Similarly to other mammalian and avian retroviruses that lack a transforming gene, moloney murine leukaemia virus (MoMuLV) causes no morphological transformation in infected tissue culture cells. However, following injection in an appropriate animal host, MoMuLV induces mainly thymic lymphomas after a long latency period. A common characteristic of neoplasms induced by retroviruses lacking transforming genes is their clonal origin. Here we have generated MoMuLV-induced rat thymic lymphomas and confirmed their clonal nature. Furthermore, we took advantage of the clonality of these tumours to investigate the specificity of provirus integration in the tumour DNA. We reasoned that if several independently derived thymic lymphomas would contain the provirus integrated in the same region of cellular DNA, this would be a strong indication that this integration event is a contributing factor in oncogenesis. The results indicate that there is indeed a cellular DNA region (termed the MLVI-1 locus) that serves as the substrate for proviral DNA integration in 5 out of 16 tumours we examined.

Lizarbe, and K. yon der Mark. 1983. Anat. Rec. 206:373-383). We have studied the expression of genes that typify osteogenic differentiation in mandibular condyles during in vitro cultivation. RNAs of the genes for collagen type I, osteonectin, alkaline phosphatase, and bone gla protein were sequentially expressed in progenitor cells and hypertrophic chondrocytes during culture. Osteopontin expression peaked in both the early and the late phase of the differentiation process. The data indicate a distinct sequence of expression of osteoblast-specific genes during osteogenic differentiation and new bone formation in mandibular condyles.

Human osteoblastic cells were grown in a three‐dimensional (3‐D) cell culture model and used to test the effects of a 20 Hz sinusoidal electromagnetic field (EMF; 6 mT and 113 mV/cm max) on collagen type I mRNA expression and extracellular matrix formation in comparison with the effects of growth factors. The cells were isolated from trabecular bone of a healthy individual (HO‐197) and from a patient presenting with myositis ossificans (MO‐192) and grown in a collagenous sponge‐like substrate. Maximal enhancement of collagen type I expression after EMF treatment was 3.7‐fold in HO‐197 cells and 5.4‐fold in MO‐192 cells. Similar enhancement was found after transforming growth factor‐β (TGF‐β) and insulin‐like growth factor‐I (IGF‐I) treatment. Combined treatment of the cells with EMF and the two growth factors TGF‐β and IGF‐I did not act synergistically. MO‐192 cells produced an osteoblast‐characteristic extracellular matrix containing collagen type I, alkaline phosphatase, and osteocalcin, together with collagen type III, TP‐1, and TP‐3, two epitopes of an osteoblastic differentiation marker. The data suggest that the effects of EMFs on osteoblastic differentiation are comparable to those of TGF‐β and IGF‐I. We conclude that EMF effects in the treatment of skeletal disorders and in orthopedic adjuvant therapy are mediated via enhancement of collagen type I mRNA expression, which may lead to extensive extracellular matrix synthesis. Bioelectromagnetics 19:222–231, 1998. © 1998 Wiley‐Liss, Inc.

Rat thymic lymphomas induced by Moloney murine leukemia virus carry DNA rearrangements due to provirus integration in at least live independent cellular DNA domains (Mlvi-1, Mlvi-2, Mlvi-3, RMoInt-1, and c-myc). We had previously shown that rearrangements in more than one of these domains could occur in the same tumor. In this report we exteid these findings by showing that, with one exception, tumors containing provirus insertions in Mlvi-) always contained provirus insertions in a second locus, Mlvi-2. To determine whether both events occurred in the same population of tumor cells, we examined the clonal nature of these tumors by taking advantage of allelic polymorphisms that occur naturally in both Mlvi-l and Mlvi-2. Tumors with provirus insertions in both Mlvi-) and Mlvi-2 arising in rats heterozygous at one of these loci were identified. DNA from these tumors was analyzed by restriction endonuclease digestion and hybridization to DNA probes derived from both Mlvi-l and Mlvi-2. Thus, we determined the clonal nature of three thymomas and showed that in these tumors both insertion events occurred in the same population of tumor cells. The concomitant appearance of provirus insertions in Mlvi-) and Mlvi-2 suggests a synergism of these two events that may be important in tumor induction and progression.

Two cellular DNA regions representing common domains for proviral DNA integration (Mlvi-l and Mlvi-2) have been identified in Moloney murine leukemia virus-induced rat thymic lymphomas. Cellular sequences which were free of repeated DNA derived from a clone that defines the Ml1i-2 integration domain (XC1228) were found to be highly conserved in a variety of vertebrate species that we examined, including mice, hamsters, cats, and humans. In this study, we identified the chromosomal map location of the Mlvi-2 homologous sequences in mice by using hamster-mouse somatic cell hybrids. The results show that Mlhi-2 is present on mouse chromosome 15 but is unrelated to the c-myc and c-sis proto-oncogenes, which map on the same chromosome. Since aberrations on chromosome 15 have been observed reproducibly in mouse thymomas, our data suggest that Mli-2 may define a novel sequence involved in the induction or progression of murine thymic lymphomas.

RFB virus is an ecotropic C-type retrovirus isolated from CF-1 mice, in which it is associated with induction of osteomas. Sequence analysis of the RFB provirus revealed no evidence for presence of an oncogene or a recombined env gene. RFB virus is a member of the murine leukemia virus (MuLV) group (RFB MuLV), sharing 97% nucleotide identity with the endogenous ecotropic provirus of AKR mice (Akv). Like Akv, expression of RFB MuLV mRNAs is inducible by dexamethasone treatment, indicating that FRB MuLV also shares transcriptional control signals with Akv. We assessed the pathogenic potential of RFB MuLV in NMRI mice, which, in contrast to CF-1 mice, do not contain endogenous ecotropic retroviruses. RFB MuLV induced osteomas, osteopetrosis, and lymphomas in newborn NMRI mice. Another CF-1 mouse-derived leukemia virus, FBJ MuLV, the helper virus of the FBJ osteosarcoma virus stock, as well as Akv, also induced osteomas, osteopetrosis, and lymphomas in NMRI mice similar to RFB MuLV. These findings indicate that endogenous retroviruses carry a pathogenic potential in hematopoietic tissues and in the skeleton.

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We report the molecular cloning of two replication-competent osteoma-inducing murine leukemia viruses from the RFB osteoma virus stock (M.