Dysregulation of Frizzled 6 Is a Critical Component of B Cell Leukemogenesis in a Mouse Model of Chronic Lymphocytic Leukemia.

Wnt/Fzd signaling is known to play a key role in development, tissue specific stem cell maintenance, and tumorigenesis, particularly through the canonical pathway involving stabilization of β-catenin. We have previously shown that Fzd9−/− mice exhibit a decrease in pre-B cells at a stage when self-renewing division is occurring in preference to further differentiation, prior to light chain immunoglobulin recombination. To determine whether pathologic usurpation of this pathway plays a role in B cell leukemogenesis, we examined the expression of Wnt/Fzd pathway genes in the Eμ-TCL1 mouse model of chronic lymphocytic leukemia (CLL). We find that in the course of leukemogenesis, the expression of Wnt16, Wnt10b, Fzd1, and most dramatically, Fzd6, are progressively upregulated in the transformed CD5+ B cells of these mice, as are β-catenin protein levels. Fzd6 expression is increased an average of 35-fold in tumor B cells in comparison to CD5- normal B cells in the same mouse. In 3 animals we were able to compare oligoclonal, preleukemic CD5+ B cells with monoclonally transformed CD5+ B cells and noted between 4 and 340 fold incremental increases in Fzd6 expression. Elimination of Fzd6 gene expression by crossing Eμ-TCL1 into Fzd6−/− mice significantly delays or eliminates development of CLL in this model, while crossing into Fzd9−/− mice has no effect. Greater than 50% of Fzd6+/+ X Eμ-TCL1 mice have evidence of CLL in the peripheral blood at 5 months of age, while in Fzd6−/− X Eμ-TCL1 mice, this occurs at 8 months (p < 0.001). Even at 1 yr. of age, >30% of Fzd6−/− X Eμ-TCL1 mice remain disease free while >95% of Fzd6+/+ X Eμ-TCL1 mice have leukemia at 7 months of age. Tumors that do arise in Fzd6−/− mice are morphologically similar to the usual Eμ-TCL1 CLL cells, but do not show upregulation of β-catenin protein by flow cytometry, suggesting that Fzd6 is responsible for this aberrant expression. Further, rather than upregulating Wnt16, Wnt10b, and Fzd1, these genes are down-modulated in tumor cells lacking Fzd6, suggesting a balance between positive and negative signals in CLL B cells mediated by the Wnt/Fzd pathway with Fzd6 acting as an oncogene and other family members potentially as tumor suppressors. We and others have noted increases in LEF-1, Wnt10b, Wnt16, and Fzd3 in human CLL B cells as well, mirroring the findings in the mouse model. Our findings suggest that the self-renewal signals mediated by Wnt/Fzd that are enlisted during B cell development may be pathologically reactivated in the neoplastic transformation of mature B cells; and that agents targeting specific members of the beta-catenin signaling pathway may have therapeutic impact that could be tested in the Eμ-TCL1 model.