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Page 759 Gmeiner et al. Cancer Drug Resist 2021;4:758-61 https://dx.doi.org/10.20517/cdr.2021.65

ubiquitination, and SUMOylation. A characteristic of cancer cells is increased or otherwise altered PTMs
[3]
modulating enzyme activity, and Top2α is no exception. In this issue, Lotz and Lamour reviewed the
interplay between Top2α PTMs and drug resistance. Top2α activity is modulated by multiple
phosphorylation sites and cancer cells phosphorylate Top2α at a number of sites not identified in non-
malignant cells including the conserved catalytic tyrosine (Y805), which is phosphorylated in Jurkat cells
and K562 acute leukemia cells. Further systematic analysis of Top2α PTM will be required to define their
role in drug resistance and to develop new therapeutic approaches targeting Top2α.

Several of the most effective and widely used anti-cancer therapeutics target Top2 and understanding the
basis for drug resistance to these is of particular concern. Etoposide and doxorubicin are Top2 poisons that
trap Top2cc resulting in persistent DNA double strand breaks that can cause cancer cells to undergo
programmed cell death. The activity of Top2 poisons requires expression of the enzyme and its nuclear
localization. Cells deficient in Top2α expression or nuclear localization are relatively resistant to Top2
[4]
poisons. Elton et al. have identified alternative splice variants of the Top2α protein that do not undergo
nuclear localization and cells expressing these variants are resistant to Top2α poisons. In this issue,
[4]
Elton et al. review Top2α splice variants and the role of alternative splicing as a cause of drug resistance
that predominates in some cancer cell lines. Alternative splicing is an important mechanism affecting drug
activity that demands consideration in addition to drug efflux mechanisms.

Human DNA topoisomerase I (Top1) is a Type IB topoisomerase that relieves both positive and negative
supercoils in DNA generated during replication and transcription. In this issue, Soren et al. review the
[5]
mechanism of human Top1 and detail how this mechanism is targeted by specific anti-cancer drugs. Top1
releases topological stress via a controlled rotation mechanism in which a reactive tyrosine (Y723) nicks the
DNA by forming a transient covalent cleavage complex (Top1cc). Relaxation of DNA supercoiling then
proceeds spontaneously within the Top1 C-type clamp that circumscribes the enzyme-bound and free DNA
ends. The enzyme subsequently catalyzes a second transesterification reaction to restore the integrity of the
DNA double helix. As is the case for Top2α, compounds that reversibly stabilize Top1cc are termed Top1
poisons since these result in persistent DNA double-strand breaks that cause cancer cells to undergo
apoptosis. Top1 poisons bind neither the DNA substrate nor the enzyme in its apo state, rather they are
specific for the cleavage complex and Soren et al. describe the structural basis for Top1 poisoning and
[5]
distinguish this mechanism from inhibitors of enzymatic activity.

Camptothecin (CPT) is a natural product identified in an anti-cancer screen by Wall et al. in 1966 and
[6]
shown to be a Top1 poison. Synthetic analogs of CPTs (e.g., irinotecan, topotecan) are widely used anti-
cancer drugs. Pommier and co-workers showed Top1 poisoning also occurs via oxidative base damage and
as a result of DNA binding by carcinogens (e.g., benzo[a]pyrene). Subsequent studies extended these
findings to demonstrate that nucleoside analogs such as cytarabine inhibit the re-ligation step of Top1
catalysis and cause DNA DSBs. Further, the importance of Top1 poisoning was demonstrated by reduced
[7]
activity in Top1-deficient cells. In this issue, Gmeiner reviews the poisoning of Top1 by nucleoside analogs
including recent findings that the Top1cc formed by nucleoside analogs shows different dependence on
repair enzymes, such as Tdp1, indicating that Top1 poisoning by nucleoside analogs is a distinct and
complementary process to the poisoning by CPTs.

The potential danger that the hallmark formation of a transient enzyme-DNA covalent complex by DNA
topoisomerases form to release the topological stress that occurs from pulling apart the double-stranded
DNA helix for DNA synthesis and transcription to occur, seemed to be a calculated risk by nature. Since the
[8]
discovery of DNA topoisomerases by Wang in 1971, and the revelation of their catalytic mechanism, the