Evidence of recombinant ecotropic provirus integration in thymic lymphomas induced by direct or indirect radiation effects

Genetics of susceptibility to radiation-induced lymphomas, leukemias and lung tumors studied in recombinant congenic strains

The genetic control of susceptibility to radiation-induced tumors in mice has been tested using the series of 20 CcS/Dem (CcS) Recombinant Congenic Strains, each carrying a different random set of 12.5% of genes of the resistant strain STS/A (STS) on the genetic background of the susceptible strain BALB/cHeA (BALB/c). Two classes of tumors were frequently observed: tumors of the haematopoietic system (lymphomas, myelocytic leukemias) and lung tumors. The results indicate that the genes controlling various aspects of tumor development were segregated in the CcS strain series. Large inter-strain differences were observed in the incidence of lung tumors. With lymphomas and leukemias, we not only observed strain differences in the incidence of tumors and in the latency of their development but also in the type of tumors (T- vs. B-cell lymphomas, myelocytic tumors) and in the frequency of their localized or disseminated (leukemic) form. Surprisingly, the myelocytic tumors, which occur very rarely or not at all in the parental strains BALB/c and STS or in their crosses, developed with high frequency in one of the CcS strains (CcS-2), indicating a unique combination of genes in this strain, which facilitates the development of myelocytic tumors. The effect of these genes is suppressed in the genetic composition of the parental strains. Tests of crosses of the resistant-strain CcS-13 with BALB/c indicated a suggestive linkage of a susceptibility gene for lymphomas to chromosome 5. These tests of the CcS strains illustrate the genetic complexity of the control of radiation-induced tumors in mice and suitability of these model systems to study their different facets.

A new leukemogenic retrovirus isolated from tumor cells derived from a radio-induced lymphoma of C57BL/6 mice: analysis of the env and LTR sequences

We report the cloning and characterization of a new ecotropic provirus encountered in a radio-induced thymic lymphoma of the C57BL/6 mouse. The provirus with an abnormally long LTR was inserted in the chromosomal DNA within the Pvt-1/MLVi-1/Mis-1 region which is a common integration site for MCF virus in mice and for Mo-MuLV in rats. This new ecotropic provirus was molecularly cloned and found to be infectious and competent for replication after transfection of murine cells. The recovered virus termed T3651/B was B-ecotropic, T-lymphotropic (in vivo) and highly leukemogenic for newborn C57BL/6 mice and for adult mice provided they were submitted to a subleukemogenic dose of irradiation. As compared to the AKV prototype N-ecotropic endogenous retrovirus, the T3651/B env proteins are only affected by few scattered point mutations. In contrast, the LTR has five repeats of enhancer sequences containing consensus motifs specific of the nuclear factors NF1-like, LVa, LVb and SEF1. Since a virus with such properties was encountered only once in 31 radio-induced tumors and isolated at a fourth tumor passage, a direct role of T3651/B virus in tumor genesis after irradiation is uncertain. Nevertheless, it is clear that T3651/B virus is a new leukemogenic retrovirus with a particular LTR structure which fits well with the model proposed by Rassart et al. (J. Virol. 58, 96-106, 1986) for the emergence of a thymotropic highly leukemogenic RadLV.

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.