Immunohistochemical distinction of ABC and GCB in extranodal DLBCL is not reflected in mutation patterns.

Gene expression profiling (GEP) separated diffuse large B-cell lymphoma (DLBCL) in two different entities, i.e. activated B cell-like (ABC) and germinal center B cell-like (GCB) lymphomas with ABC lymphomas demonstrating a less favorable outcome. NF-kB pathway activating mutations in MYD88, CD79A/B and CARD11 are predominantly found in ABC type lymphomas. Targeted therapies affecting NF-kB pathways have shown therapeutic potential in this subtype. Immunohistochemistry algorithms have been developed as a tool for distinguishing these entities in routine clinical diagnostics. To test whether this immunohistochemistry classifier would detect the biological differences between the entities 147 DLBCLs were subtyped into ABC and GCB using the Visco-Young algorithm. Mutation analysis demonstrated mutations in MYD88 or CD79 A/B in 21% (10/47) of non-GCB type but only in 3% (1/31) of GCB lymphomas (p = 0.012) in nodal lymphomas. In primary extra nodal lymphomas, however, 17.5% (4/23) of GCB type and 37.5% (15/40) of non-GCB lymphomas carried mutations in MYD88 and CD79 A/B. While the Visco-Young algorithm was sufficient to detect biological differences (i.e. mutation patterns) in nodal DLBCL it did not distinguish GCB and non-GCB type lymphomas of primary extranodal sites. Here, the morphological sites of the lymphomas seem to be more important for their molecular pattern than their immunohistochemical status.