ABL1 kinase is overexpressed and contributes to dysregulation of HR activity and genomic integrity in MM. (A) Relative expression (Log2) of ABL1 in normal and MM samples in GSE5900-GSE2658 (NPC = 22; MGUS = 44; SMM = 12; MM = 559) data set. (B) Western blot showing ABL1 expression in normal PBMC samples and MM cell lines. (C) HR activity, assessed by functional assay described in “Methods,” in MM cell lines treated with DMSO (D; control), the ABL1 inhibitor nilotinib for 48 hours (Ci) or shRNAs (C, control; sh-ABL1, ABL1-shRNA) (Cii). Error bars indicate SDs of triplicate experiment and 2-tailed P values derived by t test (∗P < .05-.001; ∗∗P < .001). (D) MM cell lines treated with DMSO or nilotinib (Nil; 5 μM) for 48 hours were evaluated for phosphorylation of RAD51 (Y315) and DNA breaks (by investigating levels of γ-H2AX) using western blotting (Di) and for RAD51 and γ-H2AX foci using fluorescence microscopy (Dii-Diii). Approximately 100 cells representing 3 different microscopic fields were investigated and cells with foci counted. Error bars indicate SDs; 2-tailed P values derived by t test (∗∗P < .001). (E) ABL1 was suppressed using shRNAs (C, control; sh-ABL1, ABL1-shRNA), and the cells were evaluated for DNA breaks (by investigating levels of γ-H2AX) using western blotting. DMSO, dimethyl sulfoxide; NPC, normal plasma cells; MGUS, monoclonal gammopathy of unknown significance; SMM, smoldering multiple myeloma; MM, multiple myeloma; NPC, normal plasma cells; SD, standard deviation; SMM, smoldering multiple myeloma.