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Key Event Title
MLL chromosomal translocation
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
Key Event Description
Chromosomal rearrangements of the mixed-lineage leukaemia (MLL) gene, located on the q23 band of chromosome 11 (11q23), are the genetic hallmark of most infant leukaemias (Meyer et al 2013; Sanjuan-Pla et al 2015). MLL is located within the fragile site FRA11G; chromosomal fragile sites are regions of the genome susceptible to breakage under conditions of replication stress; interference with TopoII may promote fragile site instability. MLL encodes a protein homologous to the Drosophila trithorax gene, which has relevant functions in embryogenesis and hematopoiesis (Ernest et al 2004, Hess et al 1997).
MLL, a human homologue of the epigenetic transcriptional regulator Trithorax of Drosophila, is an upstream transcriptional effector of HOX genes. The importance of normal MLL protein for normal axial-skeletal developmental process and HOX gene regulation has been demonstrated in the embryos of heterozygous and homozygous MLL knockout and MLL truncation mutant mice. Furthermore, expression of MLL protein is not necessary for turning on transcription of certain HOX genes, but for the maintenance of their transcription. Experiments in vitro using hematopoietic progenitors from embryos of homozygous MLL knockout mice or mice with MLL mutant showed that MLL was also critical for hematopoietic development. Recent findings suggested that MLL is required during embryogenesis for the specification or expansion of hematopoietic stem cells. As HOX genes also play a key role in the regulation of hematopoietic development, the hematopoietic dysfunction of MLL null cells is likely to be attributed to deregulated patterns of HOX gene expression in hematopoietic stem cells or progenitors. This link between MLL, HOX gene regulation, and hematopoiesis is of particular importance (Li et al. 2005).
There are many translocation and fusion partners for MLL; DNA breakage within MLL can lead to rearrangement with over 120 partner genes (Meyer et al 2013).
MLL protein (complexed with a large number of other protein factors) serves as a transcriptional activator or repressor via the binding to promoter regions of active genes, marking these regions by covalent histone modifications (Sanjuan-Pla et al 2015). Translocation and creation of fusion genes and products destroys the intrinsic control mechanisms of the MLL protein. The resulting ‘ectopic’ functions involve promoter hyper-activation and re-acquiring stem cell features (Sanjuan-Pla et al 2015). A schematic presentation of the drastic changes of the MLL product is depicted in the figure below.
Proposed model for the oncogenic conversion of MLL fusion: A. Physiological situation and B: . A chromosomal translocation, which leads to the intrinsic regulatory mechanism of MLL being destroyed. (Sanjuan-Pla et al. 2015).
How It Is Measured or Detected
MLL rearrangements can be identified following different methods. It is worthnoting that different methods will give a different information detail.
- Split-signal FISH: The split-signal FISH approach uses two differentially labeled probes, located in one gene at opposite sites of the breakpoint region. Probe sets were developed for the genes TCF3 (E2A) at 19p13, MLL at 11q23, ETV6 at 12p13, BCR at 22q11, SIL-TAL1 at 1q32 and TLX3 (HOX11L2) at 5q35. In normal karyotypes, two colocalized green/red signals are visible, but a translocation results in a split of one of the colocalized signals. Split-signal FISH has three main advantages over the classical fusion-signal FISH approach, which uses two labeled probes located in two genes. First, the detection of a chromosome aberration is independent of the involved partner gene. Second, split-signal FISH allows the identification of the partner gene or chromosome region if metaphase spreads are present, and finally it reduces false-positivity (Van der Burg et al, 2004).
- RT-PCR in combination with long-distance inverse PCR (LDI-PCR) performed on isolated genomic DNA. This method allows the identification of any kind of MLL rearrangement if located within the breakpoint cluster region. The method uses LDI-PCR to identify MLL translocations independent of the involved partner gene or other MLL aberrations that occurred within the MLL breakpoint cluster region. This method allows high-throughput analyses because genomic MLL fusion sequences can be obtained with a minimum of only four PCR reactions. Moreover, this method requires only small quantities of genomic patient DNA (1 μg) and provides relevant genetic information that can be used directly for quantitative minimal residual disease (MRD) analyses (Meyer et al. 2005).
Assays measuring chromosomal aberrations, micronuclei or DNA and chromosome damage (Comet assay) may indirectly identify the KE through its consequences in experimental systems in vitro and in vivo. FISH staining is however necessary for identification of MLL translocations.
Domain of Applicability
Although the KE deals with the general process of DNA integrity, the available evidence do not allow for evaluating whether any significant difference occurs among cell types or species. It has been shown that the mouse has an analogous fusion gene mll-af4. A recent study has shown that in utero exposure to etoposide induces mll translocations in Atm-knockout mice, which are defective in the DNA damage response, albeit not in wild-type mice; moreover, fetal liver hematopoietic stem cells were more susceptible to etoposide than maternal bone marrow mononuclear cells, pointing out the life stage-related susceptibility in regards to TopoII “poison” also in the mouse (Nanya et al., 2015).
MLL-AF4 fusion gene is present and expressed in bone marrow mesenchymal stem cells in infant patients with t(4;11) B cell-ALL (Menendez et al. 2009). However, other paediatric B cell-ALL-specific translocations/gene fusions were never found in this cell population. This suggests that the origin of the fusion gene in infant B cell-ALL is likely prehaematopoietic. Consequently, the target cell for transformation may be an early prehaematopoietic mesodermal precursor, a haematopoietic stem cell or a haematopoietic progenitor cell residing mainly in the liver (Greaves et al. 2015; sanjuan-Pla et al. 2015).
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M van der Burg, T S Poulsen, S P Hunger, H B Beverloo, E M E Smit, K Vang-Nielsen, A W Langerak and J J M van Dongen. 2004. Split-signal FISH for detection of chromosome aberrations in acute lymphoblastic leukemia. Leukemia (2004) 18, 895–908. doi:10.1038/sj.leu.2403340 Published online 25 March 2004.