Sensitivity of MLL-rearranged AML cells to all-trans retinoic acid is associated with the level of H3K4me2 in the RARα promoter region

All-trans retinoic acid (ATRA) is well established as differentiation therapy for acute promyelocytic leukemia (APL) in which the PML-RAR alpha (promyelocytic leukemia-retinoic acid receptor alpha) fusion protein causes blockade of the retinoic acid (RA) pathway; however, in types of acute myeloid leukemia (AML) other than APL, the mechanism of RA pathway inactivation is not fully understood. This study revealed the potential mechanism of high ATRA sensitivity of mixed-lineage leukemia (MLL)-AF9-positive AML compared with MLL-AF4/5q31-positive AML. Treatment with ATRA induced significant myeloid differentiation accompanied by upregulation of RAR alpha, C/EBP alpha, C/EBP epsilon and PU.1 in MLL-AF9-positive but not in MLL-AF4/5q31-positive cells. Combining ATRA with cytarabine had a synergistic antileukemic effect in MLL-AF9-positive cells in vitro. The level of dimethyl histone H3 lysine 4 (H3K4me2) in the RAR alpha gene-promoter region, PU.1 upstream regulatory region (URE) and RUNX1+24/+25 intronic enhancer was higher in MLL-AF9-positive cells than in MLL-AF4-positive cells, and inhibiting lysine-specific demethylase 1, which acts as a histone demethylase inhibitor, reactivated ATRA sensitivity in MLL-AF4-positive cells. These findings suggest that the level of H3K4me2 in the RAR alpha gene-promoter region, PU.1 URE and RUNX1 intronic enhancer is determined by the MLL-fusion partner. Our findings provide insight into the mechanisms of ATRA sensitivity in AML and novel treatment strategies for ATRA-resistant AML.