Non-identical patterns of proviral insertions around host transcription units in lymphomas induced by different strains of murine leukemia virus

AbstractIn a small sample of 57 retrovirus integration sites (RISs) isolated from 23 end-stage lymphomas induced in NMRI mice by the B-lymphotropic Akv wt or an enhancer mutant hereof, Akv1–99, we identified 14 novel RISs and defined 9 novel CISs (common insertion sites). Moreover, when comparing with RISs from tumors induced by the T-lymphomagenic SL3-3, we observed that SL3-3 targets RefSeq promoter regions with a significantly higher frequency than Akv/Akv1–99 and in an orientation-dependent way. Altogether, our results strongly emphasize the importance of host genetic background and virus type for retroviral insertion mutagenesis screens and suggest that different types of MLV may favor specific genomic regions and orientations in order exert optimal effect on target gene expression during lymphoma induction and development

Triple basepair changes within and adjacent to the conserved YY1 motif upstream of the U3 enhancer repeats of SL3-3 murine leukemia virus cause a small but significant shortening of latency of T-lymphoma induction

AbstractA highly conserved sequence upstream of the transcriptional enhancer in the U3 of murine leukemia viruses (MLVs) was reported to mediate negative regulation of their expression. In transient expression studies, negative regulation was reported to be conferred by coexpression of the transcription factor YY1, which binds to a motif in the upstream conserved region (UCR). To address the function of the UCR and its YY1-motif in an in vivo model of MLV–host interactions we introduced six consecutive triple basepair mutations into this region of the potent T-lymphomagenic SL3-3 MLV. We report that all mutants have retained their replication competence and that they all, like the SL3-3 wild type (wt), induce T-cell lymphomas when injected into newborn mice of the SWR strain. However, all mutants induced disease with slightly shorter latency periods than the wt SL3-3, suggesting that the YY1 motif as well as its immediate context in the UCR have a negative effect on the pathogenicity of the virus. This result may have implications for the design of retroviral vectors

The Icsbp locus is a common proviral insertion site in mature B-cell lymphomas/plasmacytomas induced by exogenous murine leukemia virus

AbstractICSBP (interferon consensus sequence binding protein)/IRF8 (interferon regulatory factor 8) is an interferon gamma-inducible transcription factor expressed predominantly in hematopoietic cells, and down-regulation of this factor has been observed in chronic myelogenous leukemia and acute myeloid leukemia in man. By screening about 1200 murine leukemia virus (MLV)-induced lymphomas, we found proviral insertions at the Icsbp locus in 14 tumors, 13 of which were mature B-cell lymphomas or plasmacytomas. Only one was a T-cell lymphoma, although such tumors constituted about half of the samples screened. This indicates that the Icsbp locus can play a specific role in the development of mature B-lineage malignancies. Two proviral insertions in the last Icsbp exon were found to act by a poly(A)-insertion mechanism. The remaining insertions were found within or outside Icsbp. Since our results showed expression of Icsbp RNA and protein in all end-stage tumor samples, a simple tumor suppressor function of ICSBP is not likely. Interestingly, proviral insertions at Icsbp have not been reported from previous extensive screenings of mature B-cell lymphomas induced by endogenous MLVs. We propose that ICSBP might be involved in an early modulation of an immune response to exogenous MLVs that might also play a role in proliferation of the mature B-cell lymphomas

Impairment of alternative splice sites defining a novel gammaretroviral exon within gag modifies the oncogenic properties of Akv murine leukemia virus

<p>Abstract</p> <p>Background</p> <p>Mutations of an alternative splice donor site located within the <it>gag </it>region has previously been shown to broaden the pathogenic potential of the T-lymphomagenic gammaretrovirus Moloney murine leukemia virus, while the equivalent mutations in the erythroleukemia inducing Friend murine leukemia virus seem to have no influence on the disease-inducing potential of this virus. In the present study we investigate the splice pattern as well as the possible effects of mutating the alternative splice sites on the oncogenic properties of the B-lymphomagenic Akv murine leukemia virus.</p> <p>Results</p> <p>By exon-trapping procedures we have identified a novel gammaretroviral exon, resulting from usage of alternative splice acceptor (SA') and splice donor (SD') sites located in the capsid region of <it>gag </it>of the B-cell lymphomagenic Akv murine leukemia virus. To analyze possible effects <it>in vivo </it>of this novel exon, three different alternative splice site mutant viruses, mutated in either the SA', in the SD', or in both sites, respectively, were constructed and injected into newborn inbred NMRI mice. Most of the infected mice (about 90%) developed hematopoietic neoplasms within 250 days, and histological examination of the tumors showed that the introduced synonymous <it>gag </it>mutations have a significant influence on the phenotype of the induced tumors, changing the distribution of the different types as well as generating tumors of additional specificities such as <it>de novo </it>diffuse large B cell lymphoma (DLBCL) and histiocytic sarcoma. Interestingly, a broader spectrum of diagnoses was made from the two single splice-site mutants than from as well the wild-type as the double splice-site mutant. Both single- and double-spliced transcripts are produced <it>in vivo </it>using the SA' and/or the SD' sites, but the mechanisms underlying the observed effects on oncogenesis remain to be clarified. Likewise, analyses of provirus integration sites in tumor tissues, which identified 111 novel RISs (retroviral integration sites) and 35 novel CISs (common integration sites), did not clearly point to specific target genes or pathways to be associated with specific tumor diagnoses or individual viral mutants.</p> <p>Conclusion</p> <p>We present here the first example of a doubly spliced transcript within the group of gammaretroviruses, and we show that mutation of the alternative splice sites that define this novel RNA product change the oncogenic potential of Akv murine leukemia virus.</p