Mutations in the kinase domain (KD) of BCR-ABL are the leading cause of acquired imatinib resistance. In some cases, identical mutations were detected at relapse and in pretherapeutic specimens, consistent with selection of resistant clones in the presence of drug. However, the incidence of KD mutations in imatinibnaive patients, irrespective of response to therapy, is unknown. We studied mutation frequency in 66 patients with chronic myelogenous leukemia (CML), using cDNA sequencing and allele-specific oligonucleotide-polymerase chain reaction (ASO-PCR) assays for 8 common mutations. Thirteen patients were positive by ASO-PCR only, 1 by ASO-PCR and sequencing, and 1 by sequencing only (overall frequency, 22.7%). T315I was most frequent (12% of patients). Eleven of the 14 patients with positive ASO-PCR had follow-up samples available for sequencing. Wild-type sequence was detected in 6 of 11, 2 different mutations in 1 of 11, and identical mutations in 4 of 11 patients, 2 of whom had achieved major cytogenetic response. In multivariate analysis mutation detection was associated with clonal cytogenetic evolution, exposure to 6-Thioguanine, and a low platelet count, but not with response to imatinib, event-free survival, and overall survival. KD mutants present at low levels do not invariably lead to relapse, and additional factors are required to induce a fully drug-resistant phenotype. (Blood. 2005; 106:2128-2137)

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We have identified a gene polymorphism (K247R) within or close to the P-loop of BCR-ABL, which leads to the substitution of arginine for lysine. We investigated the allelic frequency of K247R by screening 157 CML patients and 213 healthy blood donors with conventional sequencing, restriction enzyme digest and single strand conformational polymorphism analysis, and found the arginine allele to be rare. Three out of five CML patients with the arginine allele of K247R failed to achieve a major cytogenetic response to imatinib, suggesting that the arginine allele may have reduced sensitivity. However, despite K247R's position in or near to the P-loop, biochemical and cellular assays of imatinib and dasatinib sensitivity showed no alteration compared to wild type. Clinicians should be aware that possession of the arginine allele of K247R does not reflect a mutation that necessitates a change in the therapeutic strategy, unless there are other signs of inadequate response to drug.