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Inefficient DNA-DSB repair via the homologous recombination pathway in prostate cancer patients with late radiation toxicity

ABSTRACT: Purpose: Severe late normal tissue damage limits radiotherapy treatment regimens. This study aims to validate γ-H2AX foci decay ratios and induced expression levels of DNA double strand break (DSB) repair genes, found in a retrospective study, as possible predictors for late radiation toxicity. Methods and Materials: Prospectively, decay ratios (initial/residual γ-H2AX foci numbers) and genome-wide expression profiles were examined in ex vivo irradiated lymphocytes of 198 prostate cancer patients. All patients were followed ≥2 years after radiotherapy, clinical characteristics were assembled and toxicity was recorded using the Common Terminology Criteria (CTCAE) v4.0. Results: No clinical factors were correlated with late radiation toxicity. Analysis of γ-H2AX foci uncovered a negative correlation between the foci decay ratio and toxicity grade. Significantly smaller decay ratios were found in grade≥3 compared to grade 0 patients (p=0.02), indicating less efficient DNA-DSB repair in radio-sensitive patients. Moreover, utilizing a foci decay ratio threshold determined in our previous retrospective study correctly classified 23 of the 28 grade≥3 patients (sensitivity, 82%) and 9 of the 14 grade 0 patients (specificity, 64%). Grade of toxicity also correlated with a reduced induction of the homologous recombination (HR) repair gene-set. The difference in average fold induction of the HR gene-set was most pronounced between grade 0 and grade≥3 patients (p=0.008). Conclusions: Reduced responsiveness of HR repair genes to irradiation and inefficient DSB repair correlate with an increased risk of late radiation toxicity. Using a decay ratio classifier, we could correctly classify 82% of the patients with grade≥3 toxicity. Additional studies are required to further optimize and validate the foci decay assay and to assess its predictive value for late radiation toxicity in patients prostate cancer

ORGANISM(S): Homo sapiens

PROVIDER: GSE85570 | GEO | 2017/01/30

SECONDARY ACCESSION(S): PRJNA338875

REPOSITORIES: GEO

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Project description:In Saccharomyces cerevisiae, a single double-strand break (DSB) triggers extensive phosphorylation of histone H2A (known as gammaH2AX) over 50 kb on either side of the DSB. This modification is carried out by either of yeastM-^Rs checkpoint kinases, the ATM homolog, Tel1, or the ATR homolog, Mec1. In G1-arrested cells, where there is very little 5M-^R to 3M-^R processing of DSB ends, only Tel1 promotes this modification. We have recently described a second modification gammaH2B - the phosphorylation of the C terminal T129 locus of histone H2B which is also carried out by both Mec1 and Tel1 kinases. To understand in detail how gamma-H2AX and gamma-H2B spread along the chromosome from a DSB we have undertaken a high-density analysis of their occupancy where there is a DSB on three different chromosomes. gamma-H2AX and gamma-H2B modifications are similar, but there is a marked absence of gamma-H2B near telomeres. We find that there is reduced gamma-H2AX and gamma-H2B modification over strongly transcribed regions, even taking into account the reduced histone occupancy of these genes. When transcription of the galactose-regulated genes GAL1, GAL10, GAL7 are turned off by the addition of glucose, gamma-H2AX is restored within 5 min; when these genes are again induced, gamma-H2AX is rapidly lost. Regions more distal to the GAL genes have markedly reduced gamma-H2AX levels that rise rapidly when transcription is repressed, suggesting that transcription acts as a barrier to the propagation of gamma-H2AX away from the DSB. The restoration of gamma-H2AX in transcribed regions can be carried out by either Mec1 or Tel1, even 7 h after break induction, suggesting that Tel1 remains associated with damaged chromosomes for an extended time. In addition, we show that gamma-H2AX can be transferred in trans, to regions unlinked to the DSB that lie in close proximity the DSB. Specifically, if a DSB is generated 14 kb from CEN2, gamma-H2AX is transferred to regions around all the other centromeres, in keeping with observed close proximity of all centromere-adjacent chromosome arms. This transfer can be observed even in the absence of formaldehyde crosslinking of the samples.

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Inefficient DNA-DSB repair via the homologous recombination pathway in prostate cancer patients with late radiation toxicity

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