Gene for the catalytic subunit of the human DNA-activated protein kinase maps to the site of the XRCC7 gene on chromosome 8

Simultaneous inhibition of ATM, ATR, and DNA-PK causes synergistic lethality

Here we report that simultaneous inhibition of the three primary DNA damage recognition PI3 kinase-like kinases (PIKKs) -ATM, ATR, and DNA-PK- induces severe combinatorial synthetic lethality in mammalian cells. Utilizing Chinese hamster cell lines CHO and V79 and their respective PIKK mutants, we evaluated effects of inhibiting these three kinases on cell viability, DNA damage response, and chromosomal integrity. Our results demonstrate that while single or dual kinase inhibition increased cytotoxicity, inhibition of all three PIKKs results in significantly higher synergistic lethality, chromosomal aberrations, and DNA double-strand break (DSB) induction as calculated by their synergy scores. These findings suggest that the overlapping redundancy of ATM, ATR, and DNA-PK functions is critical for cell survival, and their combined inhibition greatly disrupts DNA damage signaling and repair processes, leading to cell death. This study provides insights into the potential of multi-targeted DDR kinase inhibition as an effective anticancer strategy, necessitating further research to elucidate underlying mechanisms and therapeutic applications.

ATM dysfunction in Chinese hamster XRCC8 mutants

XRCC8 is a member of the X-ray cross-complementing (XRCC) family, whose responsible gene has not been identified. Previous studies suggested ATM and other genes were potential candidates for XRCC8, but this was not confirmed. In this study, we characterized three V79-derived XRCC8 mutant cells: V-C4, V-E5, and V-G8. Western blot analysis showed reduced expression of the ATM protein in three XRCC8 mutants, and radiation-induced phosphorylated ATM foci were not detected by fluorescence immunocytochemistry. Both ATM knockout cells and XRCC8 mutants exhibited hypersensitivity to camptothecin. Through a cell fusion-based complementation test, we found that XRCC8 mutants were complemented by ATM-proficient cells, but not by ATM knockout cells, in terms of camptothecin sensitivity. Comprehensive sequencing of the ATM genome in XRCC8 mutants revealed unique mutations in each mutant. These results suggest that XRCC8 mutants carry ATM mutations, and their ATM is not properly functional, despite protein expression being detected. This is similar to missense mutations in some Ataxia Telangiectasia patients.

Non-Homologous End-Joining Pathway Genotypes Significantly Associated with Nasopharyngeal Carcinoma Susceptibility

Defects in the non-homologous end-joining (NHEJ) DNA repair pathway lead to genomic instability and carcinogenesis. However, the roles of individual NHEJ genes in nasopharyngeal carcinoma (NPC) etiology are not well-understood. The aim of this study was to assess the contribution of NHEJ genotypes, including XRCC4 (rs6869366, rs3734091, rs28360071, rs28360317, rs1805377), XRCC5 (rs828907, rs11685387, rs9288518), XRCC6 (rs5751129, rs2267437, rs132770, rs132774), XRCC7 rs7003908, and Ligase4 rs1805388, to NPC risk, with 208 NPC patients and 416 controls. Genotype-phenotype correlations were also investigated by measuring mRNA and protein expression in adjacent normal tissues and assessing the NHEJ repair capacity in blood lymphocytes from 43 NPC patients. The results showed significant differences in the distributions of variant genotypes at XRCC4 rs3734091, rs28360071, and XRCC6 rs2267437 between the cases and controls. The variant genotypes of these three polymorphisms were associated with significantly increased NPC risks. NPC patients with the risk genotypes at XRCC6 rs2267437 had significantly reduced expression levels of both mRNA and protein, as well as a lower NHEJ repair capacity, than those with the wild-type genotype. In conclusion, XRCC4 rs3734091, rs28360071, and XRCC6 rs2267437 in the NHEJ pathway were associated with NPC susceptibility. XRCC6 rs2267437 can modulate mRNA and protein expression and the NHEJ repair capacity.

DNA-PKcs: A Targetable Protumorigenic Protein Kinase

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a pleiotropic protein kinase that plays critical roles in cellular processes fundamental to cancer. DNA-PKcs expression and activity are frequently deregulated in multiple hematologic and solid tumors and have been tightly linked to poor outcome. Given the potentially influential role of DNA-PKcs in cancer development and progression, therapeutic targeting of this kinase is being tested in preclinical and clinical settings. This review summarizes the latest advances in the field, providing a comprehensive discussion of DNA-PKcs functions in cancer and an update on the clinical assessment of DNA-PK inhibitors in cancer therapy.

Genetic polymorphisms in DNA repair genes and hepatocellular carcinoma risk

Hepatocellular carcinoma (HCC) is one of the most frequent types of tumors worldwide. Its occurrence and development have been related to various risk factors, such as chronic infection with hepatitis B or C viruses and alcohol addiction. DNA repair systems play a critical role in maintaining the integrity of the genome. Defects in these systems have been related to increased susceptibility to various types of cancer. Multiple genetic polymorphisms in genes of DNA repair systems have been reported that may affect DNA repair capacity (DRC) and modulate risk to cancer. Several studies have been conducted to assess the role of polymorphisms of DNA repair genes on the HCC risk. Identifying these polymorphisms and their association with HCC risk may help to improve prevention and treatment strategies. In this study, we review investigations that evaluated the association between genetic polymorphisms of DNA repair genes and risk of HCC.

Radio-adaptive response and correlation of non-homologous end joining repair gene polymorphisms [XRRC5 (3R/2R/1R/0R), XRCC6(C/G) and XRCC7 (G/T)] in human peripheral blood mononuclear cells exposed to gamma radiation

BACKGROUND

Radio-adaptive response (RAR) is transient phenomena, where cells conditioned with a small dose (priming) of ionizing radiation shows significantly reduced DNA damage with a subsequent high challenging dose. The role of DNA double strand break repair gene polymorphism in RAR is not known. In the present study attempt was made to find out the influence of NHEJ repair gene polymorphisms [a VNTR; XRCC5 (3R/2R/1R/0R); two single nucleotide polymorphisms (SNPs); XRCC6 (C/G) and XRCC7 (G/T)] with DNA damage, repair and mRNA expression in human PBMCs in dose and adaptive response studies. Genomic DNA extracted from venous blood samples of 20 random healthy donors (16 adaptive and 4 non-adaptive) and genotyping of NHEJ repair genes was carried out using PCR amplified length polymorphism.

RESULTS

The dose response study revealed significant positive correlation of genotypes at XRRC5 (3R/2R/1R/0R), XRCC6(C/G) and XRCC7 (G/T) with DNA damage. Donors having genotypes with 2R allele at XRCC5 showed significant positive correlation with mRNA expression level (0R/2R: r = 0.846, P = 0.034; 1R/2R: r = 0.698, P = 0.0001 and 2R/2R: r = 0.831, P = 0.0001) for dose response. Genotypes C/C and C/G of XRCC6 showed a significant positive correlation (P = 0.0001), whereas, genotype T/T of XRCC7 showed significant negative correlation (r = - 0.376, P = 0.041) with mRNA expression.

CONCLUSION

Interestingly, adaptive donors having C/G genotype of XRCC6 showed significantly higher (P < 0.05) mRNA expression level in primed cells suggesting their role in RAR. In addition, NHEJ repair gene polymorphisms play crucial role with radio-sensitivity and RAR in human PBMCs.

Structural insights into the role of DNA-PK as a master regulator in NHEJ

DNA-dependent protein kinase catalytic subunit DNA-PKcs/PRKDC is the largest serine/threonine protein kinase of the phosphatidyl inositol 3-kinase-like protein kinase (PIKK) family and is the most highly expressed PIKK in human cells. With its DNA-binding partner Ku70/80, DNA-PKcs is required for regulated and efficient repair of ionizing radiation-induced DNA double-strand breaks via the non-homologous end joining (NHEJ) pathway. Loss of DNA-PKcs or other NHEJ factors leads to radiation sensitivity and unrepaired DNA double-strand breaks (DSBs), as well as defects in V(D)J recombination and immune defects. In this review, we highlight the contributions of the late Dr. Carl W. Anderson to the discovery and early characterization of DNA-PK. We furthermore build upon his foundational work to provide recent insights into the structure of NHEJ synaptic complexes, an evolutionarily conserved and functionally important YRPD motif, and the role of DNA-PKcs and its phosphorylation in NHEJ. The combined results identify DNA-PKcs as a master regulator that is activated by its detection of two double-strand DNA ends for a cascade of phosphorylation events that provide specificity and efficiency in assembling the synaptic complex for NHEJ.

DNA-PKcs: A Multi-Faceted Player in DNA Damage Response

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a member of the phosphatidylinositol 3-kinase related kinase family, which can phosphorylate more than 700 substrates. As the core enzyme, DNA-PKcs forms the active DNA-PK holoenzyme with the Ku80/Ku70 heterodimer to play crucial roles in cellular DNA damage response (DDR). Once DNA double strand breaks (DSBs) occur in the cells, DNA-PKcs is promptly recruited into damage sites and activated. DNA-PKcs is auto-phosphorylated and phosphorylated by Ataxia-Telangiectasia Mutated at multiple sites, and phosphorylates other targets, participating in a series of DDR and repair processes, which determine the cells' fates: DSBs NHEJ repair and pathway choice, replication stress response, cell cycle checkpoints, telomeres length maintenance, senescence, autophagy, etc. Due to the special and multi-faceted roles of DNA-PKcs in the cellular responses to DNA damage, it is important to precisely regulate the formation and dynamic of its functional complex and activities for guarding genomic stability. On the other hand, targeting DNA-PKcs has been considered as a promising strategy of exploring novel radiosensitizers and killing agents of cancer cells. Combining DNA-PKcs inhibitors with radiotherapy can effectively enhance the efficacy of radiotherapy, offering more possibilities for cancer therapy.

Integrated bioinformatic analysis of RNA binding proteins in hepatocellular carcinoma

RNA binding proteins (RBPs) are aberrantly expressed in a tissue-specific manner across many tumors. These proteins, which play a vital role in post-transcriptional gene regulation, are involved in RNA splicing, maturation, transport, stability, degradation, and translation. We set out to establish an accurate risk score model based on RBPs to estimate prognosis in hepatocellular carcinoma (HCC). RNA-sequencing data, proteomic data and corresponding clinical information were acquired from the Cancer Genome Atlas database and the Clinical Proteomic Tumor Analysis Consortium database respectively. We identified 406 differentially expressed RBPs between HCC tumor and normal tissues at the transcriptional and protein level. Overall, 11 RBPs (BRIX1, DYNC1H1, GTPBP4, PRKDC, RAN, RBM19, SF3B4, SMG5, SPATS2, TAF9, and THOC5) were selected to establish a risk score model. We divided HCC patients into low-risk and high-risk groups based on the median of risk score values. The survival analysis indicated that patients in the high-risk group had poorer overall survival compared to patients in the low-risk group. Our study demonstrated that 11 RBPs were associated with the overall survival of HCC patients. These RBPs may represent potential drug targets and can help optimize future clinical treatment.

Association between genetic polymorphism of XRCC7 (G6721T) and risk of acute lymphoblastic leukemia

Background

The DNA non-homologous end joining repair gene XRCC7 is one of the most important genes in the DNA double-strand break (DSBs) repair. It is supposed that DNA repair gene malfunction is the main risk factor in various malignancies. The XRCC7 G6721T (rs7003908) polymorphism impact was investigated on the splicing regulation that cause mRNA instability. The goal of the present hospital-based study was to investigate the association between the common genetic polymorphism of XRCC7 G6721T (rs7003908) and risk of acute lymphoblastic leukemia (ALL). This hospital-based case–control study was performed on 99 ALL patients versus 200 healthy children, as the control group, which were frequent matched by age with cases. The polymorphism of XRCC7 was determined using an RFLP-PCR technique.

Results

The GT (OR = 1.485, 95% CI 0.765–2.334, P = 0.243) and TT (OR = 1.655, 95% CI 00.875–3.128, P = 0.121) genotypes had no significant effect on the risk of ALL, in comparison with the GG genotype. However, TT genotype (OR = 1.996, 95% CI 1.033–3.858, P = 0.04) after adjusting for the parents’ smoking pattern showed a significant impact.

Conclusions

These findings suggest that the TT genotype may increase the ALL susceptibility in children when facing with a tobacco smoke.

DNA-PK in human malignant disorders: Mechanisms and implications for pharmacological interventions

The DNA-PK holoenzyme is a fundamental element of the DNA damage response machinery (DDR), which is responsible for cellular genomic stability. Consequently, and predictably, over the last decades since its identification and characterization, numerous pre-clinical and clinical studies reported observations correlating aberrant DNA-PK status and activity with cancer onset, progression and responses to therapeutic modalities. Notably, various studies have established in recent years the role of DNA-PK outside the DDR network, corroborating its role as a pleiotropic complex involved in transcriptional programs that operate biologic processes as epithelial to mesenchymal transition (EMT), hypoxia, metabolism, nuclear receptors signaling and inflammatory responses. In particular tumor entities as prostate cancer, immense research efforts assisted mapping and describing the overall signaling networks regulated by DNA-PK that control metastasis and tumor progression. Correspondingly, DNA-PK emerges as an obvious therapeutic target in cancer and data pertaining to various pharmacological approaches have been published, largely in context of combination with DNA-damaging agents (DDAs) that act by inflicting DNA double strand breaks (DSBs). Currently, new generation inhibitors are tested in clinical trials. Several excellent reviews have been published in recent years covering the biology of DNA-PK and its role in cancer. In the current article we are aiming to systematically describe the main findings on DNA-PK signaling in major cancer types, focusing on both preclinical and clinical reports and present a detailed current status of the DNA-PK inhibitors repertoire.

Differential Expression of DNA Repair Genes in Prognostically-Favorable versus Unfavorable Uveal Melanoma

Expression of DNA repair genes was studied in uveal melanoma (UM) in order to identify genes that may play a role in metastases formation. We searched for genes that are differentially expressed between tumors with a favorable and unfavorable prognosis. Gene-expression profiling was performed on 64 primary UM from the Leiden University Medical Center (LUMC), Leiden, The Netherlands. The expression of 121 genes encoding proteins involved in DNA repair pathways was analyzed: a total of 44 genes differed between disomy 3 and monosomy 3 tumors. Results were validated in a cohort from Genoa and Paris and the The Cancer Genome Atlas (TCGA) cohort. Expression of the PRKDC, WDR48, XPC, and BAP1 genes was significantly associated with clinical outcome after validation. PRKDC was highly expressed in metastasizing UM (p < 0.001), whereas WDR48, XPC, and BAP1 were lowly expressed (p < 0.001, p = 0.006, p = 0.003, respectively). Low expression of WDR48 and XPC was related to a large tumor diameter (p = 0.01 and p = 0.004, respectively), and a mixed/epithelioid cell type (p = 0.007 and p = 0.03, respectively). We conclude that the expression of WDR48, XPC, and BAP1 is significantly lower in UM with an unfavorable prognosis, while these tumors have a significantly higher expression of PRKDC. Pharmacological inhibition of DNA-PKcs resulted in decreased survival of UM cells. PRKDC may be involved in proliferation, invasion and metastasis of UM cells. Unraveling the role of DNA repair genes may enhance our understanding of UM biology and result in the identification of new therapeutic targets.

Single-cell imaging of normal and malignant cell engraftment into optically clear prkdc-null SCID zebrafish

Cell transplantation into immunodeficient mice has revolutionized our understanding of regeneration, stem cell self-renewal, and cancer; yet models for direct imaging of engrafted cells has been limited. Here, we characterize zebrafish with mutations in recombination activating gene 2 (rag2), DNA-dependent protein kinase (prkdc), and janus kinase 3 (jak3). Histology, RNA sequencing, and single-cell transcriptional profiling of blood showed that rag2 hypomorphic mutant zebrafish lack T cells, whereas prkdc deficiency results in loss of mature T and B cells and jak3 in T and putative Natural Killer cells. Although all mutant lines engraft fluorescently labeled normal and malignant cells, only the prkdc mutant fish reproduced as homozygotes and also survived injury after cell transplantation. Engraftment into optically clear casper, prkdc-mutant zebrafish facilitated dynamic live cell imaging of muscle regeneration, repopulation of muscle stem cells within their endogenous niche, and muscle fiber fusion at single-cell resolution. Serial imaging approaches also uncovered stochasticity in fluorescently labeled leukemia regrowth after competitive cell transplantation into prkdc mutant fish, providing refined models to assess clonal dominance and progression in the zebrafish. Our experiments provide an optimized and facile transplantation model, the casper, prkdc mutant zebrafish, for efficient engraftment and direct visualization of fluorescently labeled normal and malignant cells at single-cell resolution.

DNA double-strand break repair gene XRCC7 genotypes were associated with hepatocellular carcinoma risk in Taiwanese males and alcohol drinkers

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide, the prevalence and mortality rates of which are very high in Taiwan. The study aimed at evaluating the contribution of XRCC7 G6721T, together with cigarette smoking and alcohol drinking lifestyles, to the risk of HCC. In this hospital-based case-control study, the association of XRCC7 single nucleotide polymorphism G6721T with HCC risk was examined by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) among 298 HCC patients and 889 age- and gender-matched healthy controls. The results showed that the percentages of TT, GT, and GG XRCC7 G6721T were 53.0, 41.3, and 5.7 % in the HCC patient group and 48.9, 43.1, and 8.0 % in the non-cancer control group, respectively. We have further stratified the populations by genders, cigarette smoking, and alcohol drinking status to investigate their combinative contributions with XRCC7 G6721T genotype to HCC risk. The results showed that the GG genotype of XRCC7 G6721T conducted a protective effect on HCC susceptibility which was obvious among males and drinkers, but not females, smokers, non-smokers, or non-drinkers (p = 0.0058, 0.0069, 0.1564, 0.2469, 0.9354, and 0.3416, respectively). Our results suggested that the GG and GT genotypes of X-ray repair cross-complementing group 7 (XRCC7) G6721T had no effect on HCC risk to the whole population, but had a protective effect on HCC risk among males and alcohol drinkers.

Quality control of homologous recombination

Exogenous and endogenous genotoxic agents, such as ionizing radiation and numerous chemical agents, cause DNA double-strand breaks (DSBs), which are highly toxic and lead to genomic instability or tumorigenesis if not repaired accurately and efficiently. Cells have over evolutionary time developed certain repair mechanisms in response to DSBs to maintain genomic integrity. Major DSB repair mechanisms include non-homologous end joining and homologous recombination (HR). Using sister homologues as templates, HR is a high-fidelity repair pathway that can rejoin DSBs without introducing mutations. However, HR execution without appropriate guarding may lead to more severe gross genome rearrangements. Here we review current knowledge regarding the factors and mechanisms required for accomplishment of accurate HR.

XRCC7 rs#7003908 Polymorphism and Helicobacter pylori Infection-Related Gastric Antrum Adenocarcinoma

The X-ray repair cross-complementing group 7 (XRCC7) plays a key role in DNA repair that protects against genetic instability and carcinogenesis. To determine whether XRCC7 rs#7003908 polymorphism (XRCC7P) is associated with Helicobacter pylori (H. pylori) infection-related gastric antrum adenocarcinoma (GAA) risk, we conducted a hospital-based case-control study, including 642 patients with pathologically confirmed GAA and 927 individually matched controls without any evidence of tumours or precancerous lesions, among Guangxi population. Increased risks of GAA were observed for individuals with cagA positive (odds ratio (OR) 6.38; 95% confidence interval (CI) 5.03-8.09). We also found that these individuals with the genotypes of XRCC7 rs#7003908 G alleles (XRCC7-TG or -GG) featured increasing risk of GAA (ORs 2.80 and 5.13, resp.), compared with the homozygote of XRCC7 rs#7003908 T alleles (XRCC7-TT). GAA risk, moreover, did appear to differ more significantly among individuals featuring cagA-positive status, whose adjusted ORs (95% CIs) were 15.74 (10.89-22.77) for XRCC7-TG and 38.49 (22.82-64.93) for XRCC7-GG, respectively. Additionally, this polymorphism multiplicatively interacted with XRCC3 codon 241 polymorphism with respect to HCC risk (ORinteraction = 1.49). These results suggest that XRCC7P may be associated with the risk of Guangxiese GAA related to cagA.

DNA double strand breaks induced by the indirect effect of radiation are more efficiently repaired by non-homologous end joining compared to homologous recombination repair

The aim of this study was to investigate the relative involvement of three major DNA repair pathways, i.e., non-homologous end joining (NHEJ), homologous recombination (HRR) and base excision (BER) in repair of DNA lesions of different complexity induced by low- or high-LET radiation with emphasis on the contribution of the indirect effect of radiation for these radiation qualities. A panel of DNA repair-deficient CHO cell lines was irradiated by (137)Cs γ-rays or radon progeny α-particles. Irradiation was also performed in the presence of 2M DMSO to reduce the indirect effect of radiation and the complexity of the DNA damage formed. Clonogenic survival and micronucleus assays were used to estimate efficiencies of the different repair pathways for DNA damages produced by direct and indirect effects. Removal of the indirect effect of low-LET radiation by DMSO increased clonogenic survival and decreased MN formation for all cell lines investigated. A direct contribution of the indirect effect of radiation to DNA base damage was suggested by the significant protection by DMSO seen for the BER deficient cell line. Lesions formed by the indirect effect are more readily repaired by the NHEJ pathway than by HRR after irradiation with γ-rays or α-particles as evaluated by cell survival and the yields of MN. The results obtained with BER- and NHEJ-deficient cells suggest that the indirect effect of radiation contributes significantly to the formation of repair substrates for these pathways.

Association between exudative age-related macular degeneration and the G6721T polymorphism of XRCC7 in outdoor subjects

PURPOSE

To investigate whether the G6721T polymorphism (rs.7003908) of the non-homologous end-joining DNA repair XRCC7 gene contributes to the development of exudative age-related macular degeneration (ARMD).

METHODS

The present case-control study consisted of 111 patients with exudative ARMD and 112 sex frequency-matched healthy controls that were randomly selected from unrelated volunteers in the same clinic. Genotypes were determined by the Restriction Fragment Length Polymorphism (PCR-RFLP) based method. Logistic regression was used to calculate odds ratios (ORs) and 95% confidence intervals (CIs) for ARMD risk associated with polymorphism of XRCC7. In all analysis the GG genotype was considered to be the reference genotype.

RESULTS

There was no significant association between genotypes of XRCC7 and susceptibility to ARMD. Considering the significant difference in age distribution between cases and controls, age was used as a covariate in further analysis. After ORs were adjusted for age, the same result was observed. In the next step we stratified our subjects into outdoor and indoor groups according to their job titles. The outdoor and indoor patients were occupationally exposed to sunlight and not exposed to sunlight, respectively. Our present study showed that among indoor subjects there was no association between XRCC7 polymorphism and susceptibility to ARMD. However, among outdoor subjects, the GT + TT genotypes compared to the GG genotype increased the risk of ARMD (OR, 3.13; 95% CI, 1.04-9.39; p = 0.042).

CONCLUSIONS

Our study revealed that the T allele of the G6721T polymorphism of XRCC7 increased the risk of ARMD among outdoor subjects.

Distinct roles for DNA-PK, ATM and ATR in RPA phosphorylation and checkpoint activation in response to replication stress

DNA damage encountered by DNA replication forks poses risks of genome destabilization, a precursor to carcinogenesis. Damage checkpoint systems cause cell cycle arrest, promote repair and induce programed cell death when damage is severe. Checkpoints are critical parts of the DNA damage response network that act to suppress cancer. DNA damage and perturbation of replication machinery causes replication stress, characterized by accumulation of single-stranded DNA bound by replication protein A (RPA), which triggers activation of ataxia telangiectasia and Rad3 related (ATR) and phosphorylation of the RPA32, subunit of RPA, leading to Chk1 activation and arrest. DNA-dependent protein kinase catalytic subunit (DNA-PKcs) [a kinase related to ataxia telangiectasia mutated (ATM) and ATR] has well characterized roles in DNA double-strand break repair, but poorly understood roles in replication stress-induced RPA phosphorylation. We show that DNA-PKcs mutant cells fail to arrest replication following stress, and mutations in RPA32 phosphorylation sites targeted by DNA-PKcs increase the proportion of cells in mitosis, impair ATR signaling to Chk1 and confer a G2/M arrest defect. Inhibition of ATR and DNA-PK (but not ATM), mimic the defects observed in cells expressing mutant RPA32. Cells expressing mutant RPA32 or DNA-PKcs show sustained H2AX phosphorylation in response to replication stress that persists in cells entering mitosis, indicating inappropriate mitotic entry with unrepaired damage.

Association between polymorphisms in DNA repair genes (XRCC1 and XRCC7) and risk of preeclampsia

PURPOSE

Although the exact genes involved in preeclampsia (PE) are still not fully discovered, an important role for oxidative stress in its pathogenesis is accepted. XRCC1 (MIM: 194360) and XRCC7 (MIM: 600899) play a crucial role in the DNA repair pathways. Functional polymorphisms in XRCC1 (Arg194Trp and Arg399Gln) and XRCC7 (G6721T) may be risk factors for PE. In the present study, the association between the genetic polymorphisms of XRCC1 and XRCC7 and risk of PE is investigated.

METHODS

The present case-control study was performed on 151 preeclapmtic patients, and a total of 344 normal pregnant women, as a control group. Control women had no history of pregnancies with PE.

RESULTS

Neither polymorphism of Arg194Trp XRCC1 nor polymorphism of G6721T XRCC7 associated with the risk of PE. The Gln/Gln genotype of Arg399Gln XRCC1 polymorphism increased the risk of PE (OR=2.39, 95 % CI: 1.38-4.14, P=0.002). Statistical analysis revealed that the haplotype "194Arg-399Gln" showed higher frequency among PE patients compared to the controls (OR=1.65, 95% CI: 1.23-2.19, P=0.001).

CONCLUSIONS

The present results suggest that the 399Gln allele of the XRCC1 is significant risk factor for PE development.

Genetic variation in DNA repair gene XRCC7 (G6721T) and susceptibility to breast cancer

The human XRCC7 is a DNA double-strand break (DSBs) repair gene, involved in non-homologous end joining (NHEJ). It is speculated that DNA DSBs repair have an important role during development of breast cancer. The human XRCC7 is a NHEJ DSBs repair gene. Genetic variation G6721T of XRCC7 (rs7003908) is located in the intron 8 of the gene. This polymorphism may regulate splicing and cause mRNA instability. In the present study, we specifically investigated whether common G6721T genetic variant of XRCC7 was associated with an altered risk of breast cancer. The present study included 362 females with breast cancer. Age frequency-matched controls (362 persons) were randomly selected from the healthy female blood donors, according to the age distribution of the cases. Using RFLP-PCR based method, the polymorphism of XRCC7 was determined. The TG (OR=1.20, 95% CI: 0.83-1.74, P=0.320) and TT (OR=1.01, 95% CI: 0.67-1.53, P=0.933) genotypes had no significant effect on risk of breast cancer, in comparison with the GG genotype. Our present findings indicate that the TT and TG genotypes were not associated with an altered breast cancer risk.

Prevalence of G6721T polymorphism of XRCC7 in an Iranian population

Genetic polymorphism G6721T (rs.7003908) in gene encoding DNA-dependent protein kinase (DNA-PKcs, encoded by the XRCC7 gene) has been defined. In order to get more insight into the genetic structure of Iranian population the present study was carried out on Iranian Persian population living in Shiraz (Fars province, southwest Iran). The total study subjects consisted of 935 (195 males, 740 females) unrelated healthy individuals. Genotypes of XRCC7 G6721T were detected by PCR-RFLP based method. There was no significant difference between males and females for the XRCC7 polymorphism (χ(2)=1.275, df=2, P=0.529). Prevalence of the G allele was 0.473 (95 % CI: 0.441-0.505) in our sample. The study population was at Hardy-Weinberg equilibrium for the XRCC7 polymorphism (χ(2)=0.980, df=1, P=0.323). The allelic frequency of the G allele showed high frequency in Iranian population compared to other populations.

Interaction between polymorphisms of DNA repair genes significantly modulated bladder cancer risk

DNA repair is a primary defense mechanism against damage caused by exogenous and endogenous sources. We examined the associations between bladder cancer and 7 polymorphisms from 5 genes involved in the maintenance of genetic stability (MMR: MLH1-93G>A; BER: XRCC1--77T>C and Arg399Gln; NER:XPC Lys939Gln and PAT +/-; DSBR:ATM G5557A and XRCC7 G6721T) in 302 incident bladder cancer cases and 311 hospital controls. Genotyping was done using a polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) technique. The homozygous variant of XRCC7 G6721T (Odds Ratio [OR]: 2.36; 95% Confidence Interval [CI]: 1.13-4.92) was associated with increased bladder cancer risk. In an analysis of combined genotypes, the combination of XRCC1Arg399Gln (Gln allele) with XRCC1-77 T/T led to an increase in risk (OR: 1.61; 95% CI: 1.10-2.36). Moreover, when the XPCLys939Gln (Gln allele) (nucleotide excision repair [NER]) was present together with XRCC7 (T allele) (double strand break repair [DSBR]), the bladder cancer risk dramatically increased (OR: 4.42; 95% CI: 1.23-15.87). Our results suggest that there are multigenic variations in the DNA repair pathway involved in bladder cancer susceptibility, despite the existence of ethnic group differences.

DNA repair gene XRCC7 polymorphisms (rs#7003908 and rs#10109984) and hepatocellular carcinoma related to AFB1 exposure among Guangxi population, China

AIM

  The X-ray repair cross-complementing group 7 (XRCC7) plays an important role in the repair of DNA double-strand breaks by nonhomologous end-joining repair (NEJR) pathway. However, the role of XRCC7 polymorphisms (rs#7003908 and rs#10109984) possibly influencing NEJR capacity in hepatocellular carcinoma (HCC) induced by aflatoxin B1 (AFB1) has not been well elaborated.

METHODS

  This hospital-based case-control study, including 348 patients with newly diagnosed HCC and 597 controls without any evidence of liver diseases, was conducted to elucidate the association between these two polymorphisms and the risk of HCC related to AFB1 exposure among a Guangxi population from a high AFB1-exposure area by means of TaqMAN-polymerase chain reaction technique.

RESULTS

  We observed that HCC patients featured higher AFB1 exposure than control group (odds ratios [OR] = 6.49 and 6.75 for exposure years and exposure levels, respectively). Furthermore, these individuals with the genotypes of XRCC7 rs#7003908 G alleles (namely XRCC7-TG or -GG), compared the homozygote of XRCC7 rs#7003908 T alleles (XRCC7-TT), faced increasing risk of HCC (OR, 3.45 and 5.04; 95% confidence intervals [CIs], 2.40-4.94 and 3.28-7.76, respectively). We also found some evidence that this polymorphism interacted with AFB1-expousure years or levels in the process of HCC carcinogenesis. Additionally, XRCC7 rs#7003908 polymorphism was correlated with the levels of AFB1-DNA adducts (r = 0.142, P < 0.001). XRCC7 rs#10109984 polymorphism, however, did not modify the risk of HCC related to AFB1 exposure (P > 0.05).

CONCLUSION

  These data suggest that XRCC7 rs#7003908 polymorphism may be one of the genetic modifiers for AFB1-related HCC among Guangxi population.

Do DNA repair genes OGG1, XRCC3 and XRCC7 have an impact on susceptibility to bladder cancer in the North Indian population?

OBJECTIVE

Polymorphisms in DNA repair genes may be associated with altered DNA repair capacity, thereby influencing an individual's susceptibility to smoking-related cancers such as bladder cancer. Therefore, we sought to examine the correlation between single nucleotide polymorphisms in DNA repair genes and bladder cancer.

METHODOLOGY

We undertook a case-control study of 212 urothelial bladder cancer (UBC) cases and 250 controls to investigate the association between OGG1 (C1245G rs1052133), XRCC3 (C18067T, rs861539) and XRCC7 (G6721T, rs7003908) polymorphisms and bladder cancer susceptibility by PCR-RFLP and the ARMS method. We also investigated gene-environment interactions.

RESULTS

The OGG1 GG genotype was associated with an elevated risk of urothelial bladder cancer (UBC) (OR, 2.10; p, 0.028). XRCC7 + 6721 GG was also associated with increased susceptibility to UBC (OR, 4.45; p, 0.001). In a recessive model, the OGG1 GG genotype showed an increased risk of TaG(2,3) + T1G(1-3) tumors. Additionally, the OGG1 GG genotype in non-smokers represented a 2.46-fold greater risk (OR, 2.46; p, 0.035) in bladder cancer patients. Subsequent analysis demonstrated more pronounced association of XRCC7 with smokers (OR, 4.39; p, 0.001). XRCC7 also showed increased association with TaG(2,3) + T1G(1-3) tumors and muscle invasive tumors (OR, 3.16; p, 0.001 and OR, 4.24; p, 0.001, respectively). Multiple Cox regression analysis in non-muscle invasive bladder tumor (NMIBT) patients demonstrated an association of the OGG1 GG polymorphism with a high risk of recurrence in patients on cystoscopic surveillance (HR, 4.04; p, 0.013). Subsequently, shorter recurrence-free survival (log rank p, 0.024; CC/GG, 42/24) was observed.

CONCLUSION

Our data suggest association of a variant (GG) genotype of OGG1 with increased UBC susceptibility and a high risk of tumor recurrence in NMIBT patients on cystoscopic surveillance. XRCC7 G allele carriers (TG+GG) are also at an elevated risk for susceptibility to UBC as evidenced by a high odds ratio throughout the analysis.

The role of nonhomologous DNA end joining, conservative homologous recombination, and single-strand annealing in the cell cycle-dependent repair of DNA double-strand breaks induced by H(2)O(2) in mammalian cells

The purpose of this study was to investigate the cell cycle-dependent role of nonhomologous DNA end joining (NHEJ), conservative homologous recombination (HR), and single-strand annealing (SSA) for the repair of simple DNA double-strand breaks (DSBs) induced by H(2)O(2)-mediated OH radicals in CHO cells. Cells of the cell lines V3 (NHEJ-deficient), irs1SF (HR-deficient) and UV41 (SSA-deficient) and their parental cell line AA8 were exposed to various concentrations of H(2)O(2) in G(1) or S phase of the cell cycle and their colony-forming ability was assayed. In G(1) phase, NHEJ was the most important-if not the only-mechanism to repair H(2)O(2)-mediated DSBs; this was similar to results obtained in a parallel study of more complex DSBs induced by sparsely or densely ionizing radiation. Unlike HR (irs1SF)- and SSA (UV41)-deficient cells, the sensitivity of NHEJ-deficient V3 cells to H(2)O(2) relative to parental AA8 cells in G(1) phase is about 50 times higher compared to 200 kV X rays. This points to a specific role of the catalytic subunit of DNA-PK for efficient NHEJ of H(2)O(2)-mediated DSBs that are located at sites critical for the maintenance of the higher-order structure of cellular DNA, whereas X-ray-induced DSBs are distributed stochastically. Surprisingly, SSA-deficient cells in G(1) phase showed an increased sensitivity to high concentrations of H(2)O(2) relative to the parental wild-type cells and to HR-deficient cells, which may be interpreted in terms of a specific type of H(2)O(2)-induced damage requiring SSA for repair after its transfer into S phase. In S phase, HR is the most important mechanism to repair H(2)O(2)-mediated DSBs, followed by NHEJ. In contrast, the action of error-prone SSA may not be beneficial, since SSA-deficient cells are three times more resistant to H(2)O(2) than wild-type AA8 cells. This is likely due to channeling of DSBs into the error-free HR repair pathway or into the potentially error-prone NHEJ pathway. Cells with or without a defect in DSB repair are considerably more sensitive to H(2)O(2) in S phase compared to G(1) phase. This effect is likely due to the fact that topoisomerase II, which is expressed only in proliferating cells, is a target of H(2)O(2), resulting in enhanced accumulation of DSBs and killing of cells treated in S phase with H(2)O(2). The relative sensitivities to H(2)O(2) differ by orders of magnitude for the four cell lines. This seems to be caused mainly by H(2)O(2)-mediated poisoning of topoisomerase IIalpha rather than by a defect in DSB repair.

Five quantitative trait loci control radiation-induced adenoma multiplicity in Mom1R Apc Min/+ mice

Ionising radiation is a carcinogen capable of inducing tumours, including colorectal cancer, in both humans and animals. By backcrossing a recombinant line of Apc^Min/+ mice to the inbred BALB/c mouse strain, which is unusually sensitive to radiation–induced tumour development, we obtained panels of 2Gy-irradiated and sham-irradiated N2 Apc^Min/+ mice for genotyping with a genome-wide panel of microsatellites at ∼15 cM density and phenotyping by counting adenomas in the small intestine. Interval and composite interval mapping along with permutation testing identified five significant susceptibility quantitative trait loci (QTLs) responsible for radiation induced tumour multiplicity in the small intestine. These were defined as Mom (Modifier of Min) radiation-induced polyposis (Mrip1-5) on chromosome 2 (log of odds, LOD 2.8, p = 0.0003), two regions within chromosome 5 (LOD 5.2, p<0.00001, 6.2, p<0.00001) and two regions within chromosome 16 respectively (LOD 4.1, p  = 4×10⁻⁵, 4.8, p<0.00001). Suggestive QTLs were found for sham-irradiated mice on chromosomes 3, 6 and 13 (LOD 1.7, 1.5 and 2.0 respectively; p<0.005). Genes containing BALB/c specific non-synonymous polymorphisms were identified within Mrip regions and prediction programming used to locate potentially functional polymorphisms. Our study locates the QTL regions responsible for increased radiation-induced intestinal tumorigenesis in Apc^Min/+ mice and identifies candidate genes with predicted functional polymorphisms that are involved in spindle checkpoint and chromosomal stability (Bub1b, Casc5, and Bub1), DNA repair (Recc1 and Prkdc) or inflammation (Duox2, Itgb2l and Cxcl5). Our study demonstrates use of in silico analysis in candidate gene identification as a way of reducing large-scale backcross breeding programmes.

Single nucleotide polymorphisms in DNA repair genes and prostate cancer risk

The specific causes of prostate cancer are not known. However, multiple etiologic factors, including genetic profile, metabolism of steroid hormones, nutrition, chronic inflammation, family history of prostate cancer, and environmental exposures are thought to play significant roles. Variations in exposure to these risk factors may explain interindividual differences in prostate cancer risk. However, regardless of the precise mechanism(s), a robust DNA repair capacity may mitigate any risks conferred by mutations from these risk factors. Numerous single nucleotide polymorphisms (SNPs) in DNA repair genes have been found, and studies of these SNPs and prostate cancer risk are critical to understanding the response of prostate cells to DNA damage. A few SNPs in DNA repair genes are associated with significantly increased risk of prostate cancer; however, in most cases, the effects are moderate and often depend upon interactions among the risk alleles of several genes in a pathway or with other environmental risk factors. This report reviews the published epidemiologic literature on the association of SNPs in genes involved in DNA repair pathways and prostate cancer risk.

Genetic variants of the XRCC7 gene involved in DNA repair and risk of human bladder cancer

OBJECTIVE

To investigate the association between the polymorphisms of the KU70 and X-ray repair cross complementing group 7 (XRCC7) genes and the risk of bladder cancer.

METHODS

This hospital-based case-control study included 213 patients with newly diagnosed bladder transitional cell carcinoma and 235 cancer-free controls frequency-matched by age and sex. Two polymorphisms, KU70 and XRCC7, using a method involving polymerase chain reaction-restriction fragment length polymorphism were genotyped.

RESULTS

The risk of bladder cancer decreased in a dose-response manner as the number of XRCC76721G alleles increased (adjusted odds ratio [OR] = 0.70, 95% confident interval [CI] = 0.47-1.03 for 6721GT and OR = 0.31, 95% CI = 0.10-0.99 for 6721GG; P(trend) = 0.013). However, when we used 6721 (GT + GG) as the reference, we found a statistically significant increased risk of bladder cancer associated with the 6721TT genotype (OR = 1.53, 95% CI = 1.04-2.25). In the stratification analysis, this increased risk was more pronounced among subgroups of patients aged >65 years (OR = 2.27; 95% CI = 1.25-4.10) and ever smokers (OR = 2.06, 95% CI = 1.15-3.68). Furthermore, we observed a 3.24-fold increased risk (95% CI = 1.35-7.78) for smokers aged >65 years carrying 6721TT genotype compared with those carrying the 6721 (GG + GT) genotype. However, the KU70-61C > G polymorphism was not associated with a significantly increased risk of bladder cancer.

CONCLUSIONS

The XRCC7 but not the KU70 polymorphism appears to be involved in the etiology of human bladder cancer. Larger studies with more detailed data on environmental exposure are needed to verify these initial findings.

Targeting phosphoinositide 3-kinase: moving towards therapy

Phosphoinositide 3-kinases (PI3K) orchestrate cell responses including mitogenic signaling, cell survival and growth, metabolic control, vesicular trafficking, degranulation, cytoskeletal rearrangement and migration. Deregulation of the PI3K pathway occurs by activating mutations in growth factor receptors or the PIK3CA locus coding for PI3Kalpha, by loss of function of the lipid phosphatase and tensin homolog deleted in chromosome ten (PTEN/MMAC/TEP1), by the up-regulation of protein kinase B (PKB/Akt), or the impairment of the tuberous sclerosis complex (TSC1/2). All these events are linked to growth and proliferation, and have thus prompted a significant interest in the pharmaceutical targeting of the PI3K pathway in cancer. Genetic targeting of PI3Kgamma (p110gamma) and PI3Kdelta (p110delta) in mice has underlined a central role of these PI3K isoforms in inflammation and allergy, as they modulate chemotaxis of leukocytes and degranulation in mast cells. Proof-of-concept molecules selective for PI3Kgamma have already successfully alleviated disease progress in murine models of rheumatoid arthritis and lupus erythematosus. As targeting PI3K moves forward to therapy of chronic, non-fatal disease, safety concerns for PI3K inhibitors increase. Many of the present inhibitor series interfere with target of rapamycin (TOR), DNA-dependent protein kinase (DNA-PK(cs)) and activity of the ataxia telangiectasia mutated gene product (ATM). Here we review the current disease-relevant knowledge for isoform-specific PI3K function in the above mentioned diseases, and review the progress of >400 recent patents covering pharmaceutical targeting of PI3K. Currently, several drugs targeting the PI3K pathway have entered clinical trials (phase I) for solid tumors and suppression of tissue damage after myocardial infarction (phases I,II).

Aneuploidy, stem cells and cancer

Telomeres which protect the individual chromosomes from disintegration, end-to-end fusion and maintain the genomic integrity during the somatic cell divisions play an important role in cellular aging. Aging and cancer development are linked with each other because cancer is considered a group of complex genetic diseases that develop in old cells and, in both, telomere attrition is involved. Numeric chromosome imbalance also known as aneuploidy is the hallmark of most solid tumors, whether spontaneous or induced by carcinogens. We provide evidence in support of the hypothesis that telomere attrition is the earliest genetic alteration responsible for the induction of aneuploidy. Dysfunctional telomeres are highly recombinogenic leading to the formation of dicentric chromosomes. During cell divisions, such complex chromosome alterations undergo breakage fusion bridge cycles and may lead to loss of heterozygosity (LOH) and gene amplification. Furthermore, we have provided evidence in support of the hypothesis that all types of cancer originate in the organ- or tissue-specific stem cells present in a particular organ. Cancer cells and stem cells share many characteristics, such as, self-renewal, migration, and differentiation. Metaphases with abnormal genetic constitution present in the lymphocytes of cancer patients and in some of their asymptomatic family members may have been derived from the organ-specific stem cells. In addition, evidence and discussion has been presented for the existence of cancer-specific stem cells. Successful treatment of cancer, therefore, should be directed towards these cancer stem cells.

Radiation Response Genotype and Risk of Differentiated Thyroid Cancer: A Case-Control Analysis

BACKGROUND

Radiation is the only clear etiologic agent for differentiated thyroid cancer (DTC). Understanding the factors affecting sensitivity to gamma radiation and susceptibility to DTC will be critical to early detection and prevention of DTC.

HYPOTHESIS

Germline variants of double-strand break repair genes are markers of DTC risk.

OBJECTIVE

Determine the frequency of common single nucleotide polymorphisms of genes of the double-strand break repair pathway in patients with DTC and cancer-free controls.

STUDY DESIGN

Case-control study.

METHODS

This study included 134 patients with DTC, 79 patients with benign thyroid lesions, and 166 cancer-free control subjects. To avoid ethnic confounding, all subjects were non-Hispanic whites. Genotype analyses were performed on DNA isolated from peripheral blood lymphocytes. Multivariate logistic regression analyses were performed to estimate the risk of DTC associated with each variant genotype.

RESULTS

The XRCC3 18067T polymorphic allele was found significantly more commonly among the DTC cases than for the control subjects (P=.006). After multivariate adjustment, having the XRCC3 18067T allele was associated with an increased risk of DTC (adjusted odds ratio [OR] = 2.1; 95% confidence interval [CI] = 1.3 to 3.4; P = .004). In addition, there was a suggestion that the XRCC3 18067T polymorphic allele was more common among the patients with benign thyroid disease (P = .054), and the homozygous polymorphic genotype was associated with risk for benign thyroid disease (adjusted OR = 2.1; 95% CI = 0.9-4.9; P = .078).

CONCLUSIONS

In this case-control analysis, the XRCC3 18067T polymorphism is associated with DTC risk. However, such work needs confirmation in larger studies.

Polymorphisms of DNA Repair Genes and Risk of Glioma

DNA repair genes play a major role in maintaining genomic stability through different repair pathways that are mediated by cell cycle control genes such as p53. We found previously that glioma patients were susceptible to gamma-ray-induced chromosomal breaks, which may be influenced by genetic variation in genes involved in DNA strand breaks, such as XRCC1 in single-strand break repair, XRCC3 and RAD51 in homologous recombination repair, and XRCC7 in nonhomologous end joining double-strand break repair. Therefore, we tested the hypothesis that genetic polymorphisms in XRCC1, XRCC3, RAD51, XRCC7, and p53 were associated with risk of glioma in 309 patients with newly diagnosed glioma and 342 cancer-free control participants frequency matched on age (+/- 5 years), sex, and self-reported ethnicity. We did not find any statistically significant differences in the distributions of XRCC1 Arg399Gln, XRCC3 Thr241Met, RAD51 G135C, and P53 Arg72Pro polymorphisms between the cases and the controls. However, the XRCC7 G6721T variant T allele and TT genotype were more common in the cases (0.668 and 43.4%, respectively) than in the controls (0.613 and 38.9%, respectively), and the differences were statistically significant (P = 0.045 and 0.040, respectively). The adjusted odds ratios were 1.78 (95% confidence interval, 1.08-2.94) and 1.86 (95% confidence interval, 1.12-3.09) for the GT heterozygotes and TT homozygotes, respectively. The combined T variant genotype (GT+TT) was associated with a 1.82-fold increased risk of glioma (95% confidence interval, 1.13-2.93). These results suggest that the T allele may be a risk allele, and this XRCC7 polymorphism may be a marker for the susceptibility to glioma. Larger studies are needed to confirm our findings and unravel the underlying mechanisms.

Mechanisms of human DNA repair: an update

The human genome, comprising three billion base pairs coding for 30000-40000 genes, is constantly attacked by endogenous reactive metabolites, therapeutic drugs and a plethora of environmental mutagens that impact its integrity. Thus it is obvious that the stability of the genome must be under continuous surveillance. This is accomplished by DNA repair mechanisms, which have evolved to remove or to tolerate pre-cytotoxic, pre-mutagenic and pre-clastogenic DNA lesions in an error-free, or in some cases, error-prone way. Defects in DNA repair give rise to hypersensitivity to DNA-damaging agents, accumulation of mutations in the genome and finally to the development of cancer and various metabolic disorders. The importance of DNA repair is illustrated by DNA repair deficiency and genomic instability syndromes, which are characterised by increased cancer incidence and multiple metabolic alterations. Up to 130 genes have been identified in humans that are associated with DNA repair. This review is aimed at updating our current knowledge of the various repair pathways by providing an overview of DNA-repair genes and the corresponding proteins, participating either directly in DNA repair, or in checkpoint control and signaling of DNA damage.

Acute lymphoblastic leukemia characterized by t(8;14)(q11.2;q32)

The t(8;14)(q11.2;q32) is emerging as an uncommon, though recurrent cytogenetic finding. As of yet, too few cases of acute lymphoblastic leukemia (ALL) characterized by this translocation have been studied to determine its prognostic significance with confidence. We therefore report three new patients (two male children and one adult female) and present their hematologic, immunophenotypic, and clinical data. The clinical and laboratory characteristics of 26 other patients with t(8;14)(q11.2;q32) are summarized. The total number of patients now reported in the literature is 29 with a mean age of 14 years. Early relapse, that is, relapse within 6 months, does not appear to be a common feature of this group. The gender distribution is 19 males: 9 females (gender not reported in one case). Twenty-three t(8;14) patients show a pre-B immunophenotype and 24 of 24, on whom information is available, achieved complete remission after induction chemotherapy for B-ALL. Approximately one third of patients with t(8;14) have Down syndrome, 19 of 27 have additional acquired cytogenetic abnormalities, 5 of these have the t(9;22), and 4 show duplication of the abnormal chromosome 14, which is derived from the t(8;14). Hemoglobin and platelet counts are low at presentation in 10 of 10 and 8 of 9 patients, respectively, and the average white blood count is 38.9 x 10(9)/L. Of the 7 patients for whom IgH status has been determined, all show rearrangement of the IgH locus. Two of the present three patients are included in this group; their IgH rearrangement was demonstrated by fluorescence in situ hybridization with IgH break-apart probes.

Dimerization, translocation and localization of Ku70 and Ku80 proteins

The Ku protein is a complex of two subunits, Ku70 and Ku80, and was originally identified as an autoantigen recognized by the sera of patients with autoimmune diseases. The Ku protein plays a key role in multiple nuclear processes, e.g., DNA repair, chromosome maintenance, transcription regulation, and V(D)J recombination. The mechanism underlying the regulation of all the diverse functions of Ku is still unclear, although it seems that Ku is a multifunctional protein that works in nuclei. On the other hand, several studies have reported cytoplasmic or cell surface localization of Ku in various cell types. To clarify the fundamental characteristics of Ku, we have examined the expression, heterodimerization, subcellular localization, chromosome location, and molecular mechanisms of the nuclear transport of Ku70 and Ku80. The mechanism that regulates for nuclear localization of Ku70 and Ku80 appears to play, at least in part, a key role in regulating the physiological function of Ku in vivo.

SCID in Jack Russell terriers: a new animal model of DNA-PKcs deficiency

We recently described the incidence of a SCID disease in a litter of Jack Russell terriers. In this study, we show that the molecular defect in these animals is faulty V(D)J recombination. Furthermore, we document a complete deficit in DNA-dependent protein kinase activity that can be explained by a marked diminution in the expression of the catalytic subunit DNA-dependent protein kinase catalytic subunit (DNA-PKcs). We conclude that as is the case in C.B-17 SCID mice and in Arabian SCID foals, the defective factor in these SCID puppies is DNA-PKcs. In mice, it has been clearly established that DNA-PKcs deficiency produces an incomplete block in V(D)J recombination, resulting in "leaky" coding joint formation and only a modest defect in signal end ligation. In contrast, DNA-PKcs deficiency in horses profoundly blocks both coding and signal end joining. Here, we show that although DNA-PKcs deficiency in canine lymphocytes results in a block in both coding and signal end joining, the deficit in both is intermediate between that seen in SCID mice and SCID foals. These data demonstrate significant species variation in the absolute necessity for DNA-PKcs during V(D)J recombination. Furthermore, the severity of the V(D)J recombination deficits in these three examples of genetic DNA-PKcs deficiency inversely correlates with the relative DNA-PK enzymatic activity expressed in normal fibroblasts derived from these three species.

Frameshift mutation in PRKDC, the gene for DNA-PKcs, in the DNA repair-defective, human, glioma-derived cell line M059J

Anderson, C. W., Dunn, J. J., Freimuth, P. I., Galloway, A. M. and Allalunis-Turner, M. J. Frameshift Mutation in PRKDC, the Gene for DNA-PKcs, in the DNA Repair-Defective, Human, Glioma-Derived Cell Line M059J. Radiat. Res. 156, 2-9 (2001). The glioma-derived cell line M059J is hypersensitive to ionizing radiation, lacks DNA-PK activity, and fails to express protein for the catalytic subunit, DNA-PKcs, while a sister cell line, M059K, derived from the same tumor, has normal DNA-PK activity. Both cell lines are near pentaploid and have multiple copies of chromosome 8, the chromosome on which the DNA-PKcs gene, PRKDC, is located. Sequence analysis of PCR-amplified exons revealed the loss in M059J cells of a single "A" nucleotide in exon 32, corresponding to the first nucleotide of codon 1351 (ACC, Thr) of PRKDC. Loss of the "A" nucleotide would terminate the DNA-PKcs reading frame early in exon 33. DNA from M059K cells had only the wild-type sequence. An analysis of sequences surrounding PRKDC exon 32 from 87 unrelated individuals revealed no polymorphic nucleotides except for a triplet repeat near the 3' end of this exon; no individual had a frameshift mutation in exon 32. No other sequence differences in PRKDC between M059J and M059K cells were observed in approximately 15,000 bp of genomic sequence including the sequences of exons 5 through 38 and surrounding intron sequence, suggesting a possible reduction to homozygosity at this locus prior to acquisition of the mutation leading to the M059J cell line.

A low-pH culture condition enhances the radiosensitizing effect of wortmannin

PURPOSE

The radiosensitizing effect of wortmannin on human tumor cells in a low-pH microenvironment was compared with that in a neutral-pH environment.

METHODS AND MATERIALS

A172 human glioblastoma cells, A549 human lung adenocarcinoma cells, and HMV-1 human melanoma cells were treated with 20 microM wortmannin 2 h before irradiation, and cell survival was examined. A low-pH microenvironment was simulated by exposing cells to low-pH culture medium for 24 h before wortmannin treatment. The effects of wortmannin on the repair of DNA double-strand breaks (dsbs) after 50-Gy irradiation in both low- and neutral-pH conditions were measured by pulsed-field gel electrophoresis. Expression of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) in low-pH conditions was also compared with that in neutral-pH conditions by Western blot analysis.

RESULTS

The radiosensitizing effect of wortmannin was greater in low-pH cultures than in neutral-pH cultures for all cell lines. The fast-rejoining component of DNA dsb repair was inhibited more strongly in low-pH than in neutral-pH conditions, although there was little difference in DNA-PKcs expression between groups.

CONCLUSIONS

The low-pH culture condition, which was designed to mimic the microenvironment of the central tumor mass in actively proliferating solid tumors, enhanced the radiosensitizing effect of wortmannin by inhibiting the fast-rejoining component of DNA dsb repair and by prolonging the retention of nonrejoined DNA dsbs.

Ku autoantigen: a multifunctional DNA-binding protein

Ku is a heterodimeric protein composed of approximately 70- and approximately 80-kDa subunits (Ku70 and Ku80) originally identified as an autoantigen recognized by the sera of patients with autoimmune diseases. Ku has high binding affinity for DNA ends and that is why originally it was known as a DNA end binding protein, but now it is known to also bind the DNA structure at nicks, gaps, hairpins, as well as the ends of telomeres. It has been reported also to bind with sequence specificity to DNA and with weak affinity to RNA. Ku is an abundant nuclear protein and is present in vertebrates, insects, yeast, and worms. Ku contains ssDNA-dependent ATPase and ATP-dependent DNA helicase activities. It is the regulatory subunit of the DNA-dependent protein kinase that phosphorylates many proteins, including SV-40 large T antigen, p53, RNA-polymerase II, RP-A, topoisomerases, hsp90, and many transcription factors such as c-Jun, c-Fos, oct-1, sp-1, c-Myc, TFIID, and many more. It seems to be a multifunctional protein that has been implicated to be involved directly or indirectly in many important cellular metabolic processes such as DNA double-strand break repair, V(D)J recombination of immunoglobulins and T-cell receptor genes, immunoglobulin isotype switching, DNA replication, transcription regulation, regulation of heat shock-induced responses, regulation of the precise structure of telomeric termini, and it also plays a novel role in G2 and M phases of the cell cycle. The mechanism underlying the regulation of all the diverse functions of Ku is still obscure.

Brief communication: heat-induced accumulation of p53 and hsp72 is suppressed in lung fibroblasts from the SCID mouse

PURPOSE

To investigate how DNA-dependent protein kinase (DNA-PK) contributes to p53-dependent signal transduction after heat shock, thermosensitivity and accumulation of p53 and hsp72 after heat shock in lung fibroblasts derived from the SCID mouse were analysed.

MATERIALS AND METHODS

Thermosensitivity at 44 degrees C in colony-forming units and Western blot analysis of p53 and hsp72 were analysed.

RESULTS

The results indicated that (1) the thermosensitivity at 44 degrees C of SCID cells was higher than that of parental cells and (2) heat-induced accumulation of p53 and hsp72 was abolished and suppressed in SCID cells as compared with that in parental cells respectively.

CONCLUSIONS

The findings suggest that the catalytic subunit of DNA-PK may play an important role upstream of p53 and hsp72, which are possible determinants of cellular thermosensitivity.

Analyses of TCRB rearrangements substantiate a profound deficit in recombination signal sequence joining in SCID foals: implications for the role of DNA-dependent protein kinase in V(D)J recombination

We reported previously that the genetic SCID disease observed in Arabian foals is explained by a defect in V(D)J recombination that profoundly affects both coding and signal end joining. As in C.B-17 SCID mice, the molecular defect in SCID foals is in the catalytic subunit of the DNA-dependent protein kinase (DNA-PKCS); however, in SCID mice, signal end resolution remains relatively intact. Moreover, recent reports indicate that mice that completely lack DNA-PKCS also generate signal joints at levels that are indistinguishable from those observed in C.B-17 SCID mice, eliminating the possibility that a partially active version of DNA-PKCS facilitates signal end resolution in SCID mice. We have analyzed TCRB rearrangements and find that signal joints are reduced by approximately 4 logs in equine SCID thymocytes as compared with normal horse thymocytes. A potential explanation for the differences between SCID mice and foals is that the mutant DNA-PKCS allele in SCID foals inhibits signal end resolution. We tested this hypothesis using DNA-PKCS expression vectors; in sum, we find no evidence of a dominant-negative effect by the mutant protein. These and other recent data are consistent with an emerging consensus: that in normal cells, DNA-PKCS participates in both coding and signal end resolution, but in the absence of DNA-PKCS an undefined end joining pathway (which is variably expressed in different species and cell types) can facilitate imperfect signal and coding end joining.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

Differential stability of the DNA-activated protein kinase catalytic subunit mRNA in human glioma cells

DNA-dependent protein kinase (DNA-PK) functions in double-strand break repair and immunoglobulin [V(D)J] recombination. We previously established a radiation-sensitive human cell line, M059J, derived from a malignant glioma, which lacks the catalytic subunit (DNA-PKcs) of the DNA-PK multiprotein complex. Although previous Northern blot analysis failed to detect the DNA-PKcs transcript in these cells, we show here through quantitative studies that the transcript is present, albeit at greatly reduced (approximately 20x) levels. Sequencing revealed no genetic alteration in either the promoter region, the kinase domain, or the 3' untranslated region of the DNA-PKcs gene to account for the reduced transcript levels. Nuclear run-on transcription assays indicated that the rate of DNA-PKcs transcription in M059J and DNA-PKcs proficient cell lines was similar, but the stability of the DNA-PKcs message in the M059J cell line was drastically (approximately 20x) reduced. Furthermore, M059J cells lack an alternately spliced DNA-PKcs transcript that accounts for a minor (5-20%) proportion of the DNA-PKcs message in all other cell lines tested. Thus, alterations in DNA-PKcs mRNA stability and/or the lack of the alternate mRNA may result in the loss of DNA-PKcs activity. This finding has important implications as DNA-PKcs activity is essential to cells repairing damage induced by radiation or radiomimetric agents.

DNA-dependent protein kinase content and activity in lung carcinoma cell lines: correlation with intrinsic radiosensitivity

Intrinsic radiosensitivity and rejoining of radiation-induced DNA double-strand breaks (DNA-dsb) were analysed in five lung carcinoma cell lines: U-1285, U-1906, H-69, H-82 and U-1810. RS correlated with both the initial phase of DNA-dsb rejoining, at 15 min (r2 = 0.818) and the late phase, at 120 min postirradiation (r2 = 0.774), the most sensitive cell line (U-1285) showing least dsb rejoining and the most resistant (U-1810) showing most dsb rejoining of all five cell lines studied. As DNA-PK has been recognised as an important molecular component involved in DNA-dsb repair, we analysed content and activity of this kinase. We found that DNA-PK content and activity correlated with RS (r2 = 0.941 and r2 = 0.944, respectively). The lowest DNA-dependent content/activity was found in the most radiosensitive cells, U-1285 and H-69, whilst the highest content/activity was found in the most radioresistant cells U-1810. These results suggest a correlation between RS and DNA-PK content/activity in lung carcinoma cell lines.

Hypersensitivity of Ku-deficient cells toward the DNA topoisomerase II inhibitor ICRF-193 suggests a novel role for Ku antigen during the G2 and M phases of the cell cycle

Ku antigen is a heterodimer, comprised of 86- and 70-kDa subunits, which binds preferentially to free DNA ends. Ku is associated with a catalytic subunit of 450 kDa in the DNA-dependent protein kinase (DNA-PK), which plays a crucial role in DNA double-strand break (DSB) repair and V(D)J recombination of immunoglobulin and T-cell receptor genes. We now demonstrate that Ku86 (86-kDa subunit)-deficient Chinese hamster cell lines are hypersensitive to ICRF-193, a DNA topoisomerase II inhibitor that does not produce DSB in DNA. Mutant cells were blocked in G2 at drug doses which had no effect on wild-type cells. Moreover, bypass of this G2 block by caffeine revealed defective chromosome condensation in Ku86-deficient cells. The hypersensitivity of Ku86-deficient cells toward ICRF-193 was not due to impaired in vitro decatenation activity or altered levels of DNA topoisomerase IIalpha or -beta. Rather, wild-type sensitivity was restored by transfection of a Ku86 expression plasmid into mutant cells. In contrast to cells deficient in the Ku86 subunit of DNA-PK, cells deficient in the catalytic subunit of the enzyme neither accumulated in G2/M nor displayed defective chromosome condensation at lower doses of ICRF-193 compared to wild-type cells. Our data suggests a novel role for Ku antigen in the G2 and M phases of the cell cycle, a role that is not related to its role in DNA-PK-dependent DNA repair.