PO-372 Investigating the epigenetic changes underlying combination treatment of acute promyelocytic leukaemia with all-trans retinoic acid and arsenic trioxide

IntroductionAcute promyelocytic leukaemia (APL) results in the interrupted differentiation of granulocytes. Combination induction therapy using all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) is the most effective treatment of APL. DNA methylation and histone modifications are epigenetic mechanisms of gene regulation aberrant in APL. While ATRA treatment of APL NB4 cells restores granulocyte differentiation and induces significant genome-wide changes in histone modifications, it does not affect the aberrant DNA methylation. Similarly, ATO has comparable effects. Therefore, we hypothesise that the increased effectiveness of ATRA and ATO when combined is due to epigenetic changes at both the histone and DNA methylation level, resulting in profound changes in gene expression.Material and methodsWe identified the concentration of ATRA and ATO in combination that induced enhanced differentiation and cell death of NB4 cells in comparison to single agent treatment through fluorescence activated cells sorting (FACS) of CD11b and 7-AAD labelled cells at 72 hours and 96 hours post treatment termination. Quantitative PCR (qPCR) was performed to determine the expression levels of a panel of genes after 72 hours of treatment and 96 hours post treatment termination.Results and discussionsATRA and ATO in combination induced significant differentiation and cell death of NB4 cells after 72 hours. Interestingly, only the combination-treated cells maintained differentiation when treatment was terminated for 96 hours. Additionally,combination treatment significantly increased the expression of several genes important in APL compared to single agent treatment after 72 hours as well as 96 hours post treatment termination. Chromatin immunoprecipitation (ChIP) is currently being utilised to investigate the enrichment of chromatin with acetylated and methylated histones, and qPCR was performed to assess the enrichment of several genes of interest. Finally, bisulfite pyrosequencing is being performed to characterise the specific methylation pattern of these genes of interest.ConclusionATRA and ATO in combination synergistically induces the differentiation of APL NB4 cells. Together, the ChIP and bisulfite pyrosequencing experiments will enable us to characterise the effects of combination treatment at the epigenetic level. Ultimately, understanding the epigenetic changes induced by ATO and ATRA synergy can help improve treatment in the clinic.