Burkitt lymphoma showing expression of BCL 2 – an analysis including gene expression in formalin-fixed paraffin-embedded tissue

The differential diagnosis between Burkitt lymphoma (BL) and diffuse large B-cell lymphoma (DLBCL) can be challenging. BL has been reported to express less BCL2 than DLBCL, but this issue has not been analyzed systematically. BL expressing BCL2 can be considered to be MYC/BCL2 co-expressors, a feature that is associated with poorer outcome in DLBCL but that has not been correlated with outcome in BL so far. We analyzed the expression of BCL2 in 150 cases of conventionally diagnosed BL using two different BCL2 antibodies. BCL2 expression was detected in 22% of the cases, though the expression varied in intensity and number of positive cells. Using a subgroup of 43 cases for which detailed clinical data were available, we did not detect any relevant differences in clinical presentation and outcome between BCL2-positive and BCL2-negative BL. An independent cohort of 17 BL with expression of BCL2 were analyzed molecularly with 13 of 17 cases classified as molecularly defined BL using gene expression profiling on formalin-fixed paraffin-embedded tissues. The four lymphomas diagnosed molecularly as intermediates did not differ in clinical presentation and outcome from molecularly defined BL. intRoDuCtion The differential diagnosis of Burkitt lymphoma (BL) versus diffuse large B-cell lymphoma (DLBCL) can be challenging. In daily practice their diagnosis is based on morphological assessment, immunophenotype and detection of translocations of the MYC and absence of translocations involving the BCL2 or BCL6 gene by fluorescence in situ hybridization (FISH) (conventionally diagnosed BL). The immunophenotype, which is assessed to distinguish BL from DLBCL, frequently includes BCL2 expression (Cogliatti et al, 2006;Salaverria & Siebert, 2011;Kluin & Schuuring, 2011). The former WHO classification published in 2001 stated that BCL2 is not expressed in BL (Jaffe et al, 2001). However, gene expression profiling identified cases of molecularly defined BL (mBL) with BCL2 expression not caused by translocations of the gene (Hummel et al, 2006;Dave et al, 2006). These findings led to a new weighting of the immunophenotypic features of BL in the current WHO classification, which describes BCL2 as being expressed in up to 20% of BL cases, predominantly with a weak expression pattern (Swerdlow et al, 2008). BL expressing BCL2 must be distinguished from the group of lymphomas called “B-cell lymphomas unclassified with features intermediate between BL and DLBCL” (conventionally diagnosed intermediates), which comprises a heterogeneous group of lymphomas. Conventionally diagnosed intermediate lymphomas show at least some features reminiscent of BL, such as a monomorphic cytological picture with small to medium-sized blasts, a starry sky pattern, high proliferation and frequently translocations involving the MYC gene (Swerdlow et al, 2008). However, conventionally diagnosed intermediate lymphomas often display immunophenotypic features such as strong BCL2 expression or genetic features like BCL2 or BCL6 gene translocations that preclude a BL diagnosis. The presence of both MYC and BCL2 or BCL6 translocations is called “double hit” and is usually not compatible with the diagnosis of BL (Aukema et al, 2011;Aukema et al, 2014;Boerma et al, 2009). Intermediate lymphomas can also be identified by gene expression profiling independent of morphology, immunophenotype and FISH data. Gene expression profiling-based molecular intermediates (m-intermediates) are classified as conventionally diagnosed BL or DLBCL by conventional diagnostics. A subgroup of m-intermediates lack a genetic “double hit” constellation (Hummel et al, 2006). In children and adolescents translocations of BCL6 and BCL2, and thus “double hit” lymphomas, are exceedingly rare, but m-intermediates in the young age 4