Tyrosine kinase inhibitors induce alternative spliced BCR‐ABL (Ins35bp) variant via inhibition of RNA polymerase II on genomic BCR‐ABL

To elucidate dynamic changes in native BCR‐ABL and alternatively spliced tyrosine kinase inhibitor (TKI)‐resistant but function‐dead BCR‐ABL(Ins35bp) variant, following commencement or discontinuation of TKI therapy, each transcript was serially quantified in patients with chronic myeloid leukemia (CML) by deep sequencing. Because both transcripts were amplified together using conventional PCR system for measuring International Scale (IS), deep sequencing method was used for quantifying such BCR‐ABL variants. At the initial diagnosis, 7 of 9 patients presented a small fraction of cells possessing BCR‐ABL(Ins35bp), accounting for 0.8% of the total IS BCR‐ABL, corresponding to actual BCR‐ABL(Ins35bp) value of 1.1539% IS. TKI rapidly decreased native BCR‐ABL but not BCR‐ABL(Ins35bp), leading to the initial increase in the proportion of BCR‐ABL(Ins35bp). Thereafter, both native BCR‐ABL and BCR‐ABL(Ins35bp) gradually decreased in the course of TKI treatment, whereas small populations positive for TKI‐resistant BCR‐ABL(Ins35bp) continued fluctuating at low levels, possibly underestimating the molecular response (MR). Following TKI discontinuation, sequencing analysis of 54 patients revealed a rapid relapse, apparently derived from native BCR‐ABL (+) clones. However, IS fluctuating at low levels around MR4.0 marked a predominant persistence of cells expressing function‐dead BCR‐ABL(Ins35bp), suggesting that TKI resumption was unnecessary. We clarified the possible mechanism underlying mis‐splicing BCR‐ABL(Ins35bp), occurring at the particular pseudo‐splice site within intron8, which can be augmented by TKI treatment through inhibition of RNA polymerase II phosphorylation. No mutations were found in spliceosomal genes. Therefore, monitoring IS functional BCR‐ABL extracting BCR‐ABL(Ins35bp) would lead us to a correct evaluation of MR status, thus determining the adequate therapeutic intervention.