Abstract 904: The novel C-terminal truncated 90-kDa isoform of topoisomerase IIα, TOP2α/90, is a determinant of etoposide resistance in K562 leukemia cells via heterodimerization with the TOP2α/170 isoform

Abstract

Abstract DNA topoisomerase IIα (170 kDa, TOP2α/170) is essential in proliferating cells since it resolves DNA topologic entanglements during chromosome condensation, replication, and segregation. We previously characterized a C-terminally truncated TOP2α/90 isoform, detectable in human leukemia K562 cells but more abundantly expressed in a clonal subline, K/VP.5, with acquired resistance to the anticancer agent etoposide (J Pharmacol Exp Ther 2017;360:152-63). TOP2α/90 (786 amino acids) is the translation product of a TOP2α mRNA that retains a processed intron 19. TOP2α/90 lacks the active-site tyrosine-805 (Tyr805) required to generate double-strand DNA breaks as well as the nuclear localization signals present in the TOP2α/170 isoform (1531 amino acids). The function of TOP2α/90 is unknown. Here, we found that TOP2α/90, like TOP2α/170, was detectable in the nucleus and cytoplasm of K562 and K/VP.5 cells. Importantly, co-immunoprecipitation of endogenous TOP2α/90 and TOP2α/170 demonstrated heterodimerization of these isoforms. Forced expression of TOP2α/90 in K562 cells suppressed, while siRNA-mediated knockdown of TOP2α/90 in K/VP.5 cells enhanced, etoposide-mediated DNA strand breaks compared with similarly treated K562 or K/VP.5 cells transfected with empty vector or control siRNAs, respectively. In addition, forced expression of TOP2α/90 in K562 cells inhibited etoposide cytotoxicity assessed by soft agar colony formation assays. qPCR and immunoassays demonstrated expression of TOP2α/90 mRNA and protein in normal human tissues/cells and in leukemia cells from patients. Together, results strongly suggest that TOP2α/90 expression decreases drug-induced TOP2α-DNA covalent complexes and is a determinant of chemoresistance through a dominant-negative effect related to heterodimerization with TOP2α/170. Alternative processing of TOP2α pre-mRNA, and subsequent synthesis of TOP2α/90, may be an important mechanism regulating the formation and/or stability of TOP2α/170-DNA covalent complexes in response to TOP2α-targeting agents. Citation Format: Ragu Kanagasabai, Soumendra Karmahapatra, Yang Yu, Victor A. Hernandez, Corey A. Kientz, Evan E. Kania, Terry S. Elton, Jack C. Yalowich. The novel C-terminal truncated 90-kDa isoform of topoisomerase IIα, TOP2α/90, is a determinant of etoposide resistance in K562 leukemia cells via heterodimerization with the TOP2α/170 isoform [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 904.