Pathogenesis of Follicular Lymphoma

Human lymphoma tumor cells in the pleural fluid stained with a Defquick stain and magnified to 400x.

Source: Dr. Lance Liotta Laboratory, The Web site of the National Cancer Institute (http://www.cancer.gov).

Targeted therapies for cancer are often based on an improved understanding of the origins of the tumor. The identification of mutations, linked to transformation or progression events, or signaling pathways that become dysregulated in malignant cells, can suggest drug targets that are specific to diseased cells, sparing patients toxic side effects.

Disease Characteristics

The slow growing, and frequently asymptomatic, nature of follicular lymphoma (FL) presents a significant obstacle to clinical identification in the early stages, and thereby obscures the mechanisms that cause disease initiation and progression. Despite the challenges, a growing understanding of FL pathogenesis is resulting in new drug candidates and approaches to clinical treatment.Follicular lymphoma is the malignancy of germinal center B cells in the lymph nodes. It is the second most commonly diagnosed non-Hodgkin’s lymphoma, accounting for approximately 35% of all such cases. It is an indolent B-cell malignancy with characteristic slow progression and a high rate of response to treatment, and the median survival time is >10 years.¹Follicular lymphoma cells are principally localized to the lymph nodes and extranodal sites, most often bone marrow (70%).

Genetics of FL

The median age of diagnosis for FL is 60 years,²and a large majority of newly diagnosed patients present at the clinic with stage III or IV disease. At these stages FL is incurable, and although the initial response rate to treatment is high, most patients will develop resistance over time. Additionally, 10% to 70% of cases of FL will eventually transform into a more aggressive diffuse large B-cell lymphoma (DLBCL) at a rate of 2% to 3% per year.³By far, the most frequent genetic abnormality observed in FL cells (85%) is a translocation of chromosomes 14 and 18, t(14;18),3 which results in the activation of the B-cell CLL/lymphoma 2 (BCL-2) gene. However, the t(14;18) has also been observed in B cells from healthy individuals and patients with DLBCL. This and the fact that BCL-2 kinase has antiproliferative as well as antiapoptotic functions suggest that its overexpression is not sufficient to cause FL.

B cells from patients with FL, like many other malignant cell types, demonstrate a rich variety of somatic mutations accumulated over time in addition to t(14;18). There are therefore numerous additional gene mutations with potential relevance to the development and progression of FL.

The receptor tyrosine kinase encoded byEPHA7has been proposed as a potential tumor suppressor. In one study, EPHA7 protein staining was absent in 72% of patient samples, and the gene itself is in a chromosomal region, 6q, that is frequently deleted in FL.⁴

Deletion or mutation ofTNFRSF14, a gene that encodes a member of the tumor necrosis factor family, is another frequently observed genetic lesion in FL-affected B cells (>65%).⁵When stimulated, the cell surface receptor encoded byTNFRSF14is hypothesized to promote immunogenicity and inhibit proliferation. Many of the mutations found in FL encode a truncated form of the protein that cannot be inserted into the cell surface, thus inducing a loss of protein function.

A large number of genetic abnormalities contributing to the pathogenesis of FL include genes that affect the epigenetic status of B cells. Gain-of-function mutations affecting the histone methyltransferasesEZH2andMLL2are observed in 27%⁶and 89%⁷of patients, respectively. These histone-modifying genes affect the epigenetic regulation of other genes, and it has been shown that EZH2 protein contributes to the silencing of antiproliferative genes in healthy B cells.⁸

Microenvironment

In contrast to the previously described genes, deleterious mutations of the B cell receptor (BCR) are selected against in patients with FL,⁹suggesting that intact BCR signaling is important for the survival of the malignant cells.Like other cancers, the development and progression of FL is markedly affected by the microenvironment experienced by the neoplastic cells. The microenvironment in FL has been proposed to have 2 main influences: suppressing the antitumor immune response and supporting the growth and survival of the malignant cells.

The prosurvival contribution of the microenvironment is attested by the fact that FL cells cannot be propagated in vitro, except through the use of feeder cells or cytokines, and even then only for a short time. Follicular lymphoma cells are attracted into the nodal follicles by follicular dendritic cells (FDCs) that release cytokines such as CXCL13.¹⁰Once there, the antigen-presenting function of the FDCs stimulates BCR signaling in the FL cells, contributing to their survival.

Progression

The malignant B cells achieve functional immune suppression through a variety of mechanisms. By expressing programmed death-ligand 1 (PD-L1) on their surface, FL cells can activate PD-1 receptors on effector T cells, thereby suppressing their function. CD70 expression by the malignant cells can signal through CD27 on effector T cells to induce FoxP3 expression. This induces their conversion to regulatory T (Treg) cells¹¹and suppresses the immune response.Because of the indolent nature of FL and the lengthy time that typically elapses between the occurrence of an initiating mutation and diagnosis, patients have multiple, distinct populations of malignant B cells in various nodal and extranodal sites. These different clonal populations, once separated, acquire unique mutational profiles on which the microenvironment of each clonal population then exerts selective pressure. Disease progression in FL can therefore be understood as an evolutionary process in which multiple coexisting clones have evolved from an ancestral common progenitor clone (CPC). This model of progression by clonal evolution is borne out by the pattern of spontaneous waxing and waning of the disease in untreated patients, as well as by the unpredictable progression of FL to a more aggressive form, such as transformed FL or DLBCL.2

Conclusions

References

In a paper inNature Geneticsthis year, researchers sought to catalogue the genetic changes that occurred in patients’ lymphoma as it progressed from an indolent to an aggressive (transformed FL) stage using whole genome and whole-exome sequencing.¹²The authors sought to differentiate between the set of genetic alterations necessary for FL initiation and those involved in the switch to a more aggressive state. They identified a large degree of epigenetic pathway involvement, which they termed epigeneticaddiction, as well as alterations in B-cell development, the JAK-STAT pathway, and nuclear factor kappa B (NF-κB) signaling. No single mutation or aberration was identified in disease progression; however, “the findings confirmed our earlier suspicions that each episode of FL originated from a reservoir founder B-cell population,” said lead author, Jessica Okosun, MD, of the Centre for Haemato-Oncology, Barts Cancer Institute, Queen Mary University of London. “Knowing which genetic alterations are present within this reservoir population should allow us to refine our therapeutic approaches to target this tumor population as we believe that this will ultimately improve patients’ outcomes and lead to cures in FL,” she said.Therapeutic options available for the treatment of FL, beyond conventional radiation and chemotherapy, include immunomodulatory agents such as idiotypic vaccines, monoclonal antibodies, and kinase inhibitors. While these agents have the potential to improve disease outcomes for patients with FL, a growing understanding of the origins and progression of this cancer can offer new insights for rational treatment development. Perhaps one of these new targets will lead to the elusive goal of curing FL.

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