Regulation of normal B cell differentiation and malignant B cell survival by OCT2 (ChIP-Seq)

The requirement for the B cell transcription factor OCT2 (encoded by Pou2f2) in germinal center B cells has proved controversial. Here, we report that germinal center B cells are formed normally after depletion of OCT2 in a conditional knockout mouse but that their proliferation is reduced and in vivo differentiation to antibody-secreting plasma cells is blocked. This led us to examine the role of OCT2 in germinal center derived lymphomas. ShRNA knockdown showed that almost all DLBCL cell lines are addicted to the expression of OCT2 and its co-activator OCA-B. Genome-wide chromatin Immunoprecipitation analysis and gene expression profiling revealed the broad transcriptional program regulated by OCT2 that includes the expression of STAT3, IL10, ELL2, XBP1, MYC, TERT and ADA. Importantly, genetic alteration of OCT2 is not a requirement for cellular addiction in DLBCL. However, we detected amplifications of the POU2F2 locus in DLBCL tumor biopsies as well as a recurrent mutation of threonine 223 in the DNA binding domain of OCT2. This neomorphic mutation subtly alters the DNA binding preference of OCT2 leading to the transactivation of novel target genes including HIF1a and FCRL3. Finally, by introducing mutations designed to disrupt the OCT2-OCA-B interface we reveal a requirement for this protein-protein interface that might ultimately be exploited therapeutically. Our findings, combined with the predominantly B cell restricted expression of OCT2 and the absence of systemic phenotype in our knockout mice, suggest that an OCT2-targeted therapeutic strategy would be efficacious in both major subtypes of DLBCL whilst avoiding systemic toxicity. Overall design: To identify gene targets of OCT2 and OCA-B, we performed genome-wide chromatin immunoprecipitation analysis (ChIP-Seq) in the HBL1 ABC DLBCL line (n=6). To test whether the T223A mutant isoform alters OCT2 DNA binding, we designed "Biotag" constructs that express either wild type or T223A OCT2 fused to a peptide sequence that can be biotinylated by the bacterial enzyme BirA. We engineered the DLBCL cell line BJAB to express BirA, transduced the cells with the OCT2 Biotag constructs, and performed ChIP-seq (n=3) using streptavidin to purify chromatin bound to these OCT2 isoforms.