HS1 loss drives cytoskeletal dysregulation and reduces sensitivity to BTK inhibition in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is a B-cell malignancy driven by aberrant signaling and microenvironmental support. Cytoskeletal remodeling contributes to these mechanisms, where the hematopoietic protein HS1 appears to play a key role. Thus, we generated C57BL/6 HS1 knockout (KO) mice to assess its impact on lymphocytes development. Young HS1KO mice showed some alterations in B-cell maturation and T-cell differentiation, which appeared to normalize with age. In double-transgenic Eμ-TCL1-HS1KO mice, combining HS1 loss with the standard Eμ-TCL1 CLL mouse model, we observed a trend of reduction in the CD4/CD8 ratio, and disrupted splenic architecture with loss of follicular organization. Transcriptomic and kinase profiling of the human CLL cell line MEC1, where HS1 was knocked-out (MEC1-HS1KO), revealed a coherent impairment in terms of cell mobility as highlighted by severely impaired migration in 2D and 3D environments, reduced chemotactic responses to CXCL12 and CCL19, deregulation of cytoskeletal, adhesion, and apoptotic pathways, and increased resistance to apoptosis. We posited that these defects could reduce sensitivity to BTK inhibition, a standard therapeutic approach in CLL. Primary CLL cells from the spleen of Eμ-TCL1-HS1KO mice appeared to show reduced sensitivity to ibrutinib, a finding that was also observed in adoptive transfer experiments designed to equalize disease burden. Wild-type mice receiving Eμ-TCL1-HS1KO leukemic cells tend to be more resistant to ibrutinib, particularly in the peripheral blood and spleen. All together, these findings establish HS1 as a potential regulator linking B-cell receptor signaling, cytoskeletal remodeling, leukemic progression, and response to therapy, supporting its potential as a biomarker for BTK inhibitor sensitivity.