Multiple pre-neoplastic events and clonal selection of radiation induced mouse thymic lymphomas shown by TCR gene rearrangements

Differential effects of low- and high-dose X-rays on N-ethyl-N-nitrosourea-induced mutagenesis in thymocytes of B6C3F1 gpt-delta mice

Carcinogenesis in humans is thought to result from exposure to numerous environmental factors. Little is known, however, about how these different factors work in combination to cause cancer. Because thymic lymphoma is a good model of research for combined exposure, we examined the occurrence of mutations in thymic DNA following exposure of B6C3F1 gpt-delta mice to both ionizing radiation and N-ethyl-N-nitrosourea (ENU). Mice were exposed weekly to whole body X-irradiation (0.2 or 1.0 Gy), ENU (200 ppm) in the drinking water, or X-irradiation followed by ENU treatment. Thereafter, genomic DNA was prepared from the thymus and the number and types of mutations in the reporter transgene gpt was determined. ENU exposure alone increased mutant frequency by 10-fold compared to untreated controls and over 80% of mutants had expanded clonally. X-irradiation alone, at either low or high dose, unexpectedly, reduced mutant frequency. Combined exposure to 0.2 Gy X-rays with ENU dramatically decreased mutant frequency, specifically G:C to A:T and A:T to T:A mutations, compared to ENU treatment alone. In contrast, 1.0 Gy X-rays enhanced mutant frequency by about 30-fold and appeared to accelerate clonal expansion of mutated cells. In conclusion, repeated irradiation with 0.2 Gy X-rays not only reduced background mutation levels, but also suppressed ENU-induced mutations and clonal expansion. In contrast, 1.0 Gy irradiation in combination with ENU accelerated clonal expansion of mutated cells. These results indicate that the mode of the combined mutagenic effect is dose dependent.

Target cell frequency is a genetically determined risk factor in radiation leukaemogenesis

Whole body exposure to ionizing radiation increases the risk of radiation-induced acute myeloid leukaemia (r-AML). r-AML is the result of the accumulation of mutations in a single haemopoietic stem cell, so risk is therefore a function of the number of mutations required to transform the stem cell and the mutation rate. There is a genetic component to the risk of AML within the general population, and low penetrance variant alleles encoding DNA repair enzymes have been genetically implicated in therapy-related AML susceptibility. However, what is largely ignored is that target cell number, which defines the number of genomes at risk from DNA damaging agents, is also part of the equation that defines risk. We will review the evidence from genetic studies of inbred mouse models that target cell frequency is a risk factor in radiation leukaemogenesis. Inbred mouse strains that differ in their susceptibility to radiation-induced r-AML and thymic lymphoma (r-TL), spontaneous TL and pristane-induced plasmacytoma (PCT) have been exploited to identify susceptibility loci. The target cell in AML is the haemopoietic stem cell, whereas TLs and PCT arise from more mature lymphoid progenitor cells. Inbred mice also differ significantly in all aspects of haemopoiesis, and these differences have been used to identify quantitative trait loci (QTL) that determine the frequency of specific haemopoietic stem, progenitor or mature blood cells. The co-localization of QTL that determine risk and target cell frequency in all three haemopoietic malignancies is strong evidence that target cell frequency is a risk factor in radiation leukaemogenesis.

The mouse (Mus musculus) T cell receptor alpha (TRA) and delta (TRD) variable genes

'The Mouse (Mus musculus) T cell receptor alpha (TRA) and delta (TRD) variable genes' 'IMGT Locus in Focus' report provides the first complete list of the mouse TRAV and TRDV genes which span 1550 kb on chromosome 14 at 19.7 cM. The total number of TRAV genes per haploid genome is 98 belonging to 23 subgroups. This includes 10 TRAV/DV genes which belong to seven subgroups. The functional TRAV genomic repertoire comprises 72-82 TRAV (including 9-10 TRAV/DV) belonging to 19 subgroups. The total number of TRDV genes per haploid genome is 16 (including the 10 TRAV/DV) belonging to 12 subgroups. The functional TRDV genomic repertoire comprises 14-15 genes (5 TRDV and 9-10 TRAV/DV) belonging to 11-12 subgroups. The eight tables and three figures of this report are available at the IMGT Marie-Paule page of IMGT. The international ImMunoGeneTics information system (http://imgt.cines.fr) created by Marie-Paule Lefranc, Université Montpellier II, CNRS, France.

Myeloid, B and T lymphoid and mixed lineage thymic lymphomas in the irradiated mouse

Thymic lymphoma is a very common spontaneous and/or induced malignancy in both inbred mice and in transgenic mouse models of human cancer. Although a thymic lymphoma is defined as thymus-dependent T-cell malignancy, diagnostic criteria vary between studies and considerable heterogeneity has been reported. To define and classify the thymic lymphomas that arose in our study of X-irradiated (CBA/HxC57BL/6)F1, F1 backcross and F1 intercross mice, 66 thymic lymphomas were immunogenotyped for immunoglobulin heavy chain (IgH) and T-cell receptor beta (TCRbeta) gene rearrangements, and/or analysed for expression of lineage-specific markers and allelic loss on chromosome 4. The data indicate that 33% of the thymic lymphomas are very similar to mouse radiation-induced acute myeloid (AML) and mixed lineage (IgH(R), TCRbeta(G)) pre-B lympho-myeloid (L-MLs) leukaemias, 33% are mixed lineage (IgH(R), TCRbeta(R)) B/T lymphoid and <33% can be described as single lineage (IgH(G), TCRbeta(R)) T-cell malignancies. As the myeloid and L-ML leukaemias are not thymus-dependent this suggests that a malignant myeloid or pre-B lympho-myeloid cell can colonize the spleen to give an AML or L-ML leukaemia, or can colonize the thymus where TCRbeta gene rearrangement(s) may be induced to give the mixed lineage thymic lymphomas. Thus, assuming the single lineage T-cell thymic lymphomas fulfil the criteria of a thymus-dependent T-cell malignancy, thymic lymphomas are comprised of at least three distinct malignancies.

Clonal evolution of N-methylnitrosourea-induced C57BL/6J thymic lymphomas by analysis of multiple genetic alterations

C57BL/6J mice were treated with N-methylnitrosourea (NMU) and the evolution of leukemic T-cells clones into frank thymic lymphomas was followed in 42 animals using serology of T-cell markers, rearrangements of the T-cell receptor genes gamma, beta1 and beta2 and detection of carcinogen-induced Ki-ras mutations and trisomy of chromosome 15. During the latent period, multiple populations of T-cell clones were present in the thymus, many contained trisomy 15, but few had detectable Ki-ras mutations. Since most frank lymphomas consisted of a single T-cell clone with both a mutation of Ki-ras and trisomy 15, the results imply that these two events are critical for the evolution of T-cell clones from the preleukemic phase to a more malignant disease stage. Progression to frank lymphomas is coincident with changes in the expression pattern of the T-cell growth factor interleukin-2 receptor, which may play a role in the selection, expansion and thymus-independent growth of a T-cell clone.

A new immunobiological view of radiation-promoted lymphomagenesis

Whole-body irradiation produces T-cell leukaemias/ lymphomas (TCL) in some strains of inbred mice in an X-ray dose-related manner. Radiation biologists have related the rapid "initiation' and early appearance of preleukaemic cells in these mice to unrepaired DNA damage inflicted by radiation. Following initiation, radiation-altered thymic differentiation fosters multi-step transformation changes in proto-oncogenes and suppressor gene expression in individual clones of non-invasive preleukaemia cells as they progress to malignancy. The malignant clones arising from small numbers of initiated preleukaemia thymocytes become fully transformed only after several more months to a year after irradiation in those strains of mice which develop T-cell lymphomas. When the RFM mouse was subjected to sublethal whole-body X-ray, only 50% of the mice developed TCL by 6 months, yet nearly all developed preleukaemia thymocytes. The T-cell-mediated immune response of the irradiated host has never been substantiated to contribute to malignant TCL development. Until recently, X-ray-induced TCL were not known to carry common tumour rejection antigens TATA. However, several studies have revealed that both preleukaemia cells and fully malignant TCL express an immunogenic, common oncofoetal glycoprotein, termed 44kD OFA. OFA-activated memory CD4 Tm and CD8 Ten. T-effector cells in irradiated mice expressing OFA. As most irradiated RFM mice exhibit preleukaemia thymocytes yet only half develop tumours, this finding implicates active host T-cell effector responses in X-ray-initiated tumorigenesis. Further, the recent discovery of OFA-specific CD8 Ts clones in irradiated mice, which inhibited cytotoxicity of CD8 clones to OFA or TSTA, may explain which mice develop T-cell lymphomas.

Analysis of early initiating event(s) in radiation-induced thymic lymphomagenesis

Since the T cell receptor rearrangement is a sequential process and unique to the progeny of each clone, we investigated the early initiating events in radiation-induced thymic lymphomagenesis by comparing the oncogenic alterations with the pattern of gamma T cell receptor (TCR) rearrangements. We reported previously that after leukemogenic irradiation, preneoplastic cells developed, albeit infrequently, from thymic leukemia antigen-2+ (TL-2+) thymocytes. Limited numbers of TL-2+ cells from individual irradiated B10.Thy 1.1 mice were injected into B10.Thy 1.2 mice intrathymically, and the common genetic changes among the donor-type T cell lymphomas were investigated with regard to p53 gene and chromosome aberrations. The results indicated that some mutations in the p53 gene had taken place in these lymphomas, but there was no common mutation among the donor-type lymphomas from individual irradiated mice, suggesting that these mutations were late-occurring events in the process of oncogenesis. On the other hand, there were common chromosome aberrations or translocations such as trisomy 15, t(7F;10C), t(1A;13D) or t(6A;XB) among the donor-type lymphomas derived from half of the individual irradiated mice. This indicated that the aberrations/translocations, which occurred in single progenitor cells at the early T cell differentiation either just before or after gamma T cell receptor rearrangements, might be important candidates for initiating events. In the donor-type lymphomas from the other half of the individual irradiated mice, microgenetic changes were suggested to be initial events and also might take place in single progenitor cells just before or right after gamma TCR rearrangements.

Horse (Equus caballus) T-cell receptor alpha, gamma, and delta chain genes: nucleotide sequences and tissue-specific gene expression

Horse (Equus caballus) T-cell receptor alpha (TCRA), gamma (TCRG), and delta (TCRD) chain genes were isolated from a cDNA library and characterized. Five unique TCRAV families, including four full-length sequences, five distinct TCRAJ genes, and a single TCRAC gene were identified. TCRAV genes had closest homology with human sequences and least similarity to rat genes. Among eight horse TCRG genes, two distinct constant region genes with considerable variation in the connecting region were identified, but no variable or joining genes were present. Southern blot hybridization confirmed the presence of at least two TCRGC genes and indicated that the vast majority of horse alpha beta T cells rearrange either one or both TCRG alleles. Analysis of horse TCRD genes revealed the presence of eight unique TCRDV genes representing seven families, each having closest nucleotide homology with sheep sequences. Six unique TCRDJ genes were isolated; however, four of these sequences differed by only one base pair and thus likely represented alleles of a single gene. One horse TCRDC gene was present among fifteen clones analyzed and, based on Southern blot hybridizations, was deleted in polyclonal alpha beta T-cell populations, indicating that the TCRD locus is probably located within the TCRA locus as in other species. Polymerase chain reaction using horse-specific primers for the detection of TCRAC and TCRDC gene expression indicated that gamma delta T cells are located at numerous sites throughout the body, and with the exception of bone marrow where only TCRAC transcripts were detected, are closely associated with alpha beta T cells. This finding indicates that these two T-cell populations may be functionally interactive.