Studies on the association of the Epstein-Barr virus genome and human chromosomes

Multiple pathways for Epstein-Barr virus episome loss from nasopharyngeal carcinoma

Epstein-Barr virus (EBV) is the prototypical example for episomal persistence of genetic information. Yet, little is known about how this viral episome is lost. Episome loss occurs naturally in nasopharyngeal carcinoma (NPC) upon explantation into culture. Using whole-genome profiling, we found evidence for 2 different pathways of episome loss: (i) rapid loss of the entire episome or (ii) successive mutation/deletion of the episome until at least 1 essential cis-element is destroyed. This second phenotype was seen in a clone of HONE-1 NPC cells that maintains the EBV episome for prolonged time in culture. The conceptual insights provided by our quantitative analysis should aid our understanding of mammalian episomes, as well as lead to designs to cure latent viral infection.

Epstein-Barr-based episomal chromosomes shuttle 100 kb of self-replicating circular human DNA in mouse cells

We describe the microcell fusion transfer of 100-200 kb self-replicating circular human minichromosomes from human into mouse cells. This experimental approach is illustrated through the shutting of the latent 170 kb double-stranded DNA genome from the human herpesvirus, Epstein-Barr virus, into nonpermissive rodent cells. Using this interspecies transfer strategy, circular episomes carrying 95-105 kb of human DNA were successfully established at low copy number in mouse A9 cells. Selected episomes were stably maintained for 6 months, and unselected episomes were characterized by a 95% episomal retention per cell division. The establishment of a mouse artificial episomal chromosome system should facilitate evolutionary and therapeutic studies of large human DNA in rodent genetic backgrounds.

Epstein-Barr virus DNA in nasopharyngeal biopsies

The Epstein-Barr virus (EBV) has been closely associated with the undifferentiated form of nasopharyngeal carcinoma (NPC), which is particularly common in the high risk area in southeast China. We have examined 37 nasopharyngeal biopsies from patients within this high risk area, including 31 cases of undifferentiated NPC and 6 cases of patients with nasopharyngitis, for the presence of EBV DNA. We found that 26 of 31 biopsies from NPC patients were EBV DNA positive; 3 of the 6 biopsies from patients diagnosed with nasopharyngitis were also EBV DNA positive. Southern blot analysis of the DNAs obtained from the EBV genome positive biopsies, digested with EcoRI, showed that all preparations from the NPC tumors had only one band corresponding to the EcoRI A fragment when a BamHI W fragment was used as a probe. However, one tumor had an additional band with a molecular weight larger than EcoRI A. The presence of this novel band could indicate the integration of viral DNA into host cellular DNA. DNA from the same biopsies were restricted with BamHI and PstI restriction enzymes. The data obtained from these experiments suggest that the EBV genomes in both the NPC tumor biopsies and biopsies from nasopharyngitis patients obtained from an endemic area in South China may be similar to each other and to the B95-8 EBV isolate with respect to the BamHI Y region of the EBV genome. The data also demonstrate that infection of normal nasopharyngeal epithelial cells with EBV takes place in patients with nasopharyngitis.

Chromosome site for Epstein-Barr virus DNA in a Burkitt tumor cell line and in lymphocytes growth-transformed in vitro

The Epstein-Barr virus (EBV) genome stably persists in latently infected Burkitt tumor cells and growth-transformed B lymphocytes. These cells usually contain multiple copies of episomal viral DNA. Cytological hybridization of recombinant viral DNA fragments to metaphase chromosomes of two latently infected cell lines demonstrates that viral DNA localizes to both chromatids of one homologue of chromosome 1 in Namalwa, a Burkitt tumor cell line, and to both chromatids of one homologue of chromosome 4 in IB4, a cell line with transformed growth properties in vitro. The site of chromosome association remains stable in a clone of IB4 cells. Probes from five separate regions of the EBV genome hybridize to the same chromosome regions. A previously undescribed achromatic site is identified within the region of EBV chromosome cytological hybridization. These observations suggest that most or all of the EBV genome is integrated into the chromosomal DNA of Namalwa and IB4 cells. Although the chromosomal sites of EBV DNA association are among those regions with homology to the EBV IR3 repeated DNA sequence, EBV IR3 did not mediate recombination between EBV and chromosomal DNA.

Transcriptional activation of the translocated c-myc oncogene in burkitt lymphoma

We have previously demonstrated that translocations of V(H) genes from chromosome 14 to chromosome 8 and of the c-myc oncogene from chromosome 8 to chromosome 14 occur in Burkitt lymphomas with the t(8;14) chromosome translocation. An association of the c-myc gene with the C(mu) immunoglobulin gene has been observed in some but not all Burkitt lymphomas studied previously. In the present study, we have investigated the organization of the human heavy chain locus and of the c-myc gene in the P3HR-1 Burkitt lymphoma cell line. Becuase mouse/P3HR-1 somatic cell hybrids that retain only the 14q+ chromosome and no other human chromosome contain the human C(mu) and C(gamma) genes but not V(H) genes, we have concluded that the breakpoint on chromosome 14 in P3HR-1 cells is distal to C(mu) and between C(mu) and V(H). Thus, the breakpoint of human chromosome 14 differs in different Burkitt lymphoma cell lines. We also found that the human c-myc oncogene translocated to chromosome 14 in the P3HR-1 cell line is not recombined with the C(mu) gene. The breakpoint on human chromosome 8 may therefore also differ in different Burkitt lymphoma cell lines, because we have observed DNA rearrangement of the c-myc gene with the C(mu) gene in only some of the Burkitt lymphoma cell lines studied elsewhere. Interestingly, high levels of transcripts of the c-myc oncogene were observed in Burkitt lymphomas with translocated c-myc oncogenes both rearranged and unrearranged. Therefore, the translocation of a c-myc oncogene to the heavy chain locus on human chromosome 14 is apparently sufficient for its transcriptional activation and may be an essential step in the pathway leading to neoplasia.

Human c-myc onc gene is located on the region of chromosome 8 that is translocated in Burkitt lymphoma cells

Human sequences related to the transforming gene (v-myc) of avian myelocytomatosis virus (MC29) are represented by at least one gene and several related sequences that may represent pseudogenes. By using a DNA probe that is specific for the complete gene (c-myc), different somatic cell hybrids possessing varying numbers of human chromosomes were analyzed by the Southern blotting technique. The results indicate that the human c-myc gene is located on chromosome 8. The analysis of hybrids between rodent cells and human Burkitt lymphoma cells, which carry a reciprocal translocation between chromosomes 8 and 14, allowed the mapping of the human c-myc gene on region (q24 leads to qter) of chromosome 8. This chromosomal region is translocated to either human chromosome 2, 14, or 22 in Burkitt lymphoma cells.