Gene Expression Profiling of Persistent Polyclonal B-Cell Lymphocytosis Using Whole Genome Microarrays.

Persistent polyclonal B-cell lymphocytosis (PPBL) is a rare B-cell disorder characterised by elevated serum levels of polyclonal IgM and persistent lymphocytosis of often bi-nucleated B-cells. The molecular basis of this disorder however remains poorly understood. We therefore used whole genome expression microarrays (Affymetrix U133 plus2.0) to compare the gene expression profile of 14 cases of PPBL with normal lymphocyte populations (6 B cell (CD19⁺) and 3 T cell (CD3⁺) samples). PPBL patients (10 female, 4 male) had a median age of 43 (range 28–63) and all were smokers with an average lymphocyte count of 5.34 x 10⁷/ml (range 3.7–9) at time of presentation. Eight of these cases had +i(3q) and eight cases exhibited PCC, four cases displayed both chromosomal abnormalities. Normal lymphocyte populations were obtained from two pools of 12 individuals and individual donors.

Microarray data were first RMA pre-processed and then expression levels of patient samples’ genes compared with levels of both normal B cell and T cell samples. ANOVA (P<0.01) was used to identify differentially expressed genes. Thirty-six genes were found to be up-regulated and 157 genes were down-regulated in all patient samples by at least 2-fold.

Components of the AP-1 transcription complex (FOS, FOSB and JUN) were found to be highly up-regulated in all PPBL samples with an average of 7.2, 7.9 and 31.6 fold expression respectively. The cell cycle regulatory molecule CDC27 was also found to be up-regulated in all patient samples (8.8-fold). Down-regulated genes of interest included the B-cell specific transcription factor BACH2 (3-fold), tumor-suppressor SMAD4 (3.6-fold) and apoptotic genes DRAK1 (3-fold) and CAS (5-fold). These data are currently being validated by quantitative RT-PCR.

We propose that over-expression of AP-1 may have significance in PPBL as increased expression of components of AP-1 have previously been linked with lymphoproliferative disorders including Hodgkin’s lymphoma and splenic marginal zone lymphoma. The underlying reason for over-expression of AP-1 in PPBL remains unknown but two characteristic features of this disease could provide clues; firstly AP-1 up-regulation has been demonstrated to be tobacco-induced in animal models and secondly the LMP1 protein of EBV can result in constitutive expression of the AP-1 complex.