Chromatin compaction and tumor cell radiosensitivity at 2 gray

Mechanisms and Consequences of Double-Strand DNA Break Formation in Chromatin

All organisms suffer double-strand breaks (DSBs) in their DNA as a result of exposure to ionizing radiation. DSBs can also form when replication forks encounter DNA lesions or repair intermediates. The processing and repair of DSBs can lead to mutations, loss of heterozygosity, and chromosome rearrangements that result in cell death or cancer. The most common pathway used to repair DSBs in metazoans (non-homologous DNA end joining) is more commonly mutagenic than the alternative pathway (homologous recombination mediated repair). Thus, factors that influence the choice of pathways used DSB repair can affect an individual's mutation burden and risk of cancer. This review describes radiological, chemical, and biological mechanisms that generate DSBs, and discusses the impact of such variables as DSB etiology, cell type, cell cycle, and chromatin structure on the yield, distribution, and processing of DSBs. The final section focuses on nucleosome-specific mechanisms that influence DSB production, and the possible relationship between higher order chromosome coiling and chromosome shattering (chromothripsis).

Chromatin structure and radiation-induced DNA damage: from structural biology to radiobiology

Genomic DNA in eukaryotic cells is basically divided into chromosomes, each consisting of a single huge nucleosomal fiber. It is now clear that chromatin structure and dynamics play a critical role in all processes involved in DNA metabolism, e.g. replication, transcription, repair and recombination. Radiation is a useful tool to study the biological effects of chromatin alterations. Conversely, radiotherapy and radiodiagnosis raise questions about the influence of chromatin integrity on clinical features and secondary effects. This review focuses on the link between DNA damage and chromatin structure at different scales, showing how a comprehensive multiscale vision is required to understand better the effect of radiations on DNA. Clinical aspects related to high- and low-dose of radiation and chromosomal instability will be discussed. At the same time, we will show that the analysis of the radiation-induced DNA damage distribution provides good insight on chromatin structure. Hence, we argue that chromatin "structuralists" and radiobiological "clinicians" would each benefit from more collaboration with the other. We hope that this focused review will help in this regard.

Can the two mechanisms of tumor cell killing by radiation be exploited for therapeutic gain?

The radiation killing of tumor cells by ionizing radiation is best described by the linear-quadratic (LQ) model. Research into the underlying mechanisms of α- and β-inactivation has suggested that different molecular targets (DNA in different forms) and different microdosimetric energy deposits (spurs versus electron track-ends) are involved. Clinical protocols with fractionated doses of about 2.0 Gy/day were defined empirically, and we now know that they produce cancer cures mainly by the α-inactivation mechanism. Radiobiology studies indicate that α and β mechanisms exhibit widely different characteristics that should be addressed upfront as clinical fractionation schemes are altered. As radiation treatments attempt to exploit the advantages of larger dose fractions over shorter treatment times, the LQ model can be used to predict iso-effective tumor cell killing and possibly iso-effective normal tissue complications. Linking best estimates of radiobiology and tumor biology parameters with tumor control probability (TCP) and normal tissue complication probability (NTCP) models will enable us to improve and optimize cancer treatment protocols, delivering no more fractions than are strictly necessary for a high therapeutic ratio.

Prediction of radiosensitivity in human bladder cell lines using nuclear chromatin phenotype

BACKGROUND

Nuclear texture analysis measures phenotypic changes in chromatin distribution within a cell nucleus, while the alkaline Comet assay is a sensitive method for measuring the extent of DNA breakage in individual cells. The authors aim to use both methods to provide information about the sensitivity of cells to ionizing radiation.

METHODS

The alkaline Comet assay was performed on six human bladder carcinoma cell lines and one human urothelial cell line exposed to gamma-radiation doses from 0 to 10 Gy. Nuclear chromatin texture analysis of 40 features was then performed in the same cell lines exposed to 0, 2, and 6 Gy to explore if nuclear phenotype was related to radiation sensitivity.

RESULTS

Comet assay results demonstrated that the cell lines exhibited different levels of radiosensitivity and could be divided into a radiosensitive and a radioresistant group at >6 Gy. Using stepwise discriminant analysis, a subset of important nuclear texture features that best discriminated between sensitive and resistant cell lines were identified A classification function, defined using these features, correctly classified 81.75% of all cells into their radiosensitive or radioresistant groups based on their pretreatment chromatin phenotype. Posttreatment chromatin changes also varied between cell lines, with sensitive cell lines showing a relaxed chromatin conformation following radiation, whereas resistant cell lines exhibited chromatin condensation.

CONCLUSIONS

The authors conclude that the alkaline Comet assay and nuclear texture methodologies may prove to be valuable aids in predicting the response of tumor cells to radiotherapy.

The influence of concentration of DNA on the radiosensitivity of mammalian cells

PURPOSE

To develop relations that explicitly show the dependence of the linear-quadratic survival parameters alpha and beta on the nuclear volume, average DNA concentration, and degree to which the chromatin is condensed in zones of the nucleus.

METHODS AND MATERIALS

The microdosimetric-kinetic model of mammalian cell killing is used.

RESULTS

The relations indicate an increased tendency for lethal lesions to form by pairwise combination of potentially lethal lesions in regions of relatively greater local DNA concentration. In particular, the value of beta is proportional to the average nuclear concentration of DNA and to a parameter that reflects the condensation of DNA into a relatively concentrated phase, such as occurs in mitotic chromosomes and heterochromatin. These relations indicate local DNA concentration in the nucleus is a significant determinant of three magnitude of beta. Because alpha is composed of a term that is proportional to beta, it is also affected by DNA concentration, generally to equal or less degree.

CONCLUSION

It may be possible to establish a correlation between nuclear size and morphology, as determined from microscope slides prepared from biopsy specimens, with radiation sensitivity and the alpha/beta ratio that could be useful in radiation treatment planning.

Radiosensitization of tumour cells by cantharidin and some analogues

PURPOSE

Mammalian cells at mitosis contain chromatin in compacted form and are hypersensitive to ionizing radiation. Previous research had shown some chemicals that induce chromatin compaction within interphase cells act as radiosensitizers. Of these agents, cantharidin (LS-1), which is an inhibitor of protein phosphatases 1 (PP1) and 2A (PP2A), showed good radiosensitizing activity at non-toxic doses. Cantharidin and 13 additional structural analogues (LS-2-14) were tested for their radiosensitizing activity on tumour cells in vitro.

MATERIALS AND METHODS

Twelve of the 14 cantharidin analogues were synthesized in the authors' laboratory. Various concentrations of the drugs were screened for toxicity and radiosensitizing effectiveness with asynchronous DU-145 (human prostate carcinoma) cells. More detailed radiobiological studies of the more potent agents were performed with HT-29 (human colon carcinoma) cells since they could be readily synchronized. The radiosensitization of G1 phase HT-29 cells was measured after a 2-h exposure to the more potent drugs and reductions of the surviving fraction after an acute dose of 2 Gy (SF2Gy) served to estimate their relative effectiveness. The increase in phosphorylation of histone 1 (H1) and histone 3 (H3) induced by these drug exposures was measured by Western blotting of protein extracts. Drug-induced change in chromatin morphology was visualized by electron microscopy, and the alkaline comet assay (which measures DNA single-strand breaks) was employed to measure the radiation sensitivity of cellular chromatin in the drug-treated cells.

RESULTS

Of the 14 cantharidin analogues tested, LS-1, LS-2 and LS-5 at concentrations of 3-20 microM showed little or no toxicity, produced elevated levels of H1 and H3 phosphorylation, and effected significant radiosensitization at low radiation dose. The chromatin in tumour cells treated with LS-5 became visibly compacted and its DNA was about 1.6 times more sensitive to radiation-induced strand breakage relative to that of control cells.

CONCLUSIONS

The results confirm the authors' earlier studies that showed an increase in tumour cell intrinsic radiosensitivity by exposure to agents that promote chromatin compaction. LS-5 was identified as the optimal radiosensitizing agent of this class of compounds. Radiosensitization was correlated with chromatin compaction and elevated phosphorylation of H1 and H3. The DNA in drug-treated cells exhibited an enhanced sensitivity to radiation-induced single-strand breakage.

Incorporating clinical measurements of hypoxia into tumor local control modeling of prostate cancer: implications for the alpha/beta ratio

BACKGROUND AND PURPOSE

The recently obtained low value of approximately 1.5 for the alpha/beta of prostate cancer has led us to reexamine the optimal prostate tumor biology parameters, while taking into account everything known about the radiation response of prostate clonogens for use in a predictive dose-response model.

METHODS AND MATERIALS

Averages of the literature values of the alpha- and beta-inactivation coefficients for human prostate cancer cell lines were calculated. A robust tumor local control probability (TLCP) model was used that required average alpha and beta, as well as sigma(alpha), for the interpatient variation in single-hit killing (alpha). Median PO(2) values <or=1 mm Hg in the prostates of Fox Chase Cancer Center brachytherapy patients had been found in 21% of 115 cases. The oxygen enhancement ratios of 1.75 and 3.25 for alpha- and beta-inactivation, respectively, measured for tumor cells in vitro, were incorporated into the TLCP model, together with a clonogen density of approximately 10(5) cells/cm(3). Severe hypoxia and radioresistance were estimated for a proportion of tumors that was increased with PSA level.

RESULTS

For asynchronous human prostate cell lines irradiated in air, alpha(mean) was 0.26 +/- 0.07 (standard error) Gy(-1), sigma(alpha) = 0.06 Gy(-1), and beta(mean) was 0.0312 Gy(-2) +/- 0.0064 (standard error) Gy(-2). The TLCP data indicated that most tumors that contained aerobic cells would be cured, whereas most tumors that contained hypoxic cells would not be cured by total doses of 76 to 80 Gy. Clinical response data from the literature for external beam dose escalation, stratified by PSA value, and for low-dose-rate brachytherapy, were well predicted by the model, where the alpha/beta ratio was 8.5 and 15.5 for well-oxygenated and hypoxic clonogens, respectively.

CONCLUSIONS

Neither alpha/beta ratio nor clonogen number need be extremely low to explain the response of prostate cancer to brachytherapy and external beam therapy, contradicting other recent analyses. It is strongly suggested that severe hypoxia in the prostates of certain patients limits the overall cancer cure rate by conventional radiation therapy.