Aggressive lymphomas

Abstract

Genomewide molecular profiling has revealed new subtypes of lymphoma that originate from lymphocytes that differ in developmental stage and that use distinct oncogenic programs, yet are indistinguishable under the microscope. In this review, we discuss recent progress in the molecular genetics of aggressive lymphomas and focus on the most common form of this disease, diffuse large-B-cell lymphoma, which accounts for 30 to 40% of newly diagnosed lymphomas.

Figure 1 (facing page).. B-Cell Differentiation and Lymphomagenesis.

Malignant lymphomas can arise at multiple stages of normal B-cell development. After the stimulation of a mature naive B cell with a T-cell-dependent antigen, the germinal-center reaction is initiated. The germinal-center B cell represents a discrete, quasi-stable differentiation stage that is characterized by a unique regulatory network and the action of activation-induced cytidine deaminase (AID), which induces both immunoglobulin (Ig) somatic hypermutation and heavy-chain class switching. Several transcription factors are required to establish and maintain the identity and function of the germinal-center B cell, including BCL6, MTA3, SPIB, BACH2, OCT2, OCAB, and IRF8. Red lines indicate that a regulatory factor inhibits the indicated gene or cellular function, and blue lines indicate positive regulation. In concert, these factors block plasmacytic differentiation by repressing Blimp-1. They also promote cell-cycle progression without cell growth while blocking the DNA damage response evoked by AID-dependent mutations and DNA breaks. Within the germinal center, the rapidly proliferating centroblasts are prone to cell death. Periodically, centroblasts travel to a subcompartment of the germinal center that is rich in follicular dendritic cells and follicular helper T cells, where they become centrocytes. Centrocytes may be rescued from cell death as a result of stimulation by antigen on follicular dendritic cells and CD40 ligand on T cells and may then revert to the centroblast state and resume proliferation. IRF4 initiates plasmacytic differentiation by establishing a characteristic regulatory network, which extinguishes the mature B-cell program while promoting terminal differentiation and immunoglobulin secretion. The putative origins of various non-Hodgkin’s lymphomas — including the germinal-center B-cell—like (GCB) and activated B-cell—like (ABC) subtypes of diffuse large-B-cell lymphoma (DLBCL) — are indicated. Lymphomas that are derived from germinal-center B cells have recurrent genetic abnormalities that circumvent the normal genetic program in order to block plasmacytic differentiation, promote cell growth, and evade apoptosis. NF-κB denotes nuclear factor-κB.

Figure 2.. Oncogenic Pathways for Three Subtypes of Diffuse Large-B-Cell Lymphoma.

On the basis of gene-expression profiling, diffuse large-B-cell lymphoma can be divided into three molecular subtypes: the germinal-center B-cell–like (GCB) subtype, the activated B-cell–like (ABC) subtype, and primary mediastinal B-cell lymphoma (PMBL). These subtypes originate from various stages of B-cell differentiation and acquire distinct oncogenic abnormalities. The abnormalities that are listed are preferentially or exclusively observed in the indicated subtypes. AID denotes activation-induced cytidine deaminase, ITAM immunoreceptor tyrosine-based activation motifs, mTOR mammalian target of rapamycin, and NF-κB nuclear factor-κB. Blue lines indicate activation, and red lines indicate inhibition.

Figure 3 (facing page).. B-Cell–Receptor and Nuclear Factor-κB (NF-κB) Signaling Pathways in Normal and Malignant Lymphocytes.

Panel A shows B-cell-receptor and NF-κB signaling in normal B cells. The engagement of the B-cell receptor by antigen evokes a signaling cascade that culminates in the activation of the NF-κB, mTOR (mammalian target of rapamycin), ERK MAP kinase, and nuclear factor of activated T cells (NFAT) pathways. Signaling is initiated when a SRC-family kinase (SFK) phosphorylates tyrosines in immunoreceptor tyrosine-based activation motifs (ITAMs) of the B-cell–receptor subunits CD79A and CD79B. The tyrosine kinase SYK is recruited to the ITAMs through its tandem SH2 domains and becomes enzymatically active. SYK phosphorylates many downstream targets, including the adapter BLNK, and initiates ERK MAP kinase signaling. In parallel, phosphatidylinositol 3-kinase (PI3K) is activated by recruitment to B-cell coreceptor CD19, generating membrane lipid phosphatidylinositol 3,4,5-triphosphate (PIP3) and activating the mTOR pathway. BTK is recruited to the membrane by binding to PIP3 and forms a complex with BLNK and phospholipase Gγ (PLCγ). PLCγ generates the second messenger IP3, which initiates an influx of calcium ions through the calcium-release-activated calcium (CRAC) channel and activation of the NFAT pathway, as well as diacylglycerol (DAG), which activates protein kinase Cβ (PKCβ). The signaling scaffold protein CARD11 is present in a latent state in the cytoplasm of resting B cells but translocates to the plasma membrane after phosphorylation by PKCβ. There, CARD11 nucleates a multiprotein complex, including MALT1, BCL10, and TRAF6. In the process, the ubiquitin ligase TRAF6 is activated, leading to ubiquitination of the IκB kinase (IKK) γ subunit, a necessary step toward IKK activation. TRAF6 becomes autoubiquitinated, leading to its association with a complex consisting of TAB2 and the kinase TAK1. TAK1 phosphorylates the IKKβ subunit in its activation loop, thereby activating IKK to phosphorylate IκBα and initiate NF-κB signaling. A20, a target gene of NF-κB, inhibits NF-κB signaling through the deubiquitination of IKKγ and TRAF6. Panel B shows pathways to the activation of NF-κB in the activated B-cell-like (ABC) subtype of diffuse large-B-cell lymphoma. Roughly 10% of ABC lymphomas have activating mutations of CARD11 in its coiled-coil domain (left subpanel). These mutations cause CARD11 to form large cytoplasmic aggregates that recruit other signaling components, including IKK, leading to constitutive activation of the NF-κB pathway. Other ABC lymphomas have wild-type CARD11 and instead have a chronic active form of B-cell-receptor signaling (right subpanel). In these cases, the B-cell receptors form immobile clusters in the plasma membrane and trigger multiple downstream pathways, including NF-κB. In 21% of ABC lymphomas, chronic active B-cell-receptor signaling is associated with somatic mutations in the ITAM signaling motifs of CD79B or CD79A, the majority of which alter the first tyrosine of the CD79B ITAM. Treatment prospects for patients with the ABC subtype will depend on the mechanism by which NF-κB is activated, as indicated. Blue lines indicate activation, and red lines indicate repression.

Figure 4.. Prediction of Survival According to Gene Expression in Diffuse Large-B-Cell Lymphoma.

Panel A shows three gene-expression signatures that are associated with overall survival in patients with diffuse large-B-cell lymphoma (DLBCL) who have been treated with chemotherapy combining cyclophosphamide, doxorubicin, vincristine, and prednisone with rituximab (R-CHOP). Shown are the average expression levels of the genes in each signature in 233 biopsy specimens; each vertical line represents a single specimen. The germinal-center B-cell signature is prognostically favorable and mirrors the distinction between the two most prevalent subtypes, activated B-cell–like (ABC) and germinal-center B-cell–like (GCB) subtypes, as indicated. The stromal-1 and stromal-2 signatures reflect the character of the nonmalignant cells infiltrating the biopsy specimens. The stromal-1 signature is associated with favorable survival, and the stromal-2 signature is associated with inferior survival. These three signatures are combined in a mathematical model that provides a survival predictor score (bottom bar), which can be used to assess risk for each patient treated with R-CHOP. Gene-expression values and the survival predictor scores are shown over a range that varies by a factor of 16, according to a red-green color scale. For gene expression, red indicates high expression, and green indicates low expression. For the survival predictor score, red indicates unfavorable survival, and green indicates favorable survival. Shown are representative signature genes that are upregulated in expression in association with the indicated signature. Panel B shows survival rates for patients with a molecular diagnosis of the GCB or ABC subtype after R-CHOP therapy. Panel C shows a model for prediction of survival according to gene expression. Patients were ranked according to their survival predictor scores (as shown in Panel A) and divided into quartiles. In this Kaplan-Meier analysis, the gene-expression-based model defines differences in 3-year progression-free survival among patients treated with R-CHOP.