[Molecular analysis of the BCR-ABL1 fusion gene in a chronic myeloid leukemia patient with BCR-ABL1 International Scale-undetectable disease].

BCR-ABL1 International Scale (IS) measurement is an important technique used to monitor chronic myeloid leukemia (CML) when patients are treated with tyrosine kinase inhibitors. In very rare cases, the BCR-ABL1 IS measurement fails to detect BCR-ABL1 chimeric messages for unknown reasons. Such a situation was also experienced in our study; therefore, molecular analysis of BCR-ABL1 fusion gene was performed. A breakpoint in the ABL1 gene was identified in the middle of the a2 exon, leading to generation of chimeric messages lacking a nucleic acid sequence encoded by this exon (e14a3 chimeric message). The ABL1 a2 exon is critical for quantification of BCR-ABL1 chimeric messages with the measurement scales used in medical laboratories. Absence of this referred nucleic acid sequence was the reason why BCR-ABL1 IS measurement failed to detect the chimeric messages in our study. In the future, such rare cases of CML, with BCR-ABL1 chimeric messages not detected by standard IS measurements, in Japan should be collected and their molecular structures should be analyzed.