BCL6 Protein Expression Identifies Cases without BCL6 Gene Rearrangement in Non-Germinal Centre Diffuse Large B- Cell Lymphoma Cases

BCL6 gene rearrangements are the most frequent cytogenetic abnormality in Diffuse Large B-Cell Lymphoma (DLBCL), occurring in up to 64% of cases, suggesting a poorer prognosis and response to therapy. The aim of this study was to calculate the accuracy and predictive power of BCL6 IHC in identifying BCL6 gene rearrangements. A search for DLBCL cases was performed on the laboratory information system. We retrospectively analysed 46 cases of DLBCL, and correlated BCL6 protein expression with gene rearrangement status. Of the 46 cases, 39 (84.78%) showed positive BCL6 protein expression by IHC. In comparison, only 10 (21.74 %) samples presented with gene rearrangements. There were 8 cases positive for BCL6 IHC with gene translocations and 5 cases were negative for both protein expression and gene rearrangements. BCL6 IHC had a sensitivity of 80% and a specificity of 14%. Furthermore, the Positive Predictive Value (PPV) and Negative Predictive Value (NPV) were 21% and 71%, respectively. BCL6 rearrangement was more common in cases with a Non-Germinal Centre (GC) Cell of Origin (COO) (45.5%) as compared to the GC group (14.3%). When stratifying the analysis according to the COO, the sensitivity and NPV of BCL6 IHC in the non-GC group were both 100%, while the same parameters were considerably poorer in the GC cases. BCL6 protein expression did not correlate with the presence of BCL6 gene rearrangements in HIV related DLBCL cases. However, lack of BCL6 protein expression may be used to identify cases without a BCL6 gene rearrangement, particularly in non-GC COO.