The organization and repair of DNA in the mammaliam chromosome. I. Calibration procedures and errors in the determination of the molecular weight of native DNA

Cell-cell interactions in spheroids maintained in suspension

We have developed a system of mixed aggregates of cultured cells, to model in situ cell interactions. This three-dimensional (3D) system of floating cell aggregates, termed spheroids for their round shape, enables one to monitor their growth in both size and number of constituent clonogens and to measure survival curves for cells having 3D cell-cell interactions. This system was used to measure the three-dimensional cell-cell interactions on growth, and clonogenicity of either AG1522 fibroblasts, or HeLa cervical cancer cells (pure spheroids, or if both feeder and test cells are the same type, pseudohybrid spheroids), and/or of mixtures of both (hybrid spheroids). By following the increase or decrease in size of, or number of clonogens per, spheroid over time, one obtains growth or inhibition curves. By relating these clonogen numbers, one obtains, after a suitable growth period, relative survival. The system allows one to score the effects of irradiation and of other treatments, as well as the effect of interaction of the constituent cells on their survival. Floating pure, or pseudohybrid (composed of 10% live fibroblasts and 90% supralethally irradiated fibroblast feeder cells) spheroids, shrank to about 10-20% of their volume in three days and then remained at that size for up to six days. In contrast, pure spheroids composed of live HeLa cells increased their volume by an order of magnitude over the same period. Survival of cells in spheroids was measured by the ability of individual spheroids to grow beyond a size implying a ten-fold increase. A caveat to be observed is to correct survival for cellular multiplicity, i.e. reduce survival values to compensate for more than one colony former at the time of irradiation. The system of spheroids floating and growing in nutrient medium provides a selective system for evaluating growth of HeLa, and by implication, other neoplastic cells, without interference from (overgrowth by) normal fibroblasts. Thus it is possible to discriminate between normal and neoplastic cells by virtue of whether or not cells grow in suspension. Such a system seems ideal for testing novel strategies (radiation in combination with chemicals), in an in vivo-like environment.

A review of dsb induction data for varying quality radiations

PURPOSE

This short review summarizes the data obtained with various techniques for measuring the yields of double strand breaks (dsb) produced by particle radiations of differing linear energy transfer (LET) in order to obtain relative biological effectiveness (RBE) values.

RESULTS AND CONCLUSIONS

Studies aimed at understanding the interactions of different types of radiation with cellular DNA have monitored the yields of DNA dsb versus radiation quality. Several techniques have been used to measure dsb yields in mammalian cells, and these include: neutral sedimentation gradients, filter elution and more recently pulsed field gel electrophoresis techniques (PFGE). Recent developments in PFGE have allowed the measurement of both the yields and the distribution of breaks within the genome, which go part of the way to explaining the RBE values close to 1.0 previously measured using other approaches with various radiation qualities. It is clear that future studies to determine the effectiveness of radiations of differing LET must use techniques that determine both yields and distributions of dsb, and assays need to be developed to allow these measurements at biologically relevant doses.

Post-repair DNA damage in X-irradiated cultured human tumour cells

In three human cells lines, X-irradiation damage to DNA is quantitatively repaired in 2 hours or less. At 4 hours following single doses of 1000 rad or less, delayed or latent damage to the DNA appears in CCRF-CEM and 8402 leukaemia cells; HeLa cells under these conditions demonstrate comparatively little such post-repair DNA damage. With larger doses (2000-8000 rad), DNA in HeLa cells also exhibits post-repair damage. This damage is in the form of small acid-precipitable, double-stranded DNA fragments, and is demonstrated by batch elution from hydroxylapatite and alkaline and neutral sucrose sedimentation velocity techniques. The appearance of delayed DNA damage may be an early, critical expression of radiation-induced cytotoxic progression because the lesions are dose-dependent and are apparently not repaired.

Characterization of density gradients prepared by freezing and thawing a sucrose solution

Density gradients of sucrose can be prepared in large numbers by successive freezing and thawing of sucrose solutions. Gradients of other solute molecules, such as salt and detergents, also form and this could affect subsequent sedimentation behavior of some molecules. However, the sedimentation behavior of native and denatured DNA of bacteriophage lambda was essentially isokinetic under the conditions used thus making these gradients comparable with ones prepared manually, at least for preparative sedimentation work with nucleic acids.

An improved method for DNA alkaline gradient analysis and its application to the effect of carcinogens on mouse liver DNA

An alkaline sodium iodide density gradient technique is described, for use in sedimentation rate centrifugation studies of in vivo induction of single strand breaks in DNA. The combination of this type of gradient with a sensitive fluorometric DNA estimation makes it possible to analyze very small amounts of DNA without any need for labeling the nucleic acid with radioactive thymidine.