Purpose 53BP1 foci detection in peripheral blood lymphocytes (PBLs) by immunofluorescence microscopy (IFM) is a sensitive and quantifiable DNA double-strand break (DSB) marker. In addition, high-resolution transmission electron microscopy (TEM) with immunogold labeling of 53BP1 and DSB-bound phosphorylated Ku70 (pKu70) can be used to determine the progression of the DNA repair process. To establish this TEM method in the PBLs of patients with cancer, we analyzed and characterized whether different modes of irradiation influence the formation of DSBs, and whether accompanying chemotherapy influences DSB formation. Methods We obtained 86 blood samples before and 0.1, 0.5, and 24 h after irradiation from patients (n = 9) with head and neck or rectal cancers receiving radiotherapy (RT; n = 4) or radiochemotherapy (RCT; n = 5). 53BP1 foci were quantified by IFM. In addition, TEM was used to quantify gold-labelled pKu70 dimers and 53BP1 clusters within euchromatin and heterochromatin of PBLs. Results IFM analyses showed that during radiation therapy, persistent 53BP1 foci in PBLs accumulated with increasing numbers of administered RT fractions. This 53BP1 foci accumulation was not influenced by the irradiation technique applied (3D conformal radiotherapy versus intensity-modulated radiotherapy), dose intensity per fraction, number of irradiation fields, or isodose volume. However, more 53BP1 foci were detected in PBLs of patients treated with accompanying chemotherapy. TEM analyses showed that DSBs, indicated by pKu70, were present for longer periods in PBLs of RCT patients than in PBLs of RT only patients. Moreover, not every residual 53BP1 focus was equivalent to a remaining DSB, since pKu70 was not present at every damage site. Persistent 53BP1 clusters, visualized by TEM, without colocalizing pKu70 likely indicate chromatin alterations after repair completion or, possibly, defective repair. Conclusion IFM 53BP1 foci analyses alone are not adequate to determine individual repair capacity after irradiation of PBLs, as a DSB may be indicated by a 53BP1 focus but not every 53BP1 focus represents a DSB.

Abstract53BP1-foci detection in peripheral blood lymphocytes (PBLs) by immunofluorescence microscopy (IFM) is a sensitive and quantifiable DNA-double-strand-break (DSB) marker. In addition, high-resolution transmission-electron-microscopy (TEM) with immunogold-labeling of 53BP1 and DSB-bound phosphorylated Ku70 (pKu70) can be used to determine the progression of the DNA-repair process. Here, we analyzed whether different modes of irradiation influence the formation of DSBs in the PBLs of patients with cancer, and whether accompanying chemotherapy influences the DSB-appearance.We obtained 86 blood samples before and 0.1, 0.5 and 24 h after irradiation from patients with head and neck, or rectal cancers receiving radiotherapy (RT) or radio-chemotherapy (RCT). 53BP1-foci were quantified by IFM. In addition, TEM was used to quantify gold-labelled pKu70-dimers and 53BP1-clusters within euchromatin and heterochromatin of PBLs. During radiotherapy, persisting 53BP1-foci accumulated in PBLs with increasing numbers of administered RT-fractions. This 53BP1-foci accumulation was not influenced by irradiation technique applied (3D-conformal radiotherapy versus intensity-modulated radiotherapy), dose intensity per fraction, number of irradiation fields, or isodose volume. However, more 53BP1-foci were detected in PBLs of patients treated with accompanying chemotherapy. TEM analyses showed that DSBs, indicated by pKu70, were present for longer periods in PBLs of RCT-patients than in PBLs of RT-only-patients. Moreover, not every residual 53BP1-focus was equivalent to a remaining DSB, since pKu70 was not present at every damage site. Persistent 53BP1-clusters, visualized by TEM, without colocalizing pKu70 likely indicate chromatin alterations after repair completion, or possibly, defective repair. Therefore, IFM-53BP1-foci analyses alone are not adequate to determine individual repair capacity after irradiation of PBLs, as a DSB may be indicated by a 53BP1-focus but not every 53BP1-focus represents a DSB.The level of DNA-damage during RT is influenced by the presence of accompanying chemotherapy.