The BCL9 Oncogene Promotes Tumor Progression by Conferring Enhanced Proliferative, Metastatic and Angiogenic Properties to Myeloma Cells

Wnt signaling plays an important role in tissue development and maintenance during embryogenesis, cell differentiation, and stem cell growth. Several components of the Wnt signaling cascade have been shown to function as either tumor suppressor proteins or as oncogenes in multiple human cancers, underscoring the relevance of this pathway in oncogenesis. Deregulation of the canonical Wnt/b-catenin pathway has been implicated in numerous human epithelial malignancies as well as hematologic malignancies including multiple myeloma (MM), generating immense interest in these molecules as targets for cancer therapy. Activation of Wnt/b-catenin in cancer has been associated with mutations that enable b-catenin to escape degradation by the proteasome, thereby allowing its accumulation in the nucleus where it functions as a transcriptional regulator in conjunction with coactivators by constitutively activating target genes such as c-Myc and Cyclin D1. To date, however, no mutations in Wnt pathway have been documented in MM, suggesting that mechanisms other than gene mutation may contribute to Wnt pathway deregulation. BCL9, a key component of the Wnt pathway, is required for b-catenin transcriptional activity and resides on chromosome 1q21, a region frequently involved in secondary chromosomal aberrations associated with MM tumor progression. Here we provide evidence that dysregulation of BCL9 expression is a novel oncogenic mechanism of Wnt pathway activation in MM. Using in vitro and in vivo functional analyses, we demonstrate that BCL9 is a bonafide oncogene that is aberrantly expressed in MM and associated with survival. Using the TCF- specific luciferase reporter, we show that enforced expression of BCL9 in MM cells enhanced b-catenin mediated transcription by >12 fold, suggesting a possible role of BCL9 overexpression in the pathogenesis of MM. BCL9 enhanced proliferation (1.5 fold, P<0.02), migration (3.5 fold, P<0.0001) and the metastatic potential of MM cells. We also showed that BCL9 plays an important role in tumor progression by regulating Cyclin D1 and c-Myc mediated cell proliferation, CD44 mediated tumor metastasis, as well as VEGF mediated host angiogenesis. Importantly, BCL9 knockdown significantly increased the survival in a xenograft mouse model of human MM (P=0.001), associated with decreased tumor burden and host angiogenesis. In summary, we have demonstrated that BCL9 is a novel and potent oncogene of the Wnt pathway in MM, playing fundamental roles in tumor progression by regulating proliferation, migration, invasion, angiogenesis and the metastatic potential of tumor cells. The pleiotropic roles of BCL9 and its aberrant expression highlight its importance as an attractive and novel therapeutic target in the treatment of MM.