Genetic predisposition to lymphomas in mice

Animal Models for the Study of Childhood Leukemia: Considerations for Model Identification and Optimization to Identify Potential Risk Factors

Leukemias are the most common pediatric malignancies diagnosed in western industrialized societies. In spite of the substantial incidence of childhood leukemia in the United States and other countries, neither epidemiology studies conducted in human populations nor hazard identification studies conducted using traditional animal models have identified environmental or other factors that are directly linked to increased risk of disease. Molecular biology data and mathematical modeling of incidence patterns suggest that pediatric leukemogenesis may occur through a multistage or “multihit” mechanism that involves both in utero and postnatal events. The authors propose that pediatric leukemias can be modeled experimentally using a “multihit” paradigm analogous to the “initiation-promotion” and “complete carcinogenesis” models developed for tumor induction in mouse skin and rat liver. In this model for childhood leukemia, an initial genetic alteration occurs during in utero or early postnatal development, but clinical disease develops only upon additional genetic or nongenetic events that occur during the postnatal period. Application of this multistage or “multihit” model to hazard assessment studies conducted in transgenic or knockout mice carrying relevant molecular lesions may provide a sensitive approach to the identification of environmental agents that are important risk factors for childhood leukemia.

The source of self: genetic parasites and the origin of adaptive immunity

Stable colonization of the host by viruses (genetic parasites) can alter the systems of host identity and provide immunity against related viruses. To attain the needed stability, some viruses of prokaryotes (P1 phage) use a strategy called an addiction module. The linked protective and destructive gene functions of an addiction module insures both virus persistence but will also destroy cells that interrupt this module and thereby prevent infection by competitors. Previously, I have generalized this concept to also include persistent and lytic states of virus infection, which can be considered as a virus addiction module. Such states often involve defective viruses. In this report, I examine the origin of the adaptive immune system from the perspective of a virus addiction module. The likely role of both endogenous and exogenous retroviruses, DNA viruses, and their defective elements is considered in the origin of all the basal components of adaptive immunity (T-cell receptor, RAG-mediated gene rearrangement, clonal lymphocyte proliferation, antigen surface presentation, apoptosis, and education of immune cells). It is concluded that colonization by viruses and their defectives provides a more coherent explanation for the origin of adaptive immunity.

Svi3: A provirus common integration site in c-myc in SL/Kh pre-B lymphomas

Spontaneous pre-B lymphomas in SL/Kh mice occur by somatic acquisition of a provirus genome of endogenous murine leukemia virus (MuLV). Inverse PCR amplification and sequence analyses of a provirus and its host flanking fragment revealed a proviral insertion into c-myc in 3 out of 60 SL/Kh pre-B lymphomas, named Svi3 lymphomas (SL/Kh virus integration site-3). Southern blot analysis revealed that two lymphomas had clonal integration in c-myc exon 1 and the other, in the promoter region. In 2 out of 3 Svi3 lymphomas, a fusion transcript of provirus 3' long terminal repeat and c-myc and a normal full-length c-myc transcript were obtained, but in one Svi3 lymphoma, only the normal transcript was obtained. All three Svi3 lymphomas had increased c-myc expression, producing normal 67-kDa c-Myc protein. Svi3 lymphomas had more mature phenotypes in the steps of early B-cell differentiation than Svi1 lymphomas, in which c-myc expression was indirectly up-regulated by provirus integration into Stat5a.

Treatment with benznidazole in association with immunosuppressive drugs in mice chronically infected with Trypanosoma cruzi: investigation into the possible development of neoplasias

Benznidazole is recommended in Brazil for the treatment of Trypanosoma cruzi infection in acute and early chronic phases of Chagas' disease. Observations by others have indicated a higher incidence of neoplasias in immunosuppressed patients, presenting Chagas' disease reactivation, submitted to treatment with benznidazole. In the present study, we investigated whether there is a potentiation in the generation of lymphomas in chronically infected mice, treated with immunosuppressive drugs and benznidazole. For this, 142 Swiss mice chronically infected with the 21 SF strain of T. cruzi and 72 normal Swiss mice were used. Both infected and normal mice were divided into experimental groups and submitted to one of the following treatment regimens: benznidazole alone; immunosuppressive drugs (azathioprine, betamethasone and cyclosporin); a combination of immunosuppressive drugs and benznidazole; and untreated controls. In the infected group treated with benznidazole, one mouse developed a non-Hodgkin's lymphoma. This finding has been interpreted as a spontaneous tumor of mice. The study of the chronically infected mice treated with the combination of immunosuppressive drugs and benznidazole demonstrated an absence of lymphomas or other neoplasias. These findings support the indication of benznidazole, as the drug of choice, for immunosuppressed patients that develop a reactivation of Chagas' disease.

Leukemia and lymphoma incidence in rodents exposed to low-frequency magnetic fields

A weak association between residential or occupational exposure to electric and magnetic fields (50/60 Hz fields) and an increased incidence of leukemia has been reported. Numerous animal studies have evaluated the potential association between magnetic-field exposure and leukemia. These include long-term (up to 2(1/2) years) bioassays, initiation/promotion studies, investigations in transgenic models, and tumor growth studies. Exposure to 60 Hz circularly polarized magnetic fields at 1,400 microT for 28 months did not affect lymphoma incidence in mice. The study included over 2000 C57BL/6J mice. In another study, 1000 B6C3F(1) mice exposed to 60 Hz magnetic fields up to 1000 microT for 2 years showed no increase in lymphomas. Approximately 400 transgenic Emu-Pim1 mice exposed to 50 Hz fields up to 1000 microT for up to 18 months had no increased incidence of leukemia. Similarly, Trp53(+/-) mice and Pim1transgenic mice exposed to 60 Hz magnetic fields for 23 weeks showed no increased incidence of lymphoma. Three studies in F344 rats exposed to 50 or 60 Hz magnetic fields up to 5 mT showed no increased incidence of leukemia. The combined animal bioassay results are nearly uniformly negative for magnetic-field exposures enhancing leukemia and weaken the possible epidemiological association between magnetic-field exposures and leukemia in humans as suggested by epidemiological data.

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.

Tumour necrosis factor polymorphisms and susceptibility to follicular lymphoma

Follicular lymphoma is characterized in 85% of patients by the presence of a t(14;18) chromosomal translocation that results in overproduction of BCL2. In this study the distribution of high and low expressing TNF alleles at the TNF (-308) and LTalpha (+252) polymorphic sites in 121 patients with follicular lymphoma and 88 control individuals has been analysed. A reduction in high expressing haplotypes in patients compared to normal controls was found (P = 0.055), with no significant difference observed in response rate or overall survival between patients with high or low expressing haplotypes. These results suggest that the TNF locus, or an adjacent locus within the MHC region, is an important genetic risk factor in this disease.

Mixed phenotype lymphomas in thymectomized (SL/KhxAKR/Ms)F1 mice

Lymphomagenesis in mice is determined both by genetic and epigenetic mechanisms. The inbred strain SL/Kh mice selectively develop pre-B lymphomas and AKR/Ms, T-lymphomas. In crosses between SL/Kh and AKR/Ms, an AKR-derived dominant gene Tlsm1 (Thymic lymphoma susceptible mouse-1) determines the type of lymphoma to be a T-lymphoma. As an approach to the role of Tlsm1, we studied the effect of thymectomy at 1 week of age in (SL/KhxAKR/Ms)F1 hybrids. In intact F1 mice, the predominant type of lymphoma was of T-lineage, whereas in thymectomized mice, it was an unusual mixed-phenotype lymphoma. They were basically CD5+ B-lymphomas with a rearranged immunoglobulin gene, but carried NK1 and Mac1 on the cell surface and large lysosomal granules in the cytoplasm. Histologically, the lymphoma consisted of large lymphoblastoid cells and infiltrated the spleen, lymph node and liver. Electron microscopy and histochemistry revealed numerous cytoplasmic granules containing acid phosphatase and lysozyme. These morphological features are suggestive of large granular lymphocytes. They expressed interleukin-4, perforin, and interferon-gamma. On transplantation, these lymphoma cells grew equally well in intact and thymectomized F1 recipients.