Genetic Subtyping Using Whole Exome Sequencing across Large B Cell Lymphoma Entities in a Series of 109 DLBCL and Plasmablastic Lymphoma Cases

Background: Diffuse Large B Cell Lymphomas are still a significantly heterogeneous disease with few molecularly defined entities. Recently genomic based algorithms have been proposed for DBCL NOS disease classification, including the Lymphgen tool 1. Other large B cell lymphoma entities, including High Grade BCL Dual Hit or plasmablastic lymphoma remain poorly characterized according to this molecular based clustering methods. Here we sought to apply this tool for disease classification in a series of 109 large B cell lymphoma cases derived from GELTAMO clinical trials (NCT01848132 and NCT2015-005390-21) and retrospective multicentric series to confirm its application across different large B cell lymphoma entities. Methods: We retrieved FFPE and liquid biopsy diagnostic samples from 109 cases and performed histopathological confirmation, FISH testing and whole exome sequencing (Agilent Sureselect v6) by NGS. Sequencing data and available fusion data for BCL2 and BCL6 were used to classify the cases according to Lymphgen (https://llmpp.nih.gov/lymphgen). Results: 109 cases were classified as different large B cell lymphoma entities, including 31 DLBCL NOS GCB type, 8 HGBCL/DLBCL DH/NOS, 41 DLBCL NOS non-GCB type, 22 plasmablastic lymphoma, 3 THRLBCL, 1 PMBL, 2 DLBCL NOS and 1 3B Follicular Lymphoma. After WES median Tumor Mutation Burden was 972. Overall genetic classification by Lymphgen was obtained in 53 cases (49%) with the remaining 56 cases considered unclassified (molecular subtype other/NOS). Of the genetically identified cases, 18 were classified as ST2 (33%), 15 were considered EZB (28%), 5 BN2 (10%) and 3 MCD (5%). Interestingly 12 (22%, overall, 11%) were genetically composite, including 8, ST2/EZB; 1, ST2/BN2; 1, ST2/MCD and 2 ST2/EZB/MCD. According to histopathological subtype 17 GCB type DLBCL were classified as EZB (8, 47%), ST2 (5, 30%) and BN2 (1, 6%). 3 cases (17%) were genetically composite. 6 out of 8 cases of HGBCL/DLBCL DH/NOS were classified as either EZB (2), ST2 (2) or composite EZB/ST2 (2). 19 non-GCB type DLBCL were classified as ST2 (5, 26%), BN2 (4, 21 %), EZB (3, 16 %) and MCD (3, 16%). 4 cases were genetically composite (21%). Interestingly, plasmablastic lymphomas showed ST2 genetic features, found in 5 out of 22 (23%) and composite ST2/EZB in 2 cases. Genes overrepresented in the ST2 cluster include TET2, PRRC2C, STAT3,DOCK8 and CLTC. Plasmablastic Lymphomas classified as other showed recurrent mutations in PRRC2C, PABPC1,