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Mechanisms of Chromosomal Translocations in B Cell Lymphomas

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Mechanisms of Chromosomal Translocations in B Cell Lymphomas Oncogene (2001) 20, 5580 ± 5594 ã 2001 Nature Publishing Group All rights reserved 0950 ± 9232/01 $15.00 www.nature.com/onc Mechanisms of chromosomal translocations in B cell lymphomas Ralf KuÈ ppers*,1,2 and Riccardo Dalla-Favera1 1Institute of Cancer Genetics, Columbia University, New York, NY 10032, USA; 2Institute for Genetics and Department of Internal Medicine I, University of Cologne, Cologne, Germany Reciprocal chromosomal translocations involving the involved in control of the cell cycle, bcl-2 functions immunoglobulin (Ig) loci are a hallmark of most mature by inhibiting apoptosis, and c-myc is a major control B cell lymphomas and usually result in dysregulated factor for cellular growth (Dalla-Favera and Gaidano, expression of oncogenes brought under the control of the 2001). As will be discussed below, the frequent Ig enhancers. Although the precise mechanisms involved involvement of Ig loci in chromosomal translocations in the development of these translocations remains re¯ects the remodeling of Ig genes that occurs during B essentially unknown, a clear relationship has been cell development as a result of V gene segment established with the mechanisms that lead to Ig gene assembly, somatic hypermutation, and class switch remodeling, including V(D)J recombination, isotype recombination. This review will focus on the involve- switching and somatic hypermutation. The common ment of these processes in chromosomal translocations denominator of these three processes in the formation associated with tumors derived from mature B cells, of Ig-associated translocations is probably represented by including B cell non Hodgkin lymphomas and multiple the fact that each of these processes intrinsically myeloma. For a discussion of chromosomal transloca- generates double-strand DNA breaks. Since isotype tion in immature B cell tumors, which often lack an switching and somatic hypermutation occur in germinal association with Ig loci, the reader is refered to recent center (GC) B cells, the origin of a large number of B reviews (Harrison, 2000; Rowley, 1999). cell lymphomas from GC B cells is likely closely related to aberrant hypermutation and isotype switching activity in these B cells. Oncogene (2001) 20, 5580 ± 5594. Remodeling Ig genes: V(D)J recombination, somatic hypermutation and class switch recombination Keywords: chromosomal translocation; class switch recombination; germinal center; lymphomagenesis; V(D)J recombination somatic hypermutation; V(D)J recombination During B cell development in the bone marrow, B cell precursors form intact exons for the variable region of Introduction antibody molecules by recombining individual Ig gene segments. For the variable region exon of the Ig heavy Distinct types of cancers often show characteristic chain, V, D and J gene segments are joined, whereas genetic changes involved in tumorigenesis. A hallmark the V region exons of k and l light chains are each of mature B cell lymphomas are balanced chromoso- composed of V and J segments (Rajewsky, 1996; mal translocations, mostly involving the immunoglo- Tonegawa, 1983). These gene rearrangements occur in bulin (Ig) genes and a variety of partner genes (Dalla- an ordered fashion by a process called V(D)J Favera and Gaidano, 2001). Some of these are recombination (Figure 1). First, a DH segment is associated with speci®c subtypes of mature B cell rearranged to a JH segment. This is followed by a VH lymphoma. Prominent examples include the bcl-1/Ig to DHJH joining, which can be either in-frame (i.e. in translocation in mantle zone lymphoma, the bcl-2/Ig the correct reading frame for encoding antibody translocation in follicular lymphoma and the c-myc/Ig sequences) or out-of-frame. In the latter case, a second translocation in Burkitt's lymphoma. In most in- attempt can be made on the other heavy chain allele, stances, the translocated partner gene becomes tran- whereas in the ®rst case, the pre B cell can proceed scriptionally deregulated as a consequence of its with development by undergoing Ig light chain gene transposition into the Ig locus. The consequences of rearrangements. such dysregulated expression of proto-oncogenes can V(D)J recombination is initatiated when two often be inferred from the normal functions of their lymphocyte-speci®c endonucleases, the recombination protein products. For example, bcl-1 is a cyclin activating gene (RAG) 1 and 2 proteins cut the rearranging gene segments at speci®c recombination signal sequences (RSS) that ¯ank the coding sequences of the V, D and J segments (reviewed in Fugmann et *Correspondence: R KuÈ ppers, University of Cologne, Department of Internal Medicine I, LFI E4 R706, Joseph-Stelzmannstr. 9, D-50931 al., 2000). Each RSS is composed of a conserved Cologne, Germany; E-mail: [email protected] heptamer sequence and a conserved nonamer separated Mechanisms of chromosomal translocations in B cell lymphomas RKuÈppers and R Dalla-Favera 5581 Figure 1 Molecular processes modifying immunoglobulin genes. V(D)J recombination is a process by which gene segments coding for the variable region of antibody molecules are assembled. Three gene segments (V, D and J) are joined to create a heavy chain V region gene, the k and l light chain genes (not shown) are each composed of V and J segments. Heavy chain assembly occurs in two steps. In the ®rst rearrangment, a DH gene segment is rearranged to a JH segment, in the second rearrangement, a VH gene is rearrranged to a DHJH joint. In the human IgH locus on chromosome 14, there are ca. 50 functional VH,27DH and six JH gene segments. In class switch recombination, the expressed heavy chain constant region (CH) gene(s) (usually the originally expressed cm and cd genes) is (are) replaced by a downstream CH gene. The recombination process involves deletion of the DNA between repetitive DNA regions (switch regions, sm,sg and sa) upstream of the recombining CH genes. The speci®city of the antibody remains unaltered, but the eector functions of the antigen receptor are changed. Somatic hypermutation introduces point mutations and also some deletions and duplications at a very high rate speci®cally into the V region genes and ¯anking sequences (mutations are indicated by vertical lines) by a 12 or 23 bp long spacer. RAG1 and 2 cleave anity to the respective antigen are positively selected. precisely between the RSS and the coding sequences of GC B cells undergo several rounds of proliferation, the two rearranging gene segments in a concerted mutation and selection before they ®nally dierentiate fashion, yielding two blunt RSS signal ends and two into memory B cells or plasma cells. covalently sealed hairpin coding ends (Figure 2a). The The process of somatic hypermutation introduces signal ends are precisely joined, releasing the interven- mostly nucleotide substitutions into rearranged V ing DNA between the rearranging gene segments as a genes; deletions and duplications account for about DNA circle. The hairpin structures of the coding ends 5% of the mutation events (Goossens et al., 1998; are opened and the two ends are joined. During this Neuberger and Milstein, 1995). Mutations are intro- reaction, nucleotides may be removed from the coding duced at a very high rate (ca. 1073 to 1074/bp/ ends by exonucleolytic digestion, and non-germline generation) in a region about 1 ± 2 kb downstream of nucleotides (N sequences) may be added by the the transcriptional promoter of both productively as lymphocyte-speci®c terminal nucleotidyltransferase well as non-productively rearranged V genes. It (TdT), further increasing the diversity of V region appears that hypermutation is dependent on the genes (Figure 2a). This phase of the reaction is presence of the Ig enhancers and on transcription of catalyzed, in addition to the RAG proteins, by the Ig genes, and that in particular transcription enzymes that are also involved in non-homologous initiation is involved (Betz et al., 1994; Fukita et al., double-strand DNA break repair, including DNA ± PK 1998; Peters and Storb, 1996). The mechanism of (which is composed of the DNA-PK catalytic subunit somatic hypermutation is still unknown. However, it and the Ku70 and Ku80 proteins), DNA ligase IV and has recently been demonstrated that hypermutation is XRCC4 (Fugmann et al., 2000). associated with double-strand DNA breaks (Bross et al., 2000; Papavasiliou and Schatz, 2000). The most favored model suggests that an error-prone DNA Somatic hypermutation polymerase is involved in the introduction of mutations When mature B cells are activated through binding of in proximity of the DNA breaks. cognate antigen with their antigen receptors and interaction with T helper cells, they migrate into B Class switch recombination cell follicles of secondary lymphoid organs (like lymph nodes and spleen) and establish histological structures A fraction of GC B cells undergoes class switch called germinal centers (GC) (MacLennan, 1994). In recombination and thereby changes the isotype of the the GC, the B cells vigorously proliferate and activate expressed BCR, resulting in altered eector functions a somatic hypermutation mechanism which speci®cally of the antibody (Figure 1) (Liu et al., 1996; Maizels, introduces mutations into the V regions of Ig genes 1999). Through class switching, the Cm and Cd genes (Berek et al., 1991; Jacob et al., 1991; KuÈ ppers et al., that are originally expressed by a naive B cell are 1993) (Figure 1). Through the acquisition of somatic subsequently replaced by the Cg,Ca or Ce genes. In mutations, antibody variants are generated, and cells the human, there are nine functional IgH constant expressing a B cell receptor (BCR) with increased region genes (1 Cm,1Cd,4Cg,2Ca and 1 Ce), located Oncogene Mechanisms of chromosomal translocations in B cell lymphomas RKuÈppers and R Dalla-Favera 5582 Figure 2 V(D)J recombination and class switching (a) In V(D)J recombination, the RAG1 and RAG2 enzymes cleave the DNA at conserved recombination signal sequences (RSS) ¯anking the rearranging gene segments (here shown for a DH and JH segment).
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