Expression Profiling of ABC Transporter Genes in Chronic Myeloid Leukemia (CML) and Responsiveness to Imatinib

The deregulation of ATP-binding cassette (ABC) transporters responsible for the efflux of anticancer agents may involve mutations or single nucleotide polymorphisms (SNPs) or an increase in their expression level. Consequently, chemoresistance may develop. We have previously shown that the expression level and transcription of ABC drug transporters in CML cells is affected by c-Myc. Our results demonstrated that c-Myc is highly expressed in CD34+ cells from newly diagnosed chronic phase (CP)-CML patients, and that it can significantly upregulate the expression of several ABC genes, particularly, the ABCC1, ABCC4 and ABCG2, while it downregulates the expression of ABCC3. We have also demonstrated that c-Myc was a direct regulator and physically associated with the promoter of tested ABC genes, as assessed by Chromatin Immunoprecipitation in a cell line derived from a Ph+ CML patient. Taken together, our findings supported the model of a direct and coordinate regulation of a large set of ABC genes by the c-Myc transcription factor. Our study also supported prior findings that deregulation of specific set of ABC genes could be an important molecular mechanism altering imatinib transport. Based on these observations we have started to investigate the role of ABC transport genes expression in newly diagnosed CP-CML patients treated with imatinib. RNA extracted from white blood cells of 5 patients who achieved a stable major molecular response (MMR) by 12 months (responders) and 15 patients who didn’t show a partial cytogenetic response (CgR) by 6 months nor a complete CgR by 12 months (suboptimal responders according to European LeukemiaNet recommendations). All pts were enrolled on GIMEMA CML Working Party-sponsored clinical trials of imatinib. A panel of ABC genes including ABCB1, ABCB9, ABCC1, ABCC3, ABCC4, ABCE1 and ABCG2 was interrogated by Q-PCR for the level of expression. Results were normalized to the expression of three reference genes, i.e., GUSB, b-actin and GAPDH. Our results show that suboptimal responders display high expression levels of ABCG2 (p<0.01) and very low levels of ABCC3 (p<0.0001) as compared to patients with responders. Interestingly, when ABC expression profile of the same patient was evaluated before starting imatinib treatment and compared with a measurement obtained at the time of suboptimal response, we could not observe any significant difference between the two conditions. That suggest that this specific ABC transporter expression profile could be present at diagnosis. Although preliminary, our findings suggest that profiling of ABC drug transporter genes in CML patients could provide a novel modality of investigating their responsiveness to imatinib. Analysis of a larger series of patients is ongoing to further explore the potential predictive value of this expression profile. We speculate that ABCC3, which is strongly repressed possibly through a typical epigenetic mechanisms such as promoter hypermethylation and/or chromatin condensation, could be re-activated by means of demethylating agents or inhibitors of chromatin condensation. Thus, our study proposes that an important novel approach to optimize imatinib response in such patients merits further investigations.