DNA Methylation and Polycomb Repression in Follicular Lymphoma
C O'Riain, M Calaminici, A Clear, RM Braziel, RD Gascoyne, A Rosenwald, E Campo, EB Smeland, LM Rimsza, WC Chan, L Staudt, TA Lister, J Fitzgibbon. Barts and the London School of Medicine, London, United Kingdom; Lymphoma/Leukemia Molecular Profiling Project, Bethesda
Background: DNA methylation and trimethylation of lysine 27 on Histone H3(H3K27Me3) are key mechanisms of control of gene expression. The Polycomb Repressor Complex 2(PRC2) protein EZH2 catalyses H3K27Me3 which recruits BMI1 protein leading to transcriptional repression of target genes. Mapping studies have identified genes targeted for transcriptional control by PRC2 in embryonic stem(ES) cells. We have previously demonstrated, in an analysis restricted to 800 genes, highly significant enrichment for these PRC2 target genes among hypermethylated genes in Follicular Lymphoma(FL). In this study, we have set out to examine DNA methylation in FL on an epigenome-wide scale, confirm the link with PRC2 target genes and investigate expression of Polycomb components in FL by immunohistochemistry.
Design: Quantitative methylation values were obtained for >27000 CpG loci (>14,000 genes) for 16 FL and 4 benign lymph node DNA samples using the Illumina Infinium array. Significant FL-specific hypermethylation was identified using Illumina BeadStudio software (two-sided t test; p<0.0001). Immunohistochemistry was performed for EZH2, H3K27Me3 and BMI1 in tissue microarrays of FL(n=75) and reactive tonsil controls.
Results: 1072 loci (796 genes) showed FL-specific hypermethylation. Genes marked by H3K27Me3 in ES cells were significantly overrepresented within this group (37%; p<0.0001) compared to the entire array (6%). EZH2 was more strongly expressed in centroblasts compared to centrocytes in tonsil. This pattern contrasted with H3K27me3 and BMI1 expression which was confined to mantle cells and light zone centrocytes. While EZH2 expression in centroblasts was ubiquitous in FL, there was marked variation in expression of its product H3K27Me3 within FL cases (9 of 75 cases strongly positive). Expression of BMI1, which is recruited by H3K27Me3, was weak or absent in the majority of cases (2 of 75 FL cases strongly positive).
Conclusions: Aberrant DNA hypermethylation affects hundreds of genes in FL of which a large proportion are targeted for transcriptional repression by H3K27Me3 in ES cells. There are notable differences in the expression of Polycomb components within FL cases. Validation of these differences in a larger sample set and correlation with clinical and pathological features is warranted to ascertain the biological significance of these changes.
Monday, March 22, 2010 9:00 AM
Platform Session: Section B, Monday Morning