Abstract
NFAT is a family of highly phosphorylated proteins residing in the cytoplasm of resting cells. Upon dephosphorylation by calcineurin, NFAT proteins translocate to the nucleus where they orchestrate developmental and activation programs in diverse cell types. CLL is a clonal disorder of mature B cells characterized by the expression of CD19, CD23 and CD5. With respect to prognosis, it constitutes a heterogeneous disease with some patients exhibiting an indolent course for many years and others progressing rapidly and requiring early treatment. Expression of CD38 and ZAP70 define a subgroup of patients with enhanced responsiveness to stimulation of the B cell receptor (BCR) complex and more aggessive disease. In contrast, another subset of CLL patients with more indolent course is characterized by an anergic B cell phenotype refering to B cell unresponsiveness to IgM ligation and essential lack of phosphotyrosine induction and calcium flux. Here, we analyzed the role of NFAT2 in the pathogenesis of B-CLL and in anergy induction in CLL cells.
For this purpose, we generated mice with a conditional NFAT2 knock out allele (NFAT2fl/fl). In order to achieve NFAT2 deletion limited to the B cell lineage, we bred NFAT2fl/fl mice to CD19-Cre mice. To investigate the role of NFAT2 in the pathogenesis of CLL we made use of the Eµ-TCL1 transgenic mouse model in which the TCL1 oncogene is expressed under the control of the Eµ enhancer. TCL1 transgenic mice develop a human-like CLL at the age of approximately 14 wks to which the animals eventually succumb at an average age of 10 months. To analyze the role of NFAT2 in CLL, we generated mice (n=10) whose B cells exhibited a specific deletion of this transcription factor in addition to their transgenic expression of the TCL1 oncogene (TCL1 CD19-Cre NFAT2fl/fl). TCL1 transgenic mice without an NFAT2 deletion served as controls (n=10). To identify novel NFAT2 target genes in CLL cells, we performed a comparative gene expression analysis on CLL cells with intact NFAT2 expression and on CLL cells with NFAT2 deletion using affymetrix microarrays.
Mice with NFAT2 knock out exhibited a significantly more aggressive disease course with accelerated accumulation of CD5+CD19+ CLL cells and a significantly reduced life expectancy (200 vs. 325 days) as compared to control animals. Flow cytometric analysis at distinct time points showed a pronounced infiltration by CD5+ B cells of the peritoneal cavity, spleen, lymph nodes, liver and bone marrow which was significantly stronger in the NFAT2 ko cohort. Most of the CD5+ B cells in TCL1+NFAT2 ko mice showed high expression of ZAP70 and CD38, whereas TCL1 transgenic mice only demonstrated very few CD5+ B cells with concomitant expression of ZAP70 and CD38. To investigate the effects of an NFAT2 ko on proliferation and apoptosis of CD5+CD19+ CLL cells, we performed in vivo BrdU incorporation assays with subsequent flow cytometric analysis. Interestingly, we could show that CLL cells isolated from spleens, bone marrow and peripheral blood from mice with an NFAT ko exhibited significantly higher rates of proliferation than control animals. To identify NFAT2 target genes resonsible for the observed alterations in the disease phenotype, we subsequently peformed a gene expression analysis with CD5+CD19+ CLL cells from TCL1+NFAT2 ko mice with CLL cells from TCL1+ mice serving as controls. Here, we detected a significantly altered expression of 22 genes associated with B cell anergy in the TCL1+NFAT2 ko cohort. The vast majority of these genes was expressed significantly less in the absence of NFAT2 with Lck, Pacsin1, Hspa14 and CD166 constituting the strongest hits with up to 10fold reduced gene expression. Downregulation of the identified target genes was subsequently confirmed using RT-PCR and Western Blotting.
In summary, our data provide strong evidence that NFAT2 is a critical regulator of CD38 and ZAP70 expression and substantially controls cell cycle progression in CLL cells. In addition, we could show that NFAT2 controls the expression of several anergy-associated genes and that its absence prevents the acquisition of an anergic phenotype by the CLL cells correlating with a significantly more aggressive course of the disease. Taken together, our data demonstrate that NFAT2 plays an essential role in the pathogenesis of CLL and implicate this transcription factor as a potential target in its treatment.
Disclosures:
No relevant conflicts of interest to declare.
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