Acquired Resistance to Decitabine Associated with the Deoxycytidine Kinase A180P Mutation: Implications for the Order of Hypomethylating Agents in Myeloid Malignancies Treatment

The backbone of therapy for elderly patients with myelodysplastic syndromes and acute myeloid leukemia consists of hypomethylating agents 5-aza-2'-deoxycytidine (DAC) and 5-azacytidine (AZA). However, resistance frequently emerges during treatment. To investigate the mechanisms of resistance, we generated DAC-resistant variants of the acute myeloid leukemia cell lines, MOLM-13 and SKM-1, through their prolonged cultivation in increasing concentrations of DAC. The resistant cell variants, MOLM-13/DAC and SKM-1/DAC, exhibited cross-resistance to cytarabine and gemcitabine, but remained sensitive to AZA. Existing studies have suggested that the loss of deoxycytidine kinase (DCK) may play an important role in DAC resistance. DCK is critical for DAC activation, but the precise mechanisms of its downregulation remain incompletely understood. We identified a novel point mutation (A180P) in <i>DCK</i>, which results in acquired DAC resistance. Although the <i>DCK</i> mRNA was actively transcribed, the mutant protein was not detected in DAC-resistant cells. The transfection of HEK293 cells with the mutant <i>DCK,</i> combined with proteasomal inhibition, revealed rapid proteasomal degradation, establishing a mechanistic link between the A180P mutation and DCK loss, not previously described. This highlights the importance of also evaluating DCK at the protein and/or enzymatic activity levels in patients. The loss of functional DCK impairs the phosphorylation of deoxynucleosides, conferring resistance to DAC, gemcitabine, and cytarabine, but AZA, phosphorylated by uridine-cytidine kinase, remains effective and may represent a therapeutic alternative for patients with acquired DAC resistance.