Secondary mutation maintains the transformed state in BaF3 cells with inducible BCR/ABL expression

The BCR/ABL gene product of the Philadelphia (Ph) chromosome induces chronic myelogenous leukemia (CML). We generated a hematopoietic cell line, TonB210.1, with tetracycline-dependent BCR/ABL expression to investigate the pathways by which BCR/ABL transforms cells. TonB210.1 demonstrates conditional growth factor independence in tissue culture and rapidly forms tumors in mice fed the tetracycline analog doxycycline. The tumors regress completely upon doxycycline withdrawal, but ultimately reform in all animals. After a long latency, tumors also develop in animals never exposed to doxycycline. Subclones of TonB210.1 established from doxycycline-independent tumors demonstrate distinct mechanisms of transformation. Most subclones manifest increased basal levels of BCR/ABL expression; some have lost the capacity to augment expression upon induction, whereas others remain inducible. More interestingly, some subclones maintain tight conditional expression of BCR/ABL and are therefore transformed by secondary mechanisms that no longer require BCR/ABL expression. These subclones show constitutive phosphorylation of the STAT5 protein, suggesting that activating mutations have occurred upstream in the signaling pathway to STAT5. The tight conditional expression of BCR/ABL in the TonB210.1 cell line affords the opportunity to study several interesting aspects of the biology of BCR/ABL, including activation of critical signaling pathways and transcriptional programs, and its potential role in genomic instability.