Mechanisms of retrovirus-induced leukaemia: selected aspects

Long-distance activation of the Myc protooncogene by provirus insertion in Mlvi-1 or Mlvi-4 in rat T-cell lymphomas

T-cell lymphomas induced by Moloney murine leukemia virus frequently have proviruses integrated at the Mlvi-4 and Mlvi-1 loci, which map approximately 30 and 270 kilobases 3' of the promoter region of the Myc protooncogene, respectively. Provirus insertion in these loci is responsible for the activation of adjacent genes. To determine whether Myc expression was also affected by these provirus insertions, we constructed T-cell hybrids between two rat thymic lymphomas containing a provirus in Mlvi-4 or Mlvi-1 and the murine T-cell lymphoma line BW5147. These hybrids segregated the provirus-containing rearranged alleles from the normal nonrearranged alleles of Mlvi-4 and Mlvi-1, and they carried an intact copy of rat Myc. Using an S1 nuclease protection assay, we observed that the expression of the rat Myc cosegregated with the rearranged Mlvi-4 or Mlvi-1 locus. However, provirus insertion in these loci had no effect on promoter utilization or on the expression of the murine Myc locus. We conclude that provirus insertion exerts a long-range cis effect on the expression of Myc. Therefore, provirus integration in a single locus may affect the expression of multiple genes, some of which may be located a long distance from the site of integration.

Activation of the Mlvi-1/mis1/pvt-1 locus in Moloney murine leukemia virus-induced T-cell lymphomas

The Mlvi-1/mis-1/pvt-1 locus, located approximately 270 kilobase pairs 3' of the c-myc protooncogene, was originally discovered as a common region of provirus integration in Moloney murine leukemia virus-induced rat T-cell lymphomas. The same locus was shown subsequently to be coamplified with c-myc and to be involved in chromosomal translocations in a variety of human and animal neoplasms. Provirus integration in Mlvi-1 in Moloney murine leukemia virus-induced rat T-cell lymphomas activates the c-myc protooncogene. The studies reported here were aimed to determine whether, in addition to the activation of c-myc, provirus integration affected the expression of other neighboring genes. Provirus integration was shown to occur in three clusters separated by regions of uninterrupted DNA. The proviruses in all three clusters had integrated in a single-transcriptional orientation, and they appeared intact. Systematic hybridization of Mlvi-1 clones to rat, mouse, and human genomic DNA revealed three patches of evolutionarily conserved sequences. Two of them were mapped in regions targeted by the provirus, and the third was mapped immediately 5' to the provirus clusters. A probe derived from the conserved sequences 5' of the integrated proviruses detected a tumor-specific RNA transcript in tumors carrying a provirus in Mlvi-1 or in the neighboring Mlvi-4 and c-myc loci. The highest level of RNA transcript expression, however, was seen in a CD4+ CD8+ tumor cell line that was not carrying a provirus in this region. We conclude that provirus insertion in this region activates both c-myc and another gene that is located in the immediate vicinity of the integrated Mlvi-1 proviruses and may be developmentally regulated in T cells.

The Mlvi-1 locus involved in the induction of rat T-cell lymphomas and the pvt-1/Mis-1 locus are identical

Mlvi-1 defines a locus of proviral integration in rat thymomas induced by Moloney murine leukemia virus. pvt-1/Mis-1 represents an independently identified locus which becomes rearranged either by chromosomal translocation in murine plasmacytomas or by provirus insertion in retrovirus-induced murine and rat thymic lymphomas. Although it had been claimed that pvt-1/Mis-1 and Mlvi-1 represent two different loci, we present here evidence showing that they are identical. This finding demonstrates the need for rigorous characterization of any newly identified common regions of integration in retrovirus-induced neoplasms.

Somatic mutation and transcriptional deregulation of myc in endemic Burkitt's lymphoma disease: heptamer-nonamer recognition mistakes?

We examined the structure and expression of the myc protooncogene in DNA extracted from a primary (uncultured) endemic Burkitt's lymphoma sample designated eBL3. Dot and Northern (RNA) blot analyses demonstrated extreme levels of myc RNA in the eBL3 sample. Nearly complete sequence data of the altered myc locus isolated from eBL3 DNA demonstrated extensive mutations (duplications, insertions, and deletions) in critical myc regulatory regions. Taken together, the data support the idea that myc transcriptional deregulation in Burkitt's lymphoma disease may be a consequence of the position and number of mutations produced within and around the myc locus. Furthermore, the myc exon-1-intron-1 hypermutable PvuII site is part of a potential heptamer-nonamer recognition sequence, suggesting a mechanism for mutation in endemic Burkitt's lymphoma disease.

Somatic mutation and transcriptional deregulation of myc in endemic Burkitt's lymphoma disease: heptamer-nonamer recognition mistakes?

We examined the structure and expression of the myc protooncogene in DNA extracted from a primary (uncultured) endemic Burkitt's lymphoma sample designated eBL3. Dot and Northern (RNA) blot analyses demonstrated extreme levels of myc RNA in the eBL3 sample. Nearly complete sequence data of the altered myc locus isolated from eBL3 DNA demonstrated extensive mutations (duplications, insertions, and deletions) in critical myc regulatory regions. Taken together, the data support the idea that myc transcriptional deregulation in Burkitt's lymphoma disease may be a consequence of the position and number of mutations produced within and around the myc locus. Furthermore, the myc exon-1-intron-1 hypermutable PvuII site is part of a potential heptamer-nonamer recognition sequence, suggesting a mechanism for mutation in endemic Burkitt's lymphoma disease.