ATP Binding Domain Mutants of BCR-ABL in Patients with Chronic Myeloid Leukemia Treated with Imatinib.

Objective:Imatinib (also called STI571 or Gleevec), a competitive inhibitor at the ATP binding site of BCR-ABL, has been shown remarkable clinical activity in patients with chronic myeloid leukemia (CML). However, a significant proportion of Imatinib-treated CML patients with advanced stage disease develop resistance. In this study, we try to detect ATP binding domain mutants of BCR-ABL in Imatinib-treated CML patients and explore the possibility that ATP binding domain mutants might confer resistance to imatinib.

Patients and Methods:We analyzed bone marrow samples from 16 Imatinib-treated CML patients, including 6 chronic phase, 2 accelerated phase (AP) and 8 blast crisis (BC) patients. 8 patients were sensitive to imatinib and 8 patients resistant. A polymerase chain reaction strategy was used to amplify and sequence the ATP binding domain of BCR-ABL.

Results:Point mutations were found in the ATP binding domain of BCR-ABL in 3 of 16 patients. In 2 of 8 imatinib-resistance patients, a single nucleotide change was detected and resulted in a threonine to isoleucine substitution at position 315(Thr315Ile) of ABL kinase. This point mutation has proved to interfere with drug binding and cause resistance to imatinib. In addition, we found a mutation (Lys357Arg) remain sensitive to imatinib.

Discussion:Chronic myeloid leukemia (CML) is a hematopoietic disorder characterized by the malignant expansion of bone marrow stem cells. Its malignant clonal marker is Philadelphia chromosome (Ph) which harbors the BCR-ABL fusion gene. The latter encodes a chimeric BCR-ABL protein, identified as having a central role in the pathogenesis of CML. As a competitor for ATP binding of BCR-ABL, Imatinib selectively induces apoptosis and blocks proliferation in BCR-ABL positive cells and has been shown to have high activity in CML. Clinical studies demonstrate that Many Imatinib-treated CML patients with advanced stage disease respond initially but then relapse. Till now, the resistant mechanisms of imatinib have not all been known. In this study, we find that ATP binding domain mutants of BCR-ABL is one of the potential mechanisms of resistance to imatinib, but not all mutants associate with imatinib resistance and some mutants remain sensitive to imatinib.