The relationship between homologous recombination repair and the sensitivity of human epidermis to the size of daily doses over a 5-week course of breast radiotherapy

TY - JOUR

T1 - The relationship between homologous recombination repair and the sensitivity of human epidermis to the size of daily doses over a 5-week course of breast radiotherapy

AU - Somaiah, Navita

AU - Yarnold, John

AU - Daley, Frances

AU - Pearson, Ann

AU - Gothard, Lone

AU - Rothkamm, Kai

AU - Helleday, Thomas

PY - 2012/10/1

Y1 - 2012/10/1

N2 - PURPOSE: A molecular understanding of tissue sensitivity to radiotherapy fraction size is missing. Here, we test the hypothesis that sensitivity to fraction size is influenced by the DNA repair system activated in response to DNA double-strand breaks (DSB). Human epidermis was used as a model in which proliferation and DNA repair were correlated over 5 weeks of radiotherapy.EXPERIMENTAL DESIGN: Radiotherapy (25 fractions of 2 Gy) was prescribed to the breast in 30 women with early breast cancer. Breast skin biopsies were collected 2 hours after the 1st and 25th fractions. Samples of contralateral breast skin served as controls. Sections were coimmunostained for Ki67, cyclin A, p21, RAD51, 53BP1, and β1-integrin.RESULTS: After 5 weeks of radiotherapy, the mean basal Ki67 density increased from 5.72 to 15.46 cells per millimeter of basement membrane (P = 0.002), of which the majority were in S/G2 phase, as judged by cyclin A staining (P < 0.0003). The p21 index rose from 2.8% to 87.4% (P < 0.0001) after 25 fractions, indicating cell cycle arrest. By week 5, there was a 4-fold increase (P = 0.0003) in the proportion of Ki67-positive cells showing RAD51 foci, suggesting increasing activation of homologous recombination.CONCLUSIONS: Cell cycle arrest in S/G2 phase in the basal epidermis after a 5-week course of radiotherapy is associated with greater use of homologous recombination for repairing DSB. The high fidelity of homologous recombination, which is independent of DNA damage levels, may explain the low-fractionation sensitivity of tissues with high-proliferative indices, including self-renewing normal tissues and many cancers.

AB - PURPOSE: A molecular understanding of tissue sensitivity to radiotherapy fraction size is missing. Here, we test the hypothesis that sensitivity to fraction size is influenced by the DNA repair system activated in response to DNA double-strand breaks (DSB). Human epidermis was used as a model in which proliferation and DNA repair were correlated over 5 weeks of radiotherapy.EXPERIMENTAL DESIGN: Radiotherapy (25 fractions of 2 Gy) was prescribed to the breast in 30 women with early breast cancer. Breast skin biopsies were collected 2 hours after the 1st and 25th fractions. Samples of contralateral breast skin served as controls. Sections were coimmunostained for Ki67, cyclin A, p21, RAD51, 53BP1, and β1-integrin.RESULTS: After 5 weeks of radiotherapy, the mean basal Ki67 density increased from 5.72 to 15.46 cells per millimeter of basement membrane (P = 0.002), of which the majority were in S/G2 phase, as judged by cyclin A staining (P < 0.0003). The p21 index rose from 2.8% to 87.4% (P < 0.0001) after 25 fractions, indicating cell cycle arrest. By week 5, there was a 4-fold increase (P = 0.0003) in the proportion of Ki67-positive cells showing RAD51 foci, suggesting increasing activation of homologous recombination.CONCLUSIONS: Cell cycle arrest in S/G2 phase in the basal epidermis after a 5-week course of radiotherapy is associated with greater use of homologous recombination for repairing DSB. The high fidelity of homologous recombination, which is independent of DNA damage levels, may explain the low-fractionation sensitivity of tissues with high-proliferative indices, including self-renewing normal tissues and many cancers.

KW - Breast/metabolism

KW - Breast Neoplasms/metabolism

KW - Cell Cycle Checkpoints/radiation effects

KW - Cyclin-Dependent Kinase Inhibitor p21/metabolism

KW - DNA Breaks, Double-Stranded/radiation effects

KW - DNA Damage/radiation effects

KW - Epidermis/metabolism

KW - Female

KW - Humans

KW - Middle Aged

KW - Radiation Tolerance

KW - Recombinational DNA Repair/genetics

U2 - 10.1158/1078-0432.CCR-10-3297

DO - 10.1158/1078-0432.CCR-10-3297

M3 - SCORING: Journal article

C2 - 22855580

VL - 18

SP - 5479

EP - 5488

JO - CLIN CANCER RES

JF - CLIN CANCER RES

SN - 1078-0432

IS - 19

ER -