteins (CLPs) will hamper DNA transactions, such as replication and transcription, posing an enormous threat to cells. In bacteria, DPCs with small CLPs are eliminated by nucleotide excision repair (NER), whereas oversized DPCs are processed exclusively by RecBCD-dependent homologous recombination (HR). Here we have assessed the roles of NER and HR for DPCs in mammalian cells. We show that the upper size limit of CLPs amenable to mammalian NER is relatively small (8-10 kDa) so that NER cannot participate in the repair of chromosomal DPCs in mammalian cells. Moreover, CLPs are not polyubiquitinated and hence are not subjected to proteasomal degradation prior to NER. In contrast, HR constitutes the major pathway in tolerance of DPCs as judged from cell survival and RAD51 and gamma-H2AX nuclear foci formation. Induction of DPCs results in the accumulation of DNA double strand breaks in HR-deficient but not HR-proficient cells, suggesting that fork breakage at the DPC site initiates HR and reactivates the stalled fork. DPCs activate both ATR and ATM damage response pathways, but there is a time lag between two responses. These results highlight the differential involvement of NER in the repair of DPCs in bacterial and mammalian cells and demonstrate the versatile and conserved role of HR in tolerance of DPCs among species.

Two major factors involved in the reverse dose-rate effect for somatic mutation induction are the cell cycle position and LET value.

To study mechanisms which could be involved in the reverse dose rate effect observed during mutation induction after exposure to high LET radiation, synchronized mouse L5178Y cells were exposed to carbon 290 MeV/n beams with different LET values at the G2/M, G1, G1/S or S phases in the cell cycle. The frequency of Hprt-deficient (6-thioguanine-resistant) mutant induction was subsequently determined. The results showed that after exposure to high LET value radiation (50.8 and 76.5 keV/microm), maximum mutation frequencies were seen at the G2/M phase, but after exposure to lower LET radiation (13.3 keV/microm), the highest mutation frequencies were observed at the G1 phase. The higher LET beam always produced higher mutation frequencies in the G2/M phase than in the G1 phase, regardless of radiation dose. These results suggest that cells in the G2/M phase is hyper-sensitive for mutation induction from high LET radiation, but not to mutation induction from low LET radiation. Molecular analysis of mutation spectra showed that large deletions (which could include almost entire exons) of the mouse Hprt gene were most efficiently induced in G2/M cells irradiated with high LET radiation. These entire exon deletions were not as frequent in cells exposed to lower LET radiation. This suggests that inappropriate recombination repair might have occurred in response to condensed damage in condensed chromatin in the G2/M phase. In addition, by using a hyper-sensitive mutation detection system (GM06318-10 cells), a reverse dose-rate effect was clearly observed after exposure to carbon beams with higher LET values (66 keV/microm), but not after exposure to beams with lower LET values (13.3 keV/microm). Thus, G2/M sensitivity towards mutation induction, and the dependence on radiation LET values could both be major factors involved in the reverse dose rate effect produced by high LET radiation.