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), underscoring the importance of terminal differentiation impairment in lymphoma pathogenesis. The role of PRDM1 inactivation in Burkitt lymphoma (BL) is currently not known. Two findings, however, may suggest possible PRDM1 involvement in BL pathogenesis. First, although PRDM1 protein is consistently absent in BL, about 40% of BL cases are positive for IRF4/MUM1, suggesting asynchronous expression of these two proteins. Secondly, it has been hypothesized that microRNA-127 overexpression in EBV+ BL may promote BL development through down-regulation of PRDM1. Sequence analysis of the coding region of PRDM1 in BL cell lines and primary tumors did not demonstrate any mutational changes. To investigate whether epigenetic inactivation of PRDM1 may play a pathogenetic role in BL, BL cell lines and primary cases 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. These include (i) 11 BL cell lines, 9 EBV+ (4 latency I, 2 Wp-restricted, and 3 with latency III drift) and 2 EBV-; (ii) 7 EBV+ lymphoblastoid cell line (LCL); (iii) 62 primary BL cases, formalin-fixed, paraffin-embedded tissues (FFPE); (iv) 4 B cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL (BCL-U), FFPE. Naïve, germinal center (GC) and memory B cells sorted by flow cytometry were also analyzed as controls. Hypermethylation in PRDM1 promoter and exon 1 was seen in all EBV+ BL cell lines except one with latency III, but not in EBV- BL lines. None of the LCL demonstrated PRDM1 hypermethylation, implying that EBV is unlikely to mediate PRDM1 hypermethylation directly. Methylation status of PRDM1 was successfully determined in 39 BLs (30 sporadic, 9 HIV-related) and 5 BCL-Us (4 sporadic, 1 HIV-related). Hypermethylation was seen in 11 BLs and 1 BCL-U (28.6% of total). All the methylated cases were EBV(+) (p=0.004). Overall, 12 of 28 (43%) EBV(+) BL and BCL-U cases exhibited hypermethylation. PRDM1 hypermethaylation was independent of BL subtype and MUM1/IRF4 expression status. To determine if PRDM1 hypermethylation can potentially repress PRDM1 transcription, BL cell lines Ramos (PRDM1 unmethylated) and Mutu1 (PRDM1 hypermethylated) are treated with IL-21 (50ng/mL), a cytokine that can mediate terminal differentiation by inducing PRDM1, for 2 to 5 days. While PRDM1 is induced by IL-21 in Ramos, levels of PRDM1 are not significantly increased in Mutu1, even though STAT3, a downstream mediator of IL-21, is activated in the latter. This finding suggests that PRDM1 hypermethylation has the potential to repress PRDM1 transcription in the presence of PRDM1-inducing signals. This function, however, may be pathogenetically more relevant in a BL precursor cell than in the final BL tumor cell. EBV+ BL is thought to derive from an EBV-infected late germinal center (GC) B cell, which harbors c-myc translocation and begins, though abortively, memory B cell differentiation and adopts an EBV latency I form. BL consistently express BCL6, a known PRDM1 transcription repressor, and harbor very low levels of PRDM1 mRNA. This suggests that high BCL6 expression, rather than PRDM1 hypermethylation, may be the primary determinant of its repressed transcription seen in BL. Indeed, PRDM1 mRNA expression is 25 to 1200 fold higher in LCLs which harbor very low levels of BCL. Furthermore, reducing expression of MTA3, which forms a complex with BCL6 and mediates PRDM1 transcription repression, in Daudi increases PRDM1 transcript levels. Therefore, we hypothesize that PRDM1 hypermethylation in BL more likely represents a memory of an epigenetic event that functions in the earlier stage of BL pathogenesis. PRDM1 hypermethylation may function to repress its transcription in a BL precursor cell as it is exposed to inducing signals, e.g. IL-21, during GC transit. Our study expands the spectrum of B cell lymphomas in which PRDM1 plays a tumor suppressor role, and supports the importance of impairment of terminal differentiation in the pathogenesis of a subset of aggressive B cell lymphomas.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.