Epigenetic regulation of B-cell transcription factors during germinal center B-cell maturation and their silencing in Hodgkin lymphoma

Germinal center (GC) reaction of activated B-lymphocytes is the hallmark of T-cell dependent humoral immune response and is important for generation of high-affinity antibodies. Maturation of naïve B cells into GC B-cells and their terminal differentiation into plasma cells is very tightly regulated by transcription factors (TF) and expression these TFs is regulated meticulously during differentiation. Deregulated TFs expression results in impaired differentiation and is also implicated in lymphomagenesis. In this study we investigated the regulation of TFs expression by epigenetic modifications during normal GC-B cell maturation and plasma cell differentiation and their deregulation in Hodgkin lymphoma (HL). We used ChIP-chip technique to map the histone posttranslational modifications (PTM) at the loci of critical B cell TFs in normal primary naïve, and GC B cells and also in several Hodgkin, non-Hodgkin lymphoma and myeloma cell lines and correlated with their mRNA expression. Histone PTMs that we analyzed include Histone (H3)-acetylation (H3-Ac), H3-lysine 4 (K4)-trimethylation (me3), and H3K4me1 all of which are associated with transcription activation and H3K27me3 PTM associated with repression of transcription. H3-Ac and H3K4me3 PTMs at the promoters of TFs correlated positively while H3K27me3 at the promoters correlated negatively with gene expression. Histone PTMs profile at these critical B cell TFs loci changed significantly during cell transitions from the naïve B to GC B and then to plasma cells and correlated with change in gene expression. Detailed analysis of histone PTMs at the locus of a GC master regulator TF, BCL6, interestingly revealed reciprocal patterns of activating histone PTMs at the promoter and intron associated CpG islands that correlated inversely with BCL6 expression. We identified a cis regulatory element located about 150 Kb upstream of BCL6, which is highly active in GC B cells and in fact also interacts with the BCL6 promoter. Analysis of histone PTMs in HL cell lines identified a complete loss of activating and gain of repressive histone PTMs at several B cell TFs loci, which correlated well with the loss of B cell gene expression program. Additional DNA methylation analysis in HL cell lines also identified methylation at the promoter CpGs of some of the B-cell TFs, which in combination with repressive chromatin state may potentially result in irreversible gene silencing. Collectively our studies suggest the possible role of epigenetic modifications in regulating TFs expression during normal B cell development and also in HL.