The multidrug resistance 1 gene (MDR1) encodes P-glycoprotein (Pgp), a member of 
the ATP-binding cassette (ABC) transporter family that confers tumor drug 
resistance by actively effluxing a number of antitumor agents. We had previously 
shown that MDR1 transcription is regulated by epigenetic events such as histone 
acetylation, and had identified the histone acetylase P/CAF and the 
transcription factor NF-Y as the factors mediating the enzymatic and 
DNA-anchoring functions, respectively, at the MDR1 promoter. It has also been 
shown that MDR1 activation is accompanied by increased methylation on lysine 4 
of histone H3 (H3K4). In this study, we further investigated histone methylation 
in MDR1 regulation and function. We show that the mixed lineage leukemia 1 
(MLL1) protein, a histone methyltransferase specific for H3K4, is required for 
MDR1 promoter methylation, as knockdown of MLL1 resulted in a decrease in MDR1 
expression. The regulation of MDR1 by MLL1 has functional consequences in that 
downregulation of MLL1 led to increased retention of the Pgp-specific substrate 
DIOC(2)(3), as well as increased cellular sensitivity to several Pgp substrates. 
Regulation of MDR1 by MLL1 was dependent on the CCAAT box within the proximal 
MDR1 promoter, similar to what we had shown for MDR1 promoter acetylation, and 
also requires NF-Y. Finally, overexpression of the most prevalent MLL fusion 
protein, MLL-AF4, led to increased MDR1 expression. This is the first 
identification of a histone methyltransferase and its leukemogenic rearrangement 
that regulates expression of an ABC drug transporter, suggesting a new target 
for circumvention of tumor multidrug resistance.