Rearrangement of a DNA sequence homologous to a cell-virus junction fragment in several Moloney murine leukemia virus-induced rat thymomas

Linear and circular PVT1 in hematological malignancies and immune response: two faces of the same coin

Non coding RNAs (ncRNAs) have emerged as regulators of human carcinogenesis by affecting the expression of key tumor suppressor genes and oncogenes. They are divided into short and long ncRNAs, according to their length. Circular RNAs (circRNAs) are included in the second group and were recently discovered as being originated by back-splicing, joining either single or multiple exons, or exons with retained introns. The human Plasmacytoma Variant Translocation 1 (PVT1) gene maps on the long arm of chromosome 8 (8q24) and encodes for 52 ncRNAs variants, including 26 linear and 26 circular isoforms, and 6 microRNAs. PVT1 genomic locus is 54 Kb downstream to MYC and several interactions have been described among these two genes, including a feedback regulatory mechanism. MYC-independent functions of PVT1/circPVT1 have been also reported, especially in the regulation of immune responses. We here review and discuss the role of both PVT1 and circPVT1 in the hematopoietic system. No information is currently available concerning their transforming ability in hematopoietic cells. However, present literature supports their cooperation with a more aggressive and/or undifferentiated cell phenotype, thus contributing to cancer progression. PVT1/circPVT1 upregulation through genomic amplification or rearrangements and/or increased transcription, provides a proliferative advantage to malignant cells in acute myeloid leukemia, acute promyelocytic leukemia, Burkitt lymphoma, multiple myeloma (linear PVT1) and acute lymphoblastic leukemia (circPVT1). In addition, PVT1 and circPVT1 regulate immune responses: the overexpression of the linear form in myeloid derived suppressor cells induced immune tolerance in preclinical tumor models and circPVT1 showed immunosuppressive properties in myeloid and lymphoid cell subsets. Overall, these recent data on PVT1 and circPVT1 functions in hematological malignancies and immune responses reflect two faces of the same coin: involvement in cancer progression by promoting a more aggressive phenotype of malignant cells and negative regulation of the immune system as a novel potential therapy-resistance mechanism.

Role of the long non-coding RNA PVT1 in the dysregulation of the ceRNA-ceRNA network in human breast cancer

Recent findings have identified competing endogenous RNAs (ceRNAs) as the drivers in many disease conditions, including cancers. The ceRNAs indirectly regulate each other by reducing the amount of microRNAs (miRNAs) available to target messenger RNAs (mRNAs). The ceRNA interactions mediated by miRNAs are modulated by a titration mechanism, i.e. large changes in the ceRNA expression levels either overcome, or relieve, the miRNA repression on competing RNAs; similarly, a very large miRNA overexpression may abolish competition. The ceRNAs are also called miRNA "decoys" or miRNA "sponges" and encompass different RNAs competing with each other to attract miRNAs for interactions: mRNA, long non-coding RNAs (lncRNAs), pseudogenes, or circular RNAs. Recently, we developed a computational method for identifying ceRNA-ceRNA interactions in breast invasive carcinoma. We were interested in unveiling which lncRNAs could exert the ceRNA activity. We found a drastic rewiring in the cross-talks between ceRNAs from the physiological to the pathological condition. The main actor of this dysregulated lncRNA-associated ceRNA network was the lncRNA PVT1, which revealed a net biding preference towards the miR-200 family members in normal breast tissues. Despite its up-regulation in breast cancer tissues, mimicked by the miR-200 family members, PVT1 stops working as ceRNA in the cancerous state. The specific conditions required for a ceRNA landscape to occur are still far from being determined. Here, we emphasized the importance of the relative concentration of the ceRNAs, and their related miRNAs. In particular, we focused on the withdrawal in breast cancer tissues of the PVT1 ceRNA activity and performed a gene expression and sequence analysis of its multiple isoforms. We found that the PVT1 isoform harbouring the binding site for a representative miRNA of the miR-200 family shows a drastic decrease in its relative concentration with respect to the miRNA abundance in breast cancer tissues, providing a plausibility argument to the breakdown of the sponge program orchestrated by the oncogene PVT1.

Autophagy ameliorates cognitive impairment through activation of PVT1 and apoptosis in diabetes mice

The underlying mechanisms of cognitive impairment in diabetes remain incompletely characterized. Here we show that the autophagic inhibition by 3-methyladenine (3-MA) aggravates cognitive impairment in streptozotocin-induced diabetic mice, including exacerbation of anxiety-like behaviors and aggravation in spatial learning and memory, especially the spatial reversal memory. Further neuronal function identification confirmed that both long term potentiation (LTP) and depotentiation (DPT) were exacerbated by autophagic inhibition in diabetic mice, which indicating impairment of synaptic plasticity. However, no significant change of pair-pulse facilitation (PPF) was recorded in diabetic mice with autophagic suppression compared with the diabetic mice, which indicated that presynaptic function was not affected by autophagic inhibition in diabetes. Subsequent hippocampal neuronal cell death analysis showed that the apoptotic cell death, but not the regulated necrosis, significantly increased in autophagic suppression of diabetic mice. Finally, molecular mechanism that may lead to cell death was identified. The long non-coding RNA PVT1 (plasmacytoma variant translocation 1) expression was analyzed, and data revealed that PVT1 was decreased significantly by 3-MA in diabetes. These findings show that PVT1-mediated autophagy may protect hippocampal neurons from impairment of synaptic plasticity and apoptosis, and then ameliorates cognitive impairment in diabetes. These intriguing findings will help pave the way for exciting functional studies of autophagy in cognitive impairment and diabetes that may alter the existing paradigms.

PVT1: a rising star among oncogenic long noncoding RNAs

It is becoming increasingly clear that short and long noncoding RNAs critically participate in the regulation of cell growth, differentiation, and (mis)function. However, while the functional characterization of short non-coding RNAs has been reaching maturity, there is still a paucity of well characterized long noncoding RNAs, even though large studies in recent years are rapidly increasing the number of annotated ones. The long noncoding RNA PVT1 is encoded by a gene that has been long known since it resides in the well-known cancer risk region 8q24. However, a couple of accidental concurrent conditions have slowed down the study of this gene, that is, a preconception on the primacy of the protein-coding over noncoding RNAs and the prevalent interest in its neighbor MYC oncogene. Recent studies have brought PVT1 under the spotlight suggesting interesting models of functioning, such as competing endogenous RNA activity and regulation of protein stability of important oncogenes, primarily of the MYC oncogene. Despite some advancements in modelling the PVT1 role in cancer, there are many questions that remain unanswered concerning the precise molecular mechanisms underlying its functioning.

8q24 amplified segments involve novel fusion genes between NSMCE2 and long noncoding RNAs in acute myelogenous leukemia

The pathogenetic roles of 8q24 amplified segments in leukemic cells with double minute chromosomes remain to be verified. Through comprehensive molecular analyses of 8q24 amplicons in leukemic cells from an acute myelogenous leukemia (AML) patient and AML-derived cell line HL60 cells, we identified two novel fusion genes between NSMCE2 and long noncoding RNAs (lncRNAs), namely, PVT1-NSMCE2 and BF104016-NSMCE2. Our study suggests that 8q24 amplicons are associated with the emergence of aberrant chimeric genes between NSMCE2 and oncogenic lncRNAs, and also implicate that the chimeric genes involving lncRNAs potentially possess as-yet-unknown oncogenic functional roles.

Pvt1-encoded microRNAs in oncogenesis

BACKGROUND

The functional significance of the Pvt1 locus in the oncogenesis of Burkitt's lymphoma and plasmacytomas has remained a puzzle. In these tumors, Pvt1 is the site of reciprocal translocations to immunoglobulin loci. Although the locus encodes a number of alternative transcripts, no protein or regulatory RNA products were found. The recent identification of non-coding microRNAs encoded within the PVT1 region has suggested a regulatory role for this locus.

RESULTS

The mouse Pvt1 locus encodes several microRNAs. In mouse T cell lymphomas induced by retroviral insertions into the locus, the Pvt1 transcripts, and at least one of their microRNA products, mmu-miR-1204 are overexpressed. Whereas up to seven co-mutations can be found in a single tumor, in over 2,000 tumors none had insertions into both the Myc and Pvt1 loci.

CONCLUSION

Judging from the large number of integrations into the Pvt1 locus - more than in the nearby Myc locus - Pvt1 and the microRNAs encoded by it are as important as Myc in T lymphomagenesis, and, presumably, in T cell activation. An analysis of the co-mutations in the lymphomas likely place Pvt1 and Myc into the same pathway.

Replacement of murine leukemia virus readthrough mechanism by human immunodeficiency virus frameshift allows synthesis of viral proteins and virus replication

Retroviruses use unusual recoding strategies to synthesize the Gag-Pol polyprotein precursor of viral enzymes. In human immunodeficiency virus, ribosomes translating full-length viral RNA can shift back by 1 nucleotide at a specific site defined by the presence of both a slippery sequence and a downstream stimulatory element made of an extensive secondary structure. This so-called frameshift mechanism could become a target for the development of novel antiviral strategies. A different recoding strategy is used by other retroviruses, such as murine leukemia viruses, to synthesize the Gag-Pol precursor; in this case, a stop codon is suppressed in a readthrough process, again due to the presence of a specific structure adopted by the mRNA. Development of antiframeshift agents will greatly benefit from the availability of a simple animal and virus model. For this purpose, the murine leukemia virus readthrough region was rendered inactive by mutagenesis and the frameshift region of human immunodeficiency virus was inserted to generate a chimeric provirus. This substitution of readthrough by frameshift allows the synthesis of viral proteins, and the chimeric provirus sequence was found to generate infectious viruses. This system could be a most interesting alternative to study ribosomal frameshift in the context of a virus amenable to the use of a simple animal model.

The murine AIDS defective provirus acts as an insertional mutagen in its infected target B cells

In susceptible mice, the murine AIDS (MAIDS) defective virus can induce marked expansion of its target cells, the majority of which belong to the B-cell lineage. This expansion, which appears to be critical for the development of the immunodeficiency syndrome, is initially polyclonal but becomes oligoclonal late in the disease, suggesting the involvement of a secondary genetic event(s) during this proliferation. To determine whether integration of the MAIDS defective provirus into particular regions of the cellular genome contributes to this oligoclonal expansion, we searched for common provirus integration sites in enlarged lymphoid organs of MAIDS mice. We identified two common proviral integration sites, Dis-1 and Dis-2, which were occupied by a defective provirus at frequencies of 20 and 13%, respectively. Our analysis revealed that the Dis-1 region corresponds to the Sfpil1 (Spi-1, PU.1) locus, which maps on chromosome 2, and encodes a transcription factor. Insertion of the MAIDS defective provirus into this region led to a two- to threefold increase in the expression of Sfpi1 RNA. The Dis-2 locus was found to map to mouse chromosome 11, between Hox2 and Scya. It appears to be a novel locus probably harboring a gene involved in B-cell proliferation. The present study indicates that the MAIDS defective provirus can act as an insertional mutagen, thus contributing to the oligoclonal expansion of infected cells. The detection of two common proviral integration sites, each of which targetted at a low frequency in diseased organs, suggests that the deregulation of a unique gene through provirus insertion is essential for neither proliferation of infected B cells nor development of the immunodeficiency syndrome.

The Myb and Ahi-1 genes are physically very closely linked on mouse chromosome 10

Ahi-1 has previously been identified as a common helper provirus integration site on mouse Chromosome (Chr) 10 in 16% of Abelson pre-B-cell lymphomas and shown to be closely linked to the Myb protooncogene. By using long-range restriction mapping, we have mapped the Myb and Ahi-1 regions within a 120-kbp DNA fragment. The Ahi-1 region is located approximately 35 kbp downstream of the Myb gene. A further confirmation of this finding was obtained by screening a mouse YAC library. The three positive clones obtained contained both the Myb and Ahi-1 gene sequences. To test whether provirus integration in the Ahi-1 region enhances the expression of Myb by a cis-acting mechanism, we have also examined Myb gene expression in A-MuLV-induced pre-B-lymphomas. Our data have revealed that there is no clear evidence for such activation in the tumors we have tested, indicating that provirus insertion in the Ahi-1 region is activating a novel gene, apparently involved in tumor formation.

Long-range mapping of Mis-2, a common provirus integration site identified in murine leukemia virus-induced thymomas and located 160 kilobase pairs downstream of Myb

The nondefective Moloney murine leukemia virus (MuLV) induces clonal or oligoclonal T-cell tumors in mice or rats. The proviruses of these nondefective MuLVs have been shown to act as insertion mutagens most frequently activating an adjacent cellular gene involved in cell growth control. Mutations by provirus insertions, recognized as common provirus integration sites, have been instrumental in identifying novel cellular genes involved in tumor formation. We have searched for new common provirus integration sites in Moloney MuLV-induced thymomas. Using cellular sequences flanking a provirus cloned from one of these tumors, we found one region, designated Mis-2, which was the target of provirus integration in a low (3%) percentage of these tumors. Mis-2 was mapped on mouse chromosome 10, approximately 160 kbp downstream of myb. The Mis-2 region may contain a novel gene involved in tumor development.

A viral long terminal repeat expressed in CD4+CD8+ precursors is downregulated in mature peripheral CD4-CD8+ or CD4+CD8- T cells

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+ CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4- CD8- and CD4+ CD8+ T cells and was shut off in mature CD4+ CD8- and CD4- CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+ CD8- and CD4- CD8+ T cells and support a model of differentiation of CD4+ CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.

A viral long terminal repeat expressed in CD4+CD8+ precursors is downregulated in mature peripheral CD4-CD8+ or CD4+CD8- T cells

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+ CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4- CD8- and CD4+ CD8+ T cells and was shut off in mature CD4+ CD8- and CD4- CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+ CD8- and CD4- CD8+ T cells and support a model of differentiation of CD4+ CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.

Identification of a novel gene, Vin-1, in murine leukemia virus-induced T-cell leukemias by provirus insertional mutagenesis

The BL/VL3 radiation leukemia virus is a nondefective retrovirus which induces clonal or oligoclonal T-cell leukemia in mice. To study the role of provirus insertional mutagenesis in the development of these neoplasias, we searched for common provirus integration sites in BL/VL3 radiation leukemia virus-induced tumors. Using cellular sequences flanking a provirus cloned from one of these thymomas, we found that the viral genome was integrated into a common region, designated Vin-1, in a low percentage (5%) of these tumors. The proviruses found in this locus were integrated in the same orientation, close to a CpG-rich island, at proximity of a transcriptional unit encoding a 6-kb RNA. Vin-1 RNA was detected in several organs of the adult mouse. Vin-1 RNA levels were high in tumors having a provirus inserted within the Vin-1 region but were also high in some other tumors whose Vin-1 region was not found to be rearranged. Vin-1 was found to be well conserved among mammalian species and was mapped to mouse chromosome 6, between raf and K-ras-2. Vin-1 appears to be a novel gene which may be involved in tumor development.

Insertional mutagenesis: neoplasia arising from retroviral integration

The integration of retroviral proviruses near cellular genes can profoundly affect their expression. Painstaking analysis of insertion sites from a large number of tumors has revealed a number of previously unknown proto-oncogenes, and has elucidated new mechanisms whereby known proto-oncogenes can be activated. A number of these genes have been implicated in tumors of clinical relevance. At the time of writing a great deal remains to be learned of the normal function of these genes in the cell. While it has yet to be demonstrated that retroviral insertion mechanisms play some role in naturally occurring human neoplasms, they must be considered in the context of retroviral gene therapy protocols now being contemplated.

Effects of translocations on transcription from PVT

We have previously described a transcription unit on human chromosome 8, designated as PVT, that is consistently disrupted by the minority forms of translocations [t(2;8) and t(8;22)] in Burkitt's lymphoma. PVT begins 57 kilobase pairs downstream of the proto-oncogene MYC and is more than 200 kilobase pairs in length. In order to explore the pathogenic impact of translocations affecting PVT, we have characterized further the structure and transcription of the locus. In normal cells, PVT is transcribed into a variety of RNAs, the diversity of which remains unexplained. Alleles of PVT affected by translocations give rise to additional RNAs. These RNAs arise from a fusion of the first exon of PVT on chromosome 8 to the constant region of an immunoglobulin light chain on either chromosome 2 or chromosome 22. We have found no evidence that any of the normal or abnormal transcripts of PVT give rise to a protein. Our results suggest that the pathogenic effects of the variant translocations in Burkitt's lymphoma are not executed by a gene situated in a vicinity of the chromosomal breakpoints. Instead, our data leave open the possibility that the effects of the translocations may be mediated by activation of the relatively distant MYC gene.

The molecular biology of radiation-induced carcinogenesis: thymic lymphoma, myeloid leukaemia and osteosarcoma

In mice, external X- or gamma-irradiation may induce thymic lymphomas or myeloid leukaemias, while bone-seeking alpha-emitters may induce osteosarcomas and, to a lesser extent, acute myeloid leukaemia. The present paper aims to review briefly some of the experimental data with respect to the molecular mechanisms underlying these radiation-induced carcinogenic processes. Thymic lymphomagenesis proceeds through an indirect mechanism. Recombinant proviruses often occur in the tumour cell DNA, favouring the idea that they might be involved. However, there are indications that they might mediate tumour growth rather than induction. It is plausible that activation of ras oncogenes by somatic point mutations might play a role in the carcinogenic process, although at a yet undetermined stage. Myeloid leukaemogenesis is characterized by a very early, putative initiating event, consisting of non-random rearrangements and/or deletions of chromosome 2. These may be related to deletions in the developmentally important homeobox gene clusters and to rearrangements of the sequences flanking the IL-1 beta gene. Either a gene of the homeobox family or IL-1 beta might be considered as potentially involved in the induction process. Osteosarcomagenesis in mice is often associated with the expression of proviruses, and the tumours often contain somatically acquired proviruses. These viruses may contribute to tumour development by affecting various growth-suppressor genes. Viruses isolated from bone tumours, although non-sarcomagenic, induce osteopetrosis, osteomas and lymphomas upon infection of newborn mice. Osteogenic tumours frequently display amplification of a region on mouse chromosome 15, which encompasses c-myc and Mlvi-1 sequences. Enhanced transcription of various oncogenes is found in individual tumours, but no specificity for osteosarcomas has been identified. In vitro systems of skeletoblast differentiation are being developed to study tumour induction in vitro.

Effects of translocations on transcription from PVT

We have previously described a transcription unit on human chromosome 8, designated as PVT, that is consistently disrupted by the minority forms of translocations [t(2;8) and t(8;22)] in Burkitt's lymphoma. PVT begins 57 kilobase pairs downstream of the proto-oncogene MYC and is more than 200 kilobase pairs in length. In order to explore the pathogenic impact of translocations affecting PVT, we have characterized further the structure and transcription of the locus. In normal cells, PVT is transcribed into a variety of RNAs, the diversity of which remains unexplained. Alleles of PVT affected by translocations give rise to additional RNAs. These RNAs arise from a fusion of the first exon of PVT on chromosome 8 to the constant region of an immunoglobulin light chain on either chromosome 2 or chromosome 22. We have found no evidence that any of the normal or abnormal transcripts of PVT give rise to a protein. Our results suggest that the pathogenic effects of the variant translocations in Burkitt's lymphoma are not executed by a gene situated in a vicinity of the chromosomal breakpoints. Instead, our data leave open the possibility that the effects of the translocations may be mediated by activation of the relatively distant MYC gene.

Mutation of the core or adjacent LVb elements of the Moloney murine leukemia virus enhancer alters disease specificity

Transcriptional enhancers of replication-competent mouse C-type retroviruses are potent determinants of the distinct disease-inducing phenotypes of different viral isolates and can also strongly influence the incidence and latent period of disease induction. To study the contribution of individual protein-binding sites to viral pathogenicity, we introduced mutations into each of the known nuclear factor-binding sites in the enhancer region of the Moloney murine leukemia virus and injected viruses with these mutations into newborn NFS mice. All viruses induced disease. Viruses with mutations in both copies of the leukemia virus factor a (LVa) site, leukemia virus factor c (LVc) site, or in just the promoter proximal copy of the glucocorticoid response element (GRE) had a latent period of disease onset and disease specificity indistinguishable from that of the wild-type Moloney virus. Viruses with mutations in two or three of the GREs, in both copies of the leukemia virus factor b (LVb) site, in two of the four nuclear factor 1 (NF1) consensus motifs, or in both copies of the conserved viral core element showed a significant delay in latent period of disease induction. Strikingly, viruses with mutations in the core element induced primarily erythroleukemias, and mutations in the LVb site also resulted in a significant incidence of erythroleukemias. These and other genetic and biochemical studies suggest models for how subtle alterations in the highly conserved structure of mouse C-type retrovirus enhancers can produce a dramatic effect on disease specificity.

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.

Three breakpoints of variant t(2;8) translocations in Burkitt's lymphoma cells fall within a region 140 kilobases distal from c-myc

The variant translocations t(2;8) in Burkitt's lymphoma cells join band q24 of chromosome 8, distal from c-myc, to the Igkappa locus, with considerable variation in the location of the breakpoints on chromosome 8. We report the cloning and molecular characterization of a chromosome 8 region, distal from the c-myc locus, which encompasses the breakpoints of the Burkitt's lymphoma cell lines BL64, BL21, and LY91 within 11 kilobase pairs, termed provisionally bvr-1 (Burkitt's variants' rearranging region 1). Using probes from the c-myc, the bvr-1, and the human pvt-1 loci obtained by chromosome walking coupled with pulsed-field gel electrophoresis, we have constructed a physical map of the region 3' of c-myc. We map bvr-1 and pvt-1 about 140 and 260 kilobase pairs, respectively, distal from c-myc.

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 PVT gene frequently amplifies with MYC in tumor cells

The line of human colon carcinoma cells known as COLO320-DM contains an amplified and abnormal allele of the proto-oncogene MYC (DMMYC). Exon 1 and most of intron 1 of MYC have been displaced from DMMYC by a rearrangement of DNA. The RNA transcribed from DMMYC is a chimera that begins with an ectopic sequence of 176 nucleotides and then continues with exons 2 and 3 of MYC. The template for the ectopic sequence represents exon 1 of a gene known as PVT, which lies 50 kilobase pairs downstream of MYC. We encountered three abnormal configurations of MYC and PVT in the cell lines analyzed here: (i) amplification of the genes, accompanied by insertion of exon 1 and an undetermined additional portion of PVT within intron 1 of MYC to create DMMYC; (ii) selective deletion of exon 1 of PVT from amplified DNA that contains downstream portions of PVT and an intact allele of MYC; and (iii) coamplification of MYC and exon 1 of PVT, but not of downstream portions of PVT. We conclude that part or all of PVT is frequently amplified with MYC and that intron 1 of PVT represents a preferred boundary for amplification affecting MYC.

Retrovirally induced murine B-cell tumors rarely show proviral integration in sites common in T-cell tumors

The molecular etiology of retrovirally induced T-cell tumors has been shown in many cases to involve proviral integration near a cellular oncogene, c-myc, N-myc, Pim-1 and pvt-1 being frequent targets for insertional activation. Murine B-cell tumors induced by infection with murine leukemia virus have been studied for rearrangements in these and other loci. In contrast to the T-cell lymphomas, tumors of the B-cell lineage, either early B-cell tumors induced in nude mice or late B-cell tumors in immunocompetent mice, did not show disruption of N-myc or Pim-1 in any of the tumors studied, although those lymphomas had acquired many new proviruses. The loci c-abl, bcl-2, fis-1, c-erbB, c-myb, and neu were likewise not involved. Rearrangement of c-myc was seen in 1 out of 71 and rearrangement of the pvt-1 locus in 4 out of 73 (5%) of the B-cell tumors. Thus it appears that mechanistic differences exist in the development of T-cell tumors and B-cell tumors caused by the same etiological agent.

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.

Three breakpoints of variant t(2;8) translocations in Burkitt's lymphoma cells fall within a region 140 kilobases distal from c-myc

The variant translocations t(2;8) in Burkitt's lymphoma cells join band q24 of chromosome 8, distal from c-myc, to the Igkappa locus, with considerable variation in the location of the breakpoints on chromosome 8. We report the cloning and molecular characterization of a chromosome 8 region, distal from the c-myc locus, which encompasses the breakpoints of the Burkitt's lymphoma cell lines BL64, BL21, and LY91 within 11 kilobase pairs, termed provisionally bvr-1 (Burkitt's variants' rearranging region 1). Using probes from the c-myc, the bvr-1, and the human pvt-1 loci obtained by chromosome walking coupled with pulsed-field gel electrophoresis, we have constructed a physical map of the region 3' of c-myc. We map bvr-1 and pvt-1 about 140 and 260 kilobase pairs, respectively, distal from c-myc.

Distinct helper virus requirements for Abelson murine leukemia virus-induced pre-B- and T-cell lymphomas

Abelson murine leukemia virus (A-MuLV) can induce pre-B- or T-cell lymphomas (thymomas) in mice depending on the route and time of injection. Previous studies have shown that the choice of the helper virus used to rescue A-MuLV greatly influences its ability to induce pre-B-cell lymphomas. In this study, we investigated the role of the helper virus in A-MuLV-induced thymomas. A-MuLV rescued with the helper Moloney MuLV, BALB/c endogenous N-tropic MuLV, and two chimeric MuLVs derived from these two parents were injected intrathymically in young adult NIH Swiss mice. All four A-MuLV pseudotypes were found to be equally efficient in the induction of thymomas, whereas drastic differences were observed in their pre-B-cell lymphomagenic potential. Thymoma induction by A-MuLV was independent of the replication potential of the helper virus in the thymus, and no helper proviral sequences could be detected in the majority of thymomas induced by A-MuLV rescued with parental BALB/c endogenous or chimeric MuLVs. In the thymomas in which helper proviruses were present, none of them were found integrated in the Ahi-1 region, a common proviral integration site found in A-MuLV-induced pre-B-cell lymphomas (Y. Poirer, C. Kozak, and P. Jolicoeur, J. Virol. 62:3985-3992, 1988). In addition, helper-free stocks of A-MuLV were found to be as lymphomagneic as other pseudotypes in inducing thymomas after intrathymic inoculation, in contrast to their inability to induce pre-B-cell lymphomas when injected intraperitoneally in newborn mice. Restriction enzyme analysis revealed one to three A-MuLV proviruses in each thymoma, indicating the oligoclonality of these tumors. Analysis of the immunoglobulin and T-cell receptor loci confirmed that the major population of cells of these primary thymomas belongs to the T-cell lineage. Together, these results indicate that the helper virus has no effect in the induction of A-MuLV-induced T-cell lymphomas, in contrast to its important role in the induction of A-MuLV-induced pre-B-cell lymphomas. Our data also revealed distinct biological requirements for transformation of these two target cells by v-abl.

Identification of a human transcription unit affected by the variant chromosomal translocations 2;8 and 8;22 of Burkitt lymphoma

Chromosomal translocations in Burkitt lymphoma and mouse plasmacytomas typically lie within or near the protooncogene MYC. In some instances, however, these tumors contain variant translocations with breakpoints located more distant from and downstream of MYC, in a domain commonly known as pvt-1. Until now, there has been no evidence that pvt-1 marks the location of a functional gene. Here we report the identification of a large transcriptional unit in human DNA that includes pvt-1. We have designated this unit as PVT. PVT begins 57 kilobase pairs downstream of MYC and occupies a minimum of 200 kilobase pairs of DNA. Some of the translocations that occur downstream of MYC in Burkitt lymphoma transect PVT; others lie between the two genes. None of the translocations we have studied appear to enhance transcription from an intact allele of PVT (indeed, they may inactivate that transcription), but some are associated with the production of abundant and anomalous 0.8- to 1.0-kilobase RNAs that contain the 5' exon of PVT and sequences transcribed from the constant region of an immunoglobulin gene (the reciprocal participant in the translocation). Identification of PVT should facilitate the exploration of how translocations downstream of MYC and insertions of retroviral DNA in the vicinity of pvt-1 might contribute to tumorigenesis.

The PVT gene frequently amplifies with MYC in tumor cells

The line of human colon carcinoma cells known as COLO320-DM contains an amplified and abnormal allele of the proto-oncogene MYC (DMMYC). Exon 1 and most of intron 1 of MYC have been displaced from DMMYC by a rearrangement of DNA. The RNA transcribed from DMMYC is a chimera that begins with an ectopic sequence of 176 nucleotides and then continues with exons 2 and 3 of MYC. The template for the ectopic sequence represents exon 1 of a gene known as PVT, which lies 50 kilobase pairs downstream of MYC. We encountered three abnormal configurations of MYC and PVT in the cell lines analyzed here: (i) amplification of the genes, accompanied by insertion of exon 1 and an undetermined additional portion of PVT within intron 1 of MYC to create DMMYC; (ii) selective deletion of exon 1 of PVT from amplified DNA that contains downstream portions of PVT and an intact allele of MYC; and (iii) coamplification of MYC and exon 1 of PVT, but not of downstream portions of PVT. We conclude that part or all of PVT is frequently amplified with MYC and that intron 1 of PVT represents a preferred boundary for amplification affecting MYC.

Identification of a common helper provirus integration site in Abelson murine leukemia virus-induced lymphoma DNA

Abelson murine leukemia virus induces oligoclonal pre-B lymphoma in mice. The expression of the v-abl oncogene in target cells does not appear to be sufficient for tumor induction in several mouse strains, and additional genetic events are thought to be required. We postulated that the helper Moloney murine leukemia virus might induce these events, and its potential role as an insertional mutagen was assessed by the search of a common helper provirus integration site in Abelson murine leukemia virus lymphomas. Molecular cloning of cellular sequences adjacent to Moloney proviruses enabled us to identify a cellular region, designated Ahi-1, which was found occupied by the helper proviruses in 16% of Abelson pre-B-cell lymphomas. All proviruses for which the precise integration site within Ahi-1 could be mapped were found to be in the same orientation. Ahi-1 has been mapped to mouse chromosome 10 and represents a new common proviral integration site. These data suggest that the helper virus contributes to the induction of secondary genetic events which may be important for the development of Abelson murine leukemia virus-induced pre-B-cell lymphoma.

Fim-1, Fim-2/c-fms, and Fim-3, three common integration sites of Friend murine leukemia virus in myeloblastic leukemias, map to mouse chromosomes 13, 18, and 3, respectively

Three common proviral integration sites, Fim-1, Fim-2/c-fms, and Fim-3, have been described in mouse myeloid leukemias induced by the Friend murine leukemia virus. The nature and function of Fim-1 and Fim-3 are still unknown since no transcript from these loci has been detected so far. To identify these two loci, we undertook their chromosomal localization using restriction fragment length polymorphism detected between C57BL/6 mice and the wild-derived inbred strain of Mus spretus. Using interspecific backcross analysis, we mapped Fim-1 to mouse chromosome 13 and Fim-3 to mouse chromosome 3. Interestingly, Fim-3 is tightly linked to Evi-1, another common integration site of ecotropic virus involved in another model of mouse myeloid leukemogenesis. Fim-2 spans the 5' end of the c-fms gene, which encodes for the macrophage-colony-stimulating factor receptor. We located the c-fms gene on the D band of chromosome 18 by in situ hybridization.

Provirus tagging as an instrument to identify oncogenes and to establish synergism between oncogenes

Insertional mutagenesis is one of the mechanisms by which retroviruses can transform cells. Once a provirus was found in the vicinity of c-myc, with the concomitant activation of this gene, other proto-oncogenes were shown to be activated by proviral insertion in retrovirally-induced tumors. Subsequently, cloning of common proviral insertion sites led to the discovery of a series of new (putative) oncogenes. Some of these genes have been shown to fulfill key roles in growth and development. In this review I shall describe how proviruses can be used to identify proto-oncogenes, and list the loci, identified by this method. Furthermore, I shall illuminate the potential of provirus tagging by showing that it not only can mark new oncogenes, but can also be instrumental in defining sets of (onco)genes that guide a normal cell in a step-by-step fashion to its fully transformed, metatasizing, counterpart.

Spi-1 is a putative oncogene in virally induced murine erythroleukaemias

Retroviral insertional mutagenesis has been proposed as an efficient mechanism to turn on or to increase the expression of oncogenes in several avian or mammal models. Integration site studies of avian leukosis virus, murine leukaemia and murine mammary tumour viruses led to the coleutification of highly conserved genes whose expression is induced or increased during leukaemogenesis, probably through enhancer elements present in the retroviral long terminal repeats. This is reminiscent of the activation of cellular proto-oncogenes or putative oncogenes in numerous human tumours and leukaemias as a result of chromosomal translocations or DNA rearrangements. Here we report the characterization of a new putative oncogene isolated from a murine erythroleukaemia induced by the acute leukaemogenic retrovirus spleen focus forming virus (SFFV). An important and unusual feature of this genomic locus Spi-1 (for SFFV proviral integration) is that rearrangements due to SFFV integration were found in 95% of the erythroid tumours studied. A 4.0-kilobase messenger RNA was detected in rearranged tumours. No Spi-1 rearrangement was detected in other virally induced myeloid, lymphoid or erythroid tumours tested.

Important role of the long terminal repeat of the helper Moloney murine leukemia virus in Abelson virus-induced lymphoma

The helper virus has been shown to play a critical role in the development of lymphoma induced by the defective Abelson murine leukemia virus (A-MuLV). Indeed, A-MuLV pseudotyped with some viruses, such as the Moloney MuLV, has been shown to be highly lymphogenic, whereas A-MuLV pseudotyped with other viruses, such as the BALB/c endogenous N-tropic MuLV, has been shown to be devoid of lymphogenic potential (N. Rosenberg and D. Baltimore, J. Exp. Med. 147:1126-1141, 1978; C. D. Scher, J. Exp. Med. 147: 1044-1053, 1978). To map the viral DNA sequences encoding the determinant of the lymphogenic potential of Moloney MuLV when complexed with A-MuLV, we constructed chimeric helper viral DNA genomes in vitro between parental cloned infectious viral DNA genomes from Moloney MuLV and from BALB/c endogenous N-tropic MuLV. Chimeric helper MuLVs, recovered after transfection of NIH 3T3 cells were used to rescue A-MuLV, and the pseudotypes were inoculated into newborn NIH Swiss, CD-1, and SWR/J mice to test their lymphogenic potential. We found that a 0.44-kilobase-pair PstI-KpnI long terminal repeat-containing fragment from the Moloney MuLV was sufficient to confer some, but not complete, lymphogenic potential to a chimeric virus (p7M2) in NIH Swiss and SWR/J mice, but not in CD-1 mice. The addition of the 3'-end env sequences (comprising the carboxy terminus of gp70 and all p15E) to the U3 long terminal repeat sequences restored the full lymphogenic potential of the Moloney MuLV. Our data indicate that the 3'-end sequences of the helper Moloney MuLV are somehow involved in the development of lymphoma induced by A-MuLV. The same sequences have previously been found to harbor the determinant of leukemogenicity and of disease specificity of Moloney MuLV when inoculated alone.

Tissue-specific replication of Friend and Moloney murine leukemia viruses in infected mice

We have studied the replication of ecotropic murine leukemia viruses (MuLV) in the spleens and thymuses of mice infected with the lymphocytic leukemia-inducing virus Moloney MuLV (M-MuLV), with the erythroleukemia-inducing virus Friend MuLV (F-MuLV), or with in vitro-constructed recombinants between these viruses in which the long terminal repeat (LTR) sequences have been exchanged. At 1 week after infection both the parents and the LTR recombinants replicated predominantly in the spleens with only low levels of replication in the thymus. At 2 weeks after infection, the patterns of replication in the spleens and thymuses were strongly influenced by the type of LTR. Viruses containing the M-MuLV LTR exhibited a remarkable elevation in thymus titers which frequently exceeded the spleen titers, whereas viruses containing the F-MuLV LTR replicated predominantly in the spleen. In older preleukemic mice (5 to 8 weeks of age) the structural genes of M-MuLV or F-MuLV predominantly influenced the patterns of replication. Viruses containing the structural genes of M-MuLV replicated efficiently in both the spleen and thymus, whereas viruses containing the structural genes of F-MuLV replicated predominantly in the spleen. In leukemic mice infected with the recombinant containing F-MuLV structural genes and the M-MuLV LTR, high levels of virus replication were observed in splenic tumors but not in thymic tumors. This phenotypic difference suggested that tumors of the spleen and thymus may have originated by the independent transformation of different cell types. Quantification of polytropic MulVs in late-preleukemic mice infected with each of the ecotropic MuLVs indicated that the level of polytropic MuLV replication closely paralleled the level of replication of the ecotropic MuLVs in all instances. These studies indicated that determinants of tissue tropism are contained in both the LTR and structural gene sequences of F-MuLV and M-MuLV and that high levels of ecotropic or polytropic MuLV replication, per se, are not sufficient for leukemia induction. Our results further suggested that leukemia induction requires a high level of virus replication in the target organ only transiently during an early preleukemic stage of disease.

Identification of a new common provirus integration site in gross passage A murine leukemia virus-induced mouse thymoma DNA

The Gross passage A murine leukemia virus (MuLV) induced T-cell leukemia of clonal (or oligoclonal) origin in inoculated mice. To study the role of the integrated proviruses in these tumor cells, we cloned several newly integrated proviruses (with their flanking cellular sequences) from a single tumor in procaryotic vectors. With each of the five clones obtained, a probe was prepared from the cellular sequences flanking the provirus. With one such probe (SS8), we screened several Gross passage A MuLV-induced SIM.S mouse tumor DNAs and found that, in 11 of 40 tumors, a provirus was integrated into a common region designated Gin-1. A 26-kilobase-pair sequence of Gin-1 was cloned from two lambda libraries, and a restriction map was derived. All proviruses were integrated as a cluster in the same orientation within a 5-kilobase-pair region of Gin-1, and most of them had a recombinant structure of the mink cell focus-forming virus type. The frequency of Gin-1 occupancy by provirus was much lower in thymoma induced by other strains of MuLV in other mouse strains. Using somatic-cell hybrid DNAs, we mapped Gin-1 on mouse chromosome 19. Gin-1 was not homologous to 16 known oncogenes and was distinct from the other common regions for provirus integration previously described. Therefore, Gin-1 appears to represent a new common provirus integration region. The integration of a provirus within Gin-1 might be an important event leading to T-cell transformation, and the Gin-1 region might harbor sequences which are involved in tumor development.

Proviral integration site Mis-1 in rat thymomas corresponds to the pvt-1 translocation breakpoint in murine plasmacytomas

Two loci independently implicated in T-and B-lymphocyte neoplasia are shown to be equivalent. The Mis-1 locus is a common proviral integration site in retrovirally induced rat T lymphomas, while the pvt-1 locus on murine chromosome 15 frequently translocates to the kappa locus in plasmacytomas bearing 6;15 translocations. By comparing cloned sequences, we show that pvt-1 is the murine homolog of Mis-1.

Identification of a new common provirus integration site in gross passage A murine leukemia virus-induced mouse thymoma DNA

The Gross passage A murine leukemia virus (MuLV) induced T-cell leukemia of clonal (or oligoclonal) origin in inoculated mice. To study the role of the integrated proviruses in these tumor cells, we cloned several newly integrated proviruses (with their flanking cellular sequences) from a single tumor in procaryotic vectors. With each of the five clones obtained, a probe was prepared from the cellular sequences flanking the provirus. With one such probe (SS8), we screened several Gross passage A MuLV-induced SIM.S mouse tumor DNAs and found that, in 11 of 40 tumors, a provirus was integrated into a common region designated Gin-1. A 26-kilobase-pair sequence of Gin-1 was cloned from two lambda libraries, and a restriction map was derived. All proviruses were integrated as a cluster in the same orientation within a 5-kilobase-pair region of Gin-1, and most of them had a recombinant structure of the mink cell focus-forming virus type. The frequency of Gin-1 occupancy by provirus was much lower in thymoma induced by other strains of MuLV in other mouse strains. Using somatic-cell hybrid DNAs, we mapped Gin-1 on mouse chromosome 19. Gin-1 was not homologous to 16 known oncogenes and was distinct from the other common regions for provirus integration previously described. Therefore, Gin-1 appears to represent a new common provirus integration region. The integration of a provirus within Gin-1 might be an important event leading to T-cell transformation, and the Gin-1 region might harbor sequences which are involved in tumor development.

fim-1 and fim-2: two new integration regions of Friend murine leukemia virus in myeloblastic leukemias

The Friend helper murine leukemia virus (F-MuLV) induces in mice a high percentage of myeloblastic leukemias. Myeloblastic transformation is also observed after in vitro infection of long-term bone marrow cultures. To investigate the molecular events leading to the generation of myeloblastic leukemias, we first screened a panel of leukemic cells for rearrangement or amplification of known oncogenes or previously described specific integration sites. No modification of these genes was observed. Therefore, we searched for common integration sites by constructing a genomic library from a myeloblastic cell line harboring only five integrated proviruses. This library was screened with a virus-specific probe, and virus-host cellular junction fragments were subcloned. Two flanking cellular sequences corresponding to two different integrated proviruses were used to analyze additional myeloblastic leukemias. The first probe detected rearrangements in 2 of 42 myeloblastic leukemias, and the second probe detected rearrangements in 6 of 42. We demonstrated that, in each case, the rearrangement was the result of F-MuLV integration, with all proviruses in the same orientation and clustering in a region less than 3 kilobases long. The two regions, named fim-1 and fim-2, were different from 15 oncogenes tested. Rearrangements of these two regions were found in F-MuLV-induced myeloblastic leukemias but not in 20 lymphoid or erythroid leukemias induced by the same virus.

Proviral integration site Mis-1 in rat thymomas corresponds to the pvt-1 translocation breakpoint in murine plasmacytomas

Two loci independently implicated in T-and B-lymphocyte neoplasia are shown to be equivalent. The Mis-1 locus is a common proviral integration site in retrovirally induced rat T lymphomas, while the pvt-1 locus on murine chromosome 15 frequently translocates to the kappa locus in plasmacytomas bearing 6;15 translocations. By comparing cloned sequences, we show that pvt-1 is the murine homolog of Mis-1.

Viral sequences are associated with many histocompatibility genes

A C57BL/6By 5.5 kb Pvu II polymorphic restriction fragment which hybridizes with a spleen focus-forming env probe and maps in the H-30 region has been cloned, and a 358 bp subfragment subcloned. Hybridization and sequencing studies show that the 358 bp fragment is encoded by the region of the pol gene of murine retrovirus which codes for an endonuclease critical for viral integration. Hybridizations of digested murine genomic DNAs with the 358 bp probe generate 31 restriction fragment length polymorphisms (RFLPs); 16 of these can be placed near the following 15 minor histocompatability (H) loci: H-3, H-4, H-7, H-13, H-15, H-16, H-17, H-19, H-22, H-24, H-27, H-30, H-34, H-36, and H-38. We suggest that the proximity of viral sequences to H loci is probably evolutionarily and functionally significant and that the closeness of viral sequences and minor H loci can probably be utilized to facilitate the cloning of minor H genes. During the course of these studies, it has become possible to tentatively assign H-17, H-34, and H-38 to chromosome 12. In addition, it was observed that several H-2 congenic strains retain portions of chromosome 12 from the parental donor strains used in their derivation.

Expression of a new tyrosine protein kinase is stimulated by retrovirus promoter insertion

Tyrosine protein kinases are important both in the normal regulation of cellular proliferation and in the oncogenic transformation of cells by several tumour viruses. The LSTRA Moloney murine leukaemia virus (M-MuLV)-induced thymoma cell line contains approximately 20-fold more phosphotyrosine in protein than do typical haematopoietic cell lines; this seems to result from the expression of an abnormally high level of a cellular tyrosine protein kinase termed p56tck (refs 3, 4). This kinase is normally expressed at low levels in most, but not all, murine T cells. The elevated levels of p56tck could contribute to the malignant properties of LSTRA cells. Therefore, we have isolated cloned complementary DNAs encoding the whole of p56tck. Sequence analysis shows it to be a novel cellular tyrosine protein kinase which is distinct from all others described to date. p56tck is encoded in LSTRA cells by a hybrid messenger RNA; approximately 200 nucleotides at the 5' end of the mRNA are identical to the 5' end of the genome of M-MuLV. The three- to ninefold transcriptional activation of the gene therefore results from retroviral promoter insertion.

Common proviral integration region on mouse chromosome 7 in lymphomas and myelogenous leukemias induced by Friend murine leukemia virus

Friend murine leukemia virus (F-MuLV) induces a variety of hematopoietic neoplasms 2 to 12 months after inoculation into newborn mice. These neoplasms are clonal or oligoclonal and contain a small number of F-MuLV insertions in high-molecular-weight DNA. To investigate whether different tumors have proviral insertions in the same region, a provirus-cellular DNA junction fragment from an F-MuLV-induced myelogenous leukemia was cloned in lambda gtWES, and a portion of the flanking cellular DNA sequence was used in blot-hybridization studies of 34 additional F-MuLV-induced neoplasms. Three of these additional neoplasms (one myelogenous leukemia and two lymphomas) were found to have altered copies of the flanking cellular sequence. Restriction enzyme analysis of genomic DNA from these tumors revealed that in each case a proviral copy of F-MuLV had inserted into the same 1.5-kilobase region; all proviruses had the same orientation. Using mouse-Chinese hamster somatic cell hybrids, we mapped this common integration region, designated Fis-1, to mouse chromosome 7. Fis-1 is distinct from three oncogenes on mouse chromosome 7, Ha-ras, fes, and Int-2, based on restriction enzyme analysis and blot hybridization. Therefore, Fis-1 appears to be a novel sequence implicated in both lymphoid and myeloid leukemias induced by F-MuLV.

Genetic mapping of Pim-1 putative oncogene to mouse chromosome 17

Pim-1 is a putative oncogene activated in T-cell lymphomas induced by Moloney and AKR mink cell focus forming (MCF) viruses. We have determined the chromosomal localization of the Pim-1 gene in mice by Southern blot analysis of DNAs obtained from a panel of mouse-Chinese hamster somatic cell hybrids. The Pim-1 gene was localized on chromosome 17, a chromosome frequently aberrant in T-cell lymphomas. Two chromosomal regions, containing sequences homologous to regions within the Pim-1 locus, were localized on chromosome 6 and 16.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Insertion of an Alu SINE in the human homologue of the Mlvi-2 locus

Fifty-nine human DNA samples derived from either normal tissues or hematopoietic neoplasias were examined for rearrangements in the Mlvi-2 locus, a putative oncogene. The rearranged Mlvi-2 sequences in one of them, a B cell lymphoma, were shown to result from the insertion of an approximately 300 bp DNA fragment that hybridized to a human Alu probe. DNA sequence analysis of both the rearranged and the nonrearranged allele around the site of the insertion revealed the following: a) the insert was 88.4% homologous to the consensus sequence of the Alu family of repeats and 75% homologous to the Alu related sequence in the human 7SL RNA; b) similar to other sequenced SINES, a poly(d.A) tract was present at the 3' end of this element; c) an 8 bp direct repeat was present at both ends of the inserted element; d) this repeat was present as a single copy in the unrearranged allele. We conclude from these findings that: Alu sequences can transpose and that the direct repeats flanking certain Alu SINES may be generated by the duplication of single copy cellular sequences at the site of the insertion. Furthermore the recent nature of the Alu insertion in the Mlvi-2 locus coupled to the low degree of homology of the inserted Alu to the Alu related sequence in the 7SL RNA suggest that this event did not occur via reverse transcription and reintegration of the 7SL RNA.

Mouse chromosomal mapping of a murine leukemia virus integration region (Mis-1) first identified in rat thymic leukemia

We have previously identified a region of genomic DNA which constitutes the site of frequent provirus integration in rat thymomas induced by Moloney murine leukemia virus (Lemay and Jolicoeur, Proc. Natl. Acad. Sci. USA 81:38-42, 1984). This genetic locus is now designated Mis-1 (Moloney integration site). Cellular sequences homologous to Mis-1 are present in mouse DNA. Using a series of hamster-mouse somatic cell hybrids, we mapped the Mis-1 locus to mouse chromosome 15. Frequent chromosome 15 aberrations have been described in mouse thymomas. Mis-1 represents a putative new oncogene which might be involved in the initiation or maintenance or both of these neoplasms.