Analysis of Genetic Variants and Expression Levels of Human Organic Cation Transporter 1 (HOCT1) and Genetic Variants in MDR1 in CML: Weak Associations Were Detected but a Major Role in Clinical Response to Imatinib Resistance Is Unlikely

It has previously been shown that imatinib uptake into chronic myeloid leukemia (CML) cells is dependent on human Organic Cation Transporter 1 (hOCT1; SLC22A1). In more recent work on clinical samples it was further shown that low hOCT1 expression of this influx transporter may be an important mechanism of imatinib resistance. To further evaluate this issue we have retrospectively quantified pretreatment hOCT1 mRNA expression in 92 CML patients (pts) that responded with major molecular remission within the first year of treatment and compared these results to 19 pts with primary resistance to imatinib. We found that all 19 resistant pts had low hOCT1 expression (median: 2.032 (expressed as %hOCT1/ABL); range 0.18–4.24). Although the median hOCT1 expression at diagnosis in the responders was higher (median 8.417) the range was very heterogeneous (0.45–188.2) with only 30% of all responders having a significantly higher expression than the resistant pts. As in vitro studies have shown that genetic variants of the SLC22A1 gene that codes for hOCT1 can have a negative effect on the transport of some substrates we hypothesized that not only certain hOTC1 expression levels but also different genetic variants within the SLC22A1 gene may be associated with different efficiencies of imatinib uptake. Using high resoluting melting and subsequent sequencing we have genotyped exons 1, 2, 5, 6, 7, 9, 10, and 11 in 109 responders as well as in 55 resistant pts, thus each 326 alleles were evaluated. We detected 12 different exonic polymorphisms. Two of these, a G38D and a Y404C were so far undescribed variants. Both nonsynonymous variants were detected in heterozygeous forms, the G38D in one responder and the Y404C variant in one resistant pt. All other variants were detected in frequencies similar to those that have already been described (R61C: 0.07, L160F: 0.76, P341L: 0.01, G401S: <0.01, M408V: 0.60, delM420: 0.19, G465R: 0.05, V519I: <0.01). In addition the silent variants S51S and V501V were detected with frequencies of 0.26% and 0.01% respectively. All variants in heterozygous as well as in homozygous form were distributed equally between responders and resistant patients. Thus we did not find any correlation between SLC22A1 genotype and imatinib response. In addition there was also no correlation of any of these polymorphisms to the high expressers. We found that those polymorphisms that have been described to severely affect hOTC1 functions in vitro were very rare (P341L and G401S with <0.01% each) or even never detected (P283L and R287L) in our cohort. Thus, although in vitro studies have shown that hOCT1 polymorphisms may severely affect function with respect to substrat specificity and transport efficiency of imatinib they do not seem to play a major role in response of CML patients to imatinib. In addition, we analyzed the three most frequent polymorphisms in exons 12, 21, and 26 in the multidrug resistence gene (MDR1) that codes for an efflux transporter implicated in imatinib efflux. In total 84 responders and 38 resistant patients were analyzed. We found that the exon12 nt1236t allele is more frequently observerd in resistant patients (p=0.045) whereas there was only a week association for the exon21 nt 2677t allele (p=0.121) and the exon26 nt3435t allele (p=0.139) to resistance. In conclusion,

1. it seems to be unlikely that genetic variants of hOCT1 play a major role in imatinib resistance

2. if at all, also the hOCT1 expression levels account for the response of only a few cases.

It remains unclear whether hOCT1 plays a role in influx of imatinib or whether its function may be overwritten by other influx transporters like the very homologous and functionally redundant hOCT3 just in the vicinity of hOCT1. 3) The role of efflux transporters in imatinb resistance may be more important, however we detected only a weak association to certain polymorphisms in MDR1 to resistance in our cohort.