Distinctive Patterns of BCL6 Molecular Alterations in Different Subsets of Diffuse Large B-Cell Lymphoma and Their Functional Consequences.
Authors: Iqbal J. , Greiner T. C. , Patel K. , Ji J. , Dave B. J. , Horsman D. E. , Shen Y. , Weisenburger D. D. , Campo E. , Rosenwald A. , Gascoyne R. D. , Jaffe E. S. , Ott G. , Delabie J. , Rimsza L. , McKeithan T. , Staudt L. M. , Chan W. C. 
Published: April 2020

Abstract
Gene expression profiling of diffuse large B-cell lymphoma (DLBCL) has revealed biologically and prognostically distinct subgroups: germinal center B-cell-like (GCB), activated B cell-like (ABC), and primary mediastinal (PM) DLBCL. The BCL6 gene on chromosome 3q27 is a transcriptional repressor and is required for GC formation and function. Two molecular alterations involving the BCL6 gene are commonly observed in DLBCL: chromosomal translocations and mutations in the 5′ non-coding region.
The functional consequences of BCL6 translocation and mutation have not been studied in the context of DLBCL subgroups. Therefore, we examined the frequency of translocations and the spectrum of BCL6 mutations in exon 1 and intron 1 in different DLBCL subgroups. We correlated these findings with BCL6 mRNA and protein expression, as well as the expression of BCL6 target genes. Fluorescence in situ hybridization (FISH) using a break-apart probe detected BCL6 translocations in 24 of 112 (21%) DLBCL cases. Surprisingly, the frequency of BCL6 translocation was higher in the ABC subgroup than the GCB subgroup (10 of 40; 25% vs 5 of 44; 11%, respectively) and the PM subgroup had the highest incidence (4 of 8; 50%). Expression of BCL6 protein was detected by immunohistochemistry in 75 of 138 cases (54%), including 15 of 44 (34%) in the ABC subgroup, 46 of 57 (80%) in the GCB subgroup and 4 of 10 (40%) in the PM subgroup. A good correlation of protein and mRNA expression, as assessed by cDNA microarrays (NEJM346:1937–47; 2002) was observed in the GCB subgroup but not in the other subgroups. BCL6 mutations were detected in 71 of 128 cases (55%) of DLBCL with a higher frequency in the GCB subgroup (34 of 50; 68%) and PM subgroup (10 of 12; 90%) than the ABC subgroup (14 of 43; 32%). Interestingly, there was a distinctive pattern of distribution of BCL6 mutations in the DLBCL subgroups. For example, 11 of 100 (11%) of the mutations targeted exon 1 and 8 of these 11 mutants were observed in the GCB subgroup. They preferentially affected the BCL6, STAT1 or predicted CEBP binding sites. High BCL6 mRNA and protein expression was generally associated with mutants affecting the 3′ BCL6 binding sites. We also examined the expression of 25 known BCL6 target genes in the different subgroups of DLBCL and observed the repression of this set of genes mainly in the GCB but not in the ABC subgroup, and the presence of a translocation did not correlate with target gene suppression.
In conclusion, the frequency of BCL6 translocation and mutation is distinctly different in the DLBCL subgroups and BCL6 expression has different regulatory influences on target genes in different subgroups of DLBCL. Exon 1 mutations preferentially occur in the GCB subgroup and affect transcription factor binding sites. However, BCL6 translocations did not show good correlation with BCL6 expression or functional repression of target genes. The influence of the reciprocal partner genes in the translocations on the pathogenesis of DLBCL warrants further investigation.