Regulation of normal B cell differentiation and malignant B cell survival by OCT2 (expression)

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. Knockdown of OCT2_1196 was tested in ABC DLBCL cell lines (HBL1 n=2, OCI-Ly10 n=3, TMD8 n=2) and GCB DLBCL cell lines (BJAB n=2, HT n=2). Knockdown of OCT2_3410 was tested in ABC DLBCL cell lines (HBL1 n=2, OCI-Ly10 n=4, TMD8 n=2) and GCB DLBCL cell lines (BJAB n=2, HT n=2). Knockdown of OCA-B was tested in ABC DLBCL cell lines (HBL1 n=2, OCI-Ly10 n=2) and GCB DLBCL cell lines (BJAB n=2, HT n=2). Knockdown of OCT2_1196 was tested with and without WT rescue construct in ABC DLBCL cell line (HBL1 n=4). Knockdown of OCT2_3410 was tested with and without WT rescue construct in ABC DLBCL cell line (HBL1 n=4). Knockdown of OCT2_1196 was tested with OCT2 T223A vs. WT rescue construct in ABC DLBCL cell line (HBL1 n=2, SUDHL4 n=4) and GCB DLBCL cell line (BJAB n=4). Mouse knockout (n=4) and wild type (n=4) Pou2f2 OCT2 B cells were tested.