Oncogenes and radiosensitivity

Impact of accumulated alterations in driver and passenger genes on response to radiation therapy

OBJECTIVE

Although various single genetic factors have been shown to affect radiosensitivity, high-throughput DNA sequencing analyses have revealed complex genomic landscapes in many cancer types. The aim of this study is to elucidate the association between accumulated alterations in driver and passenger genes and radiation therapy response.

METHODS

We used 59 human solid cancer cell lines derived from 11 organ sites. Radiation-induced cell death was measured using a standard colony-forming assay delivered as a single dose ranging from 0 to 12 Gy. Comprehensive genomic data for the cell lines were acquired from the Catalogue Of Somatic Mutations In Cancer v. 80. Random forest classifiers were constructed to predict radioresistant phenotypes using genomic features. The Cancer Genome Atlas data sets were used to evaluate the clinical impact of the genomic feature following radiotherapy.

RESULTS

The 59 cancer cell lines harbored either nucleotide variations or copy number variations in a median of 157 genes per cell. Radiosensitivity of the cancer cells was correlated with neither the number of driver gene mutations nor the number of passenger gene mutations. However, the proportion of driver gene alterations to total gene alterations in gene sets selected from the Kyoto Encyclopedia Genes and Genomes predicted radioresistant cells with sensitivity of 85% and specificity of 73%. High probability of radioresistance predicted by the model was associated with worse overall survival following definitive radiotherapy in patients of The Cancer Genome Atlas data sets.

CONCLUSION

Cellular radiosensitivity was associated with the proportion of driver to total gene alterations in the selected oncogenic pathways, which may be a biomarker candidate for response to radiation therapy.

ADVANCES IN KNOWLEDGE

These findings suggest that accumulated alterations in not only driver genes but also passenger genes affect radiosensitivity.

Damage pattern as a function of radiation quality and other factors

An understanding of damage pattern in critical cellular structures such as DNA is an important prerequisite for a mechanistic assessment of primary radiation damage, its possible repair, and the propagation of residual changes in somatic and germ cells as potential contributors to disease or ageing. Important quantitative insights have been made recently on the distribution in time and space of critical lesions from direct and indirect action of ionizing radiation on mammalian cells. When compared to damage from chemicals or from spontaneous degradation, e.g. depurination or base deamination in DNA, the potential of even low-LET radiation to create local hot spots of damage from single particle tracks is of utmost importance. This has important repercussions on inferences from critical biological effects at high dose and dose rate exposure situations to health risks at chronic, low-level exposures as experienced in environmental and controlled occupational settings. About 10,000 DNA lesions per human cell nucleus and day from spontaneous degradation and chemical attack cause no apparent effect, but a dose of 4 Gy translating into a similar number of direct and indirect DNA breaks induces acute lethality. Therefore, single lesions cannot explain the high efficiency of ionizing radiation in the induction of mutation, transformation and loss of proliferative capacity. Clustered damage leading to poorly repairable double-strand breaks or even more complex local DNA degradation, correlates better with fixed damage and critical biological endpoints. A comparison with other physical, chemical and biological agents indicates that ionizing radiation is indeed set apart from these by its unique micro- and nano-dosimetric traits. Only a few other agents such as bleomycin have a similar potential to cause complex damage from single events. However, in view of the multi-stage mechanism of carcinogenesis, it is still an open question whether dose-effect linearity for complex primary DNA damage and resulting fixed critical cellular lesions translate into linearity for radiation-induced cancer. To solve this enigma, a quantitative assessment of all genotoxic and harmful non-genotoxic agents affecting the human body would be needed.

[Genetic predisposition and radiation sensitivity of tumors]

BACKGROUND

Studies on normal tissue radiation sensitivity have demonstrated profound differences of individual sensitivities. A number of genetic syndromes associated with abnormal radiation sensitivity have been described. Significant differences have also been detected in persons without known genetic disorders. The question arises as to whether tumors originating from normal tissues with abnormal radiation sensitivity share this abnormal sensitivity and as to whether a general correlation between normal tissue sensitivity and tumor tissue sensitivity can be substantiated.

METHODS

Experimental and clinical data derived from own investigations and an extensive review of the literature was used to answer the question.

RESULTS

Experimental studies on normal and tumor tissues of SCID-and C3H-mice demonstrated that the 2.7-fold enhanced radiation sensitivity of SCID normal tissues is also found in SCID tumors. Clinical investigations on cervical carcinoma and breast cancer patients revealed higher local control rates in patients with more pronounced acute side effects. A weak trend towards the same relationship was found in head and neck cancer patients. Case reports on unusually severe acute radiation side effects or unexpected tumor remissions as well as few reports on radiotherapy in ataxia telangiectasia (AT) patients suggest a correlation between normal- and tumor-tissue radiation sensitivity. Studies on fibroblasts and tumor cells from the same patient support this hypothesis in soft tissue sarcoma patients, but do not so for head and neck cancer patients. Tumor cells exhibit a considerably higher variation of radiation sensitivities than normal tissue cells.

CONCLUSIONS

Experimental and clinical data are compatible with the hypothesis that normal tissue radiation sensitivity predicts for tumor tissue sensitivity. However, in view of the larger heterogeneity of tumor cell radiation sensitivity as compared to normal tissue radiation sensitivity, the development of a clinically useful predictive test for tumor sensitivity based on normal cell sensitivity appears to be unrealistic.

Radiation studies on B cell differentiation marker CD24/SCLC Cluster-4 antigen expressing and non-expressing lung cancer cell lines and mouse fibroblasts

A correlation between CD24 expression and higher intrinsic radiation sensitivity has been described in B-lineage acute lymphoblastic leukaemia (B-ALL). We recently identified the SCLC surface antigen Cluster-4 (CL-4) to be identical to the B cell differentiation marker CD24, except for one amino acid residue. The CD24/CL-4 antigen is highly expressed on SCLC, but rarely on NSCLC cells. In order to investigate the influence of the expression of CD24/CL-4 on the radiation sensitivity in a non-leukaemic cell system, sublines of the human SCLC H249 cell line transfected with mutated ras oncogene, and differing in their CD24/CL-4 expression, were studied. In addition, we stably transfected the NSCLC A125 cell line and the mouse fibroblast NIH3T3 cell line with the CL-4 cDNA. The differential expression of CD24/CL-4 on the cells had no influence on morphology, proliferation and cloning efficiency. Radiation studies were done with cells in exponential growth phase. In the highly resistant NSCLC A125 cells no difference in radioresponsiveness was observed between CD24/CL-4 expressing and non-expressing cells. In the rather radiosensitive cells, similar responses to radiation were observed between CD24/CL-4 expressing and non-expressing SCLC H249-ras cells, whereas the CL-4 transfected NIH3T3 mouse fibroblasts showed a substantially higher radioresistance than the CD24/CL-4 non-expressing control cells. In conclusion, the correlation between CD24/CL-4 expression and radiation sensitivity is controversial and depends on the cell type.