title: Absence of global hypomethylation in promoter hypermethylated Mixed Lineage Leukaemia-rearranged infant acute lymphoblastic leukaemia.
abstract: BACKGROUND
  Mixed Lineage Leukaemia (MLL)-rearranged acute lymphoblastic leukaemia (ALL) in infants represents a highly aggressive type of leukaemia that is often characterised by severe promoter CpG island hypermethylation. Consequently, MLL-rearranged ALL cells respond well to demethylating cytosine analogue drugs. In human cancer cells, enhanced promoter methylation is typically accompanied by global loss of methylation in non-promoter regions of the genome. In turn, global hypomethylation usually leads to genomic instability, which may have contributed to cancer development.
  
  
  METHODS
  Here we examined global methylation densities in MLL-rearranged infant ALL (n=45) samples in comparison with germline MLL infant ALL (n=11), non-infant B-cell precursor ALL (n=11) and normal paediatric bone marrow (n=9) samples. For this we performed high-resolution bisulfite pyrosequencing to determine methylation levels at the repetitive elements LINE-1, Alu and satellite α (SAT-α). As an additional measure of global methylation levels we used the LUminometric Methylation Assay (LUMA).
  
  
  RESULTS
  We found that MLL-rearranged infant ALL is not characterised by global hypomethylation, despite its characteristic promoter CpG hypermethylation patterns. Instead we observed a moderate trend towards global hypermethylation and demonstrated that these methylated non-promoter sequences are responsive to demethylating agents.
  
  
  CONCLUSIONS
  MLL-rearranged infant ALL cells are characterised by an overall methylated genomic state, and both promoter and non-promoter methylation responds to demethylating agents, which may further explain the remarkable sensitivity of these cells for the methylation-inhibiting therapeutics.
authors.affiliation: Department of Pediatric Oncology/Hematology, Erasmus Medical Center - Sophia Children's Hospital, Room: Ee15-14a, Dr. Molewaterplein 50, 3015 GE, Rotterdam, The Netherlands.
authors.forename: Dominique J P M
authors.initials: DJ
authors.lastname: Stumpel
authors.forename: Pauline
authors.initials: P
authors.lastname: Schneider
authors.forename: Eddy H J
authors.initials: EH
authors.lastname: van Roon
authors.forename: Rob
authors.initials: R
authors.lastname: Pieters
authors.forename: Ronald W
authors.initials: RW
authors.lastname: Stam
mesh_terms: D019175:DNA Methylation / Q000235:genetics*; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D007223:Infant; D051788:Myeloid-Lymphoid Leukemia Protein / Q000235:genetics*; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D051788:Myeloid-Lymphoid Leukemia Protein