Impact of the Breakpoint Region on the Leukemogenic Potential and the TKI Responsiveness of Atypical BCR-ABL1 Transcripts

Chronic Myeloid Leukemia (CML) is a hematological disorder characterized by the clonal expansion of a hematopoietic stem cell carrying the Philadelphia chromosome that juxtaposes the <i>BCR</i> and <i>ABL1</i> genes. The ensuing <i>BCR-ABL1</i> chimeric oncogene is characterized by a breakpoint region that generally involves exons 1, 13 or 14 in <i>BCR</i> and exon 2 in <i>ABL1</i>. Additional breakpoint regions, generating uncommon <i>BCR-ABL1</i> fusion transcripts, have been detected in various CML patients. However, to date, the impact of these infrequent transcripts on <i>BCR-ABL1</i>-dependent leukemogenesis and sensitivity to tyrosine kinase inhibitors (TKIs) remain unclear. We analyzed the transforming potential and TKIs responsiveness of three atypical <i>BCR-ABL1</i> fusions identified in CML patients, and of two additional <i>BCR-ABL1</i> constructs with lab-engineered breakpoints. We observed that modifications in the DC2 domain of BCR and SH3 region of ABL1 affect BCR-ABL1 catalytic efficiency and leukemogenic ability. Moreover, employing immortalized cell lines and primary CD34-positive progenitors, we demonstrate that these modifications lead to reduced BCR-ABL1 sensitivity to imatinib, dasatinib and ponatinib but not nilotinib. We conclude that BCR-ABL1 oncoproteins displaying uncommon breakpoints involving the DC2 and SH3 domains are successfully inhibited by nilotinib treatment.