Autophosphorylation of oncoprotein TEL-ABL in myeloid and lymphoid cells confers resistance to the allosteric ABL inhibitor asciminib.

Abstract

Chromosomal translocations that fuse ABL1 to BCR or TEL cause human leukemias. In BCR-ABL and TEL-ABL fusion proteins, oligomerization and loss of an autoinhibitory myristoylation site in the SH3 domain of ABL lead to increased ABL tyrosine kinase activity. We assessed the ability of asciminib, an allosteric inhibitor of BCR-ABL that binds to the myristoyl-binding site in the ABL kinase domain, to inhibit these fusion proteins. Although the ABL components of the two fusion proteins have identical sequences, asciminib was much less effective against TEL-ABL than it was against BCR-ABL in cell-growth assays. In contrast, ATP-competitive tyrosine kinase inhibitors, such as imatinib and ponatinib, were equally effective against both fusion proteins. A helix in the ABL kinase domain that closes over bound asciminib was required for the sensitivity of BCR-ABL to the drug but had no effect on that of TEL-ABL, suggesting that the native autoinhibitory mechanism that asciminib engages in BCR-ABL is disrupted in TEL-ABL. Single-molecule microscopy demonstrated that BCR-ABL was mainly dimeric in cells, whereas TEL-ABL formed higher-order oligomers, which promoted trans-autophosphorylation, including of a regulatory phosphorylation site (Tyr⁸⁹) in the SH3 domain of ABL. Nonphosphorylated TEL-ABL was intrinsically susceptible to inhibition by asciminib, but phosphorylation at Tyr⁸⁹ disassembled the autoinhibited conformation of ABL, thereby preventing asciminib from binding. Our results demonstrate that phosphorylation determines whether an ABL fusion protein is sensitive to allosteric inhibition.