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

DNA topoisomerase II α (170 kDa, TOP2 α /170) is essential in proliferating cells by resolving DNA topological entanglements during chromosome condensation, replication, and segregation. We previously characterized a C-terminally truncated isoform (TOP2 α /90), 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. TOP2 α /90 (786 aa) is the translation product of a TOP2 α mRNA that retains a processed intron 19. TOP2 α /90 lacks the active-site tyrosine-805 required to generate double-strand DNA breaks as well as nuclear localization signals present in the TOP2 α /170 isoform (1531 aa). Here, we found that TOP2 α /90, like TOP2 α /170, was detectable in the nucleus and cytoplasm of K562 and K/VP.5 cells. Coimmunoprecipitation of endogenous TOP2 α /90 and TOP2 α /170 demonstrated heterodimerization of these isoforms. Forced expression of TOP2 α /90 in K562 cells suppressed, whereas siRNA-mediated knockdown of TOP2 α /90 in K/VP.5 cells enhanced, etoposide-mediated DNA strand breaks compared with similarly treated cells transfected with empty vector or control siRNAs, respectively. In addition, forced expression of TOP2 α /90 in K562 cells inhibited etoposide cytotoxicity assessed by clonogenic assays. qPCR and immunoassays demonstrated TOP2 α /90 mRNA and protein expression in normal human tissues/cells and in leukemia cells from patients. Together, results strongly suggest that TOP2 α /90 expression decreases drug-induced TOP2 α -DNA covalent complexes and is a determinant of chemoresistance through a dominant-negative effect related to heterodimerization with TOP2 α /170. Alternative processing of TOP2 α pre-mRNA, and subsequent synthesis of TOP2 α /90, may be an important mechanism regulating the formation and/or stability of cytotoxic TOP2 α /170-DNA covalent complexes in response to TOP2 α -targeting agents.