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Page 162 Elton et al. Cancer Drug Resist 2020;3:161-70 I http://dx.doi.org/10.20517/cdr.2019.117

INTRODUCTION
The human DNA topoisomerase IIα (170 kDa, TOP2α/170) enzyme functions as a homodimer with the
active site Tyr805 residues in each subunit initiating reversible transesterification reactions to generate
[1-4]
TOP2α/170-DNA covalent cleavage complexes . These transient TOP2α/170 mediated double-strand
DNA breaks are essential in proliferating cells so that entanglements which occur during DNA repair,
recombination, replication, transcription, and segregation can be resolved by allowing the passage of
double-stranded DNA segments through these openings [1-4] . Given that TOP2α/170 enzymatic activity
is necessary for cell survival, TOP2α interfacial inhibitors/poisons (e.g., etoposide, mitoxantrone,
doxorubicin, daunorubicin, and analogs) are widely exploited as anticancer drugs [5-8] . These therapeutic
agents exert their cytotoxic effects by impeding the reversal of the TOP2α/170-DNA covalent cleavage
[5-8]
complexes, which subsequently leads to the accumulation of DNA breaks and ultimately cell death .

TOP2α poisons are commonly used as chemotherapeutic agents in adults and pediatric patients to treat
a wide variety solid tumors, leukemias, and lymphomas [9-11] . For example, cisplatin/etoposide is first-line
treatment for small cell lung cancer [12,13] ; doxorubicin and epirubicin are used in combination with other
drugs as a preoperative/adjuvant therapy regimen for the treatment of breast cancer [14,15] ; and daunorubicin
and mitoxantrone are used in treating acute myeloid leukemia (AML) [16,17] .

Although TOP2α poisons are extensively utilized, the efficacy of these important drugs is often
compromised due to acquired chemoresistance [18-21] . While many chemoresistant mechanisms have been
defined [22,23] , acquired resistance to TOP2α poisons is frequently associated with decreased TOP2α/170
expression levels or altered sub-cellular localization of TOP2α/170 given that the cytotoxic activity of these
drugs is dependent upon the formation of TOP2α/170-DNA covalent cleavage complexes [18-21] . In this
review, we focus on the molecular mechanisms underlying the decreased TOP2α/170 expression levels in
chemoresistant cell lines due to alternative RNA processing.

ALTERNATIVE SPLICING
Alternative splicing is a process by which a single pre-mRNA is matured into multiple mRNA isoforms
[24]
that can contribute to transcriptomic and proteomic diversity . RNA-seq data predict that over 95% of
[24]
human genes generate at least two alternative spliced mRNA isoforms . Several modes of alternative
splicing of a pre-mRNA have been described: exon skipping, differential inclusion of an exon, alternative
[24]
splice (5’ splice or 3’ splice) site selection, and intron retention . Intron-retaining mRNA transcripts are
[25]
susceptible to nuclear intron detention , or nonsense mediated decay [26,27] , and as a consequence gene
expression is reduced at the post-transcriptional level. However, some intron-retaining mRNA transcripts
leave the nucleus and undergo translation to produce new protein isoforms with novel functions [28-31] . Such
seems to be the case with a number of documented TOP2α mRNA splice variants, which retain introns, are
translated into truncated TOP2α isoforms, and play a role in mediating TOP2α poison chemoresistance in
various cell lines [32-36] .

THE HUMAN TOP2α GENE AND TOP2α/170 PROTEIN EXPRESSION
The human TOP2α gene comprises 35 exons, spans ~30 kb (NCBI Reference Sequence: NG_027678.2)
[37]
[38]
[Figure 1A] , and has been mapped to chromosome 17q21-22 . A 5695 nucleotide (nt) mRNA (NCBI
Reference Sequence: NM_001067.4) [Figure 1A-i] is matured from the TOP2α gene and the open reading
frame encodes a protein comprising 1531 amino acids (aa), with a calculated molecular weight of 174,386 Da
[37]
[37]
(i.e., TOP2α/170) [Figure 1B-i] . TOP2α exons 1-12 encode the ATP binding domain near the N-terminus
and acts as a gate (ATP gate) [Figure 1B-i] when two TOP2α/170 subunits homodimerize [39,40] . When the ATP
gate is open, one DNA duplex (designated the G- or “gate”-segment) is loaded into the enzyme cavity and a