Submitted February 22, 2008
Accepted August 27, 2008
Mechanisms of resistance to 5-aza-2'-deoxycytidine in human cancer cell lines
Taichun Qin, Jaroslav Jelinek, Jiali Si, Jingmin Shu, and Jean-pierre J. Issa*
Department of Leukemia, The University of Texas M. D. Anderson Cancer Center, Houston, TX, United States
* Corresponding author; email: jpissa{at}mdanderson.org.
5-aza-2'-deoxycytidine is approved for the treatment of myelodysplastic syndromes, but resistance to this agent is common. In search for mechanisms of resistance, we measured the IC50 of DAC and found it differed 1000 fold among a panel of cancer cell lines. The IC50 was correlated with the doses of DAC that induced the most hypomethylation of long interspersed nuclear elements (LINE) (r=0.94, P<0.001), but not with LINE methylation or DNMT1, 3a and 3b expression at baseline. Sensitivity to DAC showed a low correlation (r= 0.44, P= 0.11 ) to that of 5-azacytidine (AZA), but a good correlation to that of Cytarabine (Ara-C) (r= 0.89, P<0.001). The five cell lines most resistant to DAC had a combination of low dCK, hENT1 and 2 transporters and high cytosine deaminase. In an HL60 clone, resistance to DAC could be rapidly induced by drug exposure, and was related to a switch from heterozygous to homozygous mutation of DCK. Transfection of wild type DCK restored DAC sensitivity. DAC induced DNA breaks as evidenced by H2AX phosphorylation and increased homologous recombination rates by 7-10 fold. These results suggest that in vitro resistance to DAC can be explained by insufficient incorporation into DNA. The lack of cross resistance between DAC and AZA is of potential clinical relevance.
