Clonal diversification of primary BALB/c plasmacytomas harboring T(12;15) chromosomal translocations

MYC inhibitors in multiple myeloma

The importance of MYC function in cancer was discovered in the late 1970s when the sequence of the avian retrovirus that causes myelocytic leukemia was identified. Since then, over 40 years of unceasing research have highlighted the significance of this protein in malignant transformation, especially in hematologic diseases. Indeed, some of the earliest connections among the higher expression of proto-oncogenes (such as MYC), genetic rearrangements and their relation to cancer development were made in Burkitt lymphoma, chronic myeloid leukemia and mouse plasmacytomas. Multiple myeloma (MM), in particular, is a plasma cell malignancy strictly associated with MYC deregulation, suggesting that therapeutic strategies against it would be beneficial in treating this disease. However, targeting MYC was - and, somehow, still is - challenging due to its unique properties: lack of defined three-dimensional structure, nuclear localization and absence of a targetable enzymatic pocket. Despite these difficulties, however, many studies have shown the potential therapeutic impact of direct or indirect MYC inhibition. Different molecules have been tested, in fact, in the context of MM. In this review, we summarize the current status of the different compounds, including the results of their clinical testing, and propose to continue with the efforts to identify, repurpose, redesign or improve drug candidates to combine them with standard of care therapies to overcome resistance and enable better management of myeloma treatment.

3'Igh enhancers hs3b/hs4 are dispensable for Myc deregulation in mouse plasmacytomas with T(12;15) translocations

Myc-deregulating T(12;15) chromosomal translocations are the hallmark cytogenetic abnormalities of murine plasmacytomas (PCTs). In most PCTs, the immunoglobulin heavy chain (Igh) locus is broken between the Eμ enhancer and the 3’ regulatory region (3’RR), making the latter the major candidate for orchestrating Myc deregulation. To elucidate the role of the Igh3’RR in tumorigenesis, we induced PCTs in Bcl-xL-transgenic mice deficient for the major Igh3’RR enhancer elements, hs3b and hs4 (hs3b-4^-/-). Contrary to previous observations using a mouse lymphoma model, which showed no tumors with peripheral B-cell phenotype in hs3b-4^-/- mice, these animals developed T(12;15)-positive PCTs, although with a lower incidence than hs3b-4^+/+ (wild-type, WT) controls. In heterozygous hs3b-4^+/- mice there was no allelic bias in targeting Igh for T(12;15). Molecular analyses of Igh/Myc junctions revealed dominance of Sμ region breakpoints versus the prevalence of Sγ or Sα in WT controls. Myc expression and Ig secretion in hs3b-4^-/- PCTs did not differ from WT controls. We also evaluated the effect of a complete Igh3’RR deletion on Myc expression in the context of an established Igh/Myc translocation in ARS/Igh11-transgenic PCT cell lines. Cre-mediated deletion of the Igh3’RR resulted in gradual reduction of Myc expression, loss of proliferative activity and increased cell death, confirming the necessity of the Igh3’RR for Myc deregulation by T(12;15).

Genetic reporter system for oncogenic Igh-Myc translocations in mice

The Myc-deregulating chromosomal T(12;15)(Igh-Myc) translocation, the hallmark mutation of inflammation- and interleukin 6-dependent mouse plasmacytoma (PCT), is the premier model of cancer-associated chromosomal translocations because it is the only translocation in mice that occurs spontaneously (B lymphocyte lineage) and with predictably high incidence (approximately 85% of PCT), and has a direct counterpart in humans: Burkitt lymphoma t(8;14)(q24;q32) translocation. Here, we report on the development of a genetic system for the detection of T(12;15)(Igh-Myc) translocations in plasma cells of a mouse strain in which an enhanced green fluorescent protein (GFP)-encoding reporter gene has been targeted to Myc. Four of the PCTs that developed in the newly generated translocation reporter mice, designated iGFP(5'Myc), expressed GFP consequent to naturally occurring T(12;15) translocation. GFP expression did not interfere with tumor development or the deregulation of Myc on derivative 12 of translocation, der (12), because the reporter gene was allocated to the reciprocal product of translocation, der (15). Although the described reporter gene approach requires refinement before T(12;15) translocations can be quantitatively detected in vivo, including in B lymphocyte lineage cells that have not yet completed malignant transformation, our findings provide proof of principle that reporter gene tagging of oncogenes in gene-targeted mice can be used to elucidate unresolved questions on the occurrence, distribution and trafficking of cells that have acquired cancer-causing chromosomal translocations of great relevance for humans.

IL-6 and MYC collaborate in plasma cell tumor formation in mice

Interleukin-6 (IL-6) plays a critical role in the natural history of human plasma cell neoplasms (PCNs), such as plasma cell myeloma and plasmacytoma (PCT). IL-6 is also at the center of neoplastic plasma cell transformation in BALB/c (C) mice carrying a transgene, H2-L(d)-IL6, that encodes human IL-6 under control of the major histocompatibility complex H2-L(d) promoter: strain C.H2-L(d)-IL6. These mice are prone to PCT, but tumor development is incomplete with long latencies ( approximately 40% PCT at 12 months of age). To generate a more robust mouse model of IL-6-dependent PCN, we intercrossed strain C.H2-L(d)-IL6 with strains C.iMyc(Emu) or C.iMyc(Calpha), 2 interrelated gene-insertion models of the chromosomal T(12;15) translocation causing deregulated expression of Myc in mouse PCT. Deregulation of MYC is also a prominent feature of human PCN. We found that double-transgenic C.H2-L(d)-IL6/iMyc(Emu) and C.H2-L(d)-IL6/iMyc(Calpha) mice develop PCT with full penetrance (100% tumor incidence) and short latencies (3-6 months). The mouse tumors mimic molecular hallmarks of their human tumor counterparts, including elevated IL-6/Stat3/Bcl-X(L) signaling. The newly developed mouse strains may provide a good preclinical research tool for the design and testing of new approaches to target IL-6 in treatment and prevention of human PCNs.

AID-deficient Bcl-xL transgenic mice develop delayed atypical plasma cell tumors with unusual Ig/Myc chromosomal rearrangements

Activation-induced cytidine deaminase (AID) is required for immunoglobulin (Ig) class switch recombination and somatic hypermutation, and has also been implicated in translocations between Ig switch regions and c-Myc in plasma cell tumors in mice. We asked if AID is required for accelerated tumor development in pristane-treated Bcl-xL transgenic BALB/c mice deficient in AID (pBxAicda−/−). pBxAicda^−/− mice developed tumors with a lower frequency (24 vs. 62%) and a longer mean latency (108 vs. 36 d) than AID-sufficient mice. The tumors appeared in oil granuloma tissue and did not form ascites. By interphase fluorescence in situ hybridization, six out of nine pBxAicda^−/− primary tumors had T(12;15) and one had T(6;15) chromosomal translocations. Two tumors were transplantable and established as stable cell lines. Molecular and cytogenetic analyses showed that one had an unusual unbalanced T(12;15) translocation, with IgH Cμ and Pvt-1 oriented head to tail at the breakpoint, resulting in an elevated expression of c-Myc. In contrast, the second was T(12;15) negative, but had an elevated N-Myc expression caused by a paracentric inversion of chromosome 12. Thus, novel mechanisms juxtapose Ig and Myc-family genes in AID-deficient plasma cell tumors.

Myc translocations in B cell and plasma cell neoplasms

Chromosomal translocations that join the cellular oncogene Myc (c-myc) with immunoglobulin (Ig) heavy-chain (Igh) or light-chain (Igk, Igl) loci are widely believed to be the crucial initiating oncogenic events in the development of B cell and plasma cell neoplasms in three mammalian species: Burkitt lymphoma (BL) in human beings, plasmacytoma (PCT) in mice, and immunocytoma in rats. Among the Myc-Ig translocations found in these neoplasms, mouse PCT T(12;15)(Igh-Myc) is of special interest because it affords a uniquely useful model system to study the fundamental outstanding questions on the mechanisms, genetics, and biological consequences of Myc translocations. Mouse T(12;15) is the direct counterpart of the human BL t(8;14)(q24;q32) translocation and thus of great relevance for human cancer. Mouse T(12;15) is the only cancer-associated translocation in mice that occurs with high incidence, spontaneity, and cell-type specificity. Due to the development of PCR methods for the detection of the underlying reciprocal Myc-Igh junction fragments, it is now known that mouse T(12;15) can be a dynamic process that begins with the genetic exchange of Myc and the Igh switch mu region (Smu), progresses by class switch recombination (CSR) just 3' of the translocation break site, and then undergoes further clonal diversification by micro-deletions in the junction flanks. The molecular pathway that subverts CSR to mediate trans-chromosomal joining of Myc and Smu (translocation origin) and secondary modification of Myc-Igh junctions (translocation "remodeling") has not been elucidated, but recent evidence indicates that it includes CSR factors, such as the activation-induced cytidine deaminase (AID), that may also be involved in the ongoing neoplastic progression of the translocation-bearing tumor precursor. Transgenic mouse models of T(12;15)/t(8;14), including newly developed "iMyc" gene-insertion mice, will be useful in elucidating the role of these CSR factors in the progression of Myc-induced B cell tumors.

Extraosseous IL-6 transgenic mouse plasmacytoma sometimes lacks Myc-activating chromosomal translocation

The cellular oncogene MYC and plasma cell growth, differentiation, and survival factor IL-6 play critical roles in the natural history of human plasma cell neoplasms such as multiple myeloma (MM). Myc and IL-6 also are at the center of neoplastic plasma cell transformation in BALB/c mice that carry a human IL-6 transgene and, therefore, predictably develop plasmacytomas (PCTs). We showed previously that, much like advanced MM or human myeloma cell lines (HMCLs), in which MYC is frequently deregulated in cis because of complex cytogenetic aberrations juxtaposing MYC to immunoglobulin enhancers, IL-6 transgenic PCTs commonly deregulate Myc in cis by chromosomal translocation, predominantly T(12;15)(Igh-Myc). In this article, we show that, analogous to primary MM in which MYC is mostly deregulated in trans by signaling pathways converging at the MYC promoter, IL-6 transgenic PCTs sometimes develop in the absence of Myc translocations, thus activating Myc in trans. We present cytogenetic and molecular evidence on two IL-6 transgenic PCTs that contained overexpressed Myc protein but lacked T(12;15)(Igh-Myc) and two related Myc--deregulating translocations that juxtapose Myc to immunoglobulin light-chain instead of heavy-chain enhancers: T(6;15)(Igkappa-Pvt1) and T(15;16)(Pvt1-Iglambda). We conclude that Myc translocations are not strictly required for IL-6-driven PCT development in mice. IL-6 transgenic PCTs may provide a valuable model system for elucidating both trans and cis mechanisms of Myc deregulation of great relevance for MYC deregulation in human MM.

AID is required for c-myc/IgH chromosome translocations in vivo

Chromosome translocations between c-myc and immunoglobulin (Ig) are associated with Burkitt's lymphoma in humans and with pristane- and IL6-induced plasmacytomas in mice. These translocations frequently involve Ig switch regions, suggesting that they might be the result of aberrant Ig class switch recombination (CSR). However, a direct link between CSR and chromosome translocations has not been established. We have examined c-myc/IgH translocations in IL6 transgenic mice that are mutant for activation induced cytidine deaminase (AID), the enzyme that initiates CSR. Here we report that AID is essential for the c-myc/IgH chromosome translocations induced by IL6.

Novel targeted deregulation of c-Myc cooperates with Bcl-X(L) to cause plasma cell neoplasms in mice

Deregulated expression of both Myc and Bcl-X(L) are consistent features of human plasma cell neoplasms (PCNs). To investigate whether targeted expression of Myc and Bcl-X(L) in mouse plasma cells might lead to an improved model of human PCN, we generated Myc transgenics by inserting a single-copy histidine-tagged mouse Myc gene, Myc(His), into the mouse Ig heavy-chain Calpha locus. We also generated Bcl-X(L) transgenic mice that contain a multicopy Flag-tagged mouse Bcl-x(Flag) transgene driven by the mouse Ig kappa light-chain 3' enhancer. Single-transgenic Bcl-X(L) mice remained tumor free by 380 days of age, whereas single-transgenic Myc mice developed B cell tumors infrequently (4 of 43, 9.3%). In contrast, double-transgenic Myc/Bcl-X(L) mice developed plasma cell tumors with short onset (135 days on average) and full penetrance (100% tumor incidence). These tumors produced monoclonal Ig, infiltrated the bone marrow, and contained elevated amounts of Myc(His) and Bcl-X(L)(Flag) proteins compared with the plasma cells that accumulated in large numbers in young tumor-free Myc/Bcl-X(L) mice. Our findings demonstrate that the enforced expression of Myc and Bcl-X(L) by Ig enhancers with peak activity in plasma cells generates a mouse model of human PCN that recapitulates some features of human multiple myeloma.

Neoplastic development in plasma cells

An increasing number of model systems of plasma cell tumor (PCT) formation have been and are being developed. Discussed here are six models in mice and multiple myeloma (MM) in humans. Each model illustrates a unique set of biological factors. There are two general types of model systems: those that depend upon naturally arising mutagenic changes (pristane-induced PCTs, 5TMM, and MM) and those that are associated with oncogenes (Emu-v-abl), growth factors [interleukin-6 (IL-6)], and anti-apoptotic factors (Bcl-xL/Bcl-2). PCTs develop in several special tissue microenvironments that provide essential cytokines (IL-6) and cell-cell interactions. In mice, the activation and deregulation of c-myc by chromosomal translocations is a major feature in many of the models. This mechanism is much less a factor in MM and the 5T model in mice. Genetically determined susceptibility is involved in many of the mouse models, but only a few genes have been implicated thus far.

E mu/S mu transposition into Myc is sometimes a precursor for T(12;15) translocation in mouse B cells

Misguided immunoglobulin (Ig) class switch recombination (CSR) has been implicated in the origin of Myc-activating chromosomal translocations, T(12;15), in BALB/c mouse plasmacytomas (PCTs). CSR has also been involved in the progression of T(12;15); for example, the approximation of Myc to the 3'-C alpha enhancer. This study provides evidence for an additional mechanism by which aberrant CSR may facilitate T(12;15): transposition of Ig heavy-chain (IgH) sequences to Myc. Five IgH transposons containing the intronic heavy-chain enhancer, E mu, and a truncated switch mu region, S mu, were found in the first intron of Myc in lymph node cells of IL-6 transgenic BALB/c mice. In two cases E mu/S mu transposition primed Myc to get involved in apparent trans-chromosomal CSR to C gamma 1, presumably leading to T(12;15). Translocations preceded by E mu/S mu transposition can sometimes be distinguished from de novo translocations by molecular fingerprints in translocation breakpoint regions (Ig switch region [S] inversions and unusual gene orders in composite S regions). The presence of such fingerprints in some PCTs suggests that the tumors sometimes evolve from transposition-bearing precursors. We propose that E mu/S mu transposition to Myc may facilitate plasmacytomagenesis by sensitizing Myc to undergo T(12;15) translocation. T(12;15), in turn, juxtaposes Myc to the 3'-C alpha enhancer, which appears to be required for deregulating Myc in a manner that is conducive to PCT development.

Lymphoma- and leukemia-associated chromosomal translocations in healthy individuals

Chromosomal translocations (CTs) are hallmark mutations of hematopoietic malignancy that result in the deregulated expression of oncogenes or the generation of novel fusion genes. The polymerase chain reaction (PCR) can be used to detect illegitimate recombinations of genomic DNA sequences as a more sensitive assay than cytogenetics for determining the presence of CTs. Both direct DNA-PCR and reverse transcriptase-PCR were used to examine healthy individuals for lymphoma- and leukemia-associated CTs. Two oncogene-activating CTs [t(14;18)(q32;q21) and t(8;14)(q24;q32)] and one fusion-gene CT [t(2;5)(p23;q35)] from lymphomas and five fusion-gene CTs from leukemia [t(9;22)(q34;q11), t(4;11)(q21;q23), t(15;17)(q22;q11), t(12;21)(p13;q22), t(8;21)(q22;q22)] were detected in such studies. The biological implication is that CTs associated with malignant tumors may also be found in cells that are not neoplastic. CTs are characteristic attributes of neoplastic clones but are by themselves insufficient to cause malignant transformation. A better understanding of the special biology of non-neoplastic CT-bearing cells will provide insight into their putative role as tumor precursors. Prospective epidemiological studies are needed to determine whether such cells in healthy individuals may, in some instances, become clonogenic founders of lymphoma or leukemia.

Isotype switch-mediated CH deletions are a recurrent feature of Myc/CH translocations in peritoneal plasmacytomas in mice

Oncogene activating chromosomal translocations that interrupt IGH switch (S) regions at 14q32 are thought to be caused by misguided IGH isotype switching in postgerminal center B-cell lymphomas and plasma cell myelomas in humans. Aberrant switching also seems to be involved in altering the fine structure of the translocation in some of these tumors, but the significance of these changes is not known. Here we report on 3 cases of IL-6 transgenic mouse plasmacytomas (PCT) that harbor T(12;15) translocations that had been modified by frustrated switch attempts that result in C(H) deletions. When considered together with 6 similar cases of PCT described previously, our observations suggest that secondary deletions in C(H) are a regular feature in the molecular evolution of T(12;15) translocations and, thereby, in the progression of PCT. We propose that the T(12;15)(+) mouse PCT offers a uniquely valuable model system for elucidating the dual role of abnormal isotype switching in causation and 'remodeling' of chromosomal translocations.

IL-6 transgenic mouse model for extraosseous plasmacytoma

Plasma cell neoplasms in humans comprise plasma cell myeloma, otherwise known as multiple myeloma, Ig deposition and heavy chain diseases, and plasmacytoma (PCT). A subset of PCT, designated extramedullary PCT, is distinguished from multiple myeloma and solitary PCT of bone by its distribution among various tissue sites but not the bone marrow. Extramedullary (extraosseus) PCT are rare spontaneous neoplasms of mice but are readily induced in a susceptible strain, BALB/c, by treatment with pristane. The tumors develop in peritoneal granulomas and are characterized by Myc-activating T(12;15) chromosomal translocations and, most frequently, by secretion of IgA. A uniting feature of human and mouse plasma cell neoplasms is the critical role played by IL-6, a B cell growth, differentiation, and survival factor. To directly test the contribution of IL-6 to PCT development, we generated BALB/c mice carrying a widely expressed IL-6 transgene. All mice exhibited lymphoproliferation and plasmacytosis. By 18 months of age, over half developed readily transplantable PCT in lymph nodes, Peyer's patches, and sometimes spleen. These neoplasms also had T(12;15) translocations, but remarkably, none expressed IgA. Unexpectedly, approximately 30% of the mice developed follicular and diffuse large cell B cell lymphomas that often coexisted with PCT. These findings provide a unique model of extramedullary PCT for studies on pathogenesis and treatment and suggest a previously unappreciated role for IL-6 in the genesis of germinal center-derived lymphomas.

Translocation remodeling in the primary BALB/c plasmacytoma TEPC 3610

Myc-activating chromosomal 12;15 translocations, the hallmark mutations of inflammation-induced BALB/c plasmacytomas, have recently been shown to undergo remodeling by isotype switch-like genetic recombinations that remove approximately 180 kb of immunoglobulin heavy-chain sequence in the vicinity of the rearranged, expressed Myc gene. Here we combine cytogenetic data on the 12;15 translocation (SKY and FISH) with the molecular analysis of key junction sites (long-range PCR followed by DNA sequencing) to demonstrate that translocation remodeling occurred as an infrequent, stepwise, and disomic tumor progression event in the tetraploid, fully transformed, and transplantable plasmacytoma TEPC 3610. This result was used, in conjunction with previously obtained molecular data on five other primary plasmacytomas, to devise a hypothesis that predicts that the selective pressure to undergo translocation remodeling may be predetermined by the location of the break site in Myc. The pressure may be low if the break occurs 5' of the normal promoter region of Myc, but it may be considerably stronger if the break occurs 3' of the Myc promoter. Published 2001 Wiley-Liss, Inc.

Efficiency alleles of the Pctr1 modifier locus for plasmacytoma susceptibility

The susceptibility of BALB/c mice to pristane-induced plasmacytomas is a complex genetic trait involving multiple loci, while DBA/2 and C57BL/6 strains are genetically resistant to the plasmacytomagenic effects of pristane. In this model system for human B-cell neoplasia, one of the BALB/c susceptibility and modifier loci, Pctr1, was mapped to a 5.7-centimorgan (cM) chromosomal region that included Cdkn2a, which encodes p16(INK4a) and p19(ARF), and the coding sequences for the BALB/c p16(INK4a) and p19(ARF) alleles were found to be polymorphic with respect to their resistant Pctr1 counterparts in DBA/2 and C57BL/6 mice (45). In the present study, alleles of Pctr1, Cdkn2a, and D4Mit15 from a resistant strain (BALB/cDAG) carrying DBA/2 chromatin were introgressively backcrossed to the susceptible BALB/c strain. The resultant C.DAG-Pctr1 Cdkn2a D4Mit15 congenic was more resistant to plasmacytomagenesis than BALB/c, thus narrowing Pctr1 to a 1.5-cM interval. Concomitantly, resistant C57BL/6 mice, from which both gene products of the Cdkn2a gene have been eliminated, developed pristane-induced plasma cell tumors over a shorter latency period than the traditionally susceptible BALB/cAn strain. Biological assays of the p16(INK4a) and p19(ARF) alleles from BALB/c and DBA/2 indicated that the BALB/c p16(INK4a) allele was less active than its DBA/2 counterpart in inducing growth arrest of mouse plasmacytoma cell lines and preventing ras-induced transformation of NIH 3T3 cells, while the two p19(ARF) alleles displayed similar potencies in both assays. We propose that the BALB/c susceptibility/modifier locus, Pctr1, is an "efficiency" allele of the p16(INK4a) gene.