Promoter and Exon 1 Hypermethylation of the Tumor Suppressor Gene PRDM 1/Blimp-1 Indicates Its Pathogenetic Role in EBV(+) Burkitt Lymphoma.
Jiong Yan, Taotao Zhang, Carlos E Bacchi, Eduardo M Queiroga, Gabriela Gualco, Kui Nie, Yifang Liu, Sharon Barouk, Attilio Orazi, Daniel M Knowles, Wayne Tam. Weill Cornell Medical College, New York, NY; Consultoria em Patologia, Botucatu, SP, Brazil
Background: PRDM1/Blimp1, a lymphoma tumor suppressor and master regulator of plasma cell differentiation, is inactivated by genomic mutation and deletion in a subset of activated B-cell-type diffuse large B-cell lymphomas (DLBCL) and down-regulated by microRNAs in Reed-Sternberg cells in Hodgkin lymphomas. PRDM1 is consistently not expressed in Burkitt lymphoma (BL). However, it is not known whether epigenetic inactivation of PRDM1 may play a pathogenetic role in BL.
Design: Formalin-fixed, paraffin-embedded tissues of 62 BL, 4 B cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL (BCL-U) and 13 EBV+ DLBCL, as well as 8 BL cell lines (6 EBV+, 2 EBV-), were bisulfite sequenced to assess the methylation status of 41 CG dinucleotides in a 601 base-pair region spanning the distal promoter (DP) and a CG island that extends from the proximal promoter to the first exon of PRDM1. Naïve B cells and germinal center B cells were analyzed as controls. The expression of PRDM1α was measured by qRT-PCR. BL cell lines were treated with 5-aza-2-deoxycytidine (5-aza) to evaluate the effect of DNA demethylation on PRDM1a transcription.
Results: Methylation status of PRDM1 was successfully determined in 38 BLs and 4 BCL-Us. Hypermethylation was seen in 11 BLs and 1 BCL-U (28.6% of total), with involvement of CG island in all 12 cases. All the methylated cases were EBV(+) (p=0.004). Overall, 12 of 28 (43%) EBV(+) BL and BCL-U cases exhibited hypermethylation. Extensive methylation was seen in 2 of 6 EBV(+) BL cell lines (Daudi and P3HR-1), which are associated with Wp-restricted latency (EBNA2-deleted, LMPs-, EBNA3s+) known to be present in a subset of EBV(+) BL. These 2 cell lines have extremely low PRDM1α levels, but 5-aza treatment resulted in >100 fold increase in its expression. In comparison, 2 of 13 EBV(+) DLBCLs demonstrated DP-only methylaton which has only minimal repressive effect on PRDM1 transcription. Normal B cells did not exhibit PRDM1 methylation.
Conclusions: PRDM1 is inactivated by methylation in a subset of EBV(+) BL and BCL-U, possibly with Wp-restricted latency. Antagonization of EBV-driven PRDM1 induction likely contributes to growth advantage of these lymphoma cells and is a potentially important pathogenetic event in this BL subset. Our study expands the spectrum of B cell lymphomas in which PRDM1 plays a tumor suppressor role, and provides a mechanistic rationale in adding demethylating agents in BL treatment regimens.
Monday, February 28, 2011 11:30 AM
Platform Session: Section B, Monday Morning