Toxicity of tritiated thymidine in P388F lymphoma cells. II. Effects of DNA molecular weight

Toxicity and mutagenicity of X-rays and [125I]dUrd or [3H]TdR incorporated in the DNA of human lymphoblast cells

We measured the toxicity and mutagenicity induced in human diploid lymphoblasts by various radiation doses of X-rays and two internal emitters, [125I]iododeoxyuridine ([125I]dUrd) and [3H]thymidine ([3H]TdR), incorporated into cellular DNA. [125I]dUrd was more effective than [3H]TdR at killing cells and producing mutations to 6-thioguanine resistance (6TGR). No ouabain-resistant mutants were induced by any of these agents. Expressing dose as total disintegrations per cell (dpc), the D0 for cell killing for [125I]dUrd was 28 dpc and for [3H]TdR was 385 dpc. The D0 for X-rays was 48 rad at 37 degrees C. The slopes of the mutation curves were approximately 75 x 10(-8) 6TGR mutants per cell per disintegration for [125I]dUrd and 2 x 10(-8) for [3H]TdR. X-Rays induced 8 x 10(-8) 6TGR mutants per cell per rad. Normalizing for survival, [125I]dUrd remained much more mutagenic at low doses (high survival levels) than the other two agents. Treatment of the cells at either 37 degrees C or while frozen at -70 degrees C yielded no difference in cytotoxicity or mutation for [125]dUrd or [3H]TdR, whereas X-rays were 6 times less effective in killing cells at -70 degrees C. Assuming that incorporation was random throughout the genome, the mutagenic efficiencies of the radionuclides could be calculated by dividing the mutation rate by the level of incorporation. If the effective target size of the 6TGR locus is 1000-3000 base pairs, then the mutagenic efficiency of [125I]dUrd is 1.0-3.0 and of [3H]TdR is 0.02-0.06 total genomic mutations per cell per disintegration. 125I disintegrations are known to produce localized DNA double-strand breaks. If these breaks are potentially lethal lesions, they must be repaired, since the mean lethal dose (D0) was 28 dpc. The observations that a single dpc has a high probability of producing a mutation (mutagenic efficiency 1.0-3.0) would suggest, however, that this repair is extremely error-prone. If the breaks need not be repaired to permit survival, then lethal lesions are a subset of or are completely different from mutagenic lesions.

DNA strand breaks, repair, and survival in x-irradiated mammalian cells

The yields of unrepairable single-and double-strand breaks in the DNA of x-irradiated Chinese hamster cells were measured by low-speed neutral and alkaline sucrose density gradient sedimentation in order to investigate the relation between these lesions and reproductive death. After maximal single-strand remoining, at all doses, the number of residual single-strand breaks was twice the number of residual double-strand breaks. Both double-strand and unrepairable single-strand breaks were proportional to the square of absorbed dose, in the range 10-50 krad. No rejoining of double-strand breaks was observed. These observations suggest that, in mammalian cells, most double-strand breaks are not repairable, while all single-strand breaks are repaired except those that are sufficiently close on complementary strands to constitute double-strand breaks. Comparison with cell survival measurements at much lower doses suggests that loss of reproductive capacity corresponds to induction of approximately one double-strand break.

Fractionation of mammalian DNA on deae-cellulose

Chromatography on a DEAE-cellulose (DE-52) column of native and denatured DNA from P388F cells has been studied. The main bulk of native DNA is eluted at 0.8M NaC1 and a minor fraction is eluted with 0.5N NaOH. The proportion of the DNA components obtained depends on the type of isotopic labelling used and the method of storing of the DNA preparation following isolation. Heat-denatured DNA elutes in 2 M NaC1 mainly within the pH gradient form 0.1 M NH4OH. When native DNA is chromatographed on DEAE-cellulose, a small fraction of the DNA elutes at 0.5 N NaOH. This fraction is double-stranded, as determined by hydroxyapatite chromatography. A similar component predominates, however, when "newly synthesised" DNA is fracionated. The profiles obtained with "newly synthesised" and "template" labelled DNA differ in their undenatured and heat-denatured configurations. The presence of formaldehyde during the chromatography of undenatured DNA leads to an increased homogeneity of the profile and during heat denaturation considerable modifications to the profile are observed. Some of the changes can be explained in terms of a decrease in the heterogeneity of the charge distribution on the DNA. The technique appears to combine a high degree of reproducibility with sensitivity to charge clustering along the DNA.