ALT-EJ GENERA REARREGLOS CROMOSÓMICOS EN RESPUESTA A ETOPÓSIDO EN CÉLULAS HUMANAS CON LOS PRINCIPALES SISTEMAS DE REPARACIÓN DE RUPTURAS DE DOBLE CADENA COMPROMETIDOS

The antitumor drug Etoposide (ETO) induces DNA double-strand breaks (DSB) and is associated with the development of secondary neoplasms in treated patients. DSB are repaired by two main mechanisms, homologous recombination (HR) and classical non-homologous end joining (c-NHEJ). When HR and c-NHEJ are defective, DSB are repaired by the PARP-1-dependent alternative end-joining (alt-EJ) pathway. The involvement of alt-EJ in the progression of DSB induced by ETO in the G2 phase of human cells was analyzed. HeLa cells deficient in HR (cohesin RAD21 inhibition, HeLa RAD21kd) and their non-silencing control (HeLa NS) were established. Cells were treated with ETO in the presence of a chemical inhibitor of DNA-PKcs (DNA-PKi, c-NHEJ). In both cell lines, ETO-induced DSB (γH2AX+) in G2 phase were increased compared to their controls. The incorrect repair of DSB in DNA-PKcs- and RAD21-deficient cells caused a synergistic augment in chromatid exchanges and dicentric chromosomes in the first and second metaphase, respectively. In contrast, the frequency of dicentric chromosomes was reduced in PARP-1-deficient cells (HeLa PARP-1kd) following ETO treatment. In HeLa RAD21kd binucleated cells, DNA-PKi/ETO increased the percentage of cells with ≥20 γH2AX foci in the G1-postmitotic phase and of micronuclei at 96 h. A greater accumulation in G2/M was observed in HeLa NS treated with DNA-PKi/ETO compared with HeLa RAD21kd at 8 h. The cell cycle restarted in HeLa NS at 16 h; however, the G2/M accumulation was maintained in HeLa RAD21kd. Chromosomal rearrangements obtained when DNA-PKcs and RAD21 were absent and their decrease in HeLa PARP-1kd cells suggest that alt-EJ contributes to their formation. Key words: chromosomal aberrations, cell cycle, cohesin, double-strand breaks, DNA repair pathways