Upstream eventBinding, topoisomerase II
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Inhibitor binding to topoisomerase II leading to infant leukaemia||adjacent||High||Not Specified|
Life Stage Applicability
Key Event Relationship Description
Certain TopoII poisons stabilize the intermediate cleavage complex and prevent the religation with appropriate DNA strands. Covalently DNA end-bound TopoII protein is digested and a hanging end is created. The same process happens in the translocation partner gene. Hanging ends of both genes are processed and subsequently joined by non-homologous end joining (Cowell and Austin 2012). Indeed, compounds that increase the rate of DNA cleavage and decrease the rate of DNA religation by topo II enzyme are often referred to as Topo II "poisons" (Nitiss 2009).Topoisomerase poisons stabilize the normally transient topoisomerase-induced DSBs and are potent and widely used anticancer drugs (Cowell and Austin 2012). They interfer with the religation step in the topoisomerase II reaction cycle, leading to the accumulation of DNA DSBs. The inhibition of the religation step will result in the formation of an unusual type of DSB called a cleavage complex, in which the topoisomerase protein remains covalently coupled to the DNA (Cowell and Austin 2012).
Evidence Supporting this KER
All cells have two major forms of topoisomerases; Type I, which make single-stranded cuts in DNA, and Type II enzymes, which cut and pass double-stranded DNA (Nitiss et al 2012). Evidence supporting the causal relationship between etoposide-induced TopoII inhibition, DNA DSB and the MLL rearrangement leading to the fusion gene is strong regarding treatment-related acute leukaemia (Cowell and Austin 2012; Pendleton et al 2014).
The KER as such is biologically plausible and strong . Type II topoisomerases are ubiquitous enzymes which are essential for a number of fundamental DNA processes. As they generate DNA strand breaks, they can potentially fragment the genome. Indeed, while these enzymes are essential for the survival of proliferating cells they can also have significant genotoxic effects by means of accumulation of DNA strand breaks.
A type II topoisomerase can introduce negative supercoils into DNA, all known eukaryotic cells can only relax DNA. The decatenation of interlocked DNA is a critical topoisomerase function, since semi-conservative DNA replication results in catenated sister chromatids. Topoisomerases are important targets for many chemotherapeutic agents. These agents convert their target topoisomerases to DNA-damaging agents. The DNA is cut in both strands and the agents prevent the subsequent DNA-resealing step normally catalyzed by topoisomerases (Nitiss 2009).
Molecular dose-response modelling of etoposide-induced DNA damage response, based on comprehensive in vitro high content imaging in the HT1080 cell model, was developed by Li et al (2014). The model was based on the hypothesis that cells are capable of clearing low-level DNA damage with existing repair capacity, but when the number of DSBs exceeds a certain value, ATM and p53 become fully activated through reversible mechanism, leading to elevated repair capacity. The model was able to capture quantitatively the dose-response relationships of a number of markers observed with etoposide. Especially interesting are the dose-response relationships for activation of p53 and the formation of micronuclei in the target cell model, which indicate point-of-departure concentrations of etoposide in the range of 0.01 to 0.1 µM (Li et al. 2014). This range is in agreement with the finding that in human fetal liver CD34+ cells an increase in DSBs was observed at a concentration of 0.14 µM and MLL translocations were detectable by FISH or flow cytometry at higher concentrations (Moneypenny et al 2006).
Uncertainties and Inconsistencies
A prerequisite for the specific outcome, i.e. creation of chromosomal rearrangement, is that TopoII inhibition has to occur in an especially vulnerable and correct hot spot in the MLL locus; however, details of this process and how it happens are not clear.
Quantitative Understanding of the Linkage
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
DNA topoisomerases are ubiquitous enzymes, which control the integrity of double-stranded DNA. They are thus key enzymes at all levels of living organisms. The available evidence suggest that important differences in sensitivity to topoisomerase inhibition might exist among different cell types, depending on the amount of proliferative burden, of the TopoII enzymes and on physiological repair processes. Mesodermal precursor or hematopoietic stem and progenitor cells (HSPCs) are rapidly dividing cells with a high content of TopoII and for these reasons they can be a sensitive target during a critical developmental window (Hernandez and Menendez 2016). In addition, evidence from micronuclei assay studies conducted in untreated and chemical-treated foetuses and newborns show that both the baseline and chemically induced micronuclei frequencies are higher in the foetuses and infants than in adults (Udroiu et al 2016). This is possibly indicating a greater sensitivity to genotoxic insult during development which can be due to the higher proliferation rate and lower ability of DNA repair of the hematopoietic stem cells. However, the role that the different microenvironments (foetal liver, infant bone marrow and adult bone marrow) during ontogenesis can exert on cell sensitivity cannot be ruled out (Udroiu et al. 2016). The existence of relevant interspecies differences is unknown, but it cannot be ruled out presently.
Cowell IG, and Austin CA. 2012. mechanism of generation of Therapy related leukaemia in response to anti-topoisomerase II agents. Int.J.Environ.Res>Public Health. 9, 20175-2091.
Hernandez Jerez AF, Menendez P. Linking pesticide exposure with pediatric leukemia: potential underlying mechanisms. Int J Mol Sci 2016; 17: 461.
Li Z, Sun B, Clewell RA, Adeleye Y, Andersen ME, Zhang Q. Dose-response modeling of etoposide-induced DNA damage response. Toxicological Sciences 2014 Feb;137(2):371-84. doi: 10.1093/toxsci/kft259.
Moneypenny CG, Shao J, Song Y, Gallagher EP. MLL rearrangements are induced by low doses of etoposide in human fetal hematopoietic stem cells. Carcinogenesis. 2006; 27(4):874–81. Epub 2005/12/27. doi: 10.1093/carcin/bgi322
Nitiss JL (2009). Targeting DNA topoisomerase II in cancer chemotherapy. Nat.Rev.Cancer 9 338-350.
Nitiss JL, Soans E, Rogojina A, Seth A, Mishina M. 2012. Topoisomerse Assays. Current Protocol Pharmacol. chapter 3: Unit 3.3.
Pendleton M, Lindsey RH Jr, Felix CA, Grimwade D, Osheroff N. Topoisomerase II and leukemia. Ann N Y Acad Sci. 2014 Mar;1310:98-110. doi: 10.1111/nyas.12358.
Udroiu I., Sgura A. Genotoxicity sensitivity of the developing hematopoietic system. 2012. Mutation Research 2012; 767: 1-7.