Key Event Title
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|topoisomerase II binding, infant leukaemia||AdverseOutcome|
|Birth to < 1 month||High|
Key Event Description
B cell ALL is the most frequent cancer in children. Infant leukaemia is a rare haematological disease with an incidence of 1 in 106 newborns, accounting for 10% of all B cell-ALLs in children younger than 15 years, manifesting soon after birth (<1 year) and displaying an intermediate prognosis except for some cytogenetic subgroups such as B cell-ALL, which remains an outlier high-risk group having a poor prognosis (Sanjuan-Pla et al., 2015). Compared with the more frequent childhood leukaemias, infant leukaemia shows distinct features (see Table 1 for a more comprehensive comparison):
- An early neonatal manifestation suggests an in utero initiation as an ‘intrauterine developmental disease’ (Greaves, 2015; Sanjuan-Pla et al., 2015);
- Rearrangements of the MLL gene on the q23 band of chromosome 11 as the hallmark genetic abnormality (Joannides and Grimwade, 2010).
- However, MLL is not the only translocation gene. Whereas about 60–80% of infant ALL carry an MLL(Sam et al., 2012; Jansen et al., 2007), for infant acute myeloid leukaemia (AML) the percentage of MLL is lower than 40%;
- The MLL-r occurs at an early stage of development, with the target cells (still unidentified) being likely theHSPCs in foetal liver and/or pre-haematopoietic mesodermal foetal precursors (Bueno et al., 2009; Menendez et al., 2009);
- Infant MLL-r leukaemia has the least number of somatic mutations among all the sequenced cancers (1.3 vs 6.5/case; Andersson et al., 2015; Dobbins et al., 2013), pointing to the lack of a “second hit” assumed in the classic carcinogenesis paradigm.
The overall scientific evidence, including the stable genome of patients, suggests that infant leukaemia originates from one “big-hit” occurring during a critical developmental window of HSPC vulnerability (Andersson et al., 2013; Greaves, 2015). In contrast to the “two-hit” model of the adult and childhood leukaemias, infant leukaemia is a developmental disorder where the differentiation arrest and clonal expansion are a direct consequence of in utero MLL translocation in target HSPCs. Even if MLL is not present in 100% of infant leukaemias, the ‘MLL rearranged (MLL-r) infant leukaemia’, especially MLL-r B-ALL, is taken here as a model for the disease principally because of the quantity of scientific evidence.
How It Is Measured or Detected
Haematological methods – identification of leukaemia cells and routine blood cell counts; observations of clinical symptoms.
Following clinical diagnosis, methods for refined diagnosis include bone marrow aspirates for immunophenotypic analyses and cytogenetic assays for molecular stratification.
The carcinogenicity assays and the extended one generation test (OECD 443) include endpoints that can potentially explore the AO; however, considerations should be made on the specificity of the disease to humans. Indeed, IFL, as such, is not an animal disease and never reported as chemically induced outcome in cancerogenesis studies. it should however be noted that cancerogenesis studis are generally performed in young adult animals and protocol including the treatment of the dams from the mating period are not common. for this reason, the sensitivity of the cancerogenesis study to capture this hazard is at its best unknown.
Domain of Applicability
Infant leukaemia is a paediatric leukaemia likely resulting from gene-environmental interactions. The limited data available suggest that dietary and environmental exposure to substances targeting topoisomerases together with reduced ability of the foetus or their mother to detoxify such compounds because of the polymorphic variants of given genes could contribute to the development of this AO (Hernadez et al. 2016).
In animals the disease is not known and artificial animal models able to reproduce the disease have limitations. Bardini et al (2015) has however developed a xenograft mouse model with patient MLL-AF4-involving leukoblasts transplanted.
Evidence for Perturbation by Stressor
Topo II is a well validated anti-cancer target and Topo II poisons are widely used and effective therapeutic agents; but they are associated with the occurence of late complications, including therapy-related acute leukaemia (Cowell and Austin, 2012). Secondary acute leukaemia carrying MLL-r is an adverse effect observed in patients treated with etoposide and a few other anticancer agents. Characteristics of the disease are in many ways analogous to those in infant leukaemia (Joannides et al., 2010, 2011). MLL rearrangement, short latency and poor prognosis, strongly suggest that infant leukaemia and treatment-related leukaemia are sufficiently similar to allow for inferences to be made regarding tentative aetiological factors, molecular events and disease progression and manifestation.
Regulatory Significance of the Adverse Outcome
Genotoxicity in general and carcinogenicity are apical endpoints in established regulatory guideline studies. TopoII poisoning has been listed as one of the potential mechanisms of genotoxicity and carcinogenicity in the ICH M7 guideline for human medicines. It is also known that some manifestations of genotoxicity in tests measuring chromosomal aberrations, micronuclei or DNA and chromosome damage (Comet assay) are partially due to double-strand breaks created by the disturbed action of TopoII enzymes.
The extended one generation test (OECD 443) includes a developmental immunotoxicity cohort. At present the cohort may identify post-natal effects of prenatal and neonatal exposures on the immune tissues and white blood cells population. However, each regulatory guideline study has potential limitations e.g. no specific parameters are in place to identify a pattern relevant to infant leukemia in humans in the extended one generation test, no treatment is occurring during the early in-utero development phase in the carcinogenicity assay and no considerations on the possible higher sensitivity of the HSC are in place for the genotoxicity assays.
Epidemiological evidence linking pesticide exposure to infant leukaemia, also suggests that pesticide exposure may have a greater impact on children than adults; though, almost all of the available evidence does not make a distinction between infant and childhood leukaemia. However, most epidemiological studies are limited because no specific pesticides have been directly associated with the risk of leukaemia, but rather the broad term “pesticide exposure” (Hernandez and Menendez 2016). In this perspective, this AOP would provide a regulatory relevant support for understanding the potential of a chemical to be involved in this toxicological pathway.
Bardini M, Woll PS, Corral L, Luc S, Wittmann L, Ma Z, Lo Nigro L, Basso G, Biondi A, Cazzaniga G, Jacobsen SE. Clonal variegation and dynamic competition of leukemia-initiating cells in infant acute lymphoblastic leukemia with MLL rearrangement. Leukemia. 2015 Jan;29(1):38-50. doi: 10.1038/leu.2014.154.
Bueno C, Catalina P, Melen GJ, Montes R, Sanchez L, Ligero G, Garcia-Perez JL, Menendez P. Etoposide induces MLL rearrangements and other chromosomal abnormalities in human embryonic stem cells. Carcinogenesis 2009; 30(9): 1628-1637. doi: 10.1093/carcin/bgp169.
Ezoe S. Secondary leukemia associated with the anti-cancer agent, etoposide, a topoisomerase II inhibitor. Int J Environ Res Public Health. 2012 Jul;9(7):2444-53. doi: 10.3390/ijerph9072444.
Gole B, Wiesmüller L. Leukemogenic rearrangements at the mixed lineage leukemia gene (MLL)-multiple rather than a single mechanism. Front Cell Dev Biol. 2015 Jun 25;3:41. doi: 10.3389/fcell.2015.00041.
Hernandez A and Menendez P. Linking pesticide exposure with pediatric leukemia: potential underlying mechanisms. Int J Mol Sci 2016; 17: 461.
Hunger SP, Mullighan CG. Acute Lymphoblastic Leukemia in Children. N Engl J Med 2015; 73: 1541-1552.
Li Z, Sun B, Clewell RA, Adeleye Y, Andersen ME, Zhang Q. Dose-response modeling of etoposide-induced DNA damage response. Toxicol Sci. 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
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.