PARENT SESSION
3:45 PM to 5:15 PM
Tuesday, April 23, 2002
Mini-Symposium 12
Cell Cycle and Apoptosis
Room: Nevada 4-5
, Co-Chair: Muschel, Ruth¹; Haimovitz-Friedman, Adriana²¹University of Pennsylvania, Philadelphia, PA²Memorial Sloan-Kettering Cancer Center, New York, NY

(MS12-3) Molecular mechanisms underlying cellular radiosensitivity: cell cycle dependent reduction of excess acentric fragments.

Buettner, Kerstin^*,1, Dikomey, Ekkehard¹, Borgmann, Kerstin¹, ¹ Institute of Biophysics and Radiobiology, Hamburg, Germany

ABSTRACT-
Purpose: The number of excess acentric fragments was found to be an excellent indicator of cellular radiosensitivity for both human lymphocytes and fibroblasts. This number was previously shown to be determined by two mechanisms: 1) DNA repair in G1 and 2) further reduction when cells proceed from G1 to metaphase. The aim of this study was to reveal the molecular mechanisms underlying that second step. Materials and Methods: Excess acentric fragments in G0/G1 were determined by premature chromosome condensation (PCC) and in M by conventional metaphase technique. Apoptosis was detected by the alkali comet assay; G1-arrest by continuous labeling with BrdU combined with flow cytometric analysis. Experiments were performed with human lymphocytes and fibroblasts as well as CHO-K1 and xrs5 (defective in Ku80) and Rat1 (p53 mutated) and R7080 (reduced Ku70/Ku80). Results: We showed that the reduction in the number of excess acentric fragments occurring when cells proceed from G0/G1 to metaphase is neither attributable to apoptosis nor to G1-arrest. Since xrs5- and R7080-cells show a similar reduction in the number of excess acentric fragments like their respective wild type counterparts, this effect cannot be attributed to a defect in non-homologous end-joining. Therefore, we postulate that the reduction in the number of excess acentric fragments is caused by homologous recombination. To address this hypothesis, we are now analyzing cell lines defective in this pathway.

KEYWORDS: cellular radiosensitivity, DNA repair, acentric fragments, apoptosis