Transient response to imatinib mesylate (STI571) in a patient with the ETV6-ABL t(9;12) translocation.

We report the transient response of a patient with the ETV6-ABL fusion gene to imatinib mesylate (STI571). A 38-year-old man was referred with an erroneous diagnosis of Philadelphia-positive chronic myeloid leukemia in blastic transformation for treatment with the ABL tyrosine kinase inhibitor, STI571. Further investigation indicated that the patient in fact had acute myeloid leukemia; no evidence of the Philadelphia translocation or BCR-ABL was found using fluorescence in situ hybridization (FISH) or reverse transcription-polymerase chain reaction. Detailed FISH analysis identified a cryptic t(9;12) translocation, and molecular studies confirmed the presence of the ETV6-ABL fusion transcript. Because the patient was gravely ill at presentation, treatment was commenced immediately with STI571 monotherapy, resulting in considerable initial improvement. However within 10 days the patient's condition again deteriorated, and he required conventional chemotherapy. This case has implications for the design of future studies using STI571 in leukemias involving ABL-encoded fusion proteins other than BCR-ABL.