SHP1 Deficiency Is Responsible for the Constitutive Activation of the BCR Pathway in GCB DLBCL

Functional and transcriptional profiling studies have identified a subset of diffuse large B cell lymphoma (DLBCL) tumors that rely on B cell receptor (BCR) signals for proliferation and survival. These BCR-dependent tumors have been identified among both the ABC and GCB DLBCL subtype. However, differences have been observed between the two subtypes in terms of the mechanism of BCR pathway activation and activity of downstream signaling molecules. BCR-dependent ABC DLBCL tumors are characterized by high basal NF-kB and PI3K/AKT activity and the presence of gain-of-function mutations in the CD79A or CD79B subunit of the BCR that are responsible for the chronic activation of the BCR pathway. In contrast, BCR-dependent GCB DLBCL tumors have low baseline NF-kB activity and lack CD79A/CD79B mutations, suggesting a different mechanism of BCR pathway activation. In this study, we investigated whether activation of the BCR pathway in GCB DLBCL is potentially caused by deficiency of the phosphatase SHP1, which is an important negative regulator of the BCR pathway and has been reported to be downregulated in approximately 40% of primary DLBCL tumors. For this purpose, we first correlated SHP1 expression with the presence of phosphorylated SYK and BLNK in the GCB DLBCL cell lines OCI-Ly1, OCI-Ly7, OCI-Ly18, SU-DHL-4, SU-DHL-6, WSU-NHL, SU-DHL-8, Toledo, BJAB, OCI-Ly19, OCI-Ly4, and Farage. Immunoblotting analysis revealed that SHP1 is expressed in SU-DHL-8, Toledo, BJAB, OCI-Ly19, OCI-Ly4, and Farage but not in the remaining 6 cell lines. Expression of SHP1 inversely correlated with expression of phosphorylated SYK (pSYK-Y352) and BLNK (pBLNK-Y84), suggesting that these two BCR-proximal signaling molecules are activated because of SHP1 deficiency. To further evaluate this possibility, we re-expressed SHP1 in OCI-Ly1 and SU-DHL-4 cells by transient transfection with a plasmid or a lentiviral SHP1 expression vector. Both approaches resulted in a substantial reduction of pSYK-Y352 and pBLNK-Y84 levels, confirming that SHP1 deficiency leads to activation of the BCR pathway.

To determine whether activation of the BCR pathway contributes to tumor cell survival, we investigated the effects of the SYK inhibitor R406 (active substance of the drug fostamatinib) on the viability of the 6 SHP1-positive and 6 SHP1-negative GCB DLBCL cell lines. At concentrations up to 2 μM, R406 displayed only minimal toxicity against each of the SHP1-positive and SHP1-negative GCB DLBCL cell lines (<20% apoptotic cells), with the exception of WSU-NHL (60% apoptotic cells). However, analysis of the effects of R406 on the expression of BCL-2 family proteins showed downregulation of the antiapoptotic protein MCL-1 and upregulation of the proapoptotic proteins BIM and HRK in all of the SHP1-negative but in none of the SHP1-positive GCB DLBCL cell lines. Such changes were also observed following siRNA-mediated knockdown of SYK or overexpression of SHP1, further suggesting that they are on-target effects of R406. In addition, treatment with the BTK inhibitor ibrutinib or siRNA-mediated knockdown of BTK also resulted in downregulation of MCL-1 and upregulation of BIM in these cell lines. Similar changes were observed in 3 of the 5 investigated primary DLBCL samples following treatment with ibrutinib or fostamatinib. Importantly, downregulation of MCL-1 by RNA interference or upregulation of BIM by mRNA transfection resulted in substantially increased sensitivity of these cell lines to the BCL-2 inhibitor venetoclax, suggesting that SYK or BTK inhibitors could potentiate the cytotoxic effects of this drug. To further evaluate this possibility, we calculated the combination index of the R406 + venetoclax or ibrutinib + venetoclax drug combination in the 12 GCB DLBCL cell lines. Significant synergistic activity was seen in all five BCL-2-positive/SHP1-negative GCB DLBCL cell lines with the R406 + venetoclax combination and in four of them with the ibrutinib + venetoclax combination. In contrast, synergistic activity was detected in only one of the 6 SHP1-positive GCB DLBCL cell lines and was not seen in the BCL-2-negative/SHP1-negative cell line OCI-Ly7. Collectively, these findings suggest that SHP1 deficiency is responsible for the constitutive activation of the BCR pathway in GCB DLBCL and identify SHP1 and BCL-2 as potential predictive markers for response to treatment with a venetoclax/BCR inhibitor combination.