hOCT1 transcript levels and single nucleotide polymorphisms as predictive factors for response to imatinib in chronic myeloid leukemia

The probability of achieving an optimal response to treatment in CML is most likely to be influenced by the variable capacity of a given patient's leukemia cells to accumulate intracellular imatinib (IM). The transport of IM into the cells is an active process mediated by the human organic cation transporter (hOCT1) protein, a membrane associated ATP-dependent protein channel.1 Recently we reported that higher levels of hOCT1 transcript levels in blood derived white cells (WBC) collected at diagnosis predicted for a 3-log reduction in BCR-ABL1 transcript levels (major molecular response, MMR) and also for greater degrees of transcript reduction in patients who had achieved complete cytogenetic responses (CCyR).2 Investigators in Liverpool also showed that higher hOCT1 transcript levels in diagnostic WBC predicted for a better response to IM.3 The mean hOCT1 mRNA level in pre-IM bone marrow mononuclear cells (MNC) was also reported to be lower in the patients who failed to achieve CCyR by 12 months of therapy.4 However a recent study of total WBC collected from newly diagnosed patients before starting IM failed to confirm any correlation between hOCT1 transcript levels and response to IM.5 These conflicting reports on the predictive value of hOCT1 transcript levels2, 3, 4, 5 might be due to the different target cells studied, to different primers and probes used to measure hOCT1 mRNA or to differing phases of the disease when the hOCT1 was measured. We isolated polymorphonuclear cells (PMNC) and mononuclear cells (MNC) from the peripheral blood of 21 normal individuals and measured hOCT1 transcript levels in both populations by Taqman real time PCR as described before.2 hOCT1 was expressed at a significantly higher level in the PMNC population than in the MNC population (Figure 1a). This suggests the investigation of hOCT1 transcript level as a prognostic factor should be consistent with cell type. We also measured hOCT1 in WBC from paired samples collected at diagnosis and at attainment of complete cytogenetic response (CCyR) in 60 patients treated with IM. The hOCT1 transcript level was significantly higher (P<0.0001) in samples collected in CCyR (range: 0.15–8.88, median: 2.2) compared to the matching diagnostic samples (range: 0.02–3.511, median: 0.14) (Figure 1b). The obvious difference at these two time points is that cells collected from blood at the time of CCyR are derived from normal rather than leukemic haemopoiesis. The relative increase in the hOCT1 transcript level in the CCyR samples could be due to an inhibitory effect of the BCR-ABL1 oncoprotein on the expression of hOCT1 in leukemic cells. As shown in Figure 1c, the level of hOCT1 transcripts in total WBC in CCyR samples (n=60) from patients on IM was not significantly different (P=0.156) from the level of hOCT1 in total WBC from normal individuals (n=21), which supports this explanation. This hypothesis is indirectly supported by the observation of a reduction in hOCT1 transcript levels in CML blast crisis samples5 which in turns is associated with increased level of BCR-ABL1 transcripts.6 In addition to the prognostic value of hOCT1 transcript levels, some investigators have focused on the possible predictive value of hOCT1 single nucleotide polymorphisms (SNPs)7 which have been previously shown to affect hOCT1 function in metformin transport.8 To study the association between the polymorphisms with significant influence on the function of hOCT1 and response to IM we investigated the frequency and association of the 4 SNPs (R61C (rs12208357), P341L (rs2282143), G401S (rs34130495) and rs622342) in 132 CML patients treated with IM in chronic phase. SNP P283L, R287G and C88R were not analysed for this association as the variation of SNP genotypes was too small to be used for any comparison (Table 1). We were unable to identify an association between the transcript level of hOCT1 and any of the individual SNPs except for SNP G401S (rs34130495) which showed a correlation with IM response (Table 1). Patients with 401(G/A) genotype (n=6) had a higher probability of achieving MMR (P=0.015). However, because patients carrying this polymorphism are rare, this result should be interpreted with caution and a larger cohort should be investigated for validation. In summary, the transcription of hOCT1 is a function of the leukocyte lineage in peripheral blood. Its level is significantly higher in PMN (granulocytes) than in MNC (which are largely composed of lymphocytes in normal individuals). Our data suggest that the BCR-ABL1 oncoprotein might reduce the transcript level of hOCT1. Based on these findings we suggest that hOCT1 as a prognostic factor for response to IM should be investigated using granulocyte-derived cDNA, prior to the start of therapy. The predictive value of hOCT1 expression in our cohort was not improved by consideration of the genotype of candidate SNPs within this gene. However a larger study may be required to validate this observation. D Marin, JF Apperley, JM Goldman, D Milojkovic and K Rezvani receive honoraria and/or research funds from Novartis Pharmaceuticals. However, Novartis had no role in the design of the study, collection of the data or the decision to publish.