Sensitivity to Wnt Pathway Inhibition in CLL Is Associated with Specific Gene Expression Signatures

Abstract 801

We have recently found that the Wnt/b-catenin signaling pathway plays a key role in chronic lymphocytic leukemia (CLL). We were, however, intrigued by the question of whether this aberrant pathway may function differently in independent leukemias, and contribute to disease heterogeneity. To assess differential activity of the Wnt pathway across patients, we tested the effects of blocking Wnt activation on CLL cell survival. We knocked down a key downstream gene, LEF1, which is the most differentially expressed gene in CLL compared to normal B cells (based on gene expression microarrays).

Addressing this question requires genetic manipulation of primary normal and malignant human B cells, and yet these cells are notoriously difficult to transfect. We therefore focused on developing a method for introducing siRNAs into normal and malignant B cells. We adapted a novel delivery system consisting of vertical silicon nanowires (SiNWs, Shalek et al PNAS 2010) that penetrate the plasma membrane in a minimally invasive fashion and deliver biomolecular cargo directly into the cytoplasm. We achieved consistent and reliable delivery of fluorescently labeled siRNAs (at 50–200 pmol) into normal and CLL B cells. siRNA was delivered to >90% of cells with >85% cell viability remaining after 48 hours.

We used this platform to knockdown LEF1 in 20 CLL-B and 5 normal CD19+ B cell samples, and examined cell survival 48 hours after siRNA delivery using an ATP-based CellTiter-Glo assay. Indeed, our studies revealed a heterogeneous response among CLL-B cells to LEF1 inhibition. As a group, CLL-B cells were significantly more sensitive to LEF1 knockdown with a survival rate of 77% (12% s.e.m) compared to 97% (13% s.e.m) in normal B cells. CLL B cells from different patients showed differential sensitivity to LEF1 knockdown, with 8 non-responders, 8 intermediate responders and 4 strong responders (i.e. significant death). Sensitivity to LEF1 inhibition did not correlate with known CLL cytogenetic prognostic factors.

To determine if the differential response to LEF1 knockdown was associated with specific gene signatures, we examined gene expression data generated from CLL-B cells from 12 (4 strong, 3 intermediate, and 5 non-responders) of the 20 CLLs tested (using the Affymetrix U133 Plus 2 Array). To increase statistical power, we used each CLL's expression profile (using only genes that showed variability across samples) to create clusters of ∼19 CLLs that showed similar expression profiles (using microarray data from our compendium of 177 additional CLLs). We further reduced the number of genes to ∼4000 genes by retaining only those whose expression levels were significantly different in at least one associated cluster relative to normal CD19+ B cell controls (T-test, FDR<10⁻⁴; p-values converted using the Benjamini-Hochberg method). These analyses led to the identification of several hundred genes whose expression correlated significantly with LEF1 knockdown's effect on cell viability. Analysis of these differentially expressed genes identified several potentially important pathways. Ongoing analyses include the identification and validation of a molecular signature for this effect. This signature could enable rapid identification of patients who would be most responsive to therapy with LEF1 inhibitors, which are under development along with other Wnt pathway inhibitors.