Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range.

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

Anaplastic plasmacytomas: relationships to normal memory B cells and plasma cell neoplasms of immunodeficient and autoimmune mice.

Anaplastic plasmacytomas (APCTs) from NFS.V(+) congenic mice and pristane-induced plasmacytic PCTs from BALB/c mice were previously shown to be histologically and molecularly distinct subsets of plasma cell neoplasms (PCNs). Here we extended these comparisons, contrasting primary APCTs and PCTs by gene expression profiling in relation to the expression profiles of normal naïve, germinal centre, and memory B cells and plasma cells. We also sequenced immunoglobulin genes from APCT and APCT-derived cell lines and defined surface phenotypes and chromosomal features of the cell lines by flow cytometry and by spectral karyotyping and fluorescence in situ hybridization. The results indicate that APCTs share many features with normal memory cells and the plasma cell-related neoplasms (PLs) of FASL-deficient mice, suggesting that APCTs and PLs are related and that both derive from memory B cells. Published in 2010 by John Wiley & Sons, Ltd.

Expression of infectious murine leukemia viruses by RAW264.7 cells, a potential complication for studies with a widely used mouse macrophage cell line.

The mouse macrophage-like cell line RAW264.7, the most commonly used mouse macrophage cell line in medical research, was originally reported to be free of replication-competent murine leukemia virus (MuLV) despite its origin in a tumor induced by Abelson MuLV containing Moloney MuLV as helper virus. As currently available, however, we find that it produces significant levels of ecotropic MuLV with the biologic features of the Moloney isolate and also MuLV of the polytropic or MCF class. Newborn mice developed lymphoma following inoculation with the MuLV mixture expressed by these cells. These findings should be considered in interpretation of increasingly widespread use of these cells for propagation of other viruses, studies of biological responses to virus infection and use in RNA interference and cell signalling studies.

Anaplastic, plasmablastic, and plasmacytic plasmacytomas of mice: relationships to human plasma cell neoplasms and late-stage differentiation of normal B cells.

We have compared histologic features and gene expression profiles of newly identified plasmacytomas from NFS.V(+) congenic mice with plasmacytomas of IL6 transgenic, Fasl mutant, and SJL-beta2M(-/-) mice. NFS.V(+) tumors comprised an overlapping morphologic spectrum of high-grade/anaplastic, intermediate-grade/plasmablastic, and low-grade/plasmacytic cases with similarities to subsets of human multiple myeloma and plasmacytoma. Microarray and immunohistochemical analyses of genes expressed by the most prevalent tumors, plasmablastic plasmacytomas, showed them to be most closely related to immunoblastic lymphomas, less so to plasmacytomas of Fasl mutant and SJL mice, and least to plasmacytic plasmacytomas of IL6 transgenic mice. Plasmablastic tumors seemed to develop in an inflammatory environment associated with gene signatures of T cells, natural killer cells, and macrophages not seen with plasmacytic plasmacytomas. Plasmablastic plasmacytomas from NFS.V(+) and SJL-beta2M(-/-) mice did not have structural alterations in Myc or T(12;15) translocations and did not express Myc at high levels, regular features of transgenic and pristane-induced plasmacytomas. These findings imply that, as for human multiple myeloma, Myc-independent routes of transformation contribute to the pathogenesis of these tumors. These findings suggest that plasma cell neoplasms of mice and humans exhibit similar degrees of complexity. Mouse plasmacytomas, previously considered to be homogeneous, may thus be as diverse as their human counterparts with respect to oncogenic mechanisms of plasma cell transformation. Selecting specific types of mouse plasmacytomas that relate most closely to subtypes of human multiple myeloma may provide new opportunities for preclinical testing of drugs for treatment of the human disease.

Expression of the cyclin-dependent kinase inhibitor p27 and its deregulation in mouse B cell lymphomas.

CDKN1B (p27) regulates cell-cycle progression at the G1-S transition by suppressing the cyclin E/CDK2 kinase complex. In normal lymphocytes and most human B cell non-Hodgkin lymphomas (NHL), there is an inverse correlation between proliferative activity and expression of p27; however, a subset of NHL with high mitotic indices expresses p27, which is inactive due to sequestration in nuclear protein complexes or due to cytoplasmic retention. Our studies of mouse B cell NHL also identified cases with high proliferative activity and high levels of p27 at a surprisingly high frequency. Here, p27 was complexed with D-type cyclins 1 and 3 and with the COPS9 protein, JAB1. In addition, we found cytoplasmic sequestration following phosphorylation by activated AKT.

Histologic and molecular characterizations of megakaryocytic leukemia in mice.

Six cases of megakaryocytic leukemia (MKL) were identified and analyzed for morphology and molecular features. MKL were composed of megakaryocyte lineage cells ranging from immature to quite mature cells. VWF, GATA1 and RUNX1 were strongly expressed in megakaryocytes in both normal spleen and MKL as analyzed by immunohistochemistry (IHC). Altered expression of Meis1, Pbx1 and Psen2 and Lef1 in MKL detected with oligonucleotide microarrays was confirmed by qPCR and IHC. This is the first report of spontaneous MKL in mice, defining VWF as a biomarker for diagnosis and suggesting possible involvement of a series of genes in disease pathogenesis.

High-throughput retroviral tagging for identification of genes involved in initiation and progression of mouse splenic marginal zone lymphomas.

Human B-cell lymphomas are frequently associated with specific genetic changes caused by chromosomal translocations that activate proto-oncogenes. For lymphomas of mice expressing murine leukemia virus, mutagenic proviral insertions are thought to play a similar role. Here we report studies designed to determine whether specific retroviral integration sites might be associated with a specific subset of mouse B-cell lymphomas and if the genes associated with these sites are regularly altered in expression. We studied splenic marginal zone lymphomas (MZL) of NFS.V(+) mice that are unusual in exhibiting frequent progression from low to high grade, potentially allowing assignment of cancer genes to processes of initiation and progression. We used inverse PCR to clone and analyze 212 retroviral integration sites from 43 MZL at different stages of progression. Sixty-two marked common integration sites and included 31 that had been marked previously. Among the new common integration sites, seven were unique to MZL. Using microarrays and real-time quantitative PCR analysis, we defined differential patterns of gene expression in association with disease progression for Gfi1, Sox4, Brca2, Snf1lk, Nfkb1, Pou2af1, Prdm1, Stat6, and Blnk. Heightened expression of Gfi1 distinguishes MZL from other lymphoma types. The combined use of proviral tagging and analyses of gene expression thus provides a powerful approach to understanding of genes that collaborate in tumorigenesis.

Characterization of a novel murine retrovirus mixture that facilitates hematopoiesis.

A new virus previously arose in BALB/c females mated repeatedly to C57BL/6 (B6) males and then injected with fixed, activated B6 male spleen cells (V. S. Ter-Grigorov, O. Krifuks, E. Liubashevsky, A. Nyska, Z. Trainin, and V. Toder, Nat. Med. 3:37-41, 1997). In the present study, BALB/cJ mice inoculated with virus-containing plasma from affected mice developed splenomegaly, which was caused by increased numbers of Sca-1(+) Lin(-) hematopoietic stem cells (HSC) and their differentiated progeny. Biological and molecular analyses of a new virus revealed a mixture of murine leukemia viruses (MuLVs). These MuLVs comprised ecotropic and mink lung cell focus-forming (MCF) virus classes and are termed Rauscher-like MuLVs because they bear numerous similarities to the ecotropic and MCF viruses of the Rauscher MuLV complex but do not include a spleen focus-forming virus. The ecotropic virus component alone transferred some disease characteristics, while MCF virus alone did not. Thus, we have described a novel virus mixture, termed Rauscher-like MuLV, that causes an increase in hematopoiesis due to activation of pluripotent HSC. Experiments using mice and a protocol that replicated the pregnancy and immunization strategy of the original experiment demonstrated that endogenous BALB/c mouse ecotropic and xenotropic MuLVs are activated by these treatments. Emv1 was expressed in the spleens of multiparous mice but not in those of virgin mice, and Bxv1Emv1-pseudotyped MuLVs were recovered following injection of fixed, activated B6 cells. Thus, multiple pregnancies and allostimuli appear to have provided the signals required for activation of and recombination among endogenous viruses and could have resulted in generation of the Rauscher-like MuLV mixture.