Multistep evolution of B-cell-derived tumors in humans and rodents

Inhibition of RNA polymerase I as a therapeutic strategy to promote cancer-specific activation of p53

Increased transcription of ribosomal RNA genes (rDNA) by RNA Polymerase I is a common feature of human cancer, but whether it is required for the malignant phenotype remains unclear. We show that rDNA transcription can be therapeutically targeted with the small molecule CX-5461 to selectively kill B-lymphoma cells in vivo while maintaining a viable wild-type B cell population. The therapeutic effect is a consequence of nucleolar disruption and activation of p53-dependent apoptotic signaling. Human leukemia and lymphoma cell lines also show high sensitivity to inhibition of rDNA transcription that is dependent on p53 mutational status. These results identify selective inhibition of rDNA transcription as a therapeutic strategy for the cancer specific activation of p53 and treatment of hematologic malignancies.

Epstein-Barr virus and its role in the pathogenesis of Burkitt's lymphoma: an unresolved issue

For several reasons Burkitt's lymphoma (BL) has become a paradigm in cancer research: for its particular geographical distribution, the presence of Epstein-Barr virus (EBV) in the cases in high incidence areas, and for the activation of the proto-oncogene c-myc by chromosomal translocation in one of the immunoglobulin gene loci. As c-MYC activates both, proliferation and apoptosis, at least two events have to cooperate in lymphomagenesis: activation of c-MYC and a shift in the balance from apoptosis towards survival. Antigenic and/or polyclonal stimulation of the B cell receptor, genetic instability imposed by activation induced deaminase (AID), as well as the viral gene products EBNA1 and several small non-coding non-polyadenylated RNAs are the main factors suspected to play an important role in the pathogenesis of BL. Despite intensive research, the role of the virus has remained largely elusive in the past decades, but the discovery of two viral microRNA clusters that are expressed in EBV associated tumors including BL has raised new hopes and expectations that EBV is going to reveal its mystery. This review focuses on the interplay between cellular and viral factors and puts special emphasis on mouse models and experimental cell culture systems that address these points.

Myc sensitizes p53-deficient cancer cells to the DNA-damaging effects of the DNA methyltransferase inhibitor decitabine

Decitabine (also referred to as 5-aza-2'-deoxycytidine) is a drug that has recently been approved by the Food and Drug Administration (FDA) for the treatment of myelodysplastic syndrome (MDS). The mechanism of action is believed to be the blocking of DNA methylation and thereby reactivating silenced genes involved in harnessing MDS. When analyzing reactivation of genes involved in Burkitt lymphoma (BL), we discovered that decitabine also sensitizes tumor cells by inducing DNA damage. This sensitization is grossly augmented by the MYC oncogene, which is overexpressed in BL, and occurs in cells lacking a functional p53 tumor suppressor pathway. In p53-deficient BL cells and p53(-/-) mouse embryo fibroblasts, Myc overrides a transient G2-block exerted by decitabine via activation of Chk1. This triggers aneuploidy and cell death that correlates with, but can occur in the absence of, Epstein-Barr virus (EBV) reactivation, caspase activation, and/or expression of the BH3-only protein Puma. In vivo modeling of Myc-induced lymphoma suggests that decitabine constitutes a potential new drug against lymphoma that would selectively sensitize tumor cells but spare normal tissue.

A non-transgenic mouse model for B-cell lymphoma: in vivo infection of p53-null bone marrow progenitors by a Myc retrovirus is sufficient for tumorigenesis

The c-Myc oncoprotein is strongly implicated in B-cell neoplasms such as human Burkitt lymphomas and mouse plasmocytomas. Transgenic mice in which the myc gene is juxtaposed to an immunoglobulin enhancer (E(mu)-myc) also develop B-cell lymphomas, but relatively late in life. In addition, these neoplasms are invariably clonal, suggesting the involvement of additional mutations. Such mutations frequently affect the p53 tumour suppressor gene or its positive regulator Arf, hinting that inactivation of the p53 pathway might be the second hit required for the progression towards malignancy. However, even tumours arising in E(mu)-myc/Arf-null animals are thought to be clonal. This observation raised doubts whether overexpression of Myc in p53-null B-cell precursors is sufficient for tumorigenesis. To address this question, we have established a new, non-transgenic mouse model of B-lymphoma. This model is based on isolation of primary bone marrow (BM) cells, admixing them with packaging cells producing a Myc-encoding retrovirus (LMycSN), and subcutaneous injection into a host with which BM cells are syngeneic. Predictably, wild type BM cells infected in vivo by LMycSN were not tumorigenic. However, LMycSN-infected p53-null BM cells readily gave rise to B-cell lymphomas composed predominantly of late pro-B/small pre-B-cells. In these tumours, heavy chain gene rearrangements were analysed using two independent PCR-based assays. All neoplasms with DJ-rearrangements were found to be polyclonal. This result suggests that inactivation of p53 and overexpression of Myc is all that is necessary for the development of full-fledged B-lymphomas. Our model would also be instrumental in assessing the transforming potential of Myc mutants and in studying cooperation between Myc and other oncogenes.

p14ARF homozygous deletion or MDM2 overexpression in Burkitt lymphoma lines carrying wild type p53

The hallmark of Burkitt lymphoma (BL) is a constitutively activated c-myc gene that drives tumor cell growth. A majority of BL-derived cell lines also carry mutant p53. In addition, the p16INK4a promoter is hypermethylated in most BL biopsies and BL cell lines, leading to silencing of this gene. Activation of c-myc and/or cell cycle dysregulation can induce ARF expression and p53-dependent apoptosis. We therefore investigated the p14ARF-MDM2-p53 pathway in BL cell lines. p14ARF was expressed and localized to nucleoli in all BL carrying mutant p53. Three out of seven BL carrying wt p53 had a homozygous deletion of the CDKN2A locus that encodes both p14ARF and p16INK4a. Three BL carrying wild type p53 retained the CDKN2A locus and overexpressed MDM2. DNA sequencing revealed a point mutation in CDKN2A exon 2 in one of these BL, Seraphine. However, this point mutation did not affect p14ARF's nucleolar localization or ability to induce p53. The Bmi-1 protein that negatively regulates the p14ARF promoter and co-operates with c-myc in tumorigenesis was expressed at low to moderate levels in all BL analysed. Our results indicate that inactivation of the ARF-MDM2-p53 pathway is an essential step during the development of Burkitt lymphoma, presumably as a mechanism to escape c-myc induced apoptosis.

Deregulated expression of c-Myc in a translocation-negative plasmacytoma on extrachromosomal elements that carry IgH and myc genes

The induced expression of c-Myc in plasmacytomas in BALB/c mice is regularly associated with nonrandom chromosomal translocations that juxtapose the c-myc gene to one of the Ig loci on chromosome 12 (IgH), 6 (IgK), or 16 (IgL). The DCPC21 plasmacytoma belongs to a small group of plasmacytomas that are unusual in that they appear to be translocation-negative. In this paper, we show the absence of any c-myc-activating chromosomal translocation for the DCPC21 by using fluorescent in situ hybridization, chromosome painting, and spectral karyotyping. We find that DCPC21 harbors c-myc and IgH genes on extrachromosomal elements (EEs) from which c-myc is transcribed, as shown by c-myc mRNA tracks and extrachromosomal gene transfer experiments. The transcriptional activity of these EEs is supported further by the presence of the transcription-associated phosphorylation of histone H3 (H3P) on the EEs. Thus, our data suggest that in this plasmacytoma, c-Myc expression is achieved by an alternative mechanism. The expression of the c-Myc oncoprotein is initiated outside the chromosomal locations of the c-myc gene, i.e., from EEs, which can be considered functional genetic units. Our data also imply that other "translocation-negative" experimental and human tumors with fusion transcripts or oncogenic activation may indeed carry translocation(s), however, in an extrachromosomal form.

Amplification of the translocated c-myc genes in three Burkitt lymphoma cell lines

Translocations of the coding exons of the human c-myc gene are consistent features of human Burkitt lymphomas (BL). In the BL cell lines CA46, JD40, and ST486, the second and third c-myc exons have been translocated into the immunoglobulin heavy chain locus. In addition to this rearrangement, in all three cell lines, we have found that the translocated c-myc exons show low-level amplification relative to restriction fragments from the germ-line c-myc gene. The patterns of hybridization of an IgM switch region probe suggest that immunoglobulin heavy chain sequences have been co-amplified with the translocated c-myc sequences. Differential sedimentation was used to determine whether the amplified sequences reside in high-molecular-weight chromosomes or low-molecular-weight extrachromosomal DNA. In JD40 and ST486 cells, the amplified c-myc sequences were found on high-molecular-weight chromosomes ST486 cells also contained translocated C-myc sequences in low-molecular-weight, extrachromosomal DNA, as did CA46 cells. These conclusions were corroborated by fluorescence in-situ hybridization (FISH) of HeLa, CA46, ST486 and JD40 metaphase chromosomes. These results suggest that there is ongoing selection for cells containing amplified copies of the expressed c-myc sequences. and that there is continuous generation of extrachromosomal copies of the translocated c-myc sequences in ST486 and CA46 cells.

Somatic hypermutation in normal and transformed human B cells

In the human, most IgM+IgD+ as well as CD5+ peripheral blood B cells express unmutated V genes and thus can be assigned to a pre-germinal centre (GC) stage of development. The memory B-cell compartment generated in the GC reaction and characterized by cells bearing somatically mutated V-region genes consists not only of class-switched cells, but also of IgM-only B cells and perhaps a subset of IgM+IgD+B cells expressing the CD27 antigen. Comparison of the rearranged V-region genes of human B-cell lymphomas with those of the normal B-cell subsets allows the identification of the progenitor cells of these tumours in terms of their stage of maturation. On this basis, most B-cell non-Hodgkin lymphomas, and in addition Hodgkin and Reed-Sternberg (HRS) cells in Hodgkin's disease (HD), are derived from B cells at a GC or post-GC stage of development. The mutation pattern indicates that the precursors of the tumour clones have been stringently selected for expression of a functional antigen receptor with one notable exception: HRS cells in classical (but not lymphocyte-predominant) HD appear to be derived from "crippled" GC B cells. Sequence analysis of rearranged V genes amplified from single tonsillar GC B cells revealed that the somatic hypermutation process introduces deletions and/or insertions into V-region genes more frequently than indicated by previous investigations. Presumably, this feature of the hypermutation mechanism is often responsible for the generation of heavy chain disease, and also several types of chromosomal translocations of oncogenes into immunoglobulin loci in human B-cell lymphomas.

Frequent occurrence of deletions and duplications during somatic hypermutation: implications for oncogene translocations and heavy chain disease

Human naive and germinal center (GC) B cells were sorted by flow cytometry and rearranged VH region genes were amplified and sequenced from single cells. Whereas no deletions or insertions were found in naive B cells, approximately 4% of in-frame and >40% of out-of-frame rearrangements of GC B cells harbored deletions and/or insertions of variable length. The pattern of deletions/insertions and their restriction to mutated V genes strongly suggests that they result from somatic hypermutation. Deletions and insertions account for approximately 6% of somatic mutations introduced into rearranged VH region genes of GC B cells. These deletions/insertions seem to be the main cause for the generation of heavy chain disease proteins. Furthermore, it appears that several types of oncogene translocations (like c-myc translocations in Burkitt's lymphoma) occur as a byproduct of somatic hypermutation within the GC-and not during V(D)J recombination in the bone marrow as previously thought.

Detection of EBV RNA (EBER-1 and EBER-2) in malaria lymph nodes by in situ hybridization

Acute Plasmodium falciparum malaria in African children allows expansion of latent Epstein-Barr virus infection, leading to colonization of lymph nodes by virus-infected lymphoblasts in 60% of cases as demonstrated by in situ hybridization for the detection of EBER-1 and EBER-2 RNA. This probably arises against a background of malaria-induced immunosuppression to EBV and concurrent lymphoid activation. The relevance of the results to the pathogenesis of African endemic Burkitt's lymphoma is discussed.

Integration of an Epstein-Barr virus episome 3' into the gene encoding immunoglobulin heavy-chain alpha 1 in a lymphoblastoid cell line

For the first time we have characterized an unoccupied site of Epstein-Barr (EBV) virus integration in a lymphoblastoid cell line, RGN1. The site of integration is about 1.5 kb downstream from the gene encoding the heavy chain constant alpha 1, specifying immunoglobulin A (IgA). Sequence and Southern analysis allowed us to hypothesize that integration occurred via a double exchange involving the viral latent origin of DNA replication (oriP) and the human DNA. The region involved in the integration is transcribed into poly(A)+ RNA in all the tested lymphoid lines, but not in the RGN1 line. We suggest a mechanism of integration primed by interactions between oriP and cell ori and its potential role in the establishment and/or evolution of EBV-carrying lines.