Induction of genomic instability in TK6 human lymphoblasts exposed to 137Cs gamma radiation: comparison to the induction by exposure to accelerated 56Fe particles.

The induction of genomic instability in TK6 human lymphoblasts by exposure to (137)Cs gamma radiation was investigated by measuring the frequency and characteristics of unstable clones isolated approximately 36 generations after exposure. Clones surviving irradiation and control clones were analyzed for 17 characteristics including chromosomal aberrations, growth defects, alterations in response to a second irradiation, and mutant frequencies at the thymidine kinase and Na(+)/K(+) ATPase loci. Putative unstable clones were defined as those that exhibited a significant alteration in one or more characteristics compared to the controls.

Characteristics of genomic instability in clones of TK6 human lymphoblasts surviving exposure to 56Fe ions.

Genomic instability in the human lymphoblast cell line TK6 was studied in clones surviving 36 generations after exposure to accelerated 56Fe ions. Clones were assayed for 20 characteristics, including chromosome aberrations, plating efficiency, apoptosis, cell cycle distribution, response to a second irradiation, and mutant frequency at two loci. The primary effect of the 56Fe-ion exposure on the surviving clones was a significant increase in the frequency of unstable chromosome aberrations compared to the very low spontaneous frequency, along with an increase in the phenotypic complexity of the unstable clones.

Diverse delayed effects in human lymphoblastoid cells surviving exposure to high-LET (56)Fe particles or low-LET (137)Cs gamma radiation.

To obtain information on the origin of radiation-induced genomic instability, we characterized a total of 166 clones that survived exposure to (56)Fe particles or (137)Cs gamma radiation, isolated approximately 36 generations after exposure, along with their respective control clones. Cytogenetic aberrations, growth alterations, responses to a second irradiation, and mutant frequencies at the Na(+)/K(+) ATPase and thymidine kinase loci were determined. A greater percentage of clones that survived exposure to (56)Fe particles exhibited instability (defined as clones showing one or more outlying characteristics) than in the case of those that survived gamma irradiation.

Differential antimutagenicity of WR-1065 added after irradiation in L5178Y cell lines.

The purpose of this study was to determine the antimutagenicity of WR-1065 added after irradiation of cells of cell lines differing in their ability to rejoin radiation-induced DNA double-strand breaks (DSBs). The postirradiation antimutagenicity of WR-1065 at the thymidine kinase locus was demonstrated for L5178Y (LY)-S1 cells that are deficient in repair of DNA DSBs. Less postirradiation antimutagenicity of WR-1065 was observed in LY-R16 and LY-SR1 cells, which are relatively efficient in DSB repair.

Mutagenicity of photodynamic therapy as compared to UVC and ionizing radiation in human and murine lymphoblast cell lines.

The mutagenicity of photodynamic therapy (PDT) using red light and either Photofrin (porfimer sodium) (PF) or aluminum phthalocyanine (AlPc) as the photosensitizer was determined at the thymidine kinase (TK) locus in the human lymphoblastic cell lines, TK6 and WTK1, and was compared to the mutagenicity of UVC and X-radiation in these cells as well as the mutagenicity of PDT in murine L5178Y lymphoblastic cell lines. Photodynamic therapy was found not to be mutagenic in TK6 cells, which possess an active p53 gene and which are relatively deficient in recombination and repair of DNA double-strand breaks. In contrast, PDT with either sensitizer was significantly mutagenic in WTK1 cells, which harbor an inactivating mutation in the p53 gene and are relatively efficient in recombination and double-strand break repair as compared to TK6 cells.

Induction of multilocus mutations at the Tk1 locus after X irradiation of L5178Y cells at different times in the mitotic cycle.

TK1+/- L5178Y-R16 cells were separated into G1, S and G2/M-phase populations by centrifugal elutriation and were treated with 1.5 Gy X radiation. Cells irradiated in the G1 and G2/M phases were most sensitive to the cytotoxic effects of radiation, while cells irradiated in the G2/M phase showed the highest mutant frequency at the thymidine kinase (Tk1) locus.

Characterization of multilocus lesions in human cells exposed to X radiation and radon.

Human TK6 lymphoblasts were exposed to X radiation or radon, and thymidine kinase negative (TK-/-) mutants were selected, isolated and harvested for analysis of structural changes in the TK gene. A large majority (82%) of the radon-induced mutants, 74% of the X-radiation-induced mutants and 45% of the spontaneous mutants lost the entire active TK allele. To analyze these mutants further we measured the loss of heterozygosity at several loci neighboring the TK locus on chromosome 17q.

Cytotoxic and mutagenic effects of radon and radon daughters on murine L5178Y lines differing in DNA repair.

The effects of 222Rn were measured in mouse L5178Y (LY) lymphoblasts that differ in repair capabilities. Line LY-S1 is deficient in the repair of X-radiation-induced DNA doublestrand breaks, while lines LY-R16 and LY-R83 are presumed to be deficient in the excision of UV-radiation-induced pyrimidine dimers. Line LY-R83 is hemizygous while the other two lines are heterozygous at the thymidine kinase (tk) locus.

The sensitivity to DNA topoisomerase inhibitors in L5178Y lymphoma strains is not related to a primary defect of DNA topoisomerases.

DNA topoisomerase-targeting antitumor drugs are potent inducers of protein-concealed strand breaks in mammalian cells and act by trapping DNA topoisomerases on chromosomal DNA in the form of drug-enzyme-DNA cleavable complexes. It has been proposed that the cleavable complex is an unusual form of DNA damage that elicits cellular responses analogous to those caused by DNA damaging agents. The relationship between topoisomerase-targeting drug-induced damage and radiation-induced damage has been investigated by analyzing the properties of DNA topoisomerases in mouse L5178Y lymphoma strains that are cross-sensitive to topoisomerase I-II inhibitors and to UV light or X-ray irradiation.

DNA double-strand break rejoining deficiency in TK6 and other human B-lymphoblast cell lines.

TK6, WI-L2, SB and three other B-lymphoblast lines were deficient in the rejoining of DNA double-strand breaks (DSBs) induced by ionizing radiation. Cells of these cell lines rejoin less than 50% of the breaks in 60 min after exposure, as assayed by filter elution at pH 9.6.

Induction of multilocus lesions by UVC-radiation in mouse L5178Y lymphoblasts.

The survival, the mutant frequency and the nature of the DNA alteration responsible for the inactivation of the thymidine kinase (tk) locus were investigated in 5 strains of mouse L5178Y lymphoblasts exposed to UVC radiation. The nature of the DNA alteration was investigated in independent TK-/- mutants using Southern blot analysis. The concomitant loss of galactokinase (GK) activity in homogenates of individual TK-/- mutants was taken as an indication that the lesion inactivating the tk allele extended to the neighboring galactokinase (gk) allele.

Mutagenicity of 2-amino-N6-hydroxyadenine at the tk locus in L5178Y strains differing in repair capabilities and karyotype.

The cytotoxicity and mutagenicity of 2-amino-N6-hydroxyadenine (AHA) were measured in strains of L5178Y differing in repair capabilities and karyotype. Strain LY-R83 is monosomic for chromosome 11 and is therefore hemizygous for the tk gene, while strains LY-R16 and LY-S1 are TK+/- heterozygotes. Both strain LY-R83 and LY-R16 are sensitive to UV light and are presumed to be deficient in the excision of pyrimidine dimers as shown for the parental strain, LY-R (Hagen et al.

The effect of dose rate on X-radiation-induced mutant frequency and the nature of DNA lesions in mouse lymphoma L5178Y cells.

The induction of mutants at the heterozygous tk locus by X radiation was found to be dose-rate dependent in L5178Y-R16 (LY-R16) cells, but very little dose-rate dependence was observed in the case of strain L5178Y-S1 (LY-S1), which is deficient in the repair of DNA double-strand breaks. Induction of mutants by X radiation at the hemizygous hprt locus was dose-rate independent for both strains. These results are in agreement with the hypothesis that the majority of X-radiation-induced TK-/- mutants harbor multilocus deletions caused by the interaction of damaged DNA sites.

Relationship between topoisomerase II and radiosensitivity in mouse L5178Y lymphoma strains.

The cytotoxic and mutagenic effects of topoisomerase II inhibitors were measured in closely related strains of mouse lymphoma L5178Y cells differing in their sensitivity to ionizing radiation. Strain LY-S is sensitive to ionizing radiation relative to strain LY-R and is deficient in the rejoining of DNA double-strand breaks induced by this agent, whereas 2 radiation-resistant variants of strain LY-S have regained the ability to rejoin these double-strand breaks. We have found that the sensitivity of these cells to m-AMSA, VP-16, and ellipticine is correlated to their sensitivity to ionizing radiation.

Deficiency in DNA repair in mouse lymphoma strain L5178Y-S.

The production and repair of radiation-induced DNA damage were measured by filter elution in strains of mouse lymphoma L5178Y cells differing in their sensitivity to ionizing radiation. The induction of radiation-induced damage, as measured by filter elution at pH 12.1, 9.

Locus specificity in the mutability of L5178Y mouse lymphoma cells: the role of multilocus lesions.

Mouse L5178Y strain LY-S and its parental strain LY-R differ in their comparative sensitivities to the cytotoxic effects of various mutagenic agents--i.e., strain LY-S has been found to be more sensitive, less sensitive, or similarly sensitive to individual agents in comparison to strain LY-R.