DNA Methyltransferase 1 Contributes to Epigenetic Signatures and Biological Phenotype during Normal B-Cell Differentiation and Lymphomagenesis.

Normal hematopoiesis requires incremental changes in gene expression in order to establish cellular phenotypes with specialized functions. We are particularly interested in the transcriptional and epigenetic programming of germinal center (GC) B-cells, which acquire unusual biological features normally associated with cancer. Specifically, GC B-cells (i.e. centroblasts - CB) undergo rapid DNA replication while at the same time undergoing genetic recombination, and give rise to a majority of B-cell lymphomas. We hypothesized that epigenetic programming would play a critical role in the CB stage of development, and that gene-specific and genome-wide DNA methyltransferase activity is critical for these cells. We first examined the CpG methylation levels of 24,000 gene promoters in five sets of primary human B-cells just prior to (i.e. naive B-cells - NBC) and upon entering the GC reaction (i.e. CBs). This was achieved using the HELP (HpaII tiny fragment Enrichment by Ligation-mediated PCR) assay, which relies on differential digestion of genomic DNA by the isoschizomer enzymes HpaII and Msp. HELP is a robust and reproducible method that provides accurate and quantitative measurement of DNA methylation levels throughout the genome. Remarkably, we found that the DNA methylation profile of B-cells undergoes a significant shift as readily appreciated by hierarchical clustering. The epigenetic signatures of NBC and CB are differentiation-stage dependent and do not vary significantly between individuals. The coefficient of correlation between individuals was 0.98, as compared to the NBC vs. CB fractions 0.92–0.95. Supervised analysis demonstrated that 266 genes (P<0.001) were differentially methylated upon entry of NB-cells into the GC reaction. We further correlated the methylation status of these genes with their gene expression level. The most heavily affected pathways by differential methylation and concordant expression in naive B-cells were the Jak/STAT and MAP3K signaling pathways, while in CBs the p38 MAPK pathway and Ikaros family of genes were most affected. Given the epigenetic reprogramming observed in CBs vs. NBCs, along with the need for maintenance of methylation during rapid replication, we predicted that DNA methyltransferase (DNMT) enzymes play a critical role in centroblasts. By performing QPCR and Western blots on isolated fractions of human tonsilar lymphocytes and anatomical localization by immunohistochemistry, we found that DNMTs have a complex temporal and combinatorial expression pattern whereby DNMT1 was the main methyltransferase detectable in centroblasts. Additionally we studied 10 DLBCL cell lines and a panel of primary DLBCL (n=176 for mRNA and 70 for protein) for DNMTs expression. Spearman Rank correlation analysis revealed that DNMT1 was preferentially highly expressed in GCB vs. ABC primary DLBCLs, as well as in BCR vs. OxPhos DLBCLs. Taken together, our data suggest that i) dynamic changes in epigenetic programming contribute to formation of GCs, ii) that DNMT1 may play both a de novo and maintenance methylation role in GC cells, iii) that DNMT1 is markedly upregulated in normal centroblasts and in DLBCLs with the BCR or GCB gene expression profiles and iv) specific therapeutic targeting of DNMT1 rather than non-specific global inhibition of DNA methylation could be a useful anti-lymphoma strategy for germinal center-derived DLBCLs.