Dominant-Negative Impact of PAX5/TEL on Downstream Targets of PAX5 and Essential Pre-B Cell Receptor Genes

Abstract 3231

B cell lineage acute lymphoblastic leukemia (ALL) is a common malignancy in childhood, and the pre-B cell receptor (pre-BCR) signalling pathway was previously demonstrated to function as a tumor suppressor. The transcription factor PAX5, a key regulator of B cell development, is frequently involved in chromosomal rearrangements of leukemic blasts. Using high resolution single nucleotide polymorphism (SNP) genomic microarray by us and other groups, several candidate partner genes fused to PAX5 have been detected in samples of pediatric ALL, such as TEL, FOXP1, AUTS2, and C20orf112. Recently, we studied the fusion gene PAX5/TEL and its role in leukemic evolution by Affymetrix HG-U133 plus 2.0 Array of the ALL cell line Nalm6 transfected with a PAX5/TEL construct. PAX5/TEL reduced the expression of PAX5 and its downstream target genes (e.g. CD79A, BACH2, CD19). Moreover, we demonstrated a dominant negative impact of the PAX5/TEL-fusion protein on the binding affinity of wild-type PAX5 to the promoter of CD79A (Iwanski et al., 2009 ASH Abstract No. 3455).

To expand our findings, we analyzed the gene expression profile of pediatric ALL samples carrying PAX5/TEL (PAX5/TEL+, n=2) compared to samples with normal PAX5 (n=7) from a genomic ALL study. Samples with normal PAX5 were selected from among 95 B-ALL patients with normal PAX5 status, based on characteristics that most closely matched the two PAX5-TEL+ patients including cytogenetics and current risk stratification. Gene expression data were compiled using the Affymetrix HG-U133A Array, and a heatmap based on the Top 200 probes with the highest expression levels from both sample sets was generated (TIBCO Software Inc.). Notably, the downregulated genes included Bruton agammaglobulinemia tyrosine kinase (BTK; -2.8 fold, FDR < 0.2), an important regulator of pre-BCR signaling, Spleen tyrosine kinase (SYK, -2.3 fold, FDR < 0.2), and IGHM (-5,9 fold; FDR < 0.1), but also significantly up-regulated expression of genes involved in myeloid differentiation, namely Myeloperoxidase (MPO, +24.2 fold, FDR < 0.2), and CCAAT/enhancer binding protein alpha (CEBPA, +3.2 fold, FDR < 0.3), as well as the erythroid genes Aminolevulinate delta-dehydratase (ALAD, +12.2 fold, FDR < 0.2) and the Erythropoietin receptor (EPOR, +7.0, FDR < 0.3).

Additionally, we performed a meta-analysis comparing deregulated genes detected in the Nalm6-microarray (PAX5/TEL-MIGR vs. empty vector) to the data from the human B-ALL samples (PAX5/TEL+ vs. normal PAX5). Overall, we identified a set of 35 overlapping genes (FDR < 0.3) that were deregulated in both data sets (21 downregulated, 14 up-regulated). Notably, 10 out of the 21 (47%) downregulated genes are known to be involved in B cell development and BCR signaling, some of them well-recognized as direct PAX5 target genes (e.g. CD79A, CD19, BACH2).

Moreover, reporter gene assay with a luciferase reporter construct containing cDNA of the CD19 promoter with PAX5 binding sites (luc-CD19) was performed in Nalm6 cells. Since these cells already express a high level of endogenous PAX5, transcriptional activity of the luc-CD19 reporter plasmid was relatively high in the Nalm6 cells transfected with empty vector, as compared to 293T cells. In contrast, PAX5/TEL-transduced Nalm6 cells displayed a significantly reduced transcriptional activation of the reporter construct (P < 0.01).

We also explored if mutation and/or deletion of PAX5 (mut/del PAX5) may have an impact on genes involved in B cell development and the pre-BCR/BCR pathway. Hence, expression files from human ALL samples with mut/del PAX5 (n=50) and B-ALL samples with normal PAX5 (n=95) were analyzed. Notably, only two genes that are known to be involved in B cell development and the pre-BCR pathway were significantly downregulated in samples with mut/del PAX5 compared to normal PAX5, namely CD72, a B cell specific repressor gene activated by PAX5 (-1.51 mean fold, FDR = 0.05), and immunoglobulin heavy constant delta (IGHD), a gene involved in pre-BCR signalling (-1.6 mean fold, FDR = 0.18). These findings suggest no strong influence of mut/del PAX5 on the expression of downstream genes involved in pre-BCR signaling.

In conclusion, our results provide further insights into the dominant-negative role of PAX5/TEL and link this fusion gene with the pre-BCR pathway.