Epstein-Barr virus-transformed pro-B cells are prone to illegitimate recombination between the switch region of the mu chain gene and other chromosomes

Construction of Japanese BAC library Yamato-2 (JY2): a set of 330K clone resources of damage-minimized DNA taken from a genetically established Japanese individual

A bacterial artificial chromosome (BAC) library referred to as Yamato-2 (JY2), was constructed from a Japanese individual and contained 330,000 clones. Library construction was based on 2 concepts: Japanese pedigree and non-immortalization. Genomic DNA was extracted from white blood cells from umbilical cord blood of a Japanese male individual. Four traits of the sample, (1) amelogenin DNA, (2) short tandem repeat (STR), (3) mitochondrial DNA (mtDNA), and (4) HLA-allele typing, were investigated to verify attribution of the donor. One of the samples with quite good Japanese characteristics was named JY2 and used as a resource for construction of a BAC library. Amelogenin DNA indicated male. STR indicated Mongoloid. MtDNA suggested haplogroup B, which is different from any other diploid whose sequence has been reported. The HLA gene was classified into east-Asian specific haplotype. These results revealed that JY2 was obtained from a Japanese male. We sequenced both ends of 185,012 BAC clones. By using the BLAST search, BAC end sequences (BESs) were mapped on the human reference sequence provided by NCBI. Inserts of individual BAC clones were mapped with both ends properly placed. As a result, 103,647 BAC clones were successfully mapped. The average insert size of BAC calculated from the mapping information was 130 kb. Coverage and redundancy of the reference sequence by successfully mapped BAC clones were 96.4% and 3.9-fold, respectively. This library will be especially suitable as a Japanese standard genome resource. The availability of an accurate library is indispensable for diagnostics or drug-design based on genome information, and JY2 will provide an accurate sequence of the Japanese genome as an important addition to the human genome.

Somatic mutations and activation-induced cytidine deaminase (AID) expression in established rheumatoid factor-producing lymphoblastoid cell line

Epstein-Barr virus (EBV) transforms human peripheral B cells into lymphoblastoid cell lines (LCLs), allowing the production of specific antibody-secreting cell lines. We and others have previously found that in contrast to peripheral blood B cells, EBV-transformed lymphoblastoid cell lines express the activation-induced cytidine deaminase (AID) gene. The opposite is true for the germinal center-specific BCL6 gene: it is expressed in adult peripheral blood B cells and is no longer expressed in LCLs. The present work extends our findings and shows that whereas AID expression is rapidly induced following EBV infection, BCL6 expression is gradually down-regulated and is fully extinguished in already established LCLs. The question of whether AID activation induces the process of somatic hypermutation (SHM) was investigated in adult-derived LCLs. It was found that the VH gene from the rheumatoid factor-producing RF LCL (derived from a rheumatoid arthritis patient), accumulated somatic point mutations in culture. Overall, nine unique mutations have accumulated in the rearranged VH gene since the generation of the RF cell line. Four additional intraclonal mutations were found among 10 cellular clones of the RF cells. One out of the four was in CDR1 and could be correlated with loss of antigen-binding activity in three out of the 10 clones. Altogether, these 13 mutations were preferentially targeted to the DGYW motifs and showed preference for CG nucleotides, indicating that they were AID-mediated. By contrast, mutations were not detected among 3700-4000 nucleotides each of the Vlambda, Cmu and GAPDH genes derived from the same RF cell cultures and the cellular clones. Our results thus show that AID may generate point mutations in the rearranged Ig VH during in vitro cell culture of adult-LCLs and that these mutations may be responsible, at least in part, for the known instability and occasional loss of antigen-binding activity of antibody-secreting LCLs.

DNA demethylation and pericentromeric rearrangements of chromosome 1

Rearrangements in the vicinity of the centromere of chromosome 1 are over-represented in many types of human cancer and are a characteristic feature of a rare genetic disease called ICF (immunodeficiency, centromeric heterochromatin instability, and facial anomalies). Evidence is presented that implicates DNA hypomethylation in the formation of these pericentromeric chromosomal anomalies. The DNA methylation inhibitors 5-azadeoxycytidine and 5-azacytidine, but not other tested genotoxins, induced the preferential formation of pericentromeric rearrangements of chromosome 1 at a very high frequency in a pro-B-cell line (FLEB14) and at a lower frequency in a mature B-cell line (AHH-1). These abnormal chromosomes appear identical to the diagnostic chromosomal aberrations in the ICF syndrome. A major component of the pericentromeric DNA in chromosome 1, satellite 2, was shown to be hypomethylated in an ICF B-cell line, although DNA from this cell line did not display detectable overall hypomethylation. It is hypothesized that demethylation in certain DNA regions, including in pericentromeric satellite DNA, helps lead to pericentromeric chromosomal rearrangements in lymphocytes from ICF patients and in normal lymphoblastoid cells incubated in vitro with DNA demethylating agents.

Juxtaposition of N-myc and Ig kappa through a reciprocal t(6;12) translocation in a mouse plasmacytoma

Nearly all mouse plasmacytomas (MPCs) carry an Ig/myc translocation. Any one of the three Ig loci may participate, while the myc contribution has been limited to c-myc, excluding other members of the myc gene family. The same is true for the other two known Ig/myc translocation-carrying tumors, Burkitt's lymphoma and rat immunocytoma. It is believed that the Ig/myc juxtaposition plays a decisive, rate limiting role in the genesis of the three tumors, acting through the constitutive activation of myc that makes it refractory to normal regulation. Here we describe the molecular analysis of a unique t(6;12)(CI;B) translocation that we previously identified in an exceptional MPC that expressed N-myc but not c-myc. We now show that the translocation led to the juxtaposition of N-myc and Ig kappa. This is the first case of an Ig/myc-carrying tumor that involves N-myc rather than c-myc. These findings suggest that the translocation may already have occurred at the pro- or pre-B cell stage at which N-myc is open for transcription. According to this interpretation, constitutive activation of N-myc would suppress the expression of c-myc, but would not interfere with the differentiation of the pro-B cell into a fully mature plasma cell. Its tumorigenic influence would become manifest only at the time when the cell would normally be programmed to leave the cycling compartment, in connection with its terminal differentiation.

Documentation of Burkitt lymphoma with t(8;14) (q24;q32) in X-linked lymphoproliferative disease

BACKGROUND

Acquired hypogammaglobulinemia or agammaglobulinemia, aplastic anemia, chronic or fatal infectious mononucleosis (IM), virus-associated hemophagocytic syndrome, and a variety of B-cell malignant lymphomas (ML) develop in boys with X-linked lymphoproliferative disease (XLP) after infection by the Epstein-Barr virus (EBV). They have an inherited immunodeficiency to EBV. Approximately 80% of the patients die during childhood and 100% by the age of 40. The ML occurring in patients with XLP are different from those of other populations in that there is a maternal family history of males with phenotypes of XLP, particularly ML involving the ileocecal region.

METHODS

This article describes two brothers with XLP in whom ML developed. Also, a maternally related male cousin had died of aplastic anemia complicating IM.

RESULTS

A Burkitt lymphoma (BL)-specific translocation of t(8;14) (q24;q32) was observed in the BL cells of the younger brother. The histopathologic appearance and rapid relapse after complete remission in the patient also are suggestive of this aggressive phenotype.

CONCLUSIONS

This tumor in the patient documents that the BL of patients with XLP probably arises from characteristic tumor-specific chromosomal translocations, as hypothesized in 1980.

Host-cell-phenotype-dependent control of the BCR2/BWR1 promoter complex regulates the expression of Epstein-Barr virus nuclear antigens 2-6

Epstein-Barr virus nuclear antigens (EBNAs) are expressed in a cell-phenotype-dependent manner. EBNA 1 is regularly expressed in all Epstein-Barr virus-carrying cells, whereas EBNAs 2-6 are only expressed in Epstein-Barr virus-carrying cells with a lymphoblastoid phenotype including group III Burkitt lymphoma (BL) lines positive for B-cell activation markers. Transcripts are initiated at the BCR2 or exceptionally at one BWR1 promoter in lymphoblastoid cell lines and group III BL lines. In group I BL lines, nasopharyngeal carcinoma, and the somatic cell hybrids, where EBNAs 2-6 are downregulated, the BCR2/BWR1 promoter complex is inactive or switched off. Upregulation of EBNAs 2-6 in group III BL cells and in 5-azacytidine-treated group I BL cells accompanies the activation of the silent BCR2/BWR1 promoters. Activation of BCR2 parallels demethylation of at least one CpG pair in the same promoter region. The activity of BCR2/BWR1 promoter complex depends on a particular B-cell phenotype. EBNA 1 transcription must be initiated at another promoter in cells that express only EBNA 1.

An exceptional mouse plasmacytoma with a new kappa/N-myc [T(6; 12) (C1; B)] translocation expresses N-myc but not c-myc

Mouse plasmacytomas (MPC) carry one of three reciprocal translocations that juxtapose c-myc to one of the three immunoglobulin (Ig) loci. Here we describe an exceptional MPC, induced by pristane oil and Abelson (A-MuLV) virus. It does not carry any of the three c-myc/Ig translocations, but contains a previously unknown reciprocal T(6;12) translocation affecting the bands known to carry the IgK (6C/1) and N-myc (12B) loci, respectively. Northern blot analysis showed high N-myc but no c-myc expression. This is consistent with the constitutive activation of N-myc by a juxtaposition of the IgK and N-myc loci. Reciprocal translocation in B-cell derived tumors are believed to involve the Ig loci by the action of some enzyme that participates in the physiological rearrangement of the Ig loci. Only transcriptionally active chromatin regions are accessible to such recombinases (Alt et al. 1987). N-myc is not expressed in B-cells, but it is transcriptionally active during the early pro- and pre-B cell stage, whereafter it and the surrounding chromatin region becomes inactive (Smith et al. 1992). It is therefore most likely that the N-myc/Kappa translocation has arisen at an early stage of B-cell differentiation. This would imply that the myc/Ig translocations do not block B-cell differentiation. They also reaffirm the functional equivalence of N- and c-myc in relation to B-cell carcinogenesis, as shown by our previous work on tumor induction in N-myc transgenic mice (Wang et al. 1992).

Viruses and oral cancer

Oral cancer is a disease with a complex etiology. There is evidence for important roles of smoking, drinking, and genetic susceptibility, as well as strong indications that DNA viruses could be involved. The herpes simplex virus type 1 has been associated with oral cancer by serological studies, and animal models and in vitro systems have demonstrated that it is capable of inducing oral cancer. Papillomaviruses are found in many oral cancers and are also capable of transforming cells to a malignant phenotype. However, both virus groups depend on co-factors for their carcinogenic effects. Future research on viruses and oral cancer is expected to clarify the role of these viruses, and this will lead to improvements in diagnosis and treatment of the disease.

SCID mouse model of Epstein-Barr virus-induced lymphomagenesis of immunodeficient humans

Immunodeficient humans are at very high risk of developing Epstein-Barr virus (EBV)-induced lymphomagenesis. Severe combined immunodeficient mice (SCID) have been shown to develop lymphoproliferative disease (LPD) following transfer of peripheral blood leukocytes (PBL) with latent EBV. To study mechanisms of lymphomagenesis, we compared results of engraftment of PBL from normal donors and immunodeficient donors with X-linked lymphoproliferative disease (XLP). Graft-versus-host disease (GVHD) developed in 6 of 10 SCID mice 4 to 8 weeks following transfer of PBL from normal donors. In contrast, none of 9 mice engrafted with PBL from XLP patients with T-cell defects showed GVHD. LPD developed in mice regardless of the immune competence of the donors. The expression of EBV-encoded proteins, results of immunophenotyping, and karyotyping of the LPD lesions revealed lethal oligoclonal LPD owing to transfer of latent EBV in B cells in mice engrafted with PBL from seropositive donors. Polyclonal LDP developed in mice engrafted with PBL from seronegative patients which were infected with B95-8 virus 6 weeks after transfer of the cells. This model is useful for investigating mechanisms of EBV-induced LDP in immunodeficient patients.

Effect of transforming growth factor-beta 1 and -beta 2 on the proliferation of Burkitt lymphoma and lymphoblastoid cell lines

We tested the effect of transforming growth factor (TGF)-beta 1 and TGF-beta 2 on the proliferation of human B cell lines. The panel was selected to give information whether (1) their origin, (2) their phenotype, (3) their Epstein-Barr virus (EBV) carrier state, influence their responsiveness. The growth of lymphoblastoid cell lines (LCL) was not inhibited by TGF-beta 1. The EBV-carrying Burkitt lymphoma (BL) lines, Daudi, Jijoye, Rael but not Raji were inhibited. Three EBV-negative BL lines and the majority of their converted sublines were sensitive. The cell lines tested expressed TGF-beta receptors and TGF-beta 1 transcripts. The proliferation of EBV-infected B cells was inhibited by TGF-beta, their sensitivity decreased, however, after 3 days. The results suggest that the activation state of the B cells is decisive for TGF-beta sensitivity and EBV influences it indirectly by changing the cell phenotype.

Immortalizing genes of Epstein-Barr virus

EBV immortalizes human B lymphocytes efficiently. Ten of its approximately 100 genes are expressed in these proliferating lymphoblasts and are candidates for mediating the changes central to the immortalization of the cell. Enough has been learned now about three of these viral genes to indicate that they are likely to be required for immortalization. As more is learned, additional genes of EBV will probably be found to support the process of immortalization of the host cell. EBNA-2 has been shown genetically to be required for EBV to immortalize an infected B lymphocyte. The biochemical activities of EBNA-2 that constitute this requirement have not been identified. Many experiments indicate that EBNA-2 affects the accumulation of specific viral and cellular RNAs. These effects, however, can be detected only in certain EBV-negative B-lymphoblastoid cells. It is, therefore, not clear that the known effects of EBNA-2 adequately explain its ubiquitous requirement in the immortalization of primary human B lymphocytes. LMP is likely to be required for immortalization because it can affect the growth properties of established human lymphoid and epithelial cells and can transform at least two established rodent cells to proliferate in an anchorage-independent manner. The structure of this viral protein, its position in the plasma membrane, many of its biochemical properties, as well as studies of its mutant derivatives are consistent with its acting as a growth factor receptor or affecting the activity of such a receptor. However, no biochemical activity has been assigned directly to LMP, and both its mechanism of action and its possible contribution to immortalization by EBV remain enigmatic. EBNA-1 presumably is required for EBV to immortalize a B lymphocyte because it is essential for the initiation of plasmid DNA replication by EBV. Circumstantial observations indicate also that EBNA-1 is probably necessary for sustaining viral DNA replication in the proliferating cell population. EBNA-1 may well affect the regulation of transcription of viral genes that themselves are required for immortalization. These roles of EBNA-1 are performed in part by its site-specific binding to the elements of oriP required in cis for the replication of EBV plasmid DNAs. It is probable that EBNA-1 also binds both to a set of cellular proteins that function in transcription and to a nonidentical set of cellular proteins that function in replication. EBV effects a fascinating phenotypic change in B lymphocytes it infects. It does so by using several viral genes that alter the physiology of the cell by different means.(ABSTRACT TRUNCATED AT 400 WORDS)

Clonal lymphoid progenitor cell lines expressing the BCR/ABL oncogene retain full differentiative function

The early stages of hematopoiesis have been difficult to study due to problems in obtaining homogeneous populations of progenitor cells that retain both self-renewal and differentiative capacities. We have developed an in vitro system in which transformation of murine bone-marrow cells with the BCR/ABL oncogene, a gene associated with stem-cell leukemias, leads to the outgrowth of clonal lines that have an early lymphoid progenitor cell phenotype. The progenitor cells retain immunoglobulin heavy and light chain genes in a germ-line configuration. These cells give rise in vitro to pre-B cells that have diverse diversity-joining (D-J) region rearrangements, and on transfer to mice with severe combined immune deficiency, differentiate to surface IgM+, immunoglobulin-secreting B cells that respond to T-cell help and function in an antigen-specific fashion. Although their growth is stimulated by BCR/ABL, the progenitor cells depend for continued growth on a stromal cell-derived soluble factor distinct from the pre-B-cell growth factor, interleukin 7. These findings show that BCR/ABL can promote proliferation of an early hematopoietic progenitor cell without preventing its differentiation. This system provides a means of studying the complete B-cell developmental process from clonal progenitor cell to end-stage plasma cell.