AT cells show dissimilar hypersensitivity to heavy-ion and X-rays irradiation.

Ataxia telangiectasia (AT) cells, with their defective double-strand break (DSB) repair processes, exhibit high sensitivity to low-LET radiation such as X-rays irradiation and gamma beams. Since heavy ion beam treatment for cancer is becoming increasingly common in Japan and elsewhere, it is important to also determine their sensitivity to high-LET radiation. For this purpose we irradiated AT and normal human cells immortalized with the human telomerase gene using high- (24-60 keV/microm carbon and 200 keV/microm iron ions) or low-LET (X-rays) radiation in non-proliferative conditions. In normal cells the RBE (relative biological effectiveness) of carbon and iron ions increased from 1.19 to 1.81 in proportion to LET. In contrast, their RBE in AT cells increased from 1.32 at 24 keV/microm to 1.59 at 40 keV/microm, and exhibited a plateau at over 40 keV/microm. In normal cells most gamma-H2AX foci induced by both carbon- and iron-ion beams had disappeared at 40 h. In AT cells, however, a significant number of gamma-H2AX foci were still observed at 40 h. The RBEs found in the AT cells after heavy-ion irradiation were consistent with the effects predicted from the presence of non-homologous end joining defects. The DSBs remaining after heavy-ion irradiation suggested defects in the AT cells' DSB repair ability.

Introduction of a normal human chromosome 8 corrects abnormal phenotypes of Werner syndrome cells immortalized by expressing an hTERT gene.

Werner syndrome (WS) is an autosomal recessive disease characterized by premature aging and caused by mutations of the WRN gene mapped at 8p12. To examine functional complementation of WS phenotypes, we introduced a normal human chromosome 8 into a strain of WS fibroblasts (WS3RGB) immortalized by expressing a human telomerase reverse transcriptase subunit (hTERT) gene. Here, we demonstrate that the abnormal WS phenotypes including cellular sensitivities to 4-nitroquinoline-1-oxide (4NQO) and hydroxy urea (HU), and chromosomal radiosensitivity at G(2) phase are corrected by expression of the WRN gene mediated by introducing a chromosome 8. This indicates that those multiple abnormal WS phenotypes are derived from a primary, but not secondary, defect in the WRN gene.

Opposite impact of NKG2D genotype by lifestyle exposure to risk of aerodigestive tract cancer among Japanese.

It was reported that there are 2 haplotypes in natural killer complex (NKC) region. One of them could be divided by NKG2D polymorphism into 2 haplotype alleles (high and low natural killer (NK) cell activity) and were associated with overall cancer risks. However, its impact on a specific cancer is unclear. Therefore, by a case-control study, we analyzed the association between NKG2D genotype and aerodigestive tract cancer risk. Subjects were 502 aerodigestive tract cancer patients (276 with head and neck, 226 with esophageal) and 1,004 sex-age matched noncancer controls. Exposures to 2 lifestyle factors, smoking and drinking, were evaluated by a self-administered questionnaire. The genotype of NKG2D was determined by the TaqMan method, and its impact was assessed by multivariable logistic regression models. Association strength was measured by the odds ratio (OR) and its confidence intervals (CI). An overall analysis revealed no statistically significant association between NKG2D genotype and the risk of aerodigestive tract cancer. However, we found protective effects of G allele among never smokers (OR 0.35; 95% CI 0.15-0.84) and never drinkers (0.42; 0.19-0.94). In contrary, increased risks were observed for G allele among heavy smokers (5.92; 3.23-10.85) and heavy drinkers (4.13; 2.29-7.47). Interactions between NKG2D genotype and lifestyle exposure were statistically significant (interaction p = 0.001 for smoking, 0.005 for drinking). The same trends were observed in both sexes, and in head and neck cancer and esophageal cancer independently. These results suggest an opposite impact of NKG2D genotype by lifestyle exposure to the risk of aerodigestive tract cancer among a Japanese population.

Decreased risk of colorectal cancer with the high natural killer cell activity NKG2D genotype in Japanese.

BACKGROUND: NKG2D is an immune receptor on natural killer (NK) and other cells active in the immune system. It recognizes ligands expressed on mainly transformed cells and plays a role in their elimination through the so-called 'cancer immune surveillance'. It was reported that there are two haplotypes of NKG2D, HNK1 (high NK activity) and LNK1 (low NK activity). Harboring the HNK1 is reported to reduce the overall cancer risk. To elucidate its impact on colorectal cancer (CRC), we conducted the present case-control study. METHOD: The subjects were 379 CRC patients and 1137 sex-age-matched non-cancer controls. Data on lifestyle factors including diet were obtained by self-administered questionnaire. The NKG2D genotypes (rs1049174: G-C, LNK1/LNK1:CC; LNK1/HNK1:CG and HNK1/HNK1:GG) were assessed by the TaqMan method. Associations were then assessed by multivariate logistic regression models, considering potential confounders. The measure of association was the odds ratio (OR) and its confidence intervals (CIs). RESULTS: We found a reduced risk of CRC with the NKG2D HNK1. Adjusted ORs were 0.77 for LNK1/HNK1 (95% CI: 0.60-0.99) and 0.48 for HNK1/HNK1 (0.32-0.72) relative to LNK1/LNK1. The same association was consistently observed with stratified analyses across all confounders except regular exercise and body mass index (BMI). Thus, the impact of harboring HNK1 was more evident among those with BMI >/= 25 and those exercising regularly, suggesting possible interactions between NKG2D genotype and these factors. CONCLUSION: We found that the HNK1 genotype, associated with high NK cell activity, might be an independent protective factor for CRC among the Japanese population. This possibility warrants further analysis.

Pretreatment with 5-FU enhances cisplatin cytotoxicity in head and neck squamous cell carcinoma cells.

PURPOSE: In the treatment of head and neck malignancy, cisplatin and 5-FU have been used the most as chemotherapeutic agents. The difference in efficacies of these is unclear and controversial. To investigate more effective schedule, we analyzed the cytotoxicity in different treatment sequence with two agents in vitro and the mechanism for different effectiveness. METHODS: UM-SCC-23 and UM-SCC-81B, head and neck squamous cell carcinoma cell lines, were analyzed for cellular killing in alternative sequence treatment with cisplatin and 5-FU. The treatment schedule was designed based on the clinical regimen. To determine the mechanism for the difference of cytotoxicity with each schedule, cell cycle distributions of both cells after 5-FU treatment with various durations were analyzed by flow-cytometry and immunostaining with anti-PCNA and anti-BrdU. RESULTS: 5-FU pretreatment followed by cisplatin treatment showed higher cell killing in both types of cells than the reverse treatment schedule. In the cell cycle analysis and immunostaining after the treatment of 5-FU, the rate of PCNA-positive cells was increased from 24 to 144 h in both cells. The rate of BrdU-positive cells of UM-SCC-81B in flow-cytometry was also increased, while that of UM-SCC-23 was gradually decreased. These data suggested that the cells treated with 5-FU for more than 144 h were still in the S-phase with or without DNA synthesis. CONCLUSIONS: In head and neck carcinoma cells, we showed 5-FU pretreatment enhanced cisplatin cytotoxicity. The result of cell cycle analysis and immunostaining showed S-phase arrest by treatment of prolonged 5-FU treatment. The very long arrest in S-phase might be a mechanism to enhance cisplatin cytotoxicity by 5-FU pretreatment. We thus suggest pretreatment with 5-FU to enhance the effectiveness of cisplatin-based chemotherapy.

Hypersensitivity to cisplatin after hRev3 mRNA knockdown in head and neck squamous cell carcinoma cells.

Resistance to platinum-based chemotherapy frequently poses a significant problem in the treatment of head and neck squamous cell carcinomas (HNSCCs). In this study, we isolated cisplatin-resistant UM-SCC-23 CDDP/R cells from a UM-SCC-23 head and neck squamous cell carcinoma cell line. The UM-SCC-23 CDDP/R cells were approximately 3.5-fold more resistant to cisplatin than the UM-SCC-23 cells. Translesion DNA synthesis (TLS) is one of the major pathways involved in post-replication repair. To ascertain whether TLS is involved in cisplatin resistance in human cancer cells, we analyzed expression of the Rev3 gene, which encodes the catalytic subunit of DNA polymerase ζ (Polζ), using quantitative PCR. Expression of hRev3 mRNA in the UM-SCC-23 CDDP/R cells was approximately 1.4-fold higher than in the UM-SCC-23 cells. In both types of cells, expression of hRev3 mRNA was suppressed using siRNAs. Cisplatin sensitivity after hRev3 mRNA knockdown increased approximately 2-fold in UM-SCC-23 cells and approximately 3-fold in UM-SCC-23 CDDP/R cells. This study suggests that hRev3 knockdown with siRNAs increases sensitivity to cisplatin. Inhibition of the hRev3 gene may therefore prove to be a novel clinical strategy for reducing resistance to platinum-based chemotherapy in HNSCC.

Narrow-band UVB induces more carcinogenic skin tumors than broad-band UVB through the formation of cyclobutane pyrimidine dimer.

Phototherapy with narrow-band UVB (NB-UVB), with a peak exclusively at 311 nm wavelength, has been found to be more effective in treating a variety of skin diseases than conventional broad-band UVB (BB-UVB). To assess the difference in carcinogenic activity between NB-UVB and BB-UVB, we investigated skin tumor formation by irradiating albino hairless, Ogg1 knockout mice and C57BL/6J wild counterparts with these two UV sources. We found that the ratio of malignant skin tumors induced by NB-UVB was significantly higher than that induced by BB-UVB. There was no significant difference in carcinogenicity of skin tumor induced by NB-UVB between Ogg1 knockout and wild-type mice. To investigate the possible cause of different carcinogenic activity by the different UV sources, we examined three types of DNA damage: cyclobutane pyrimidine dimer (CPD), (6-4) photoproduct, and 8-oxoguanine (8-oxoG) induced by each UV source. We found that CPD formation following a minimum erythema dose (MED) by NB-UVB was significantly higher than that following 1 MED by BB-UVB, whereas the formation of (6-4) photoproducts and 8-oxoG following BB-UVB was significantly higher than those following NB-UVB exposure. These results suggest that CPD formation is closely related to the higher carcinogenic characteristics of NB-UVB. JID JOURNAL CLUB ARTICLE: For questions, answers and open discussion about this article please go to http://network.nature.com/.

Perturbed gap-filling synthesis in nucleotide excision repair causes histone H2AX phosphorylation in human quiescent cells.

Human histone H2AX is rapidly phosphorylated on serine 139 in response to DNA double-strand breaks and plays a crucial role in tethering the factors involved in DNA repair and damage signaling. Replication stress caused by hydroxyurea or UV also initiates H2AX phosphorylation in S-phase cells, although UV-induced H2AX phosphorylation in non-cycling cells has recently been observed. Here we study the UV-induced H2AX phosphorylation in human primary fibroblasts under growth-arrested conditions. This reaction absolutely depends on nucleotide excision repair (NER) and is mechanistically distinct from the replication stress-induced phosphorylation. The treatment of cytosine-beta-D-arabinofuranoside strikingly enhances the NER-dependent H2AX phosphorylation and induces the accumulation of replication protein A (RPA) and ATR-interacting protein (ATRIP) at locally UV-damaged subnuclear regions. Consistently, the phosphorylation appears to be mainly mediated by ataxia-telangiectasia mutated and Rad3-related (ATR), although Chk1 (Ser345) is not phosphorylated by the activated ATR. The cellular levels of DNA polymerases delta and epsilon and proliferating cell nuclear antigen are markedly reduced in quiescent cells. We propose a model that perturbed gap-filling synthesis following dual incision in NER generates single-strand DNA gaps and hence initiates H2AX phosphorylation by ATR with the aid of RPA and ATRIP.

DNA repair defect in AT cells and their hypersensitivity to low-dose-rate radiation.

Ataxia telangiectasia (AT) and normal cells immortalized with the human telomerase gene were irradiated in non-proliferative conditions with high- (2 Gy/min) or low-dose-rate (0.3 mGy/min) radiation. While normal cells showed a higher resistance after irradiation at a low dose rate than a high dose rate, AT cells showed virtually the same survival after low- and high-dose-rate irradiation. Although the frequency of micronuclei induced by low-dose-rate radiation was greatly reduced in normal cells, it was not reduced significantly in AT cells. The number of gamma-H2AX foci increased in proportion to the dose in both AT and normal cells after high-dose-rate irradiation. Although few gamma-H2AX foci were observed after low-dose-rate irradiation in normal cells, significant and dose-dependent numbers of gamma-H2AX foci were observed in AT cells even after low-dose-rate irradiation, indicating that DNA damage was not completely repaired during low-dose-rate irradiation. Significant phosphorylation of ATM proteins was detected in normal cells after low-dose-rate irradiation, suggesting that the activation of ATM plays an important role in the repair of DNA damage during low-dose-rate irradiation. In conclusion, AT cells may not be able to repair some fraction of DNA damage and are severely affected by low-dose-rate radiation.

Gene-environment interaction involved in oral carcinogenesis: molecular epidemiological study for metabolic and DNA repair gene polymorphisms.

BACKGROUND: Exposure to environmental carcinogens leads to oral squamous cell carcinoma (OSCC); however, the impact of genetic variations in carcinogen metabolisms and DNA repair on OSCC risk considering environmental exposures has not been clearly elucidated. METHODS: We conducted a case-control study with 122 cases and 241 controls. The risk of OSCC was evaluated in 10 genetic polymorphisms of nine genes, such as CYP1A1, CYP2E1, GSTM1, GSTT1, XPA, XPC, XPC, XPF and ERCC1. Gene-environment interaction was also evaluated. RESULTS: We found that CYP2E1 and XPA polymorphisms significantly affected the OSCC risk. Gene-environment interactions with smoking were significant for CYP2E1 and ERCC1 polymorphisms. Odds ratios for gene-environment interaction were 7.98 (P = 0.036), 9.67 (P = 0.017) and 8.49 (P = 0.031) for CYP2E1RsaI, DraI and ERCC1 polymorphisms, respectively. No interaction was observed with heavy drinking and any polymorphisms. CONCLUSION: CYP2E1, XPA and ERCC1 polymorphisms may affect the risk of OSCC.

Activation of ataxia telangiectasia-mutated DNA damage checkpoint signal transduction elicited by herpes simplex virus infection.

Eukaryotic cells are equipped with machinery to monitor and repair damaged DNA. Herpes simplex virus (HSV) DNA replication occurs at discrete sites in nuclei, the replication compartment, where viral replication proteins cluster and synthesize a large amount of viral DNA. In the present study, HSV infection was found to elicit a cellular DNA damage response, with activation of the ataxia-telangiectasia-mutated (ATM) signal transduction pathway, as observed by autophosphorylation of ATM and phosphorylation of multiple downstream targets including Nbs1, Chk2, and p53, while infection with a UV-inactivated virus or with a replication-defective virus did not. Activated ATM and the DNA damage sensor MRN complex composed of Mre11, Rad50, and Nbs1 were recruited and retained at sites of viral DNA replication, probably recognizing newly synthesized viral DNAs as abnormal DNA structures. These events were not observed in ATM-deficient cells, indicating ATM dependence. In Nbs1-deficient cells, HSV infection induced an ATM DNA damage response that was delayed, suggesting a functional MRN complex requirement for efficient ATM activation. However, ATM silencing had no effect on viral replication in 293T cells. Our data open up an interesting question of how the virus is able to complete its replication, although host cells activate ATM checkpoint signaling in response to the HSV infection.

Cytotoxic and mutagenic effects of chronic low-dose-rate irradiation on TERT-immortalized human cells.

To analyze the genetic effects of low-dose-rate radiation on human cells, we used human telomere reverse transcriptase (TERT)-immortalized fibroblast cells obtained from normal individuals. We studied the effect of low-dose-rate (0.3 mGy/ min) and high-dose-rate (2 Gy/min) radiation on cells in a confluent state. Survival and micronucleus induction frequency showed higher resistance after irradiation at low dose rate than at high dose rate. The survival after 5 Gy of high-dose-rate radiation was 0.01 compared to 0.3 after low-dose-rate irradiation at the same dose. In accordance with this, the level of HPRT mutation induction by low-dose-rate radiation decreased to approximately one-eighth that for high-dose-rate radiation. We then characterized the mutants by multiplex PCR analysis, which showed that the fraction of deletion mutations was lower in the mutant cells induced at low dose rate than at high dose rate. Furthermore, the size of the deletions in mutant cells induced by low-dose-rate radiation appeared to be smaller than those in mutant cells irradiated at high dose rate. Only a few exons were deleted in the former mutants while all exons were deleted in most of the latter mutants. The present study indicates that the genetic effects of low-dose-rate radiation on nonproliferating normal human cells are quantitatively and qualitatively less severe than the effect of high-dose-rate radiation.

Two haplotypes of 5' untranslated region in L-myc gene with different internal ribosome entry site activity.

We found two polymorphisms, A or C, at position 63 from the L-myc transcription start site, and 8- or 9-base Gs stretch between positions 82 and 89 or 90, in L-myc 5' UTR, which exhibited a strong linkage disequilibrium as two haplotypes, A-9G and C-8G. We analyzed whether both haplotypes of 5' UTR of the L-myc gene, each of which is approximately 200 nt long, are involved in translational regulation of the L-myc gene. Each haplotype of 5' UTR was inserted between Renilla luciferase and firefly luciferase genes to construct a bicistronic vector which was transcribed as a single mRNA. A translation of the downstream cistron was increased 70-100-fold in cos 7 cells, and 7-10-fold in HeLa S3 cells compared with a control vector without insertion. However, in this bicistronic assay system, both haplotypes of the L-myc 5' UTR showed similar internal ribosome entry site (IRES) activity. When a stable hairpin loop with a palindromic sequence was inserted into a 5'-end of bicistronic mRNA, the hairpin loop decreased only the activity of an upstream luciferase in both cos 7 and HeLa S3 cells, supporting the presence of an IRES. We also inserted 5' UTR of the L-myc gene into the 5'-end of reporter mRNA. C-8G showed approximately 2-fold higher IRES activity than A-9G in both cos 7 and HeLa S3 cells, suggesting that two polymorphisms in the L-myc exon 1 might be involved in regulation of L-myc protein expression.

No induction of p53 phosphorylation and few focus formation of phosphorylated H2AX suggest efficient repair of DNA damage during chronic low-dose-rate irradiation in human cells.

Human fibroblast cells obtained from a normal individual and immortalized by introduction of the hTERT gene were irradiated with 0 to 5 Gy of acute high-dose-rate radiation (1.8 Gy/min) or chronic low-dose-rate radiation (0.3 mGy/min) in the G0 phase, and p53 activation was studied. After high-dose-rate irradiation, a dose-dependent induction of Ser15 phosphorylation was observed, whereas after low-dose-rate irradiation almost none was observed. Then we analyzed the focus formation of phosphorylated histone H2AX protein, which is closely correlated with the induction of double-strand breaks. High-dose-rate radiation induced a significant number of foci in a dose-dependent manner, whereas, low-dose-rate radiation could induce only a few foci even at the highest dose. These results strongly suggest that DNA damage induced by low-dose-rate radiation such as a double-strand break is efficiently repaired during chronic irradiation.

Frequent somatic mutations of mitochondrial DNA in esophageal squamous cell carcinoma.

Recent studies of various cancers, such as those of the breast, head and neck, bladder and lung, reported that 46-64% of somatic mutations in the D-loop region of mitochondrial DNA (mtDNA) are observed. However, in esophageal cancer, only a low rate (5%) of somatic mutations has so far been reported in one article (Hibi, K. et al., Int J Cancer 2001;92:319-321). Thus, to confirm this we analyzed the somatic mutations for hypervariable regions (HVR-I and HVR-II) in the D-loop of mtDNA to reevaluate the possibility of mitochondrial genetic instability in this cancer. We amplified both HVRs by PCR and DNA samples obtained from 38 esophageal tumors and matched normal tissues, and then sequenced them. Comparing the sequences of tumors to those of normal tissues, we found 14 somatic mutations in 13 patients (34.2%). Eleven mutations were at the C consecutive stretch from position 303 to 309 of MITOMAP in the mitochondria databank (http://www.mitomap.org/), 1 at position 215 in HVR-II and 2 at positions 16,304 and 16,324 in HVR-I. There were 41 types of germ line variations in HVR-I including 2 not so far recorded in the mtDNA databank and 17 in HVR-II including 1 not yet recorded. We also determined nuclear genome instability of these 38 specimens by analyzing 3 independent microsatellite sequences. While 4 specimens showed a single microsatellite change, which is tumor specific, we did not find any co-relation between a somatic mtDNA mutation and microsatellite instability of nuclear genome DNA. These results suggest that mtDNA mutations might show a genetic instability in esophageal cancer independently from a nuclear genome instability.

Low incidence of p53 mutations in well-differentiated tongue squamous cell carcinoma in Japan.

BACKGROUND: Despite an increasing number of patients suffering from squamous cell carcinomas of the tongue, little is known about the molecular mechanisms involved in the origin and development of these neoplasms. METHODS: We screened microdissected tongue squamous cell carcinoma (TSC) specimens from 28 consecutive, previously untreated, Japanese patients for mutations in the p53 tumor-suppressor gene single-strand conformation polymorphism analysis (exons 5, 6, 7, 8) and direct genomic sequencing. RESULTS: Among them, 24 tumor specimens were well differentiated, three moderately and one poorly differentiated, according to the WHO classification. Mutations in the p53 tumor-suppressor gene were detected in only two out of the 28 (7%) tumor specimens. One was well differentiated and the other was poorly differentiated. CONCLUSIONS: Our results suggest that p53 gene mutations are less frequent in well differentiated TSC. These results indicate that mutations in the p53 gene may not be strongly involved in the development of well differentiated TSC.

Frequent polymorphic changes but rare tumor specific mutations of the LATS2 gene on 13q11-12 in esophageal squamous cell carcinoma.

We previously reported that loss of heterozygosity on 13q12-13 is related with poor prognosis of esophageal cancer. However, a target tumor-suppressor gene on this region is not yet identified. Recently, LATS2, a new human homologue of the Drosophila tumor suppressor gene (lats/warts) was identified on 13q11-12. We therefore screened esophageal tumor cell lines and tumor tissues to detect tumor specific mutations of the LATS2 gene. Although we found 5 different polymorphisms in this gene (4 kinds of single-base changes and a 6-bp insertion), a tumor specific mutation was identified in only one out of 60 tumor tissues. These results indicated that the LATS2 gene is inactivated in only a small part of esophageal tumors, if any.

Establishment of immortal normal and ataxia telangiectasia fibroblast cell lines by introduction of the hTERT gene.

To establish immortal human cells, we introduced the human catalytic subunit of telomerase (hTERT) gene into skin fibroblast cells obtained from normal and ataxia telangiectasia (AT) individuals of Japanese origin. After hTERT introduction, these cells continue to grow beyond a population doubling number of 200 while maintaining their original radiosensitivity. Inductions of p53, phosphorylation of Ser15 in p53, and induction of p21 by X-ray irradiation in immortal cells derived from normal individual were not affected by the hTERT introduction. Both normal and AT immortal cells exhibited an apparent inhibition of growth as original primary cells when they reached confluence. Karyotype analysis has revealed that they are in a diploid range. These results suggest that cells immortalized by hTERT introduction retain their original characteristics except for immortalization, and that they may be useful for analyzing various effects of radiation on human cells.

DNA damage response pathway in radioadaptive response.

Radioadaptive response is a biological defense mechanism in which low-dose ionizing irradiation elicits cellular resistance to the genotoxic effects of subsequent irradiation. However, its molecular mechanism remains largely unknown. We previously demonstrated that the dose recognition and adaptive response could be mediated by a feedback signaling pathway involving protein kinase C (PKC), p38 mitogen activated protein kinase (p38MAPK) and phospholipase C (PLC). Further, to elucidate the downstream effector pathway, we studied the X-ray-induced adaptive response in cultured mouse and human cells with different genetic background relevant to the DNA damage response pathway, such as deficiencies in TP53, DNA-PKcs, ATM and FANCA genes. The results showed that p53 protein played a key role in the adaptive response while DNA-PKcs, ATM and FANCA were not responsible. Wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K), mimicked the priming irradiation in that the inhibitor alone rendered the cells resistant against the induction of chromosome aberrations and apoptosis by the subsequent X-ray irradiation. The adaptive response, whether it was afforded by low-dose X-rays or wortmannin, occurred in parallel with the reduction of apoptotic cell death by challenging doses. The inhibitor of p38MAPK which blocks the adaptive response did not suppress apoptosis. These observations indicate that the adaptive response and apoptotic cell death constitute a complementary defense system via life-or-death decisions. The p53 has a pivotal role in channeling the radiation-induced DNA double-strand breaks (DSBs) into an adaptive legitimate repair pathway, where the signals are integrated into p53 by a circuitous PKC-p38MAPK-PLC damage sensing pathway, and hence turning off the signals to an alternative pathway to illegitimate repair and apoptosis. A possible molecular mechanism of adaptive response to low-dose ionizing irradiation has been discussed in relation to the repair of DSBs and implicated to the current controversial observations on the expression of adaptive response.

L-myc genotype is associated with different susceptibility to lung cancer in smokers.

We have shown that L-myc genotype is associated with the risk of esophageal cancer from smoking and heavy drinking. In this study, we have analyzed the relationship between the L-myc genotypes and lung cancer risk from smoking in 191 Japanese lung-cancer patients and 241 non-cancer controls. The odds ratios (ORs) were markedly higher in SS and LS genotypes than in LL genotype; age-sex-adjusted ORs were 3.19, 2.30 and 0.92, respectively. This result suggests that the L-myc polymorphism may affect the induction of lung cancer by smoking. The OR for smoking in SS-genotype patients diagnosed within 2 years was higher than that in other SS patients, suggesting that smoking-related lung cancer in SS genotype might exhibit a poorer prognosis.