A novel insertion mutation of K294RGG within BCR-ABL kinase domain confers imatinib resistance: sequential analysis of the clonal evolution in a patient with chronic myeloid leukemia in blast crisis

BCR-ABL kinase domain mutations were sequentially analyzed in a patient with chronic myeloid leukemia (CML) who exhibited repeated B-lymphoid blast crisis (CML-BC) during treatment with imatinib and dasatinib. We first identified five mutant BCR-ABL clones: Y253H, G250E, F311L, F317L and K294RGG, which was generated by two-nucleotide mutations and six-nucleotide insertion, at the third BC during the imatinib treatment, and retrospectively found that three of them (Y253H, G250E, K294RGG) were already present at the second BC. The in vitro analysis using K294RGG mutant BCR-ABL-expressing 32D cells revealed that K294RGG mutation was imatinib resistant but dasatinib sensitive. Consistent with the in vitro data, the clone with K294RGG mutation was eliminated by the dasatinib treatment in this patient. During the imatinib treatment, several mutant clones emerged and expanded, while additional mutations on the same allele were not acquired. However, after the dasatinib treatment, wild-type BCR-ABL clone disappeared and T315I or F317L mutation was acquired in G250E and Y253H mutant clones on the same allele without the emergence of each sole mutant clone. Cytogenetic and immunoglobulin heavy chain gene rearrangement analysis revealed that all mutant clones that appeared in this patient might be derived from the same CML clone.