Gene Expression Analysis Of Plasmablastic Lymphoma Identifies Down Regulation Of B Cell Receptor Signaling and Additional Unique Transcriptional Programs

Plasmablastic lymphoma (PBL) is currently recognized as a distinct sub-type of diffuse large B-cell lymphoma (DLBCL), but remains a poorly characterized B-cell malignancy. We conducted gene expression profiling of 15 PBL, 10 DLBCL, and 5 EOP (extraosseous isolated plasmacytoma). 864 genes were significantly over- or under-expressed (at<1% false discovery rate) uniquely in one of these diseases relative to the other two. Of these, 102 were highly expressed in PBL relative to DLBCL and EOP, while 166 showed low expression in PBL. This set of 268 genes defined a distinct transcriptional program operating in PBL. Among these were surface markers such as CD320, CD300A, and IL6 receptor, as well as the cytokine Oncostatin M, which was highly expressed in PBL but almost never expressed in DLBCL or EOP. CD320 plays a role in generation and proliferation of plasma cells in the germinal center in response to IL-10 stimulation, while the immunoglobulin superfamily member CD300 is variably expressed across the hematopoietic hierarchy. The apoptosis-inducer BAX (Bcl-2 associated X protein) was highly expressed in PBL, similar to reports in plasma cell neoplasms. In addition the CpG methyltransferase gene Dnmt3b showed high expression levels in PBL.

By comparing malignancy-specific gene expression patterns to known biological pathways, we found that expression of components of the B-cell receptor signaling pathway (Cd79a, Cd79b, Blk, Lyn, Syk, Ptprc, Csk, Pik3cd, Swap70, and Rel) were repressed by 2-fold or more on average in PBL relative to DLBCL. We observed a similar pattern in EOP relative to DLBCL. In contrast, mitochondrial genes were more highly expressed in PBL than in DLBCL. Analysis against a large compendium of sets of transcription factor targets from motif and ChIP analyses identified that targets of MYB, a major transcriptional regulator of hematopoietic differentiation, were up-regulated in PBL; whereas targets of NFKB1 were repressed relative to DLBCL.

Both PBL and EOP highly expressed genes that have previously been described as up-regulated in plasmacytomas. To further investigate the potential cell of origin of these malignancies, we compared genes expressed in PBL, DLBCL, and EOP to genes that are highly expressed in specific sub-types of B-cells. Genes highly expressed in plasma cells relative to other types of B-cells were highly expressed in PBL and EOP compared to DLBCL. Notably, this included the transcription factor XBP1 (X-box binding protein 1), which is a critical regulator of plasma cell differentiation. The plasma cell marker CD138 (syndecan-1, encoded by the Sdc1 gene) was also over-expressed in the PBL and EOP samples.

We have validated the array data by reanalyzing expression of four candidate genes (Lyn, Syk, SPIB, and Swap70) by real time PCR. These four genes were highly expressed in DLBCL, but their expression was low in both in PBL and EOP. The observed overexpression of Swap70 in DLBCL as compared to PBL and EOP was subsequently validated by immunohistochemistry (IHC). Immunostaining for Swap70 was performed in all 30 cases used for array analysis and was negative in all cases of PBL (0/15 positive) and EOP (0/5 positive) but was diffusely positive in all but one of the DLBCLs (9/10 positive). Swap70 analysis by IHC was subsequently performed in 7 additional cases of DLBCL and was diffusely positive in all of these cases (7/7), thus suggesting that immunohistochemical analysis for Swap70 may be useful in differentiating PBL and EOP from DLBCL.

Overall our results provide insight into the unique transcriptional programs distinguishing PBL from morphologic and clinical mimics DLBCL and EOP, as well as identify similarities between them. Most notably, we observed that B-cell receptor signaling pathway genes are significantly down-regulated in PBL and EOP compared to DLBCL. These findings corroborate the downregulation of surface immunoglobulin expression as seen in PBL and EOP and suggest a biologic similarity between these two neoplasms. Among normal B-cell sub-populations, PBL and EOP were most similar in their expression patterns to plasma cells and plasmablasts in that they expressed several well-known plasma cell markers. These findings additionally identify novel candidate genes that provide opportunities for further phenotypic and functional characterization of these neoplasms.