Prevalence of IRF4 rearrangement in large B-cell lymphomas of the Waldeyer's ring in adults

Diffuse large B-cell lymphoma (DLBCL) is the most common B-cell non-Hodgkin lymphoma (B-NHL) in adults. These lymphomas are classified according to gene expression profiling (GEP) into germinal center B-cell (GCB) and activated B-cell type (ABC). Recent studies have suggested new subtypes of large B-cell lymphoma, based on genetic and molecular alterations, among them is large B-cell lymphoma with IRF4-rearrangement (LBCL-IRF4). We used fluorescence in situ hybridization (FISH), GEP (using the DLBCL COO assay by HTG Molecular Inc), and next generation sequencing (NGS) to comprehensively characterize 30 cases of LBCLs located in Waldeyer's ring in adult patients and to identify LBCL-IRF4. FISH revealed breaks of IRF4 in 2/30 cases (6.7%), BCL2 breaks in 6/30 cases (20.0%), and IGH breaks in 13/29 cases (44.8%). GEP classified 14 cases each as GCB or ABC subtype, and 2 cases remained unclassified; this was concordant with the immunohistochemistry (IHC) in 25/30 cases (83.3%). A subgrouping, based on GEP, was performed: group 1 included 14 GCB cases with the most frequent mutations in BCL2 and EZH2 in 6/14 cases (42.8%). The two cases with IRF4 rearrangement were assigned to this group by GEP and showed IRF4 mutations, supporting the diagnosis of LBCL-IRF4. Group 2 included 14 ABC cases; the most frequent mutations were CD79B and MYD88 identified in 5/14 patients (35.7%). Group 3 included 2 unclassifiable cases in which no molecular patterns were detected. Overall, LBCLs of Waldeyer's ring in adult patients are a heterogeneous group, including LBCL-IRF4, which shares several features with cases in the pediatric population.