Pathobiology of Burkitt lymphoma

INTRODUCTION — Burkitt lymphoma (BL) is a highly aggressive B cell neoplasm characterized by the translocation and deregulation of the MYC gene on chromosome 8. Three distinct clinical forms of BL are recognized: endemic, sporadic, and immunodeficiency-associated. Although they are histologically identical and have similar clinical behavior, there are differences in epidemiology, clinical presentation, and genetic features between the three forms. (See "Epidemiology, clinical manifestations, pathologic features, and diagnosis of Burkitt lymphoma", section on 'Clinical features'.)

BL with tissue-only disease and BL with peripheral blood involvement are considered different manifestations of the same disease in the World Health Organization (WHO) classification of hematologic malignancies [1]. The WHO classification also recognizes the existence of tumors with features intermediate between BL and diffuse large B cell lymphoma, which remain difficult diagnostic challenges. (See "Classification of hematopoietic neoplasms", section on 'Lymphoid neoplasms'.)

The pathogenesis of BL will be discussed here. Information regarding the diagnosis and treatment of BL is presented separately. (See "Epidemiology, clinical manifestations, pathologic features, and diagnosis of Burkitt lymphoma" and "Treatment of Burkitt leukemia/lymphoma in adults".)

CELL OF ORIGIN — BL is derived from germinal or post-germinal center B cells. The germinal or post-germinal center ancestry of these cells is principally supported by the identification of somatic mutations in the variable region of the immunoglobulin genes (IgV), which serve as a marker of germinal center transit, as well as the tumor cell immunophenotype. Some studies have demonstrated ongoing somatic mutation, a process that is normally restricted to germinal center B cells. (See "Normal B and T lymphocyte development" and "Epidemiology, clinical manifestations, pathologic features, and diagnosis of Burkitt lymphoma", section on 'Other genetic abnormalities'.)

The different clinical subtypes of BL (ie, endemic, nonendemic, and immunodeficiency-related) likely arise from B cells at different stages of development. Specifically, Epstein-Barr virus (EBV)-negative BLs show a low level of somatic hypermutation of their variable region heavy chain genes and no signs of antigen selection, whereas EBV-positive (generally endemic or AIDS-related) BLs have significantly higher levels of somatic hypermutation and evidence of antigen selection [2]. These findings suggest that EBV-negative BL may arise from an early centroblast, while EBV-positive BL may arise from a memory B cell or late germinal center B cell. This difference in cell of origin may also relate to the observed difference in MYC translocation breakpoints described below. Despite these differences, these clinical subtypes may have a common pathogenesis, based on similarities in gene expression [3]. (See 'Chromosomal translocations' below.)

PATHOGENESIS

Overview — The development of BL is dependent on the constitutive expression of the MYC gene located at chromosome 8q24, which encodes the MYC protein transcription factor. MYC modulates the expression of target genes that regulate many cellular processes, including cell growth, division, death, metabolism, adhesion, and motility [4,5]. Cells can generate high levels of MYC protein through various mechanisms, as described in the sections below.

The vast majority of BL have rearrangements of the MYC gene. However, additional factors beyond MYC translocation must occur for the development of BL. In support of this idea, a small percentage of HIV-positive persons and healthy controls have MYC translocations in B lymphocytes isolated from enlarged lymph nodes, a finding that does not appear to predict the later emergence of lymphoma [6]. (See "Epidemiology, clinical manifestations, pathologic features, and diagnosis of Burkitt lymphoma", section on 'Diagnosis'.)

MYC in normal cells — The MYC gene encodes a ubiquitously expressed nuclear phosphoprotein that functions as a pleiotropic transcriptional regulator of cell proliferation, differentiation, and apoptosis [4]. In vivo, MYC is found mainly in heterodimeric complexes with the related protein MAX; these heterodimers bind to the "E box consensus sequence" and directly activate transcription. The MYC-MAX interaction is required for MYC to stimulate transcription and cell proliferation [7-9]. Remarkably, there appear to be thousands of bona fide MYC/MAX binding sites in the genome that permit MYC to regulate the expression of numerous genes, leading to the idea that it may be better viewed as a global regulator of chromatin and gene expression than as a conventional transcription factor [10,11]. As a result, cells that express high levels of MYC produce far more RNA per cell than those that do not [12]. The net effect of this enhanced gene expression is to support both cell cycle progression and Warburg metabolism, which act together to maximize cell growth and proliferation [13].

MYC activity is normally regulated by the amount of MAX available to form MYC-MAX heterodimers and by competition from complexes formed by MAX with other proteins. MAX can form homodimers (MAX-MAX) and heterodimers with MAD family proteins (ie, MNT, MGA) that are negative regulators of transcription and block MYC-dependent transformation [14,15]. It is postulated that MAD-MAX or MNT-MAX heterodimers inhibit MYC function by either sequestering MAX and preventing the formation of MYC/MAX heterodimers or by competing with MYC/MAX heterodimers for binding to the same target sites. Upregulation of MYC favors MYC-MAX over MNT-MAX complexes at the promoters of target genes, thereby promoting cell cycle progression, while overexpression of MNT inhibits MYC-induced cell cycle entry, consistent with a hypothesis of competitive binding to MAX [16]. In non-Hodgkin lymphomas carrying MYC translocations, it is likely that constitutive expression of MYC increases the ratio of MYC/MAX complexes relative to inhibitory heterodimers, leading to upregulation of a program of gene expression that promotes growth.

The role of MYC in normal germinal center B cells was initially obscure, as most germinal center B cells exhibit low levels of MYC expression. However, conditional Myc knockout mice have established that MYC is essential for the initial proliferative burst that occurs following T cell-dependent antigen-mediated activation of B cells and for the establishment of germinal centers [17,18]. Shortly after entry into germinal centers, antigen-activated B cells downregulate MYC, but a subset of cells re-express MYC, which may contribute to the process of proliferation-associated somatic hypermutation of germinal center B cells.

MYC overexpression — BL is characterized by inappropriately high levels of MYC expression, which may occur through at least one of the following mechanisms [4]:

●Chromosomal translocations place MYC under the control of immunoglobulin gene enhancers that are constitutively active in mature B cells.

●5' regulatory regions normally flanking the MYC gene body are mutated or removed as a direct result of the translocation.

●Mutations of the MYC coding sequence result in amino acid substitutions that stabilize MYC protein by decreasing its proteosome-mediated degradation, thereby increasing its half-life.

Substantial experimental evidence has shown that constitutive expression of MYC influences the growth of B cells in vitro and in vivo, consistent with a role in B cell lymphomagenesis. As examples:

●In vitro, the expression of a MYC gene in Epstein-Barr virus (EBV)-immortalized human B cells leads to their malignant transformation [19].

●In vivo, transgenic overexpression of MYC in B cell lineage cells in mice leads to the development of B cell malignancy [20].

●Gene expression profiling of primary BL has confirmed the overexpression of MYC target genes identified in experimental models [21,22].

Chromosomal translocations — In virtually all cases, BL is associated with a translocation between the long arm of chromosome 8, the site of the MYC gene (8q24), and one of three Ig genes [23]:

●The Ig heavy chain gene on chromosome 14 – Resulting in the t(8;14)(q24;q32) found in 80 percent of BL.

●The kappa light chain gene on chromosome 2 – Resulting in the t(2;8)(p11;q24) found in 15 percent of BL.

●The lambda light chain gene on chromosome 22 – Resulting in the t(8;22)(q24;q11) found in 5 percent of BL.

The common effect of these translocations is that the translocated MYC allele is expressed constitutively in tumor cells, as opposed to the tight regulation of MYC levels in normal B cells [24]. (See 'MYC in normal cells' above.)

Although fairly homogeneous at the microscopic level, these translocations display a high degree of molecular heterogeneity. The specific gene breakpoint sites vary not only by translocation but also by clinical context (ie, endemic versus sporadic cases):

●The breakpoint sites on chromosome 8 found in t(8;14) are located 5' and centromeric to MYC, whereas the sites found in t(2;8) and t(8;22) map 3' to MYC [23].

●In endemic (African) cases, the breakpoint on chromosome 14 involves the heavy chain joining region and the breakpoint in chromosome 8 usually lies adjacent to MYC, while in non-endemic cases, the breakpoint on chromosome 14 involves the heavy chain class-switch region and the breakpoint in chromosome 8 often lies in intron 1 of MYC [25,26].

This molecular heterogeneity precludes the development of sensitive polymerase chain reaction (PCR) based testing for these translocations. Instead, these translocations are identified either by karyotyping of metaphase chromosomes or by fluorescence in situ hybridization (FISH).

Work using mouse models has shed light on the possible mechanisms that lead to MYC translocations. Translocations involving the class-switch region of the immunoglobulin heavy chain gene and MYC occur with surprisingly high frequency in activated B cells undergoing class-switch recombination. These "mistakes" occur during the recombination event that allows B cells to switch from the expression of IgM to the expression of other immunoglobulin types and require the enzyme activation-induced cytidine deaminase (AID) [27], which is an essential co-factor for normal class-switch recombination. In further support of a role for AID in chromosomal rearrangements that lead to lymphomagenesis, infection of mice with malaria (a risk factor for human endemic BL) provokes sustained expansion of AID-expressing germinal center B cells and increases the development of aggressive B cell lymphomas with chromosomal rearrangements bearing the molecular signatures of AID-mediated DNA damage [28].

Other work has shown that MYC tends to lie in close proximity to the immunoglobulin heavy chain locus in the nuclei of B cells, which may explain in part why MYC is often involved in such translocations. It is hypothesized that most cells bearing these translocations die rapidly, as unbridled MYC expression is a potent inducer of apoptosis (programmed cell death) in normal cells. How BL cells escape this fate is largely unknown; in some tumors, however, loss of TP53 function may abrogate MYC-induced apoptosis. (See 'Other genetic lesions' below.)

Other mechanisms — As mentioned above, up to 5 percent of tumors with features that are otherwise typical of BL lack MYC rearrangements. Other mechanisms leading to the increased expression of MYC include interference with the 5' regulatory regions and/or stabilization of the protein product.

5' regulatory regions normally present within the MYC DNA sequence can be mutated or moved away from MYC as a direct result of the translocation. Release of MYC from normal regulation can result in increased expression. The MYC first exon/first intron border, where MYC regulatory sequences are located, can be either decapitated by the translocations described in the section above or undergo mutation in translocated alleles [23,29].

Alternatively, the transactivation domain of MYC (encoded in exon 2) can acquire mutations that interfere with modulation of MYC activity by p107 [30,31]. Other mutations may stabilize MYC by decreasing proteasomal degradation.

Consequences of MYC overexpression — BL may be the fastest growing human cancer and MYC overexpression is believed to be responsible for many of the alterations that support the rapid growth of BL tumors cells [32]. Included among these MYC-dependent activities are:

●Increased glucose utilization and a switch to so-called Warburg metabolism, in which cells mainly rely on glycolysis for ATP production instead of oxidative phosphorylation.

●Increased glutamine metabolism, which creates metabolic intermediates that are needed for membrane biogenesis.

●Upregulation of rRNA expression and ribosome biogenesis, leading to increased protein synthesis.

Other genetic lesions — In addition to the presence of MYC translocations in the vast majority of BL cases, most tumors contain additional chromosomal abnormalities, the molecular implications of which are poorly understood [33]. Of interest, BL cases appear to have fewer additional genetic lesions than DLBCL or unclassifiable aggressive B cell lymphomas [34].

Other pathogenic mutations in BL have emerged from genomic sequencing:

●A study that reported whole genome sequencing of 101 tumors, which included all three categories of BL (ie, endemic, sporadic, immunodeficiency-associated) reported likely driver mutations in 72 genes; these alterations involved both coding and non-coding regions [35]. In vitro assays were used to confirm a functional role for a subset of these genes (eg, MYC, IGLL5, BACH2, ID3, and BTG2).

●In one study, high-throughput RNA sequencing and RNA interference screening identified recurrent mutations in components of the p53, cyclin D3/CDK6, PI(3) kinase, and tonic BCR signaling pathways [36]. Approximately 70 percent of sporadic BL cases had mutations in TCF3 or ID3. Mutations in CCND3 (a gene encoding cyclin D3, a key regulator of cell cycle progression) were present in 38 percent.

●In another study, whole genome sequencing of BL tumor tissue and germline tissue from the same individual identified mutations of potential interest that were further evaluated with exome sequencing of 59 BL cases and 94 DLBCL cases [37]. Mutations in ID3 were identified in 34 percent of BL, but were not present in DLBCL.

●In a third study, whole genome and transcriptome sequencing of four BL cases identified seven recurrently mutated genes [38]. Further evaluation of these genes in an extension cohort demonstrated ID3 mutations in 36 of 53 (68 percent) of patients with BL but only 6 of 47 (13 percent) of other B cell lymphomas with Ig-MYC translocation.

●The Burkitt Lymphoma Genome Sequencing Project employs whole genome sequencing coupled with transcriptome analyses in an effort to comprehensively characterize the genetics of BL [39]. This ongoing work has identified 22 genes with mutations that are deemed to be significant, as judged by their presence in multiple tumors and their likely disruptive effects on gene function. Newly identified mutated genes include DDX3X (which encodes a putative RNA helicase linked to cell cycle progression) and a number of chromatin factors, including chromatin remodeling factors (ARID1A), chromatin "writers" (KMT2D), and even histones (HIST1H1E). How perturbation of these factors contribute to BL pathobiology remains to be determined.

A common theme emerging from these studies is the frequent occurrence in BL of loss of function mutations in ID3 and gain of function mutations in TCF3, which encodes an E-box type transcription factor previously known as E2A that has important roles in lymphoid development. TCF3 appears to stimulate survival through the BCR/PI3K signaling pathway, which upregulates CCND3 (cyclin D3) [40]. Normally, TCF3 also induces its own inhibitor (ID3), but this negative feedback loop is broken in BL by mutations that activate TCF3 and inactivate ID3. TCF3 appears to augment B cell receptor signaling, thereby enhancing pro-survival signaling mediated through the PI-3K pathway [40].

Other tumors with MYC translocations — Other lymphoid tumors may have MYC translocations similar or identical to those found in BL, including diffuse large B cell lymphoma, multiple myeloma, and a subset of unclassifiable B cell lymphomas with some features of BL. In several studies of the latter tumors, cases with "Burkitt-like" features frequently had dual MYC and B cell leukemia/lymphoma 2 (BCL2) translocations (ie, "double hit" lymphomas) and were associated with an extremely poor clinical outcome [41,42]. Another study found that these "double hit" lymphomas could be distinguished from BL by their expression of BCL-2, expression of IRF4/MUM1, a Ki-67 staining fraction <95 percent, and the absence of Epstein-Barr virus [43]. Many of these cases may represent "transformed" follicular lymphomas, in which the MYC translocation is acquired as a secondary event. (See "Pathobiology of follicular lymphoma", section on 'Translocations involving BCL-2'.)

Other Burkitt-like tumors that lack MYC translocations — The 2016 World Health Organization (WHO) classification of lymphoid neoplasms includes the provisional entity Burkitt-like lymphoma with 11q aberration [1]. These tumors lack MYC translocations, tend to occur in children and young adults, and have a gene expression profile and an immunophenotype that resemble those seen in BL. However, Burkitt-like lymphoma with 11q aberration has lower levels of MYC expression and appears to lack mutations in TCF3 and ID3 [44], indicating that it is genetically distinct from BL. Other tumors resembling BL that lack MYC rearrangements and 11q aberrations are best considered variants of diffuse large B-cell lymphoma or high-grade B-cell lymphoma, not otherwise specified, per the WHO classification [1].

Infections

General — Chronic EBV infection appears to play a role in virtually all cases of endemic (African) BL and a minority of sporadic and immunodeficiency-associated BL. However, data suggest that EBV infection alone cannot account for the development of endemic BL and that the etiology is likely polymicrobial [45].

Patients with some other infections are more likely to have a persistent acute phase of initial EBV infection. This prolonged acute phase with its associated B cell expansion may increase the likelihood of acquisition of translocations involving the MYC locus. Proposed organisms associated with persistent EBV infection and BL development include HIV, malaria, and arboviruses [46]. The specific role of these organisms in the pathogenesis of BL remains to be determined.

Initial epidemiologic data demonstrated a high incidence of both Plasmodium falciparum malaria and endemic BL in equatorial Africa and Papua New Guinea [47,48]. Further evidence came from a study that demonstrated that, when compared with age-, sex-, and location-matched controls, cases of childhood endemic BL were more likely to have serologic evidence of a recent malaria infection (anti-HRP-II antibodies) and less likely to have serologic evidence of a long-term malaria infection (anti-SE36 antibodies) [49].

Plausible explanations directly linking falciparum malaria to BL have emerged from several studies. Isolation of tonsillar germinal center B cells from individuals who are chronically infected with P. falciparum reveal an increase in cells expressing activation-induced cytosine deaminase (AID), an enzyme that may cause DNA damage that can lead to chromosomal translocations, including MYC translocations. Furthermore, a higher incidence of EBV-positivity was observed in germinal center B cells isolated from individuals with chronic P. falciparum infections [50]. Mouse models using recurrent P. chabaudi infection demonstrated expansion of germinal center B cells expressing AID and development of aggressive lymphomas in a p53-deficient background [28].

Epstein-Barr virus (EBV) infection — Monoclonal EBV infection is present in virtually all cases of endemic BL, approximately 30 percent of sporadic BL, and 40 percent of immunodeficiency-associated BL. The pathobiologic role of EBV infection in the development of BL is poorly understood.

One hypothesis is that EBV infection stimulates B cell expansion during which process translocations may occur, leading to the overexpression of MYC. Once BL emerges, the EBV infection is thought to have little effect on tumor maintenance. Of interest, EBV infection in BL displays a particular latent infection phenotype characterized by the lack of expression of the EBV transforming proteins LMP-1 and EBNA-2. (See "Virology of Epstein-Barr virus", section on 'Transformation and latent proteins'.)

Whole-genome analysis of 106 cases of endemic and sporadic BL reported that, compared with EBV-negative tumors, EBV-positive tumors had a striking increase in somatic hypermutation and fewer driver mutations (eg, CCND3 mutations) [51]. The increase in somatic hypermutation in EBV-positive cases is consistent with prior work showing that EBV infection increases the expression of activation-induced cytosine deaminase in B cells [52]. These distinctions may also be related to different cells of origin for EBV-positive and EBV-negative BLs. (See 'Cell of origin' above.)

SUMMARY — Burkitt lymphoma (BL) encompasses a group of highly aggressive B cell neoplasms with three distinct clinical forms: endemic, sporadic, and immunodeficiency-associated. (See "Epidemiology, clinical manifestations, pathologic features, and diagnosis of Burkitt lymphoma", section on 'Clinical features'.)

●BL is derived from germinal or post-germinal center B cells. (See 'Cell of origin' above.)

●The development of BL is dependent on the constitutive expression of the MYC gene located at chromosome 8q24, which encodes the MYC transcription factor. This transcription factor modulates the expression of target genes that regulate many cellular processes, including cell growth, division, and death. Tumor cells maintain high expression of MYC through various mechanisms:

•Chromosomal translocations place the DNA coding sequences for MYC under the control of immunoglobulin gene enhancers that are constitutively active in mature B cells. The translocations include: t(8;14), t(2;8), and t(8;22). (See 'Chromosomal translocations' above.)

•5' regulatory regions normally present within the MYC DNA sequence are mutated or moved as a direct result of the translocation or other mutations of the MYC gene result in an increased half-life of its protein product. (See 'Other mechanisms' above.)

●How and why B cells acquire the chromosomal translocations and other genetic changes that result in the increased expression of MYC and the resultant development of BL remains incompletely understood, but the inherent genomic instability of germinal center B cells is very likely an important contributing factor. Chronic Epstein-Barr virus infection appears to play a role in virtually all cases of endemic (African) BL and a minority of sporadic and immunodeficiency-associated BL. (See 'Infections' above.)