Diffuse large B cell lymphoma (DLBCL) is one of the most common and aggressive hematological malignancies and is classified as a heterogeneous disease based on clinical and biological definitions. DLBCL is the most common type of non-Hodgkin’s lymphoma, and the nature of the heterogeneity of DLBCL has been under investigation for more than 20 years. Current treatment options have improved clinical outcomes for DLBCL patients, but a large fraction of patients remain refractory or relapse following treatment. Advances in high-throughput genome sequencing in the last five years has been critical to identifying genetic alterations in B cells that drive pathogenesis and have been instrumental in identifying biomarkers that can predict responses to targeted treatments.
Early gene expression studies in DLBCL indicated that malignancies can emerge from B cells at different stages of differentiation, and treatment outcomes differed based on the B cell of origin1. There are three major subtypes of DLBCL that have been classified based on cell of origin: germinal center B cell-like subtype (GBC), activated B cell-like subtype (ABC), and primary mediastinal B cell lymphoma. Whole exome sequencing (WES), transcriptomics, and copy number variation measurements have been particularly important to understanding the wide range of genetic abnormalities that drive different DLBCL subtypes. Early studies were limited by small sample sizes and challenges in linking genetic mutations to clinical outcomes. More recent studies have included larger cohorts and used paired tumor and normal tissue samples to define the landscape of genetic drivers associated with DLBCL. A 2017 study by Reddy et al. used integrative analysis of WES and transcriptome data from 1,001 DLBCL patients to identify 150 genetic drivers and described a genomic risk model that included genetic alterations, DLBCL subtype, International Prognostic Index (IPI) score and MYC/BCL2 dual expression.
Transcriptome (RNAseq) data distinguished ABC and GBC subtypes, and whereas most driver genes were shared between these subtypes, 20 genes were differentially mutated. This study also validated earlier studies that showed a direct correlation between high expression of MYC and BCL2 and a poor prognosis, and mutations in MYC, CD79B and zinc finger and AT-hook domain containing (ZFAT) were associated with poor overall survival. A 2020 study of more than 1,400 previously untreated CD20+ DLBCL patients (Phase III GOYA study) confirmed that alterations in BCL2 was the only factor associated with reduced progression-free survival (PFS) and was independent of cell of origin subtype, IPI, or treatment regimen. Many of the mutations they detected occurred at low frequencies, once again highlighting the heterogeneity of DLBCL. Interestingly, BCL2 translocations were significantly associated with shorted PFS in the GCB subtype independent of other factors, but no single genetic alteration had a similar prognostic value for the ABC subtype. A 2020 study examined driver genes with relatively low mutation frequencies to better understand biological processes involved in lymphomagenesis. This study identified high expression of EIF3B, MLH1, PPP1CA and RECQL4 being associated with decreased overall survival, but high expression of XPO1 and LYN correlated with improved overall survival.
Translating Mutations into Treatment
The combination of cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) was the standard of care for DLBCL, but outcomes were improved dramatically with the addition the anti-CD20 antibody rituximab (R-CHOP). In general, patients with ABC subtype have inferior outcomes with R-CHOP as does the GCB subtype with BCL2 overexpression. Poor prognoses have also been linked to lymphomas with translocations in MYC, BCL2 and/or BCL6. Combining R-CHOP with treatments that target these molecules is currently of great interest, and a 2021 phase II study (CAVALLI) examined the efficacy and safety of combining venetoclax, a selective BCL2 inhibitor with R-CHOP, and results indicate that addition of venetoclax can improve PFS and overall survival. As more targeted inhibitors are being examined in clinical trials, new options for improved combined therapies or alternative therapies are likely to become available, especially for DLBCL subtypes with poor prognoses. Continued preclinical and clinical studies are needed to identify and evaluate potential therapeutic targets.
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