DSB Repair (dsb + repair)

Distribution by Scientific Domains

Selected Abstracts

Expression of DNA repair gene Ku80 in lymphoid neoplasm

Tsai-Yun Chen
Abstract:,Objectives:,Ku, a heterodimer of KU70 and Ku80 that binds to double-strand DNA breaks (DSBs) and activates the catalytic subunit (DNA-PKcs) when DNA is bound, is essential in DSB repair and V(D)J recombination. Ku80 is a putative tumor suppressor gene that might play an important role in drug resistance. Our aim was to determine the role of Ku80 in lymphoid malignancy. Patients and methods:,Competitive reverse transcription-polymerase chain reaction assays were performed and the expression levels of Ku80 were measured in normal peripheral blood mononuclear cells (n = 9) and malignant cells from 25 patients with acute lymphoblastic leukemia (ALL) (14 children, 11 adults), and chronic lymphoproliferative disorders (n = 6). The Ku80 transcripts were sequencing for the possibility of mutation. Results:,No mutation or Ku80 variant at the RNA level was seen in any patient samples or in the Raji or CCRF-CEM cell lines. In Ku80 expression, 8.8-, 1.9-, and 6.2-fold mean increases were seen in adult, pediatric ALL, and chronic lymphoid malignancies compared with the control. The Ku80 was significantly higher in adult than in pediatric ALL (P = 0.02). The amount of Ku80 expression in ALL was moderately correlated with peripheral white blood cell counts, but not with Ki67 labeling index. High Ku80 expressers (higher than the mean of all patients with ALL) tended to respond poorly to therapy: Only 22% of high Ku80 expressers achieved durable complete remission compared to 62% of low expressers. Conclusions:,Our study suggests that Ku80 might contribute to generally poor prognoses in adult ALL. [source]

Papillary and muscle invasive bladder tumors with distinct genomic stability profiles have different DNA repair fidelity and KU DNA-binding activities

Johanne Bentley
Low-grade noninvasive papillary bladder tumors are genetically stable whereas muscle invasive bladder tumors display high levels of chromosomal aberrations. As cells deficient for nonhomologous end-joining (NHEJ) pathway components display increased genomic instability, we sought to determine the NHEJ repair characteristics of bladder tumors and correlate this with tumor stage and grade. A panel of 13 human bladder tumors of defined stage and grade were investigated for chromosomal aberrations by comparative genomic hybridization and for NHEJ repair fidelity and function. Repair assays were conducted with extracts made directly from bladder tumor specimens to avoid culture-induced phenotypic alterations and selection bias as only a minority of bladder tumors grow in culture. Four noninvasive bladder tumors (pTaG2), which were genetically stable, repaired a partially incompatible double-strand break (DSB) by NHEJ-dependent annealing of termini and fill-in of overhangs with minimal loss of nucleotides. In contrast, four muscle invasive bladder cancers (pT2-3G3), which displayed gross chromosomal rearrangements, repaired DSBs in an error-prone manner involving extensive resection and microhomology association. Four minimally invasive bladder cancers (pT1G3) had characteristics of both repair types. Error-prone repair in bladder tumors correlated with reduced KU DNA-binding and loss of TP53 function. In conclusion, there were distinct differences in DSB repair between noninvasive papillary tumors and higher stage/grade invasive cancers. End-joining fidelity correlated with stage and was increasingly error-prone as tumors became more invasive and KU binding activity reduced; these changes may underlie the different genomic profiles of these tumors. 2008 Wiley-Liss, Inc. [source]

Role and regulation of human XRCC4-like factor/cernunnos,

Kirsten Dahm
Abstract In mammalian cells, non-homologous end joining (NHEJ) is the major double strand break (DSB) repair mechanism during the G1 phase of the cell cycle. It also contributes to DSB repair during the S and G2 phases. Ku heterodimer, DNA PKcs, XRCC4 and DNA Ligase IV constitute the core NHEJ machinery, which joins directly ligatable ends. XRCC4-like factor/Cernunnos (XLF/Cer) is a recently discovered interaction partner of XRCC4. Current evidence suggests the following model for the role of XLF/Cer in NHEJ: after DSB induction, the XRCC4-DNA Ligase IV complex promotes efficient accumulation of XLF/Cer at DNA damage sites via constitutive interaction of the XRCC4 and XLF/Cer head domains and dependent on components of the DNA PK complex. Ku alone can stabilise the association of XLF/Cer with DNA ends. XLF/Cer stimulates ligation of complementary and non-complementary DNA ends by XRCC4-DNA Ligase IV. This activity involves the carboxy-terminal DNA binding region of XLF/Cer and could occur via different, non-exclusive modes: (i) enhancement of the stability of the XRCC4-DNA Ligase IV complex on DNA ends by XLF/Cer, (ii) modulation of the efficiency and/or specificity of DNA Ligase IV by binding of XLF/Cer to the XRCC4-DNA Ligase IV complex, (iii) promotion of the alignment of blunt or other non-complementary DNA ends by XLF/Cer for ligation. XLF/Cer promotes the preservation of 3, overhangs, restricts nucleotide loss and thereby promotes accuracy of DSB joining by XRCC4-DNA Ligase IV during NHEJ and V(D)J recombination. J. Cell. Biochem. 104: 1534,1540, 2008. 2008 Wiley-Liss, Inc. [source]

Intracellular redistribution and modification of proteins of the Mre11/Rad50/Nbs1 DNA repair complex following irradiation and heat-shock

Joshua D. Seno
Mre11, Rad50, and Nbs1form a tight complex which is homogeneously distributed throughout the nuclei of mammalian cells. However, after irradiation, the Mre11/Rad50/Nbs1 (M/R/N) complex rapidly migrates to sites of double strand breaks (DSBs), forming foci which remain until DSB repair is complete. Mre11 and Rad50 play direct roles in DSB repair, while Nbs1 appears to be involved in damage signaling. Hyperthermia sensitizes mammalian cells to ionizing radiation. Radiosensitization by heat shock is believed to be mediated by an inhibition of DSB repair. While the mechanism of inhibition of repair by heat shock remains to be elucidated, recent reports suggest that the M/R/N complex may be a target for inhibition of DSB repair and radiosensitization by heat. We now demonstrate that when human U-1 melanoma cells are heated at 42.5 or 45.5C, Mre11, Rad50, and Nbs1 are rapidly translocated from the nucleus to the cytoplasm. Interestingly, when cells were exposed to ionizing radiation (12 Gy of X-rays) prior to heat treatment, the extent and kinetics of translocation were increased when nuclear and cytoplasmic fractions of protein were analyzed immediately after treatment. The kinetics of the translocation and subsequent relocalization back into the nucleus when cells were incubated at 37C from 30 min to 7 h following treatment were different for each protein, which suggests that the proteins redistribute independently. However, a significant fraction of the translocated proteins exist as a triple complex in the cytoplasm. Treatment with leptomycin B (LMB) inhibits the translocation of Mre11, Rad50, and Nbs1 to the cytoplasm, leading us to speculate that the relocalization of the proteins to the cytoplasm occurs via CRM1-mediated nuclear export. In addition, while Nbs1 is rapidly phosphorylated in the nuclei of irradiated cells and is critical for a normal DNA damage response, we have found that Nbs1 is rapidly phosphorylated in the cytoplasm, but not in the nucleus, of heated irradiated cells. The phosphorylation of cytoplasmic Nbs1, which cannot be inhibited by wortmannin, appears to be a unique post-translational modification in heated, irradiated cells, and coupled with our novel observations that Mre11, Rad50, and Nbs1 translocate to the cytoplasm, lend further support for a role of the M/R/N complex in thermal radiosensitization and inhibition of DSB repair. J. Cell. Physiol. 199: 157,170, 2004 2004 Wiley-Liss, Inc. [source]

WRN, the protein deficient in Werner syndrome, plays a critical structural role in optimizing DNA repair

AGING CELL, Issue 4 2003
Lishan Chen
Summary Werner syndrome (WS) predisposes patients to cancer and premature aging, owing to mutations in WRN. The WRN protein is a RECQ-like helicase and is thought to participate in DNA double-strand break (DSB) repair by non-homologous end joining (NHEJ) or homologous recombination (HR). It has been previously shown that non-homologous DNA ends develop extensive deletions during repair in WS cells, and that this WS phenotype was complemented by wild-type (wt) WRN. WRN possesses both 3, , 5, exonuclease and 3, , 5, helicase activities. To determine the relative contributions of each of these distinct enzymatic activities to DSB repair, we examined NHEJ and HR in WS cells (WRN,/,) complemented with either wtWRN, exonuclease-defective WRN (E,), helicase-defective WRN (H,) or exonuclease/helicase-defective WRN (E,H,). The single E, and H, mutants each partially complemented the NHEJ abnormality of WRN,/, cells. Strikingly, the E,H, double mutant complemented the WS deficiency nearly as efficiently as did wtWRN. Similarly, the double mutant complemented the moderate HR deficiency of WS cells nearly as well as did wtWRN, whereas the E, and H, single mutants increased HR to levels higher than those restored by either E,H, or wtWRN. These results suggest that balanced exonuclease and helicase activities of WRN are required for optimal HR. Moreover, WRN appears to play a structural role, independent of its enzymatic activities, in optimizing HR and efficient NHEJ repair. Another human RECQ helicase, BLM, suppressed HR but had little or no effect on NHEJ, suggesting that mammalian RECQ helicases have distinct functions that can finely regulate recombination events. [source]

Limited concentration of RecA delays DNA double-strand break repair in Deinococcus radiodurans R1

Edmond Jolivet
Summary To evaluate the importance of RecA in DNA double-strand break (DSB) repair, we examined the effect of low and high RecA concentrations such as 2500 and 100 000 molecules per cell expressed from the inducible Pspac promoter in Deinococcus radiodurans in absence or in presence of IPTG respectively. We showed that at low concentration, RecA has a negligible effect on cell survival after ,-irradiation when bacteria were immediately plated on TGY agar whereas it significantly decreased the survival to ,-irradiation of ,ddrA cells while overexpression of RecA can partially compensate the loss of DdrA protein. In contrast, when cells expressing limited concentration of RecA were allowed to recover in TGY2X liquid medium, they showed a delay in mending DSB, failed to reinitiate DNA replication and were committed to die during incubation. A deletion of irrE resulted in sensitivity to ,-irradiation and mitomycin C treatment. Interestingly, constitutive high expression of RecA compensates partially the ,irrE sensitization to mitomycin C. The cells with low RecA content also failed to cleave LexA after DNA damage. However, neither a deletion of the lexA gene nor the expression of a non-cleavable LexA(Ind,) mutant protein had an effect on survival or kinetics of DNA DSB repair compared with their lexA+ counterparts in recA+ as well as in bacteria expressing limiting concentration of RecA, suggesting an absence of relationship between the absence of LexA cleavage and the loss of viability or the delay in the kinetics of DSB repair. Thus, LexA protein seems to play no major role in the recovery processes after ,-irradiation in D. radiodurans. [source]

Mutagenicity of non-homologous end joining DNA repair in a resistant subset of human chronic lymphocytic leukaemia B cells

Ludovic Deriano
Summary Non-homologous end joining (NHEJ) is an important determinant of genomic stability in mammalian cells. This DNA repair pathway is upregulated in a subset of B-cell chronic lymphocytic leukaemia (B-CLL) cells resistant to DNA damage-induced apoptosis. Using an in vitro assay for double-strand breaks (DSB) end ligation, we studied the fidelity of DSB repair in B-CLL cells which were resistant or sensitive to in vitro DSB-induced apoptosis with concomitant patients' resistance or sensitivity to chemotherapy, respectively. The fidelity of DNA repair was determined by DNA sequencing of polymerase chain reaction products cloned in pGEM-T vector. Sequence analysis of DNA end junctions showed that the frequency of accurate ligation was higher in sensitive B-CLL cells and control cell lines, than in resistant cells where end joining was associated with extended deletions. Upregulated and error-prone NHEJ in resistant cells could be a quite possible mechanism underlying both genomic instability and poor clinical outcome. [source]

Impairment of double-strand breaks repair and aberrant splicing of ATM and MRE11 in leukemia,lymphoma cell lines with microsatellite instability

CANCER SCIENCE, Issue 3 2006
Maria Francisca Ham
Mutations of DNA double-strand breaks (DSB) repair genes, ATM, MRE11, RAD50, NBS1 and ATR, are postulated to play a role in the development of gastrointestinal malignancies with an impaired mismatch repair (MMR) function. In the present study, mutations of these genes together with the presence of microsatellite instability (MSI) were examined in 50 leukemia,lymphoma cell lines. MSI was detected in 13 (26%) lines. Mutations of intronic mononucleotide repeats in ATM and MRE11 were found in nine and six lines, respectively, whereas mutations of mononucleotide repeats of RAD50 were found in only one line, and none were found in either NBS1 or ATR. Frequencies of ATM and MRE11 mutations were significantly higher in MSI-positive than MSI-negative lines. These mutations generated aberrant splicing in both genes. The intensity of the aberrant transcript of ATM (497del22) was stronger in five lines harboring mononucleotide mutations of 2 bp or more than in the lines without or with a 1-bp mutation. The intensity of the aberrant transcript of MRE11 (315del88) was stronger in four lines with mononucleotide mutations than in lines without. The expression levels of ATM and MRE11 transcripts in MSI-positive lines were significantly higher than those in MSI-negative lines. MSI-positive cell lines showed delay or abrogation of DSB repair. The present study suggests that impairment of the MMR system causes aberrant transcripts in the DSB repair genes ATM and MRE11. This might result in inactivation of the DSB repair system, thus inducing an acceleration of genome instability and accumulation of genetic damage. (Cancer Sci 2006; 97: 226,234) [source]