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Prepublished online as a Blood First Edition Paper on September 12, 2002; DOI 10.1182/blood-2002-07-2009.
REVIEW ARTICLE
From the Lymphoma Group, Molecular Pathology Program,
Centro Nacional de Investigaciones Oncológicas (CNIO), Madrid,
Spain.
Disruption of the physiologic balance between cell proliferation
and death is a universal feature of all cancers. In general terms,
human B-cell lymphomas can be subdivided into 2 main groups, low- and
high-growth fraction lymphomas, according to the mechanisms through
which this imbalance is achieved. Most types of low-growth fraction
lymphomas are initiated by molecular events resulting in the inhibition
of apoptosis, such as translocations affecting BCL2, in
follicular lymphoma, or BCL10 and API2/MLT1, in
mucosa-associated lymphoid tissue (MALT) lymphomas. This
results in cell accumulation as a consequence of prolonged cell
survival. In contrast, high-growth fraction lymphomas are characterized
by an enhanced proliferative activity, as a result of the deregulation
of oncogenes with cell cycle regulatory functions, such as
BCL6, in large B-cell lymphoma, or c-myc, in
Burkitt lymphoma. Low- and high-growth fraction lymphomas are both able
to accumulate other alterations in cell cycle regulation, most
frequently involving tumor suppressor genes such as
p16INK4a, p53, and
p27KIP1. As a consequence, these tumors behave
as highly aggressive lymphomas. The simultaneous inactivation of
several of these regulators confers increased aggressivity and
proliferative advantage to tumoral cells. In this review we discuss our
current knowledge of the alterations in each of these pathways, with
special emphasis on the deregulation of cell cycle progression, in an
attempt to integrate the available information within a global model
that describes the contribution of these molecular changes to the
genesis and progression of B-cell lymphomas.
(Blood. 2003;101:1220-1235) Although studies in experimental models and
analyses of virus-induced cell transformation have shown that cell
cycle subversion is a key step in tumorigenesis, the available
information on human tumors has only recently reached the critical
threshold of knowledge that allows a reasonably clear understanding of
the mechanisms of cell cycle inactivation and their contribution to the
genesis and progression of human cancer. Here, we have chosen B-cell
lymphoproliferative lesions on the understanding that the accumulation
of data concerning alterations in specific key genes makes it possible
to propose a general model that describes the role of these specific
alterations in cell cycle regulators in the initiation and progression
of these tumors.
Lymphoma/leukemia is a group of different types of cancer of the
lymphoid system, in which numerous entities feature distinctive molecular alterations (Table 1).
The most frequent types of lymphoma are collectively denominated B-cell
lymphomas (BCLs), a term that encompasses different entities with
variable clinical behavior and diverse molecular features.
Nevertheless, in general terms, it is possible to segregate BCLs into 2 main groups, defined as low- and high-growth fraction lymphomas that
roughly overlap with previous definitions of low- and high-histologic
grade.
Low-growth fraction BCLs, including follicular lymphoma (FL), marginal
zone lymphomas (MZLs), mantle cell lymphoma (MCL), and B-cell chronic
lymphocytic leukemia (B-CLL), are distinguished by a relatively low
proliferative index, small cell size, formation of large tumoral masses
in lymph nodes, bone marrow or extranodal locations, and a paradoxical
combination of advanced clinical stages associated with low clinical
aggressivity (for a review see Seng and Peterson32 and
Capello and Gaidano33). This clinicopathologic presentation seems to be the final consequence of significant advantages to cell accumulation as a result of alterations in apoptosis
regulators rather than in cell cycle control genes. The situation is
the opposite of that commonly observed in high-growth fraction BCLs,
including large B-cell lymphoma (LBCL) and Burkitt lymphoma (BL), in
which an increased proliferative index and larger cell size are
associated with more frequently localized clinical stages but a higher
clinical aggressivity, as a consequence of alterations in cell cycle
regulators such as BCL6 or c-Myc that usually occur in combination with
the more common defects in apoptosis.
A fraction of both low- and high-growth fraction lymphomas are
characterized by the acquisition of additional alterations in cell
cycle control, usually involving cyclin-dependent kinase inhibitors
(CKIs), such as p16INK4a, p21CIP1, or
p27KIP1. These tumors, irrespective of their histologic
grade, show an increased clinical aggressivity and more frequent
treatment resistance.
Alterations in genes controlling apoptosis have been recognized in
most of these tumors, although the nature of the observed changes seems
to be more complex than initially expected. Apoptosis can be initiated
in cells by 2 alternative convergent pathways (reviewed in
Igney and Krammer34): the "extrinsic" pathway
involves death receptors, such as tumor necrosis factor (TNF)
receptors or FAS, whose binding to their ligands leads to the
activation of caspase-8. The mitochondrial or "intrinsic" pathway
is thought to be triggered by the translocation into mitochondria of
BCL2 family members, which causes alterations in mitochondrial
permeability and the consequent release of cytochrome c, which,
in association with apoptotic protease-activating factor-1
(APAF1), activates caspase-9 (Figure 1).
Caspases-8 and -9 (initiator caspases) both subsequently activate
caspases-3, -6, and -7 (executioner caspases), which, in turn, cleave
substrates involved in the regulation of cell death. Alterations in the
proteins involved in these pathways could lead to tumor initiation;
moreover, the suppression of apoptosis facilitates the accumulation of
further oncogenic lesions, eventually leading to uncontrolled
proliferation.
BCL2 protein family
Chromosomal translocations involving the BCL2 gene are the characteristic type of genetic alteration of follicular lymphoma. The translocation t(14;18)(q32;q21), which is present in the large majority of FL cases (70%-90%), places BCL2 under the control of the immunoglobulin heavy chain (IgH) Eµ enhancer, leading to high expression of the BCL2 protein.1-7 Chromosomal breakpoints occur in 3'-noncoding regions of BCL2 locus and cluster in 3 regions: the major breakpoint region (MBR) (50%-70% of translocations), the intermediate cluster region (icr), and the minor cluster region (mcr).8,9 Generally, close associations have been found between the presence of translocation and the expression of BCL2 protein, although in some cases BCL2 expression seems to be independent of translocations.35-37 The absence of BCL2 expression in some FL cases suggests that tumoral cells may acquire apoptosis inhibition through mechanisms other than that of BCL2 overexpression; for instance, an important role has been postulated for BCL-XL.38 In fact, BCL2 overexpression alone is apparently not sufficient to induce a malignant phenotype, because clonal BCL2-Ig rearrangement products are detectable in normal B cells.39 Moreover, in transgenic mice mimicking t(14;18), BCL2 overexpression results in prolonged B-cell survival without an increase in cell cycling; as a consequence, these mice are susceptible to developing autoimmune diseases and follicular hyperplasia40-43 that progresses to malignant large-cell lymphoma only after a long latency and through the acquisition of secondary alterations.42 Therefore, other oncogenic events in addition to BCL2 deregulation appear to be necessary in the genesis of FL. As a final step, when alterations in key cell cycle regulators, such as p53, p16INK4a, or c-myc, are added to the initial antiapoptotic lesions, FL undergoes progression to more aggressive variants.44-48 A paradigm of a neoplasm caused by a failure in the mechanisms regulating apoptosis without an increase in cell proliferation is provided by B-cell chronic lymphocytic leukemia. Data obtained by different experimental approaches has demonstrated that the nondividing cells that constitute the hallmark of this entity carry on an increased expression of the antiapoptotic proteins BCL2 (in virtually all cases) and MCL1 (in approximately one half of the cases)49-52 and show down-regulation of the proapoptotic gene BID, as has been observed in expression profiling experiments.50 NF-  B (nuclear factor B) refers to dimeric
transcription factors belonging to the REL family (for a review see
Karin and Lin53). Studies in mice defective in NF-B
activation suggest a role for this family in adaptive immunity,
inflammation, and lymphoid organ development.54 Another
function assigned to these transcription factors is the inhibition of
apoptosis: NF-B induces the expression of antiapoptotic factors such
as the inhibitor of apoptosis proteins (IAP),
c-FLIP, and TRAF1-2, as well as members of the
BCL2 family such as BCL-XL and
A1.53
Mammals express 5 REL proteins, encoded by the genes RELA,
RELB, c-REL, NFKB1, and
NFKB2, whose activity is regulated by specific inhibitors
(I The IAP gene family encodes a group of proteins involved in the suppression of programmed cell death, in addition to other cellular functions. Their ability to suppress apoptosis is mediated, at least in part, by their ability to block caspase activity. Some IAPs (XIAP, c-IAP1, c-IAP2) bind and directly inhibit certain caspases (-3, -7, and -9)65,66 and can act as E3-ligases, catalyzing the ubiquitination of the caspases they bind.67,68 IAPs may also have caspase-independent functions, such as cell cycle regulation. In mucosa-associated lymphoid tissue (MALT) lymphomas, the most common translocation is t(11;18)(q21;q21), which occurs in 30% to 50% of these lymphomas.12-17 The genes involved in this translocation are API2 (c-IAP2) on 11q21, a member of the IAP family with caspase-inhibitory functions, and MLT1 (on 18q21), encoding the human paracaspase (a protein with certain sequence similarity to caspases whose precise function is not fully understood).69 The translocation results in a chimeric transcript encoding an API2/MLT1 fusion protein, whose action may lead to increased inhibition of apoptosis and, therefore, confer a survival advantage on tumoral cells.70 BCL10, an apoptosis-regulatory molecule containing an amino-terminal
caspase-recruitment domain, is overexpressed as a result of
t(1;14)(p22;q32), a translocation that is also recurrently detected in
a group of MALT lymphomas.10,11 In normal cells, BCL10
weakly promotes apoptosis, induces NF- Death receptors: FAS FAS (CD95, Apo-1) is a death receptor belonging to the TNF-receptor family. The binding of FAS to its physiologic ligand (FASL) leads to the assembly of a death-inducing signaling complex, which consists of trimerized FAS, Fas-associated via death domain (FADD), and procaspase-8. When recruited to this complex, caspase-8 is activated by autocleavage.FAS-induced apoptosis mediates the process of negative selection of B cells within the germinal center. Additionally, it has been suggested that FAS acts as a tumor suppressor gene. Approximately 20% of BCLs derived from germinal center or postgerminal center B cells (FL, MALT, LBCL, MM, HL) have somatic mutations in the FAS gene,78-82 which are especially frequent in extranodal lymphomas. Most of these mutations are deleterious and mainly affect the death domain, giving rise to mutant proteins that may behave as dominant-negative forms. In addition, germline mutations of FAS are associated with BCLs in both mice and humans. The mouse strain MRL/lpr-lpr, harboring an inactivating mutation in the FAS gene, develops lymphadenopathy and is prone to autoimmunity and the development of BCL.83 Germline mutations of FAS in humans lead to the autoimmune lymphoproliferative syndrome, which is characterized by symptoms that are similar to those in MRL/lpr-lpr mice and by a high predisposition to the development of BCL.84 FAS mutations have not been detected in BCLs derived from immature or pregerminal center B cells (B-cell acute lymphoblastic leukemia [ALL], MCL, B-CLL).79 However, the FAS apoptotic pathway might be disrupted in these lymphomas through different mechanisms. B-CLL cells show down-regulation of FAS and are resistant to FAS-induced apoptosis85,86; this may contribute to the extended in vivo survival of B-CLL cells. Similarly, MCL cells express low or null levels of FAS and high levels of CD40, thereby favoring the CD40 cell survival pathway, and exogenous FAS ligand has no effect on MCL cells.87 Other alterations observed in low-growth fraction lymphomas The hallmark of mantle cell lymphoma is t(11;14)(q13;q32); this translocation juxtaposes the cyclin D1 gene with an IgH enhancer,18,19 resulting in up-regulation of cyclin D1. Virtually 100% of MCLs overexpress cyclin D1, in most cases as a consequence of a t(11;14).20,21 Despite the role of cyclin D1 as a positive regulator of G1-phase progression, cyclin D1 expression in MCL is not related to proliferative activity; moreover, overexpression of cyclin D1 in Eµ-cyclin D1 mice is not sufficient to promote malignant transformation, although it strongly cooperates with other oncogenes such as myc alleles.88 These observations imply that additional alterations in MCL development are needed, although the precise nature of these molecular changes has not been completely defined. Gene expression profiling in MCL has suggested the existence of a broad spectrum of alterations in pathways regulating apoptosis, including the mitochondrial apoptotic pathway (up-regulation of BCL2, down-regulation of cytochrome C1 and caspase-9) and death-receptor signaling pathways (down-regulation of FADD, DAXX, and RAIDD).89 A different study found down-regulation of FAS and resistance to FAS-induced apoptosis in MCL cells (see "Death receptors: FAS").87 At the same time, several genes stimulating cell cycle progression were found to be up-regulated (CDK4, MDM2, SKP2, myc, E2F5/DP2, some growth factors and their receptors).89 Ataxia telangiectasia mutated gene (ATM), a kinase involved in p53 activation in response to DNA damage, is inactivated by mutation in a significant proportion of MCLs.90 A subset of MCL has been shown to accumulate alterations in p16INK4a and p53, leading to the generation of blastoid variants characterized by higher proliferative activity and more aggressive clinical behavior (see "Alterations in the Rb pathway in lymphomas").A subset of MM also features the t(11;14) translocation and displays overexpression of cyclin D1.91 In addition, cyclin D1 expression is frequent in hairy cell leukemia (HCL), although it is often weaker than it is in MCL, and it appears to be unrelated to translocations or amplification of the cyclin D1 gene.92 Alterations in the ATM gene (loss of heterozygosity [LOH] and mutations, ~15% each) have also been described in a group of B-CLL cases and are associated with impaired p53 function and resistance to the induction of apoptosis93-97; p53 itself is mutated in a similar percentage of B-CLL tumors.98 p53 dysfunction because of p53 or ATM mutation is an adverse prognostic factor in B-CLL99 and is independent of known predictors of poor outcome (CD38 expression and absence of IgVH mutations100). Interestingly, p53 dysfunction occurs exclusively in the subset of B-CLL with unmutated IgVH.99 PAX5, a member of the paired-box gene family, encodes a B-cell-specific transcription factor that regulates B-cell genes (CD19, Ig genes) and plays a key role in B-lymphoid differentiation. In addition, PAX5 stimulates B-cell proliferation and is able to transcriptionally repress p53 in vitro.101 PAX5 is juxtaposed with the IgH locus by the t(9;14)(p13;q32) translocation, which is characteristic of a subset of LPLs23,24 and has rarely been detected in other BCLs such as splenic marginal zone lymphoma (SMZL).102 The inappropriately increased expression of PAX5 in terminally differentiated B cells, in which this gene is normally down-regulated, may contribute to the pathogenesis of LPL.
In the case of lymphomas with a high-growth fraction, the most common chromosomal translocations seem to affect genes involved in proliferation control such as BCL6, in LBCL, or c-myc, in BL. Contribution of BCL6 to lymphomagenesis BCL6 acts as a multifunctional regulator involved in cell cycle control and other critical cell functions such as lymphocyte differentiation and immune response (Figure 2). It was initially identified because of its involvement in chromosomal translocations affecting 3q27, present in a group of FLs and LBCLs.103 Although BCL6 mRNA can be detected in different cell types,104,105 the protein seems to be expressed at higher levels in lymphocytes. Therefore, the oncogenic properties of BCL6 have mainly been explored in the lymphoid system, where it is expressed by B and T cells in the germinal center, isolated cells in the lymph node interfollicular area, and cortical thymocytes.106-108 Knockout mice studies have demonstrated that BCL6 is required for germinal center formation and T-cell-dependent antigen responses.109 The selective expression of BCL6 by germinal center B cells has led to its use as a surrogate marker for malignancies derived from the germinal center; thus, BCL6 is expressed in those tumors assumed to arise from follicular center cells, such as FL (almost 100%), BL (100%), a majority of LBCLs (> 80%), and nodular lymphocyte predominance-Hodgkin lymphoma (NLP-HL) (> 80%).106-108 In general terms, BCL6 expression is a finding associated with cells with a high proliferation index. Thus, BCL6 is not expressed by low-growth fraction lymphomas, with the exception of FL, in which BCL6 is detected in proliferating cells and down-regulated in the interfollicular compartment, where the growth fraction is also diminished. A similar exception is provided by NLP-HL, in which proliferating tumoral cells are distinguished by BCL6 expression. In general terms, the pattern of expression of BCL6 in normal and tumoral lymphoid tissue is the opposite of that of p27KIP1; thus, it seems that BCL6, probably as a consequence of its ability to down-regulate p27KIP1 expression,110 is a key marker in B cells leading to proliferation (Figure 3).
BCL6 rearrangements (chromosomal translocations involving the BCL6 5'-noncoding region in 3q27) occur at a frequency of 20% to 40% in large B-cell lymphoma and 6% to 14% in FL.25-27 In contrast, rearrangements of BCL6 (as well as those affecting BCL2 or c-myc) are very rare in primary mediastinal B-cell lymphoma, which suggests that this tumor is a separate type of LBCL arising from different pathogenetic mechanisms.111 These translocations contribute to BCL6 deregulation; however, we know that the level of BCL6 expression seems to depend on the partner involved in the translocation, because tumors in which BCL6 is translocated to an Ig locus express higher levels of BCL6 mRNA than those with non-Ig/BCL6 translocation, and also that overexpression of BCL6 is often independent of chromosomal translocations.112 In this respect, it has recently been claimed that somatic mutations involving the 5'-noncoding region of the BCL6 gene can contribute to the deregulation of BCL6 expression.113 In LBCL, somatic mutations in the major mutational cluster have been described as playing a role in the regulation of BCL6 expression. Mutations within the 423 to 443 hotspot in the first intron of BCL6 are associated with increased protein levels, suggesting that this region may include a regulatory element whose mutation may modify the binding of hypothetical BCL6-regulating transcription factors. The BCL6 gene encodes a sequence-specific transcriptional repressor.114-117 Expression profiling of B-cell lines in which BCL6 was positively or negatively modulated revealed that BCL6 represses a group of genes involved in B-cell activation and terminal differentiation (such as Blimp-1) and cell cycle control (cyclin D2, p27KIP1), and it has been hypothesized that this indirectly induces c-myc expression.110 Therefore, the lack of germinal center formation, as observed in BCL6 knockout mice,109 could be at least partially a consequence of the absence of the proliferative signal that is dependent on BCL6. Blimp-1 (B-lymphocyte-induced maturation protein 1) is a transcriptional repressor with a key role in the differentiation of B cells into plasma cells.118 One of the targets of Blimp-1 repression is c-myc.119 In vitro studies have shown that inhibition of BCL6 function induces Blimp-1 and down-regulates c-myc expression, causing cell cycle arrest in G1.110 Conversely, it might be hypothesized that overexpression of BCL6 because of translocations or somatic mutations could indirectly activate c-myc through repression of Blimp-1 by BCL6, although this has not been formally demonstrated in human tumors. The altered expression of BCL6 could also repress genes involved in apoptosis, thereby favoring neoplastic cell growth. One candidate target is PDCD2,120 previously found to be associated with programmed cell death in thymocytes. However, the role of BCL6 in apoptosis is controversial because the report cited earlier, and others, suggest that BCL6 may be an inhibitor of apoptosis,121,122 whereas other studies suggest that it induces cell death. For example, it has been proposed that AFX (a forkhead transcription factor) activates apoptosis by induction of BCL6, which in turn represses BCL-XL.123 This dual role of BCL6 in the regulation of apoptosis could be dependent on factors such as cell type or level of expression. BCL6 has also been identified as a potent inhibitor of senescence in murine embryo fibroblasts and primary B cells, making cells unresponsive to negative proliferation control by the p19ARF-p53 pathway through a mechanism mediated by up-regulation of cyclin D1.124 This finding may be potentially relevant, but we need to bear in mind that cyclin D1 expression in B cells is mainly restricted to MCL. The prognostic significance of 3q27 translocation and BCL6 expression has been analyzed in different series. BCL6 rearrangement has been described as an independent marker of favorable clinical outcome in LBCL.26 The partner in BCL6 translocation is predictive of survival, because non-Ig/BCL6 translocations are an indicator of poor prognosis in LBCL compared with Ig/BCL6.125 Most studies coincide in showing that a high level of BCL6 expression, as determined by real-time polymerase chain reaction (PCR) or immunohistochemistry, is a favorable prognostic marker. These observations may be linked by the finding that non-Ig/BCL6 translocations lead to much lower BCL6 mRNA levels than do Ig/BCL6 translocations.112 Mutations in the 423 to 443 cluster, which are associated with high BCL6 levels, are also associated with a better clinical outcome.113 The increased expression of BCL6 was one of the features that made it possible to define a subset of germinal center B-cell-like LBCLs, characterized by lower aggressivity, increased frequency of c-REL amplification, and translocations involving the BCL2 gene.57,126,127 Thus, Rosenwald et al57 have recently identified 3 different LBCL subgroups (germinal center B-cell-like, activated B-cell-like, and type 3 LBCL) through the use of gene expression profiling. The different clinical behavior of the germinal center B-cell-like group and the fact that some molecular alterations appear to occur exclusively in this concrete subset may support the idea that this LBCL subgroup actually represents a distinct entity with specific mechanisms of transformation. The c-Myc transcription factor c-Myc, a helix-loop-helix leucine zipper transcription factor, has been proposed as being involved in multiple cellular functions such as cell cycle regulation, apoptosis, cell growth, metabolism, cell adhesion, and differentiation. However, much of the information available is still controversial, and, to some extent, the precise involvement of c-Myc in these functions might be cell type- and context-dependent (reviewed in Eisenman,128 Boxer and Dang,28 Dang,129 and Amati et al130). The ability of c-Myc to bind to specific DNA sequences (E-boxes) and activate transcription requires heterodimerization with Max. Max also heterodimerizes with other Myc family proteins, such as Mad or Mxi1; Mad-Max dimers mediate transcriptional repression in E-boxes and might antagonize the activity of Myc-Max, at least in some promoters.131,132 Myc-Max complexes can also exert repressing activity on initiator (Inr) elements, in conjunction with Miz-1.133Myc promotes proliferation through the induction of genes involved in cell cycle control such as CDK4, Id2, CDC25A, and cyclins D1, D2, A, and E. Additionally, Myc suppresses growth-arresting genes such as p15INK4b, p21CIP1, GADD45, or GAS1.128-130,133-135 p27KIP1 is negatively regulated by Myc at different levels, including transcriptional repression,136 the increase of its degradation, and its sequestration by CDK4/cyclin D2 (Figure 2). c-Myc also induces cellular immortalization, a function that may be mediated by induction of the catalytic subunit of telomerase.137 Overexpression of c-Myc in primary cells results in cell cycle arrest or apoptosis, in contrast to what happens in immortalized cells. High levels of expression of c-Myc result in induction of p19ARF, which sequesters MDM2, leading to activation of p53-mediated arrest or apoptosis.138 Other targets that contribute to Myc-mediated apoptosis may be FAS, FASL, and BAX.28 The ability of Myc to induce apoptosis explains why, in transgenic mice, tumors overexpressing Myc frequently select for alterations that neutralize its apoptotic program through inactivation of the p19ARF/p53 pathway139 or BCL2 overexpression.140 The c-myc gene is subject to complex transcriptional and
posttranscriptional regulation. Four promoters have been identified, although one of them (P2) usually contributes to 80% to 90% of c-myc RNA in normal cells.28 Expression of
c-myc is positively regulated by multiple factors, such as
BCL6,110 epidermal growth factor (EGF),
platelet-derived growth factor (PDGF), and interleukin 7 (IL-7),141 and negatively controlled by
Blimp-1,119 transforming growth factor The c-myc gene is translocated to the Ig loci in
virtually all Burkitt lymphomas.28-31 The most frequent
translocation, c-myc/IgH (t(8;14)(q24;q32)), is detected in
about 80% of BLs. Each of the variant translocations, t(2;8)(p12:q24)
and t(8;22)(q24;q11), involving IgK and IgL,
respectively, occur at a frequency of approximately 10%. These or
other translocations involving c-myc have also been detected
in LBCL (6%)143 and other hematologic malignancies, such
as T-cell ALL and MM. These translocations are believed to place
c-myc under the control of an intronic and/or 3'
Ig enhancer, leading to a deregulated expression from the
translocated allele (the normal allele remains silent). This expression
could be explained by a combination of events, including a switch from
P2 to P1 promoters and the release of the block on transcription
elongation. NF- Most translocated c-myc alleles in BL are subjected to somatic hypermutation.145 Mutations tend to cluster in either the exon 1-intron 1 region, where they may cancel the block on transcription elongation, or the transactivation domain (Thr58 is a hot spot), possibly interfering with phosphorylation and thereby contributing to the stabilization of the protein. Similarly distributed somatic mutations were detected in LBCL (32%).146 Deregulation of c-myc by alternative mechanisms, such as
amplification, occurs in diverse types of tumors and has been
implicated in the pathogenesis of these diseases; in LBCL
c-myc amplification has been found in up to 16% of
cases.56 Increased expression or activation of upstream
c-myc regulators (NF- Alterations in c-myc seem to constitute one of the primary events in malignant transformation in BL, but they could also be a secondary event in tumor progression. In fact, translocations of c-myc have been observed in transformed FL.47,48 According to experimental models, overexpression of Myc alone may not be sufficient to induce cellular transformation. Eµ-myc mice develop clonal B-cell malignancies (not comparable with BL, however) that usually accumulate alterations in the p19ARF/p53 pathway or BCL2, thereby allowing the cancellation of Myc-induced apoptosis.139 This mimics the situation observed in samples of progressed FL and aggressive B-cell lymphoma with double c-myc and BCL2 translocation, in which simultaneous BCL2 and c-Myc expression gives a critical advantage to the tumoral cells.47,48 There are few conclusive studies regarding the relationship between c-Myc and clinical or biologic variables in lymphomas, and there is some variability in the findings among these reports concerning protein levels, cell distribution, and subcellular localization. Nevertheless, these analyses seem to coincide in showing that c-Myc overexpression is common in aggressive lymphomas and appears to be related to advanced clinical stage, high International Prognostic Index, or shorter overall survival.147-149
Both low- and high-growth fraction lymphomas can evolve to highly aggressive tumors; the process takes place through the acquisition of alterations in the major tumor suppressor pathways, most frequently affecting CKI. In fact, inactivation of CKI is a marker of highly aggressive lymphomas, irrespective of their histologic grade. CKI inactivation takes place in a concerted manner to confer a proliferative advantage to tumoral cells and, at the same time, determines increased treatment resistance. The G1/S transition of the cell cycle is regulated by 3 main pathways that might be altered in virtually 100% of aggressive tumors and that are represented by the proteins p16INK4a-cyclin D-CDK4-Rb, p14ARF-MDM2-p53-p21CIP1, and p27KIP1-cyclin E-CDK2 (reviewed in Sherr150). Alterations in the Rb pathway in lymphomas In normal cells, the transition through the restriction point is negatively regulated by the retinoblastoma (Rb) protein (Figure 4A). Rb binds to transcription factors (E2Fs) whose activity is necessary for the expression of S-phase genes, and it assembles a transcriptionally repressing complex that keeps E2F-responsive promoters inactive. Under conditions of mitogenic stimulation, accumulation of D-type cyclins allows the formation of active CDK4/6-cyclin D complexes, which phosphorylate and inactivate Rb, thus promoting E2F-mediated transcription and subsequent progression through the cell cycle. An additional level of regulation is provided by the INK4 (inhibitors of cdk4) family of CKI (p16INK4a, p15INK4b, p18INK4c, p19INK4d). These 4 homologous proteins inhibit the kinase activity of CDK4/6 by preventing its interaction with the activating cyclins D. The integrity of this regulatory cascade, termed the "Rb pathway" (INK4-CDK4,6/cyclin D-Rb-E2F), appears to be compromised in most human cancers. This inactivation can be achieved by an increased activity of CDK-cyclin complexes and/or by inactivation of the tumor suppressor genes INK4a or Rb.
The distribution of Rb protein in reactive lymphoid tissue follows a pattern that is parallel to that of Ki67 (a proliferation marker), with preferential expression in most germinal center B cells and cortical thymocytes.151 This normal regulation of Rb in relation to proliferative activity is frequently preserved in different types of lymphoma, in which genetic alterations of the Rb gene appear to be relatively rare.152-154 Levels of Rb are low in low-growth fraction BCLs, whereas they are higher in most high-growth fraction tumors.151,155 NLP-HL also displays this direct correlation between Rb and proliferation,156 and in blastoid MCL the expression and phosphorylation level of Rb are higher than in classical types.152 A deviation from this pattern, consisting of partial or total loss of Rb expression in highly proliferative tumors, was detected in a group of high-growth fraction non-Hodgkin lymphomas (NHLs)151,157 and classical HL156; this may be, at least in part, a consequence of genetic alterations, which have been found at low frequencies in different lymphoid malignancies. Loss of Rb was found to be an adverse prognostic factor in LBCL, in which there was a correspondence between high Rb protein levels and extended overall survival time,158 and in HL, in which Rb loss was related to failure to achieve complete remission159; monoallelic deletions of Rb predict poor outcome in MM.160 Notably, the Rb-like p130 protein is mutated in most cases of endemic BL.161 Of the 3 D-type cyclins (D1, D2, D3), cyclin D3 is the most ubiquitously expressed in lymphoid tissue. It is detected mainly in proliferating lymphocytes, either in germinal centers or interfollicular areas.162,163 Several observations point to a possible oncogenic role of cyclin D3 in lymphomagenesis. Immunohistochemical analysis of aggressive BCL revealed that a significant number of these tumors overexpressed cyclin D3, a finding that was more frequent in BL.162 Recent work points to an association between high levels of cyclin D3 expression and poor clinical outcome in LBCL.164 It is interesting to note that, in addition to the direct activation of CDK4/6, cyclin D3 appears to play another role in the promotion of cell proliferation, ie, the indirect activation of CDK2-cyclin E achieved by the sequestration of p27KIP1 (see "p27KIP1 alterations in lymphomas"). Recent data suggest that cyclin D3 overexpression may have a genetic basis in some cases, because t(6;14)(p21;q32) translocations involving cyclin D3 and IgH genes have been detected in several hematologic malignancies (myeloma, plasma cell leukemia, LBCL, SMZL) and result in the deregulated expression of cyclin D3.165 In contrast, the level of cyclin D2 expression is low in normal and tumoral lymphoid tissue (except in B-CLL and LPL in which it is often overexpressed166), and cyclin D1 is virtually absent167 (with the exception of MCL, HCL, and a subset of MM; see "Other alterations observed in low-growth fraction lymphomas"). Molecular studies concerning the status of CDK4/6 in lymphomas are scarce; amplification of the CDK4 gene has been identified in 11% of LBCLs,56 and translocations affecting CDK6 (t(7;14) or t(2;7)) have been described in splenic lymphoma with villous lymphocytes.168 Despite the fact that the INK4 proteins share structural and functional homology, only p16INK4a has gained credence as a bona fide tumor suppressor gene in vivo. The p16INK4a and p15INK4b genes are located in 9p21, which is, after p53, the most frequently altered locus in human cancer and which also encodes p14ARF; in contrast, p18INK4c and p19INK4d are very rarely altered in human tumors, including lymphomas.169,170 In reactive lymphoid tissue, p16INK4a protein is detected
in all compartments, with a slight increase in the most proliferative ones (germinal centers, interfollicular areas) and a decrease in the
mantle zone, which mainly consists of resting
lymphocytes.171 Low-growth fraction BCLs display normal
p16INK4a expression by small- and medium-sized tumoral
cells. There is a dramatic difference in high-growth fraction BCLs,
more than 50% of which exhibit partial or total loss of expression of
p16INK4a protein by tumoral cells. This finding is often
associated with alterations in the p16INK4a gene
(Table 2). Thus, although
p16INK4a alterations are rare in low-growth
fraction BCLs (< 5%), genetic or epigenetic inactivation of
p16INK4a occurs in 30% to 50% of high-growth
fraction BCLs,173,178,179,183,184 predominantly by 9p21
deletion/LOH or promoter hypermethylation (the most frequent mechanism
in LBCL); point mutations of p16INK4a are
infrequent. It has been shown that p15INK4b is
either frequently codeleted with p16INK4a or
hypermethylated at variable frequency and that
p15INK4b hypermethylation can occur
independently of that of
p16INK4a.172,181,185 Inactivation
of p16INK4a may also play a role in the development of HL,
because many HL cases show selective loss of p16INK4a
expression by Reed-Sternberg (RS) cells; hypermethylation of the
p16INK4a promoter appears to be the underlying
molecular mechanism in most of these tumors.182
There is ample evidence supporting a role for
p16INK4a inactivation in the transformation of
low-growth fraction lymphomas into their aggressive variants. The
histologic progression of FL to aggressive large cell lymphoma is
frequently accompanied by 9p21 deletions (absent in the low-grade
tumor) that often result in reduction or loss of p16INK4a
expression.44 In MCL, p16INK4a
deletions are associated with aggressive variants,174 a
finding that interestingly suggests the possibility that cyclin D1
overexpression and p16INK4a inactivation might
not be completely redundant alterations. In composite tumors (lymphomas
consisting of a low-grade The relatively high frequency of p16INK4a/Rb
alterations Alterations in the p53 pathway in lymphomas p53 is a transcription factor that induces the expression of genes involved in cell cycle arrest or apoptosis in response to a variety of toxic or oncogenic stimuli, the final cellular response depending on the biologic context. Negative regulation of cell cycle progression by p53 is partially mediated by induction of p21CIP1, a CKI of the CIP/KIP family that neutralizes the kinase activity of CDK2-cyclin E complexes. One of the target genes of p53 is MDM2, which inhibits p53 and targets it for degradation by virtue of its E3-ubiquitin ligase activity. In response to DNA damage or hyperproliferative stimulation, the p53-MDM2 interaction is disrupted by phosphorylation or antagonized by p14ARF induction, respectively, resulting in the activation of p53.150 This "p53 pathway" is also frequently altered in most human cancers, either by inactivation of the tumor suppressor genes p14ARF or p53 or by hyperactivation of the MDM2 oncogene. In fact, p53 is the most frequently mutated gene in human cancer (> 50% of all cases).Mutations of p53 are detected in 17% to 25% of cases
of LBCL and BL.178,180,186-190 This frequency is
relatively low compared with other types of cancer but is still
significantly higher than that observed in most low-growth fraction
BCLs, which supports the hypothesis that p53 inactivation is
one of the events leading to high-grade progression (Table
3). Mutations of p53 in
lymphomas tend to be clustered in conserved regions that are part of
the sequence encoding the DNA binding domain. In aggressive BCL, the inability of mutant p53 to transactivate its target genes is reflected by the p53+p21
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Abstract
Introduction
, have
the ability to activate either cell survival (through IAP proteins,
components of the TNFR complex) or apoptosis. The intrinsic or
mitochondrial apoptotic pathway is controlled by BCL2 family members,
including proapoptotic BID (activated by caspase-8) or BAX, or
antiapoptotic proteins, such as BCL2, MCL1, and BCL-XL.
Activation of the mitochondrial pathway involves release of cytochrome
c from mitochondria and its association with APAF-1 and procaspase-9 to
form the apoptosome. Caspase-9 is activated within this complex. IAP
(inhibitor of apoptosis proteins) are, in turn, inhibited by second
mitochondria-derived activator of caspase (SMAC). The
extrinsic and intrinsic pathways are connected via BID and caspase-3.
Some of the receptors also exert prosurvival functions through NF-
(TGF
FL or MALT
immunophenotype in contrast
with the p53