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Elton et al. Cancer Drug Resist 2020;3:161-70 I http://dx.doi.org/10.20517/cdr.2019.117 Page 163

A

B

C

Figure 1. Schematic representation of the human TOP2α gene, TOP2α mRNAs, TOP2α protein, and visualization of RNA-seq results.
A: the human TOP2α gene is comprised of 35 exons. At least three mature mRNA transcripts (i-iii) can be transcribed from the human
TOP2α gene. Two of these mRNAs harbor retained and processed introns; B: the three TOP2α mRNAs encode three distinct TOP2α
protein isoforms. Depicted are the ATP gate, which harbors the ATPase domain; the DNA gate, which includes the winged-helix domain
and harbors the active site tyrosine, Tyr805; the C gate, which comprises the coiled-coil region (coiled domain) and the characterized
dimerization sequences, DD1053-1069 and DD1121-1143 [41-45] ; and the C-terminal domain, which contains the defined nuclear localization
signal NLS1454-1497 [46,47] ; C: visualization of retained intron 19 of TOP2α RNA-seq genome coverage tracks showing the intron 19
retention event in K/VP.5 cells. RNA-seq raw reads from K562 and K/VP.5 RNA samples were mapped to the human reference genome
GRCh38 using Hierarchical Indexing for Spliced Alignment for Transcripts v.2.1.0 [48] and visualized using the Integrative Genomics
Viewer [49] . Reduced coverage denoted for Exon 20 indicates fewer full length TOP2α/170 reads in K/VP.5 cells. (A, B) Images adapted in
part from Figures 1A, B published originally in the Journal of Pharmacology and Experimental Therapeutics; Kanagasabai et al. [35] , 2017.
TOP2α: topoisomerase IIα; WHD: winged-helix domain; CTD: C-terminal domain; DD: dimerization domains

transient double-strand DNA break is generated (i.e., TOP2α/170-DNA covalent cleavage complex) [39,40] within
[37]
the DNA gate [Figure 1B-i], which is encoded by TOP2α exons 13-27 . The transesterification reaction,
[37]
which is mediated by the active site Tyr805 residue on each monomer, is encoded by exon 20 . Subsequently,
the “transfer”-segment (T-segment) is captured within the ATP gate upon ATP binding and is transported