Molecular analysis of the BCR-ABL1 kinase domain in chronic-phase chronic myelogenous leukemia treated with tyrosine kinase inhibitors in practice: Study by the Nagasaki CML Study Group

Abstract

An appropriate trigger for BCR-ABL1 mutation analysis has not yet been established in unselected cohorts of chronic-phase chronic myelogenous leukemia patients. We examined 92 patients after 12 months of tyrosine kinase inhibitor (TKI) treatment in Nagasaki Prefecture, Japan. Univariate analysis revealed that significant factors associated with not attaining a major molecular response (MMR) were the presence of the minor BCR-ABL1 fusion gene, a low daily dose of TKI, and the emergence of BCR-ABL1 kinase domain mutations conferring resistance to imatinib. Factors associated with the loss of sustained MMR were a low daily dose of TKI and the emergence of alternatively spliced BCR-ABL1 mRNA with a 35-nucleotide insertion. Taken together, our results suggest that the search for BCR-ABL1 mutations should be initiated if patients have not achieved MMR following 12 months of TKI treatment.

Persistent detection of alternatively spliced BCR‐ABL variant results in a failure to achieve deep molecular response

“…Several reports have demonstrated that BCR-ABL Ins35bp is detected mainly in patients who have achieved hematological/cytogenetic response, but not DMR, despite long-term TKI treatment. (6,12,15,43) In contrast, extremely low amounts of BCR-ABL Ins35bp were detected in patients with newly-diagnosed, previously untreated CML (data not shown). We found that TKI treatment inhibits recruitment of RNA polymerase a toward genomic BCR-ABL, which positively regulates both transcription and pre-mRNA intron splicing, resulting in a relative increase in the amount of BCR-ABL Ins35bp with a decrease of total BCR-ABL transcript level.…”

“…It has remained unclear how alternatively spliced BCR‐ABL Ins35bp variants can be acquired in CML cells under TKI treatment, and why mis‐splicing reproducibly occurs at the specific sites of intronic 35 bp in ABL intron 8. Several reports have demonstrated that BCR‐ABL Ins35bp is detected mainly in patients who have achieved hematological/cytogenetic response, but not DMR, despite long‐term TKI treatment . In contrast, extremely low amounts of BCR‐ABL Ins35bp were detected in patients with newly‐diagnosed, previously untreated CML (data not shown).…”

“…Recent studies have shown that alternatively spliced BCR‐ABL variants cause failure in achieving optimal molecular response to TKI, especially in patients under long‐term TKI treatment . Alternatively spliced BCR‐ABL variants have been detected in approximately 20% of CML patients who have gained hematologic/cytogenetic response but have failed to achieve DMR through the conventional Sanger sequencing method . A representative alternatively spliced BCR‐ABL Ins35bp variant, in which retention of 35 intronic nucleotides at the splice junction of ABL exons 8 and 9 introduces a stop codon, results in a frameshift leading to the addition of 10 intron‐encoded residues and truncation of 653 residues (Fig.…”

The clinical outcomes of chronic myeloid leukemia patients harboring alternatively spliced BCR-ABL variants

“…Of note, Case #3 exhibited an infrequent type of mutation and responded less well to nilotinib and better to dasatinib treatment. The respective resistance patterns could have resulted from individual patient factors such as medical history, therapy adherence, amino acid substitutions, or other unidentified mechanisms but it might also result from the structural affinity of each TKI to each mutation [5]. Because of the limited number of clinical reports, it remains uncertain whether the exon 6 frameshift acts differently against dasatinib and nilotinib.…”

“…Recently, alternative splicing variants such as an exon 7 deletion, insertion of 35 intronic nucleotides at the junction of exon 8/9, and an exon 6 frameshift; i.e. a CAGG transnucleotide insertion at the junction of exon 5/6, have been proposed as mechanisms of TKI resistance [3,4], although the clinical significance of these mutations in the effectiveness of TKIs remains controversial [2,3,[5][6][7][8][9].…”