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 ¦Á (SAT-¦Á). As an additional measure of global methylation levels we used the LUminometric Methylation Assay (LUMA).
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.
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.