Cytotoxicity of restriction enzyme-induced DNA strand breaks in radiosensitive and radioresistant human tumor cell lines

Reprint of: Deciphering and simulating models of radiation genotoxicity with CRISPR/Cas9 systems

This short review explores the utility and applications of CRISPR/Cas9 systems in radiobiology. Specifically, in the context of experimentally simulating genotoxic effects of Ionizing Radiation (IR) to determine the contributions from DNA targets and 'Complex Double-Stranded Breaks' (complex DSBs) to the IR response. To elucidate this objective, this review considers applications of CRISPR/Cas9 on nuclear DNA targets to recognize the respective 'nucleocentric' response. The article also highlights contributions from mitochondrial DNA (mtDNA) - an often under-recognized target in radiobiology. This objective requires accurate experimental simulation of IR-like effects and parameters with the CRISPR/Cas9 systems. Therefore, the role of anti-CRISPR proteins in modulating enzyme activity to simulate dose rate - an important factor in radiobiology experiments is an important topic of this review. The applications of auxiliary domains on the Cas9 nuclease to simulate oxidative base damage and multiple stressor experiments are also topics of discussion. Ultimately, incorporation of CRISPR/Cas9 experiments into computational parameters in radiobiology models of IR damage and shortcomings to the technology are discussed as well. Altogether, the simulation of IR parameters and lack of damage to non-DNA targets in the CRISPR/Cas9 system lends this rapidly emerging tool as an effective model of IR induced DNA damage. Therefore, this literature review ultimately considers the relevance of complex DSBs to radiobiology with respect to using the CRISPR/Cas9 system as an effective experimental tool in models of IR induced effects.

Deciphering and simulating models of radiation genotoxicity with CRISPR/Cas9 systems

This short review explores the utility and applications of CRISPR/Cas9 systems in radiobiology. Specifically, in the context of experimentally simulating genotoxic effects of Ionizing Radiation (IR) to determine the contributions from DNA targets and 'Complex Double-Stranded Breaks' (complex DSBs) to the IR response. To elucidate this objective, this review considers applications of CRISPR/Cas9 on nuclear DNA targets to recognize the respective 'nucleocentric' response. The article also highlights contributions from mitochondrial DNA (mtDNA) - an often under-recognized target in radiobiology. This objective requires accurate experimental simulation of IR-like effects and parameters with the CRISPR/Cas9 systems. Therefore, the role of anti-CRISPR proteins in modulating enzyme activity to simulate dose rate - an important factor in radiobiology experiments is an important topic of this review. The applications of auxiliary domains on the Cas9 nuclease to simulate oxidative base damage and multiple stressor experiments are also topics of discussion. Ultimately, incorporation of CRISPR/Cas9 experiments into computational parameters in radiobiology models of IR damage and shortcomings to the technology are discussed as well. Altogether, the simulation of IR parameters and lack of damage to non-DNA targets in the CRISPR/Cas9 system lends this rapidly emerging tool as an effective model of IR induced DNA damage. Therefore, this literature review ultimately considers the relevance of complex DSBs to radiobiology with respect to using the CRISPR/Cas9 system as an effective experimental tool in models of IR induced effects.

Relationships between the cytotoxic effects of restriction endonucleases and radiation on mammalian cells

Wild-type Chinese hamster ovary (CHO) cells, a radiosensitive mutant CHO line (xrs6), and two human cervical carcinoma cell lines, MS751 and ME180, differ in sensitivity to ionizing radiation with surviving fractions at 2 Gy (SF2) of 0.84, 0.06, 0.90 and 0.24 respectively. Restriction endonucleases (REs) were introduced into the cells by treatment with streptolysin O (SLO) and the effects of this on clonogenic cell survival were compared. A comparison was made of REs inducing either blunt- (AluI) or cohesive-ended (Sau3AI) double-strand breaks. Whilst MS751 cells were resistant to the effects of both REs, AluI caused significantly greater cell killing than Sau3AI in the other three lines (p < 0.05 for all). Both ME180 and xrs6 were significantly more sensitive to REs than their radioresistant counterparts, MS751 and CHO (p < 0.05 for both). In order to investigate the effect of DNA methylation on dsb induction, the isoschizomers MspI and HpaII (cohesive-ended dsb inducers) were introduced into the cell lines. Both REs recognize the same sequence but HpaII cannot cleave if the internal cytosine is methylated. MS751 was also resistant to the effects of both of these enzymes and MspI was more cytotoxic than HpaII in the other three lines (p < 0.03 for all). The differential sensitivity to the two REs was more marked in the radiosensitive cell lines, suggesting that there may be a greater degree of DNA methylation in radiosensitive cells. The variation in sensitivities to REs between the cell lines could not be explained in terms of differences in cell poration following SLO treatment because, although MS751 was resistant to SLO (25% of cells porated), the other three lines showed the same level of cell poration (98% of cells). With these four cell lines, there was a significant correlation between sensitivity to RE and radiosensitivity for AluI, Sau3AI and MspI but not for HpaII.

Molecular structural analysis of 417 HPRT mutations induced by restriction endonucleases in Chinese hamster ovary (CHO) cells

CHO cells were exposed to 11 different restriction endonucleases by electroporation and their mutagenicity was measured. Nine of them have one or more recognition sites within exons of the HPRT gene, whereas the remaining two cut in introns only. The mutagenic efficiency of the various enzymes varied markedly; mutagenicity of Sau3AI was considerably higher than that of the other enzymes. Neither cytotoxicity nor mutagenicity could be related to the number or location of recognition sites within the cDNA. A total of 417 independent restriction enzyme induced mutant clones were isolated from 20 separate experiments for molecular analysis; all nine exons of the HPRT gene were analyzed by a modified multiplex deletion screening method with polymerase chain reaction (PCR) amplification. Among spontaneously arising mutants, 70.8% showed no change in PCR pattern, indicating a small scale change (point mutation), whereas partial deletions were observed in 24.7%, and total deletions in 4.5% of mutant clones. In contrast, approximately 70% of restriction enzyme induced mutants showed partial or total deletions. There was no obvious relationship between type of break (blunt versus staggered ends), and the DNA structure of the mutations induced. For partial deletions, the distribution of breakpoints within introns appeared to occur at random, and did not correlate with the mutagenicity of a given enzyme. Thus, though DNA double-strand breaks appear to be important mutagenic lesions that can induce a high frequency of deletion mutants, no specific relationship of mutagenic potential to the type of breaks, their sites within the HPRT gene or the molecular structure of the mutations induced could be identified.