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Mutations in the kinase domain (KD) of BCR-ABLare 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)

Background: The two common types of BCR-ABL fusion protein, p190 and p210, are associated with Ph-positive ALL and CML, respectively. Compared to p190, p210 retains the Dbl-like, CDC24 homology and PH domains. Dbl-like and CDC24 homology domains are thought to serve as a guanidine exchange factor (GEF) for small G proteins, including Rac, CDC42 and RhoA. It has been suggested that the loss of GEF activity may be responsible for the more aggressive phenotype of p190-positive leukemia (McWhirter and Wang, Oncogene 1997; 15(14):1625–34). PH domains bind phosphoinositides and have been implicated in targeting proteins to cellular membranes as well as modulating the activity of the adjacent Dbl-like GEFs. It is not known, however, if the PH domain of BCR plays a role in determining disease features. We therefore decided to study the biology of p200BCR-ABL, a naturally occurring variant identified in several CML patients that lacks the PH domain but retains the Dbl-like and CDC24 homology domains.