Ambivalent Role of BCL6 in Cell Survival and Transformation

Olivier Albagli-Curiel*

CNRS UPR 1983, Institut Andre´ Lwoff, 7 rue Guy Moˆquet, BP8, 94801 Villejuif, France

The BCL6 gene is often structurally altered and probably sequence in vivo (Figure 1) (for a recent review on ‘misregulated’ in two different types of human B-cell non- transcriptional repression by BCL6, see Dent et al., Hodgkin lymphomas (BNHL) thought to arise from 2002). It does so in part by recruiting, directly and germinal centre B cells. BCL6 encodes a BTB/POZ and indirectly, class I and II histone deacetylases (HDACs) zinc finger protein whose biochemical properties support a (Dent et al., 2002; Lemercier et al., 2002). Despite these role as a DNA-binding transcriptional repressor and important findings, its cellular function, including how, disclose, in part, the underlying mechanisms. In contrast, and even whether, it contributes to lymphomagenesis the study of the ‘oncogenic’ structural alterations of BCL6 remains unclear. Indeed, the many biological activities in BNHL and of its cellular functions gives rise to much reported for BCL6 appear highly diverse and even more heterogeneous data with no obvious unifying picture sometimes contradictory at first glance. In this review, so that how and even whether BCL6 contributes to the results regarding the cellular functions of BCL6 will lymphomagenesis remains unclear. This review will be summarized and an attempt will be made to reconcile summarize the current knowledge about the ‘oncogenic’ these functions beyond their diversity, and to under- alterations and cellular functions of BCL6 and, based on stand the possible reasons of their apparent discrepan- some results, will propose the following hypotheses: (1) In cies. Moreover, bearing in mind the structural various systems, including in memory T cells and also in alterations of BCL6 in BNHL, this overview will germinal centre B cells and possibly in certain postmitotic highlight what these results bring to the understanding cells, BCL6 may act by stabilizing a particular stage of of the possible relationship between BCL6 and human differentiation. (2) Both its ambivalent effects on cell oncogenesis. survival and the heterogeneous consequences of its alterations in BNHL suggest that BCL6 can be oncogenic not only upon overexpression or persistent expression, as often proposed, but also, similar to some of its relatives, Alterations of the BCL6 50 noncoding region in normal upon ‘accidental’ downregulation. and transformed B cells Oncogene (2003) 22, 507–516. doi:10.1038/sj.onc.1206152 The BCL6 gene is originally linked to human oncogen- Keywords: BCL6; B-cell lymphoma; Germinal centre; esis. It has been cloned because it maps to a major cell survival; apoptosis translocation breakpoint cluster affecting the 3q27 chromosomal region in two types of BNHL: follicular lymphomas (FL) and diffuse large B-cell lymphomas (DLCL), which both presumably derive from germinal Introduction centre (GC) B cells (Kerckaert et al., 1993; Lo Coco et al et al et al The BCL6 gene, also known as BCL5 and LAZ3, was ., 1994; Ye ., 1993, 1997; Miki ., 1994; Ohno and Fukuhara, 1997; Alizadeh et al., 2000; Lossos and initially cloned by virtue of its disruption in certain types Levy, 2000). As a result of numerous 3q27 transloca- of human B-non an Hodgkin lymphomas (BNHL). It tions, BCL6 fuses to one of at least 20 ‘partner’ loci, encodes one of the most studied so-called ‘POK’ including three Ig gene loci, which generally gives rise to proteins, harbouring N-terminal POZ (or BTB) domain a chimeric X-BCL6 transcript (Chen et al., 1998b; that mediates protein/protein interactions including self- Galiegue-Zouitina et al., 1999; Yoshida et al., 1999a; interactions, and a varying number (six in the case of Ueda et al., 2002a). Importantly, the breakpoints spare BCL6) of C-terminal Kru¨ppel-like C2H2-type zinc the BCL6 coding region, which presumably remains fingers. Like many other POK proteins, BCL6 is able unchanged, as they often are clustered in a conserved to bind DNA through its C-terminal zinc fingers 4 kb region – called MTC or MBR (major translocation (thereafter referred to as BCL6DBD, DNA-binding- cluster/major breakpoint region) – spanning the BCL6 domain) with a strong affinity for a particular sequence promoter, its first noncoding exon and the intronic in vitro, and represses transcription when bound to this region immediately downstream (Kerckaert et al., 1993; Ye et al., 1993; Otsuki et al., 1995). In addition, in some *Correspondence: O Albagli-Curiel; E-mail: [email protected] or [email protected] cases of BNHL but independent of the translocations, Received 16 July 2002; revised 11 October 2002; accepted 21 the MTC/MBR region undergoes ‘large’ deletions, October 2002 which overlap in a short region of the 50 side of the Ambivalent role of BCL6 O Albagli-Curiel 508

Figure 1 The BCL6 protein and its functional domains. Several functional domains of BCL6 mediate protein/protein interactions, transcriptional repression, post-translational regulation and specific binding to DNA. * The binding to SMRT, NcoR and BcoR involves mostly a conserved charged pocket spanning aa 25–54. **The isolated BTB/POZ domain of BCL6 is not sufficient to bind PLZF or LRF but may, however, contribute to their interaction in cooperation with other domain(s), including the DBD region. *** Evi9 has been shown to bind in vitro to a BCL6 fragment spanning amino acids 1–180, which is slightly longer than the BTB/POZ domain. ****Binding to mSIN3A has been demonstrated using yeast two-hybrid assays and requires the N-terminus of this region (aa 191–264). Another subdomain (aa 370–386) completely conserved in the highly related BAZF protein also appears essential for the repressive activity of this region, and this activity is counteracted by the HDAC inhibitor, trichostatin A. Note that the IRF4 transcription factor, which is able to interact with – and is inhibited by – BCL6, is not mentioned here, as the interacting domains are not mapped on BCL6 (Gupta et al., 2001). BTB/POZ: Bric a` brac, Tramtrack, Broad complex/Pox virus and Zinc finger. DBD region: DNA-binding region. HDACs: histone deacetylases. (References: Dhordain et al., 1995, 1997, 1998, 2000; Chang et al., 1996; Niu et al., 1998; Okabe et al., 1998; Davies et al., 1999; Huynh et al., 2000; Nakamura et al., 2000; Zhang et al., 2001; Dent et al., 2002; Lemercier et al., 2002; Melnick et al., 2002; Vasanwala et al., 2002)

first intron, while the other copy of BCL6 may also be lucci et al., 2001). Thus, the high proportion of BNHL altered or lost (Nakamura et al., 1996; Bernardin et al., cases with ‘microalterations’ in BCL6 reflects the 1997). Overall, a lesion of BCL6 revealed by a frequent GC or post-GC origin of these tumours and rearrangement in MTC/MBR occurs in 5–15% of FL not their malignant nature. However, it remains possible and 25–35% of DLCL, and is therefore one of the most that a subset of these ‘microalterations’ alters BCL6 frequent genetic abnormalities in BNHL (Ohno and regulation and hence may somehow bias the fate of GC Fukuhara, 1997). However, a 3q27 translocation does B cells, possibly toward lymphomagenesis (Zan et al., not necessarily mean a BCL6 structural alteration: other 2000). Accordingly, it was reported that FL transforma- breakpoints exist, such as in ABR (alternative break- tion into aggressive DLCL is sometimes associated with point region) located 200–270 kb telomeric to MTC/ the accumulation of new point mutations in the 50 non- MBR, and therefore outside the defined BCL6 locus coding region of BCL6 (Lossos et al., 2001) (Figure 2). (Ohno and Fukuhara, 1997; Chen et al., 1998a). In addition, human BCL6 is also frequently subject to multiple somatic point mutations and, more rarely, microdeletions (up to 8 bp) in its first exon and in a BCL6 ‘misregulation’ and lymphomagenesis 700 bp region – termed MMC for major mutations cluster – again in the 50 side of its first intron (Migliazza The incidence and nature of BCL6 alterations suggest a et al., 1995). Although located in a subdomain of MTC/ causal implication of its ‘misregulation’ in BNHL. MBR, these ‘microalterations’, which often affect both BCL6 undergoes two changes upon translocations: its BCL6 alleles, are independent of lymphomagenesis, as 50 untranslated region is altered (which is also caused by they also occur in vivo in normal GC B cells and are deletions and ‘microalterations’), and it is placed under induced upon appropriate stimuli in a model of GC B- the control of a heterologous promoter (Ye et al., 1995; cell differentiation in vitro (Pasqualucci et al., 1998; Shen Ohno and Fukuhara, 1997). The initial analysis of et al., 1998; Zan et al., 2000). In fact, they share typical BCL6 expression in both rearranged and nonrearranged features of those affecting the IgV sequences in GC B BNHL showed similar levels of BCL6 transcript (Otsuki cells, which strongly suggests that BCL6 and IgV are et al., 1995; Ohno and Fukuhara, 1997). However, the concomitantly targeted by the same (or closely related) fact that two differently altered BCL6 alleles are somatic hypermutation machinery (Zan et al., 2000, differentially expressed in some DLCL-derived cells 2001). So far, although multiple genes were shown to be indicates that at least some alterations indeed affect the aberrantly targeted by the hypermutation machinery in regulation of BCL6 expression (Ye et al., 1995; Chen some malignant derivatives of GC B cells, only two et al., 1998a; Dalla-Favera et al., 1999). But what does genes, – in addition to IgV – BCL6 and CD95/Fas, ‘misregulation’ precisely mean given the diversity of the undergo such physiological ‘microalterations’ during the alterations? Numerous partner-genes (hereafter referred transit of normal human B cells through GC (Pasqua- to as the ‘X’ genes) providing the ‘host’ promoter to

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 509 DeletionsTranslocations Microalterations their consequences on BCL6 expression. Indeed, mutations in another more proximal intronic hotspot have no Promoter substitution effect on BCL6 expression (Artiga et al., 2002). More- over, it was recently reported that BCL6 translocations in Ig and non-Ig loci lead to high and low BCL6 Impairment in BCL6 transcriptional regulation Ectopic expression of BCL6 at late stages of B cell differentiation expression, respectively, which may correspond to two kinds of DLCL with distinct phenotypic and clinical features (Alizadeh et al., 2000; Ueda et al., 2002b). This important result reveals that the ‘X’ partner gene does Other oncogenic alterations (including within BCL6) not necessarily impart its transcriptional control to BCL6 – probably because the translocation mingles

B cell lymphoma heterologous regulatory elements – and suggests that abnormally low BCL6 expression can also contribute to Figure 2 BCL6 oncogenic activation and B lymphomagenesis. In B lymphomagenesis (Ueda et al., 2002a). Finally, the BNHL, BCL6 expression may be deregulated through transloca- occasional occurrence of the reciprocal BCL6-X tran- tion and promoter substitution, as well as deletions and particular ‘microalterations’ (point mutations and more rarely short dele- scripts indicates that, upon BCL6 translocation, the ‘X’ tions) affecting its first noncoding exon and an intronic region genes can be placed under the control of the BCL6 immediately downstream. Although independent, these three kinds promoter and might thereby also be ‘misregulated’ of alterations can be combined and result in biallelic multiple (Galiegue-Zouitina et al., 1999; Preudhomme et al., 0 alterations. Note that the 5 noncoding region of BCL6 is also 2000). This could contribute to differential phenotypic subject to ‘microalterations’ in normal GC B cells (see text). Sometimes, these events may lead to persistent BCL6 expression at or clinical features among BNHL cases, or even, as late stages of B-cell differentiation, where it is normally shut off, suggested when the ‘X’ gene encodes the GTP-binding thereby possibly preventing terminal differentiation and increasing RhoH/TTF protein, to the tumorigenic process itself cell survival. This may in turn make the B cells more prone to (Preudhomme et al., 2000). In fact, as transgenic mice undergo ‘successfully’ other genetic alterations, and to progress toward a transformed phenotype. These secondary alterations can expressing lymphoma-associated BCL6 alleles in B cells include ‘microalterations’ within BCL6 that would give a further are still missing and given that no lymphomas have been selective advantage to the cells. Indeed, it has been reported that reported in the short-lived BCL6À/À mice nor in mice the transformation from ‘indolent’ FL to ‘aggressive’ DLCL is overexpressing BCL6 in T cells (see below), it remains sometimes associated with the accumulation of new point unclear as to whether BCL6 misregulation, and if so mutations in the 50 noncoding region of BCL6. Note that some oncogenic events may precede the alterations of BCL6. Moreover, which misregulation, causes BNHL. The cases of 3q27 other genetic alterations may lead to BCL6 downregulation, which breakpoints far from the defined BCL6 locus raise could also contribute to the genesis of another subset of BNHL further questions and suggest the involvement of other cases (see text) unknown genes that may be subject to rearrangement (Chen et al., 1998a). translocated BCL6 have been cloned (reviewed recently in Chen et al., 2001; Ueda et al., 2002a). Beyond their Other POK proteins and human cancers diversity, the ‘X’ genes often display throughout the B- cell lineage an expression broader than that of the Given such uncertainties, it is noteworthy that three unaltered BCL6 (Chen et al., 1998b, 2001), suggesting other genes encoding POK proteins are also altered in that ‘misregulation’ means an abnormally persistent malignancies. The PLZF gene is fused to the retinoic expression of the translocated BCL6 at the late stages of acid receptor alpha in some cases of acute promyelocytic B cell differentiation (see below). This was directly leukaemia (Chen et al., 1993). The APM1 gene is altered supported by several data. First, BCL6 translocations upon human papillomavirus integration in one case of abrogate its normal transcriptional shut-off upon cervical cell line (Reuter et al., 1998). Finally, the HIC1 stimulation of CD40 receptor, a regulator of GC gene may be involved in many human solid and differentiation, in several DLCL-derived cell lines (All- haematopoietic tumours, in addition to its proposed man et al., 1996; Cattoretti et al., 1997; Gupta et al., role in a developmental disorder known as Miller– 1999). Similarly, other BCL6 alterations associated with Dieker syndrome (Carter et al., 2000). An important BNHL, including some ‘microalterations’, are suspected common feature for these three genes is that they to relieve BCL6 from negative (possibly auto-) regula- appear, based on both expression and functional data, tions, although studies based on transiently transfected somehow inactivated, and not hyperactive, in malig- artificial reporters do not necessarily recapitulate the nancies (as well as in Miller–Dieker syndrome for transcriptional control of endogenous BCL6 (Kikuchi HIC1). The RARa/PLZF chimera, which cooperates et al., 2000; Pasqualucci et al., 2000; Ueda et al., 2002a). with the reciprocal PLZF/RARa chimera to induce Finally, a study of several DLCL cases reveals that point leukaemia in transgenic models, binds DNA onto mutations in a short intronic mutational ‘hotspot’ are PLZF-binding sites. RARa/PLZF probably acts by associated with a higher BCL6 expression (Artiga et al., competitively inhibiting PLZF, since deletion of PLZF 2002). However, it appears increasingly clear that BCL6 phenocopies its effect on leukaemogenesis (Licht et al., alterations in BNHL are not equivalent with respect to 1996; He et al., 2000). APM1 is expressed in normal

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 510 cervical keratinocytes but not in the majority of cervical it is rescued by concomitant overexpression of BCL6 carcinoma cell lines and inhibits cell growth of Hela and (Kumagai et al., 1999). Moreover, knockout experi- Caski cells when overexpressed (Reuter et al., 1998). ments demonstrate that heterozygous and, to a greater Finally HIC1, which also induces growth arrest upon extent, homozygous loss of BCL6 abnormally prolongs overexpression, is hypermethylated and underexpressed the wave of apoptosis normally affecting the differen- in many cancers (hence its name, Hypermethylated in tiating male germ cells at the spermatocyte stage in Cancer) but is upregulated by the tumour suppressor young adult animals (Kojima et al., 2001). This p53 (Wales et al., 1995; Carter et al., 2000; Guerardel increased apoptosis is accompanied by the induction et al., 2001). Overall, these data suggest that POK of the BAX protein and mimics that induced by a transcription factors appear as ‘antioncoproteins’ whose hyperthermic shock on the testes of normal mice. This loss or inactivation contributes to various forms of might be an intrinsic defect, since spermatocytes express cancers. BCL6 (Yoshida et al., 1996; Kojima et al., 2001). Consistent with these ﬁndings, PCDC2 (which is highly homologous to RP8), a rat gene rapidly induced BCL6 as a pro cell survival gene following proapoptotic stimuli in thymocytes, has been proposed to be a BCL6 target gene in B cells, although it BCL6 can inhibit apoptosis has been shown to be responsive to a BCL6DBD-VP16 chimera (Baron et al., 2002) but not, so far, to BCL6 Many overexpressed proto-oncogenes involved in hae- itself. Altogether, these data were interpreted as the matopoietic malignancies act by preventing apoptosis indication that BCL6 shields the cells against various (Adams and Cory, 1998; Nason-Burchenal et al., 1998). proapoptotic ‘stresses’, including trophic and thermic That such a mechanism may occur when BCL6 loses stresses, an activity that may be relevant to explain its negative regulations in BNHL has received experimental proposed role in BNHL persistently expressing BCL6 support. Indeed, C2C12 mouse myocytes were more (Kojima et al., 2001) (Figure 3). prone to undergo apoptosis upon serum starvation, which is a culture condition favouring terminal differ- Other protective roles of BCL6 entiation, when BCL6 expression was lowered by the mean of antisense RNA (Kumagai et al., 1999). This Conﬁrming the notion that BCL6 is a protective protein, phenotype, which appears to be dependent on caspase 3 the heart muscle has been reported to degenerate in activation, is clearly caused by the BCL6 ‘depletion’, as BCL6À/À mice after 4 weeks of age (Yoshida et al.,

Increased survival Cell death (resistance to stress?) - Spermatocytes - NIH3T3 - Cardiac myocytes - U2OS - HeLa - Differentiating C2C12 skeletal myocytes - CV1 "High" - Primary B cells in culture - Primary macrophages in culture BCL6 - Mouse embryo fibroblasts expression - GC B cells?

Stabilisation of a particular growth and/or differentiation state - Memory T cells - Differentiated skeletal and cardiac myocytes? - Differentiated monocytic cells? - GC B cells? - Differentiated keratinocytes?

Figure 3 Multiple roles of BCL6 in cell survival and differentiation. BCL6 has been shown to exert contrasting inﬂuences on cell survival. Indeed, in many cell lines, enforced expression of BCL6 leads to apoptosis. Moreover, in one case of primary cells, namely macrophages cultured in vitro, endogenous BCL6 decreases cell survival. On the contrary, in some other primary cells, enforced expression of BCL6 improves cell survival and endogenous BCL6 protects established skeletal muscle cells against apoptosis upon serum deprivation in vitro, as well as spermatocytes against apoptosis in vivo. Moreover, endogenous BCL6 has also been proposed to shield cardiac myocytes against degeneration in vivo, and appears to be essential for GC B cells, although the reasons for the lack of GC in BCL6À/À mice are not fully understood. The protective role of BCL6 may partly correspond to a stabilization of a particular differentiation state. Such a ‘maintenance’ activity of BCL6 has been evidenced in memory T cells but can be envisioned in various other cell types, including GC B cells

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 511 1999b). Importantly, although myocarditis and heart several studies have reported that enforced BCL6 destruction in these animals were initially supposed to expression triggers apoptosis. This has been shown in be an indirect effect, occurring as a result of the Th2- various cell lines, such as U2OS osteosarcoma type inflammation and eosinophilic infiltration affecting cells using a tetracycline-regulated system (Albagli many tissues (Dent et al., 1997; Fukuda et al., 1997; Ye et al., 1999), CV1 and HeLa cells using an adenoviral et al., 1997, see below), recent electron microscopy vector (Yamochi et al., 1999), NIH 3T3 cells studies show that cardiac myocytes exhibit several upon transient transfection (Zhang et al., 2001) and defects prior to the eosinophilic infiltration. Moreover, was also reported in a pro-B cell line (DIF9) as well chimeric animals, in which all organs including the heart as in mouse L cells (Zhang et al., 2001). In CV1 cells, the are BCL6+/+ while the lymphocytes are BCL6À/À induction of apoptosis by BCL6 is preceded by the (these animals were obtained by transferring bone downregulation of the genes encoding the apoptosis marrow from BCL6À/À mice into sublethally irradiated repressors BCLX(L) and BCL2. Altogether, these BCL6+/+/RAG1À/À ones), show no myocarditis, data have led some authors to suggest that high although a massive eosinophilic infiltration is still BCL6 expression may kill all the cells in which it is observed in other tissues. Finally, in wild-type mice, naturally not (or barely) expressed (Dent et al., 2002). BCL6 expression is induced 2 weeks after birth in heart Moreover, enforced BCL6 expression has also been myocytes. Altogether, these data suggest that heart shown to impair cell cycle progression, although in a myocytes lack an intrinsic prosurvival activity in different manner depending on the cell line. Indeed, BCL6À/À mice, with the myocarditis being therefore U2OS cells induced to express BCL6 hardly progress merely the consequence of this primary defect (Yoshida throughout the S phase, while enforced BCL6 et al., 1999b). However, these results do not completely expression leads CV1 cells to accumulate in the G2/M rule out the possibility that myocarditis could be phase (Albagli et al., 1999; Yamochi et al., 1999). indirectly mediated by nonlymphoid BCL6À/À haema- The existence of a link between the cell cycle alterations topoietic cells, especially macrophages. The latter indeed and the induction of apoptosis by BCL6 is uncertain, overproduce various chemokines when BCL6 is dis- since it has been reported that BCL6-mediated rupted and thus are likely to contribute to the Th2-type apoptosis of CV1 cells does not require cell cycle arrest inflammation in BCL6À/À mice (Toney et al., 2000). at the G2/M phase (Yamochi et al., 1999). Obviously, More recently, a screen in mouse embryo fibroblasts the physiological relevance of such studies based on (MEF) carrying a thermosensitive version of SV40 T dramatically enforced BCL6 expression in established antigen has established that the retrovirus-driven cell lines is questionable. However, a similar enforced expression of BCL6 efficiently abrogates a p19ARF/ expression of truncated BCL6 derivatives showing a p53-mediated senescence pathway and cooperates with reduced ability to repress transcription (Figure 1) activated ras (which alone induces premature MEF does not lead to apoptosis (Albagli et al., 1999; senescence) to fully transform these cells. Enforced Yamochi et al., 1999; Zhang et al., 2001) and has BCL6 expression does not appear to affect the expres- little effect on the cell cycle (Albagli et al., 1999). sion level of p19ARF nor its ability to activate p53. Moreover, endogenous BCL6 may be an important Rather, enforced BCL6 expression upregulates (pre- mediator of the apoptosis triggered in HeLa cells by an sumably indirectly) the expression of the cyclin D1 gene, activated version of the transcription factor AFX which is a critical target, since cyclin D1 knockout MEF (AFX*). Indeed, AFX* upregulates the expression of are resistant to BCL6-induced immortalization (Shvarts endogenous BCL6, which in turn might directly repress et al., 2002). This prosurvival effect of BCL6 is also BCLX(L) expression in these cells (Tang et al., 2002). demonstrated in primary human tonsillar B cells whose However, it has not been demonstrated that the replicative life span in vitro is dramatically extended upregulation of the endogenous BCL6 expression upon infection with a BCL6-encoding retroviral vector following AFX* expression is sufficient to trigger (Shvarts et al., 2002). According to this study, BCL6 is a apoptosis (Tang et al., 2002). Other pathways indepen- bonafide immortalizing oncogene. Together with the dent of BCLX(L) repression seem to exist, since possible role of BCL6 as an antiapoptotic factor, these overexpressed BCLX(L) does not counteract BCL6- results are consistent with other studies reporting that induced apoptosis in NIH3T3 cells (Zhang et al., 2001) expression of a dominant negative form of BCL6 in B- and overexpressed BCL6 does not affect BCLX(L) cell lines promotes cell death and cell cycle arrest at the transcription in C2C12 myocytes (Kumagai et al., 1999). G1 phase (Shaffer et al., 2000; Baron et al., 2002). These However, and most importantly, it has been shown data also suggest that in certain BNHL, BCL6 persistent that primary macrophages derived from mice expression improves the survival of the GC B cells, lacking BCL6 show enhanced survival in vitro com- thereby making them more prone to ‘successfully’ pared to their wild-type counterparts (Tang et al., 2002). undergo other oncogenic genetic changes (Figure 3). Thus, the endogenous expression of BCL6 impairs cell survival in some nonestablished cells grown BCL6 as a death-inducing gene in vitro, suggesting that it could also sometimes have a negative influence on the cell survival in vivo. Hence, Not too surprisingly, the above conclusions are not its ‘accidental’ downregulation could contribute to valid in all cellular systems, suggesting a possible some BNHL cases, possibly at a late step (Ueda et al., ambivalent role of BCL6 in transformation. Indeed, 2002b).

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 512 BCL6 and cell differentiation factor that plays a prominent role in promoting B-cell terminal differentiation (Reljic et al., 2000). Accord- Role of BCL6 in GC B cells ingly, the expression of a dominant negative version of BCL6 in Raji B cells downregulates several genes that The first indication of a role for BCL6 in normal B cells are characteristically expressed in GC cells, while it came from studies showing that BCL6 expression is induces some, but not all, markers of terminal plasma- tightly regulated during their differentiation. Indeed, cytic differentiation (Shaffer et al., 2000). It is therefore immunohistochemical studies of normal lymphoid likely that BCL6 also prevents premature terminal tissues show that BCL6 is selectively abundant in GC differentiation of GC B cells in vivo, thereby allowing B cells (Cattoretti et al., 1995; Onizuka et al., 1995; affinity maturation of the immune response to occur, Allman et al., 1996), possibly, in part, as a consequence until it is shut off as the cells exit from GC (Reljic et al., of a (post)-translational regulation (Allman et al., 1996). 2000). Therefore, alterations in BNHL that abrogate Accordingly, BCL6 is generally expressed at both BCL6 downregulation would stabilize the GC pheno- mRNA and protein levels in the transformed counter- type by delaying or preventing terminal differentiation, parts of GC B cells, hence in many BNHL, but not in B thereby contributing to lymphomagenesis. cell lines exhibiting a less (pre B) or more (plasma cells) et al differentiated phenotype (Cattoretti ., 1995; Onizu- Role of BCL6 in memory T cells ka et al., 1995; Allman et al., 1996). More directly, BCL6À/À mice show a normal lymphoid organ devel- BCL6 expression has been observed early in T cells opment and B lymphogenesis as well as T-cell-indepen- (Fukuda et al., 1995). Moreover, BCL6À/À mice dent B-cell responses, but lack GC and consequently overproduce Th2 cytokines and suffer Th2-type inflam- exhibit impaired T-cell-dependent antibody responses. mations in multiple organs (Dent et al., 1997; Fukuda Chimeric mice demonstrated that this defect is intrinsic et al., 1997; Ye et al., 1997). Whether BCL6 plays an to B cells, indicating that BCL6 selectively plays an intrinsic role in T cells yet remains unclear, as the essential role in GC B cells (Dent et al., 1997; Fukuda disruption of BCL6 only in lymphoid cells (in either T, et al., 1997; Ye et al., 1997; Toney et al., 2000). The in B, or both lineages) was shown to be insufficient to vivo tracking of splenic B cells expressing an inactive but trigger the full-scale Th2-type inflammatory disease detectable BCL6 truncated version produced by one (Toney et al., 2000). However, it has recently been type of ‘BCL6À/À’ mice suggests that BCL6 is essential reported that BCL6 negatively regulates IL5 transcrip- for follicular B cells to differentiate into GC B cells and tion in Th2 cells, which may explain, in part, the over- to proliferate (Ye et al., 1997). However, BCL6 is expression of this cytokine in BCL6À/À mice (Dent strongly expressed in most, if not all, GC B cells, in both et al., 1997; Arima et al., 2002). It has also been very centroblasts and centrocytes that are, respectively, recently reported that mice lacking BCL6 have a actively proliferating and resting cells (Cattoretti et al., reduced number of CD8+ (and CD4+) T-cells with a 1995; Ye et al., 1997). Accordingly, a search for BCL6 memory phenotype (CD44+, Ly6C+) and that this target genes in B cell lines reported that it represses defect could be rescued upon expression of a BCL6 genes encoding presumed cell cycle promoters (cyclin D2 transgene under the control of a T cell-specific promoter and cyclin A2) as well as inhibitor (p27kip1) (Shaffer (Ichii et al., 2002). The expression of BCL6 has not yet et al., 2000; Hosokawa et al., 2001). In fact, although it been examined throughout the differentiation of mem- has been proposed that BCL6 controls affinity matura- ory CD8+ T-cells, but transfer experiments of purified tion and apoptosis of GC B cells, possibly in part by precursor ‘naı¨ ve’ BCL6À/À CD8+ T-cells into normal regulating BCL2 and PCDC2 expression (Cattoretti mice strongly suggest that this defect is cell-autonomous et al., 1995; Yamochi et al., 1999; Baron et al., 2002; (Ichii et al., 2002). Note however that indirect effects Tang et al., 2002), no clear correlation has been involving other types of T cells cannot be completely observed in vivo between BCL6 expression and any ruled out. Indeed, BCL6 plays a potent role in particular fate of GC B cells (Fukuda et al., 1997; Ye regulating the differentiation of CD4+ Th2 cells, which et al., 1997). This leads again to the idea that high BCL6 themselves strongly affect CD8+ T-cell differentiation expression might buffer some general stresses – which (A Dent, personal communication), so that an even remain to be defined – experienced by GC B cells small fraction of contaminating BCL6À/À CD4+ Th2 (Fukuda et al., 1997) and/or is required for them to cells might alter the fate of the injected CD8+ acquire and maintain their phenotype (Figure 3). precursors. The too low number of memory T cells In vitro studies provide further evidences for a arising from the injected ‘naı¨ ve’ BCL6À/À CD8+ T cells ‘maintenance’ role of the GC phenotype by BCL6. is not due to an increased apoptosis or (for a small part) Retrovirus-driven BCL6 expression inhibits the terminal to a reduced proliferation of these precursors (Ichii et al., differentiation of the BCL1 B cells as well as that of 2002). Rather, it appears that BCL6 is important for primary B cells (Reljic et al., 2000; Shaffer et al., 2000). CD8+ memory T cells to maintain their characteristic It does so, in part, by antagonizing the activity of the phenotype. Indeed, upon deletion of BCL6, this STAT3 transcription factor – a mediator of IL2 phenotype appears abnormally transient – being rapidly signalling that shares DNA-binding specificity with ‘crossed’ and replaced by the CD44À, Ly6C+ one – BCL6 – and hence by repressing the transcription of whereas, conversely, it is too stable in BCL6-over- the gene encoding Blimp, a zinc finger transcription expressing cells. Interestingly, the stabilization of this

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 513 particular stage of differentiation by BCL6 is again not survival. This could help to understand its role in strictly linked to the cessation of cell proliferation as transformation, especially if both BCL6 ‘overexpression’ some CD44+, Ly6C+ CD8+ T cells still incorporate (i.e. abnormally persistent expression through B-cell bromodeoxyuridine (BrdU) (Ichii et al., 2002). differentiation) and BCL6 ‘underexpression’ turn out to be indeed oncogenic. However, our ‘historical’ propen- Is BCL6 involved in permanent exit from the cell cycle of sity to consider all the reported cellular functions of other cell types? BCL6 in the light of its proposed role in BNHL, a tendency certainly enhanced by the lack of definitive In contrast with what occurs in GC B cells, the proof causally linking BCL6 alterations to BNHL, is expression of BCL6 does not drop but rather con- probably not entirely legitimate, given the diversity of tinuously rises upon terminal differentiation of numer- the model systems. In fact, as emphasized above, by ous cell types. This has been shown in keratinocytes, several aspects, GC B cells appear somewhat atypical skeletal myocytes, male germ cells and myeloid/mono- with respect to BCL6 regulation. Presumably, the cytic cells upon monocytoid differentiation. In at least accelerated molecular evolution specifically undergone some of these cases, the increase in BCL6 protein level by human BCL6 regulatory regions in these cells parallels the observed upregulation of the mRNA through somatic hypermutations generates a particular (Yoshida et al., 1996; Yamochi et al., 1997; Albagli- plasticity to their fate, including towards neoplasia. The Curiel et al., 1998; Kumagai et al., 1999) (Figure 3). recent suggestion that BCL6 is expressed in certain solid Reciprocally, early observations also suggested that tumours (Shvarts et al., 2002) may offer other frame- outside GC, BCL6 may contribute to stabilize lymphoid works to consider the available data about its cellular cells in the G0/G1 phase of the cell cycle. Indeed, the functions. BCL6 mRNA level is high in both B and T resting Another possible link between some data is that lymphocytes from mouse spleen and human blood, but BCL6 stabilizes particular states of differentiation. This strongly drops upon their mitogenic activation in vitro is demonstrated in memory T cells whose phenotypic and becomes stably minimal a few hours before their stability depends to some degree on BCL6 (Ichii et al., entry into the S phase (Allman et al., 1996). Moreover, 2002). It is probably even clearer in B cells, in which at least in skeletal myocytes and keratinocytes, the BCL6 might well be absolutely required for these cells to upregulation of BCL6 expression is not the consequence stay at the GC stage, despite extensive proliferation. of a reversible growth arrest, but rather is linked with This might also explain the frequent increase of BCL6 the definitive exit from the cell cycle accompanying expression upon permanent exit of the cell cycle and also terminal differentiation (Yoshida et al., 1996; Albagli- partially account for the protective effect of endogenous Curiel et al., 1998; Kumagai et al., 1999). This suggests BCL6 expression in some cells in vivo as well as in some that a ‘high’ BCL6 expression contributes to induce and/ primary and established cells in vitro. Interestingly, this or maintain a terminally differentiated state in various aspect of BCL6 cellular function resembles what has cell types. Accordingly, the fact that BCL6-depleted been shown for various POK proteins in vertebrates and myocytes undergo apoptosis does not reveal a protective Drosophila. Indeed, PLZF appears to be crucial to role of BCL6 against apoptosis induced by serum maintain the quiescent, undifferentiated state of haema- starvation. It might rather reflect a more fundamental topoietic progenitors (Shaknovich et al., 1998). Simi- impairment of the ability of these cells to coordinate larly, in Drosophila, TTK, whose degradation upon cessation of their growth and terminal differentiation, MAPK kinase activation strikingly parallels that under- and/or to exit irreversibly from the cell cycle once they gone by BCL6, is required to maintain glial differentia- have started to differentiate (Albagli-Curiel et al., 1998). tion (Moriyama et al., 1997; Dickson, 1998; Niu et al., Indeed, BCL6-depleted skeletal myocytes exhibit a 1998; Badenhorst, 2001). Moreover, the Drosophila phenotype similar to those lacking RB, which is required GAGA factor as a component of some Polycomb for differentiating myocytes to irreversibly lock their complexes is also involved in the stabilization of postmitotic state (Wang et al., 1997). These data, transcriptional regulations and differentiation patterns together with the proapoptotic activity shown by (Shaknovich et al., 1998; Horard et al., 2000; Busturia BCL6 under some circumstances, further suggest how et al., 2001). BCL6, like at least three other related proteins, could act In addition, as often proposed, BCL6 may play a as an ‘antioncoprotein’ whose insufficiency would also more direct protective role against some ‘stresses’ cause cancer. In some DLCL cases, BCL6 insufficiency encountered by cells in vivo as well as in vitro.As may cause hyperactivation of IRF4, a transcription pointed out by the high expression and essential factor normally bound and repressed by BCL6 function of BCL6 in GC B cells, including when these (Figure 1), thereby inducing unchecked proliferation cells are very actively proliferating and undergo (Alizadeh et al., 2000; Gupta et al., 2001). hypermutations, such stresses could be related to DNA damages. Reinforcing this notion, somatic hypermutations are associated with DNA double-strand Conclusion and perspectives breaks (Pasqualucci et al., 2001), which interestingly also occur upon meiotic crossing over in spermatocytes I Several ideas can be generated from this overview. The precisely when BCL6 appears to also have a vital first one is that BCL6 exerts ambivalent effects on cell function (Kojima et al., 2001). Moreover, cellular

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 514 senescence, which is counteracted by BCL6 in an in vitro does not affect the survival of CD8+ T cells, and even model, has been proposed to result, at least partly, from increases that of primary macrophages in vitro. In fact, it the progressive accumulation of genetic damage (Sherr is increasingly clear that any protein, rather than having and DePinho, 2000; Rubin, 2002). Finally, both BCL6 ‘intrinsic’, stable, activities, acts according to what and another POK protein FAZF (TZFP) have been chance and intracellular context make it ‘meets’ shown to associate with BrdU-containing DNA, and the (McKnight, 2001). It is thus likely that the diverse interaction between FAZF and FANCC, encoded by a activities of BCL6 on cell survival and transformation gene linked to the Fanconi Anemia, further suggests its may reflect different interactions at least with proteins involvement in DNA repair (Albagli et al., 2000; Dai and DNA, as it has been exemplified for GAGA to et al., 2002). Another, nonmutually exclusive hypothesis explain its involvement in numerous aspects of DNA is that BCL6 would be connected to redox pathways and metabolism and transcriptional regulations (Tsukiyama would protect the cells against metabolically induced and Wu, 1995; Strutt et al., 1997; Espinas et al., 2000; oxidative injuries. In this regard, it is noteworthy that Horard et al., 2000; Xiao et al., 2001). The links between class I HDACs, which are partners of BCL6, form the activities of BCL6 and its interactions, including complexes containing a protein homologous to FAD- those presumed to mediate transcriptional repression, dependent oxidoreductases (Humphrey et al., 2001). are still largely unknown and deserve further work in the This hypothesis is consistent with the particular future. sensitivity of cardiac – but not of skeletal – myocytes to BCL6 deletion in vivo. Indeed, the heart requires strong antioxidant defences and is more severely damaged when manganese superoxide dismutase, an intramitochondrial free radical scavenging protein Acknowledgments maintaining the integrity of mitochondrial enzymes, is I warmly thank Dr Alex Dent for insightful comments and lacking (Li et al., 1995; Yoshida et al., 1999b). sharing unpublished data, Drs Takeshi Tokuhisa, Alex Dent, Interestingly, mitochondria, which generate the majority Yuichi Nakamura, He´ le` ne Pelczar and Patrick Martin for a of oxygen radicals and play crucial functions in critical reading of the manuscript, and to Dr Christophe Pivot- Pajot for suggestions. I am also very grateful to Dr Sophie apoptosis and senescence, are altered in both damaged Green-Rousseaux for her precious help with the language. This cardiac myocytes and sperm cells lacking BCL6 work was supported by grants from Centre National de la (Yoshida et al., 1999b; Kojima et al., 2001). However, Recherche Scientifique (CNRS), Association pour la Re- it should be recalled that not all the current data support cherche sur le Cancer (ARC) and Ligue contre le Cancer, a protective role of BCL6, as for instance its deletion comite´ du Val de Marne.

References

Adams JM and Cory S. (1998). Science, 281, 1322–1326. Zeleznik-Le N and McKeithan TW. (2002). Proc. Natl. Albagli O, Lantoine D, Quief S, Quignon F, Englert C, Acad. Sci. USA, 99, 2860–2865. Kerckaert JP, Montarras D, Pinset C and Lindon C. (1999). Bernardin F, Collyn-d’Hooghe M, Quief S, Bastard C, Oncogene, 18, 5063–5075. Leprince D and Kerckaert JP. (1997). Oncogene, 14, 849– Albagli O, Lindon C, Lantoine D, Quief S, Puvion E, Pinset C 855. and Puvion-Dutilleul F. (2000). Mol. Cell Biol., 20, 8560– Busturia A, Lloyd A, Bejarano F, Zavortink M, Xin H and 8570. Sakonju S. (2001). Development, 128, 2163–2173. Albagli-Curiel O, Dhordain P, Lantoine D, Aurade F, Quief S, Carter MG, Johns MA, Zeng X, Zhou L, Zink MC, Kerckaert JP, Montarras D and Pinset C. (1998). Differ- Mankowski JL, Donovan DM and Baylin SB. (2000). entiation, 64, 33–44. Hum. Mol. Genet., 9, 413–419. Alizadeh AA, Eisen MB, Davis RE, Ma C, Lossos IS, Cattoretti G, Chang CC, Cechova K, Zhang J, Ye BH, Falini Rosenwald A, Boldrick JC, Sabet H, Tran T, Yu X, Powell B, Louie DC, Ofﬁt K, Chaganti RS and Dalla-Favera R. JI, Yang L, Marti GE, Moore T, Hudson Jr J, Lu L, Lewis (1995). Blood, 86, 45–53. DB, Tibshirani R, Sherlock G, Chan WC, Greiner TC, Cattoretti G, Zhang J, Clery AM, Lederman S, Gaidano G, Weisenburger DD, Armitage JO, Warnke R, Leoy R, Carbone A and Chaganti RS. (1997). Blood, 90, Wyndham W, Greoer MR, Byrd JC, Botstein D, 175a. Brown PO and Staudt LM. (2000). Nature, 403, 503–511. Chang CC, Ye BH, Chaganti RS and Dalla-Favera R. (1996). Allman D, Jain A, Dent A, Maile RR, Selvaggi T, Kehry MR Proc. Natl. Acad. Sci. USA, 93, 6947–6952. and Staudt LM. (1996). Blood, 87, 5257–5268. Chen SJ, Zelent A, Tong JH, Yu HQ, Wang ZY, Derre J, Arima M, Toyama H, Ichii H, Kojima S, Okada S, Hatano M, Berger R, Waxman S and Chen Z. (1993). J. Clin. Invest., 91, Cheng G, Kubo M, Fukuda T and Tokuhisa T. (2002). J. 2260–2267. Immunol., 169, 829–836. Chen W, Butler M, Rao PH, Chaganti SR, Louie DC, Dalla- Artiga MJ, Saez AI, Romero C, Sanchez-Beato M, Mateo MS, Favera R and Chaganti RS. (1998a). Oncogene, 17, 1717– Navas C, Mollejo M and Piris MA. (2002). Am. J. Pathol., 1722. 160, 1371–1380. Chen W, Iida S, Louie DC, Dalla-Favera R and Chaganti RS. Badenhorst P. (2001). Development, 128, 4093–4101. (1998b). Blood, 91, 603–607. Baron BW, Anastasi J, Thirman MJ, Furukawa Y, Fears S, Chen W, Itoyama T and Chaganti RS. (2001). Genes Kim DC, Simone F, Birkenbach M, Montag A, Sadhu A, Chromosomes Cancer, 32, 281–284.

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 515 Dai MS, Chevallier N, Stone S, Heinrich MC, McConnell M, Kumagai T, Miki T, Kikuchi M, Fukuda T, Miyasaka N, Reuter T, Broxmeyer HE, Licht JD, Lu L and Hoatlin ME. Kamiyama R and Hirosawa S. (1999). Oncogene, 18, 467– (2002). J. Biol. Chem., 1, 1. 475. Dalla-Favera R, Migliazza A, Chang CC, Niu H, Pasqualucci Lemercier C, Brocard MP, Puvion-Dutilleul F, Kao HY, L, Butler M, Shen Q and Cattoretti G. (1999). Curr. Albagli O and Khochbin S. (2002). J. Biol. Chem., 19, 19. Top Microbiol. Immunol., 246, 257–263; discussion Li Y, Huang TT, Carlson EJ, Melov S, Ursell PC, Olson JL, 263–265. Noble LJ, Yoshimura MP, Berger C, Chan PH, Wallace DC Davies JM, Hawe N, Kabarowski J, Huang QH, Zhu J, Brand and Epstein CJ. (1995). Nat. Genet., 11, 376–381. NJ, Leprince D, Dhordain P, Cook M, Morriss-Kay G and Licht JD, Shaknovich R, English MA, Melnick A, Li JY, Zelent A. (1999). Oncogene, 18, 365–375. Reddy JC, Dong S, Chen SJ, Zelent A and Waxman S. Dent AL, Shaffer AL, Yu X, Allman D and Staudt LM. (1996). Oncogene, 12, 323–336. (1997). Science, 276, 589–592. Lo Coco F, Ye BH, Lista F, Corradini P, Offit K, Knowles Dent AL, Vasanwala FH and Toney LM. (2002). Crit. Rev. DM, Chaganti RS and Dalla-Favera R. (1994). Blood, 83, Oncol. Hematol., 41, 1–9. 1757–1759. Dhordain P, Albagli O, Ansieau S, Koken MH, Deweindt C, Lossos IS, Jones CD, Zehnder JL and Levy R. (2001). Leuk. Quief S, Lantoine D, Leutz A, Kerckaert JP and Leprince D. Lymphoma, 42, 1343–1350. (1995). Oncogene, 11, 2689–2697. Lossos IS and Levy R. (2000). Blood, 95, 1400–1405. Dhordain P, Albagli O, Honore N, Guidez F, Lantoine D, McKnight SL. (2001). Cell, 107, 259–261. Schmid M, The HD, Zelent A and Koken MH. (2000). Melnick A, Carlile G, Ahmad KF, Kiang CL, Corcoran C, Oncogene, 19, 6240–6250. Bardwell V, Prive GG and Licht JD. (2002). Mol. Cell Biol., Dhordain P, Albagli O, Lin RJ, Ansieau S, Quief S, Leutz A, 22, 1804–1818. Kerckaert JP, Evans RM and Leprince D. (1997). Proc. Migliazza A, Martinotti S, Chen W, Fusco C, Ye BH, Natl. Acad. Sci. USA, 94, 10762–10767. Knowles DM, Offit K, Chaganti RS and Dalla-Favera R. Dhordain P, Lin RJ, Quief S, Lantoine D, Kerckaert JP, (1995). Proc. Natl. Acad. Sci. USA, 92, 12520–12524. Evans RM and Albagli O. (1998). Nucleic Acids Res., 26, Miki T, Kawamata N, Hirosawa S and Aoki N. (1994). Blood, 4645–4651. 83, 26–32. Dickson BJ. (1998). Curr. Biol., 8, R90–R92. Moriyama M, Yamochi T, Semba K, Akiyama T and Mori S. Espinas ML, Canudas S, Fanti L, Pimpinelli S, Casanova J (1997). Oncogene, 14, 2465–2474. and Azorin F. (2000). EMBO Rep., 1, 253–259. Nakamura T, Yamazaki Y, Saiki Y, Moriyama M, Largae- Fukuda T, Miki T, Yoshida T, Hatano M, Ohashi K, spada DA, Jenkins NA and Copeland NG. (2000). Mol. Cell Hirosawa S and Tokuhisa T. (1995). Oncogene, 11, 1657– Biol., 20, 3178–3186. 1663. Nakamura Y, Miki T, Miura I, Hashimoto K, Miura A, Fukuda T, Yoshida T, Okada S, Hatano M, Miki T, Ishibashi Akimoto K, Hirosawa S, Saito K, Enokihara H, Furusawa S K, Okabe S, Koseki H, Hirosawa S, Taniguchi M, Miyasaka and Shishido H. (1996). Leukemia, 10, 658–661. N and Tokuhisa T. (1997). J. Exp. Med., 186, 439–448. Nason-Burchenal K, Takle G, Pace U, Flynn S, Allopenna J, Galiegue-Zouitina S, Quief S, Hildebrand MP, Denis C, Martin P, George ST, Goldberg AR and Dmitrovsky E. Detourmignies L, Lai JL and Kerckaert JP. (1999). Genes (1998). Oncogene, 17, 1759–1768. Chromosomes Cancer, 26, 97–105. Niu H, Ye BH and Dalla-Favera R. (1998). Genes Dev., 12, Guerardel C, Deltour S, Pinte S, Monte D, Begue A, Godwin 1953–1961. AK and Leprince D. (2001). J. Biol. Chem., 276, 3078–3089. Ohno H and Fukuhara S. (1997). Leuk. Lymphoma, 27, 53–63. Gupta S, Anthony A and Pernis AB. (2001). J. Immunol., 166, Okabe S, Fukuda T, Ishibashi K, Kojima S, Okada S, Hatano 6104–6111. M, Ebara M, Saisho H and Tokuhisa T. (1998). Mol. Cell Gupta S, Jiang M, Anthony A and Pernis AB. (1999). J. Exp. Biol., 18, 4235–4244. Med., 190, 1837–1848. Onizuka T, Moriyama M, Yamochi T, Kuroda T, Kazama A, He LZ, Bhaumik M, Tribioli C, Rego EM, Ivins S, Zelent A Kanazawa N, Sato K, Kato T, Ota H and Mori S. (1995). and Pandolfi PP. (2000). Mol. Cell, 6, 1131–1141. Blood, 86, 28–37. Horard B, Tatout C, Poux S and Pirrotta V. (2000). Mol. Cell Otsuki T, Yano T, Clark HM, Bastard C, Kerckaert JP, Jaffe Biol., 20, 3187–3197. ES and Raffeld M. (1995). Blood, 85, 2877–2884. Hosokawa Y, Maeda Y and Seto M. (2001). Biochem. Biophys. Pasqualucci L, Migliazza A, Fracchiolla N, William C, Neri A, Res. Commun., 283, 563–568. Baldini L, Chaganti RS, Klein U, Kuppers R, Rajewsky K Humphrey GW, Wang Y, Russanova VR, Hirai T, Qin J, and Dalla-Favera R. (1998). Proc. Natl. Acad. Sci. USA, 95, Nakatani Y and Howard BH. (2001). J. Biol. Chem., 276, 11816–11821. 6817–6824. Pasqualucci L, Migliazza A, Ye BH, Cattoretti G, Chaganti Huynh KD, Fischle W, Verdin E and Bardwell VJ. (2000). RS and Dalla-Favera R. (2000). Blood, 96, 58a. Genes Dev., 14, 1810–1823. Pasqualucci L, Neumeister P, Goosens T, Nanjangud G, Ichii H, Sakamoto A, Hatano M, Okada S, Toyama H, Taki S, Chaganti RSK, Ku¨ ppers R and Dalla-Favera R. (2001) Arima M, Kuroda Y and Tokuhisa T. (2002). Nat. Nature, 412, 341–346. Immunol., 20, 20. Preudhomme C, Roumier C, Hildebrand MP, Dallery- Kerckaert JP, Deweindt C, Tilly H, Quief S, Lecocq G and Prudhomme E, Lantoine D, Laı¨ JL, Daudignon A, Adenis Bastard C. (1993). Nat. Genet., 5, 66–70. C, Bauters F, Fenaux P, Kerckaert JP and Galiegue- Kikuchi M, Miki T, Kumagai T, Fukuda T, Kamiyama R, Zouitina S. (2000). Oncogene, 19, 2023–2032. Miyasaka N and Hirosawa S. (2000). Oncogene, 19, 4941– Reljic R, Wagner SD, Peakman LJ and Fearon DT. (2000). J. 4945. Exp. Med., 192, 1841–1848. Kojima S, Hatano M, Okada S, Fukuda T, Toyama Y, Yuasa Reuter S, Bartelmann M, Vogt M, Geisen C, Napierski I, S, Ito H and Tokuhisa T. (2001). Development, 128, Kahn T, Delius H, Lichter P, Weitz S, Korn B and Schwarz 57–65. E. (1998). EMBO J., 17, 215–222.

Oncogene Ambivalent role of BCL6 O Albagli-Curiel 516 Rubin H. (2002). Nat. Biotechnology, 20, 675–681. Xiao H, Sandaltzopoulos R, Wang HM, Hamiche A, Ranallo Shaffer AL, Yu X, He Y, Boldrick J, Chan EP and Staudt LM. R, Lee KM, Fu D and Wu C. (2001). Mol. Cell, 8, 531–543. (2000). Immunity, 13, 199–212. Yamochi T, Kaneita Y, Akiyama T, Mori S and Moriyama M. Shaknovich R, Yeyati PL, Ivins S, Melnick A, Lempert C, (1999). Oncogene, 18, 487–494. Waxman S, Zelent A and Licht JD. (1998). Mol. Cell Biol., Yamochi T, Kitabayashi A, Hirokawa M, Miura AB, 18, 5533–5545. Onizuka T, Mori S and Moriyama M. (1997). Leukemia, Shen HM, Peters A, Baron B, Zhu X and Storb U. (1998). 11, 694–700. Science, 280, 1750–1752. Ye BH, Cattoretti G, Shen Q, Zhang J, Hawe N, de Waard R, Sherr CJ and DePinho RA. (2000). Cell, 102, 407–410. Leung C, Nouri-Shirazi M, Orazi A, Chaganti RS, Rothman Shvarts A, Brummelkamp TR, Scheeren F, Koh E, Daley GQ, P, Stall AM, Pandolfi PP and Dalla-Favera R. (1997). Nat. Spits H and Bernards R. (2002). Genes. Dev., 16, 681–686. Genet., 16, 161–170. Strutt H, Cavalli G and Paro R. (1997). EMBO J., 16, 3621– Ye BH, Chaganti S, Chang CC, Niu H, Corradini P, Chaganti 3632. RS and Dalla-Favera R. (1995). EMBO J., 14, 6209–6217. Tang TT, Dowbenko D, Jackson A, Toney L, Lewin DA, Ye BH, Lista F, Lo Coco F, Knowles DM, Offit K, Chaganti Dent AL and Lasky LA. (2002). J. Biol. Chem., 2, 2. RS and Dalla-Favera R. (1993). Science, 262, 747–750. Toney LM, Cattoretti G, Graf JA, Merghoub T, Pandolfi PP, Yoshida S, Kaneita Y, Aoki Y, Seto M, Mori S and Moriyama Dalla-Favera R, Ye BH and Dent AL. (2000). Nat. M. (1999a). Oncogene, 18, 7994–7999. Immunol., 1, 214–220. Yoshida T, Fukuda T, Hatano M, Koseki H, Okabe S, Tsukiyama T and Wu C. (1995). Cell, 83, 1011–1020. Ishibashi K, Kojima S, Arima M, Komuro I, Ishii G, Ueda C, Akasaka T, Kurata M, Maesako Y, Nishikori M, Miki T, Hirosawa S, Miyasaka N, Taniguchi M, Ochiai T, Ichinohasama R, Imada K, Uchiyama T and Ohno H. Isono K and Tokuhisa T. (1999b). Cardiovasc. Res., 42, (2002a). Oncogene, 21, 368–376. 670–679. Ueda C, Uchiyama T and Ohno H. (2002b). Blood, 99, 2624– Yoshida T, Fukuda T, Okabe S, Hatano M, Miki T, Hirosawa 2625. S, Miyasaka N, Isono K and Tokuhisa T. (1996). Biochem. Vasanwala FH, Kusam S, Toney LM and Dent A. (2002). J. Biophys. Res. Commun., 228, 216–220. Immunol., 169, 1922–1929. Zan H, Komori A, Li Z, Cerutti A, Schaffer A, Flajnik MF, Wales MM, Biel MA, el Deiry W, Nelkin BD, Issa JP, Diaz M and Casali P. (2001). Immunity, 14, 643–653. Cavenee WK, Kuerbitz SJ and Baylin SB. (1995). Nat. Med., Zan H, Li Z, Yamaji K, Dramitinos P, Cerutti A and Casali P. 1, 570–577. (2000). J. Immunol., 165, 830–839. Wang J, Guo K, Wills KN and Walsh K. (1997). Cancer Res., Zhang H, Okada S, Hatano M, Okabe S and Tokuhisa T. 57, 351–354. (2001). Biochim. Biophys. Acta., 1540, 188–200.

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