Abstract
Background: B-chronic lymphocytic leukemia (B-CLL) is the most common leukemia in the Western world but its pathogenesis remains largely unknown. Our strategy was to employ gene expression profiling (GEP) to discover genetic differences between CLL B cells and blood B cells from healthy adults. Previously, we and others identified the transcription factor (TF) lymphocyte enhancer factor-1 (LEF-1) as one of several genes significantly over expressed in CLL B cells suggesting a role for the oncogenic Wnt signaling pathway. LEF-1 acts as a central mediator of Wnt signaling and is crucial for the proliferation and survival of pro-B cells during development. Recently, deregulated LEF-1 activation has been directly linked with leukemogenesis.
Methodology: The goals of our study were to:
validate LEF-1 over expression in CLL B cells in a new cohort of B-CLL patients and normal controls; study CLL B cell LEF-1 protein expression; and investigate CLL B cell expression levels of other genes linked to Wnt signaling.
To accomplish these goals, Affymetrix U133 GEP was performed on B cells from 41 B-CLL patients who were enrolled on a trial of combined pentostatin, cyclophosphamide, and rituximab and 11 healthy adults over age 60. Only the perfect match data were utilized for all analyses; the non-background corrected perfect match data were normalized using an intensity-dependent procedure and analyses were done using the base-2 logarithm transformed normalized values.
Results: GEP analysis confirmed CLL B cells expressed the LEF-1 gene at ~28-fold higher levels than normal B cells (p<.0001). These results were validated by quantitative PCR and all CLL B cells studied expressed LEF-1 mRNA. By contrast, LEF-1 mRNA expression was undetectable in blood B cells from healthy adults both before and after in vitro mitogenic stimulation suggesting aberrant LEF-1 expression in B-CLL. Moreover, LEF-1 protein was readily detected by flow cytometry in CLL B cell samples and all clonal B cells expressed a uniform level of LEF-1. Full length LEF-1 is more oncogenic and predominates in certain cancers, while a dominant negative short isoform is found at greater levels in normal lymphocytes. Of interest, western blot analysis revealed that CLL B cells predominantly expressed full length oncogenic LEF-1. Ongoing studies are focused on silencing LEF-1 expression to determine LEF-1 target genes. In this regard, IGFBP4 (IGF binding protein 4) mRNA expression levels are 19-fold higher in CLL vs control B cells (p<0.0001) and the IGFBP4 protein was recently shown to antagonize Wnt signaling. A scan of the IGFBP4 promoter element identified 3 possible LEF/TCF consensus sites. Thus, there is a possible negative feedback loop in CLL, with activation of the Wnt pathway leading to expression of a negative Wnt regulator. Finally, we queried the GEP data for evidence of differential expression of 63 other Wnt-related genes. Surprisingly, in addition to LEF1, only five genes met our selection threshold of ≥1.5 fold change in expression between CLL and control B cells and p value <.001. These genes were CSNK1D (casein kinase, delta 1; 1.9-fold higher in control vs CLL; p<0.0001); CCND2 (cyclin D2; 2.4-fold higher in CLL vs control; p<0.0001); JUN (3.2-fold higher in control vs CLL); WNT3 (6.4-fold higher in CLL vs control; p<0.0001); and TCF4 (transcription factor 4, alias ITF-2; 4-fold higher in CLL vs control; p<0.0001). To our knowledge, we are the first to report that the TCF4 (ITF-2) gene is over expressed in B-CLL. This TF is of great interest because it is a known downstream target of the Wnt/TCF pathway, is activated in human cancers with b-catenin defects, and it promotes neoplastic transformation. Using Spearman correlation analysis to explore the relationship between LEF1, TCF4, and WNT3 expression levels, we observed a marginal correlation between WNT3 expression and LEF1 (ρ=0.31; p value=0.05) and TCF4 (ρ=0.29; p value=0.07). Summary: These studies add new support for a critical role of the Wnt pathway in CLL. The WNT3 results are consistent with the literature; however, it is notable that we failed to corroborate other reports demonstrating that other Wnts and Fzd genes are differentially expressed in CLL B cells vs normal B cells. It is striking that CLL B cells over express two TFs in this pathway that have been linked with neoplastic transformation. Ongoing studies are aimed at further elucidating the roles of these genes in B-CLL.
Targeting Wnt signaling in colorectal cancer. A Review in the Theme: Cell Signaling: Proteins, Pathways and Mechanisms
