Functional Somatic and Germline Variants in the NF-κB Pathway in Chronic Lymphocytic Leukemia

Abstract 560

Nuclear factor kappa B (NF-κB) encompasses a family of transcription factors involved in oncogenic processes including cellular proliferation and apoptotic inhibition. Constitutive activation of NF-κB has been observed in hematologic malignancies and is thought to confer resistance to chemotherapeutic agents. Here, we examine the role of the NF-κB pathway in chronic lymphocytic leukemia (CLL).

Whole-exome sequencing was performed using tumor and matched germline DNA from 167 CLL patients. We identified 51 patients (30%) carrying 53 non-silent somatic variants in genes of the canonical NF-κB pathway, which consists of 272 genes as defined by the Ingenuity Pathway Analysis tool. Of the 99 patients whose germline sequences have been analyzed to date, 27 patients (27%) carry 34 non-silent germline variants in NF-κB pathway genes. A total of 67 patients (40%) have at least one non-silent somatic or germline variant. Variants in the NFKB1 gene, itself, were also observed: a somatic variant, H66R, found in two patients, and two germline variants, Y89F and R849W, each found in one patient.

To evaluate the functional consequences of the NFKB1 variants, we performed site-directed mutagenesis to generate full-length NFKB1 cDNAs encoding these variants. We subsequently measured transcriptional activity of wild-type and mutant NFKB1 via luciferase assays in HEK293T cells using reporter cassettes containing the NFKB1 response element. Transcriptional activity of the three NFKB1 variants was found to be at least 2-fold higher than that of wild-type NFKB1 (p<0.0001). We further hypothesized that this increased transcriptional activity would lead to increased expression of NFKB1 downstream target genes. Analysis of gene expression profiles from Affymetrix HG-U133 Plus 2.0 Arrays of 65 CLL patient samples showed that the NFKB1 downstream targets CCL3, CCL4, and CD69 are upregulated in NFKB1 variants. To validate these results, we performed quantitative RT-PCR in patients with (n=3) or without (n=9) NFKB1 variants and confirmed upregulation of CCL3 (p=0.0286), CCL4 (p=0.0384), and CD69 (p=0.0263). Direct transfection of HEK293T cells with NFKB1 variants also resulted in a 3.3-fold upregulation of CCL3 (p=0.05).

To test the hypothesis that deregulation of the NF-κB pathway is a key mechanism in CLL, we compared gene expression profiles of NF-κB pathway genes between CLL patient samples (n=146) and normal B cells (n=16) and found overall upregulation of the NF-κB pathway in CLL (Kolmogorov-Smirnov test, p=2.2e-16). K-means clustering and principal component analysis (PCA) further revealed that CLL patients can be divided into two subgroups exhibiting differential magnitude of NF-κB pathway upregulation. Studies in progress aim to identify the clinical significance of these subgroups.

Finally, we assessed the effect of inhibiting the NF-κB pathway using the cell permeant NF-κB inhibitor, SN50. We performed Annexin V/PI staining 24 hours post-treatment in CLL cells with (n=9) or without (n=3) NF-κB pathway variants. SN50 increased cell death 1.8-fold in all cells tested (p<0.0001). Quantitative RT-PCR also showed a 59% decrease in expression of CCL3 one hour post-treatment, confirming inhibition of the NF-κB pathway.

In conclusion, our findings demonstrate that a high proportion of CLL patients harbor somatic and germline variants in NF-κB pathway genes, some of which appear to be functional. Furthermore, the NF-κB pathway is upregulated in CLL and pharmacological inhibition of the pathway leads to increased cancer cell death. Functional characterization of NF-κB pathway variants offers mechanistic insight into the disease, providing novel targets for therapy.