Allelic loss mapping and physical delineation of a region harboring a thymic lymphoma suppressor gene on mouse chromosome 16

Susceptibility loci and chromosomal abnormalities in radiation induced hematopoietic neoplasms in mice

Genetics of susceptibility to radiation-induced hematopoietic neoplasms and somatic chromosomal aberrations were analyzed in 305 backcross (CcS-17xCcS-2)xCcS-2 mice of two CcS/Dem recombinant congenic strains. Irradiated CcS-2 mice were previously shown to exhibit high frequency of myeloid neoplasms whereas irradiated CcS-17 mice were susceptible to T-cell lymphomas. Mice were exposed to four whole-body irradiation doses of 1.7 Gy at one week intervals, which resulted in 139 hematopoietic neoplasms. The hematopoietic neoplasms were classified according to the Bethesda proposals for classification of lymphoid and nonlymphoid hematopoietic neoplasms in mice. Genotyping of mice with 24 microsatellite markers and subsequent statistical analysis indicated linkage of the radiation induced T-lymphomas to two loci on chromosome 10 (D10Mit134) and chromosome 12 (D12Mit52). T-lymphoma susceptibility appeared to be linked to D10Mit134 in a sex dependent way. In contrast, the myeloid-granulocytic leukemias susceptibility is linked to combined effects of chromosome 5 (D5Mit179) and 16 (D16Mit34). Cytogenetic analysis was performed according to the standard G-bands procedure and confirmed using FISH method. We found non-random numerical and structural chromosomal changes in lymphoid neoplasms. Cytogenetic analysis indicated chromosomal aberrations presumably associated with lymphomagenesis, no specific cancer-related rearrangements were observed.

Analysis of changes in DNA copy number in radiation-induced thymic lymphomas of susceptible C57BL/6, resistant C3H and hybrid F1 Mice

Radiation-induced thymic lymphoma in mice is a useful model for studying both the mechanism of radiation carcinogenesis and genetic susceptibility to tumor development. Using array-comparative genomic hybridization, we analyzed genome-wide changes in DNA copy numbers in radiation-induced thymic lymphomas that had developed in susceptible C57BL/6 and resistant C3H mice and their hybrids, C3B6F1 and B6C3F1 mice. Besides aberrations at known relevant genetic loci including Ikaros and Bcl11b and trisomy of chromosome 15, we identified strain-associated genomic imbalances on chromosomes 5, 10 and 16 and strain-unassociated trisomy of chromosome 14 as frequent aberrations. In addition, biallelic rearrangements at Tcrb were detected more frequently in tumors from C57BL/6 mice than in those from C3H mice, suggesting aberrant V(D)J recombination and a possible link with tumor susceptibility. The frequency and spectrum of these copy-number changes in lymphomas from C3B6F1 and B6C3F1 mice were similar to those in C57BL/6 mice. Furthermore, the loss of heterozygosity analyses of tumors in F(1) mice indicated that allelic losses at Ikaros and Bcl11b were caused primarily by multilocus deletions, whereas those at the Cdkn2a/Cdkn2b and Pten loci were due mainly to uniparental disomy. These findings provide important clues to both the mechanisms for accumulation of aberrations during radiation-induced lymphomagenesis and the different susceptibilities of C57BL/6 and C3H mice.

DNA copy number alterations and expression of relevant genes in mouse thymic lymphomas induced by gamma-irradiation and N-methyl-N-nitrosourea

The genetic mechanism for the development and progression of a lymphoma is unclear. This study investigated the alterations in the DNA copy number and the expression profiles of the genes located in the altered regions in mouse thymic lymphomas that were induced by two mutagens, gamma-irradiation and N-methyl-N-nitrosourea (MNU). Microarray-based comparative genomic hybridization was used to precisely delineate the boundaries of the altered region. The copy number gains of chromosomes 4 and 5 were observed only in the radiation-induced lymphomas, and gains of chromosomes 10 and 14 were observed only in the MNU-induced lymphomas. Regional copy number losses in chromosomes 11, 16, and 19 appeared frequently in the radiation-induced lymphomas. The cancer-related genes Pten, Ikaros/Znfn1a1, Ercc4, and Top3b were located in the minimal deletion regions. In particular, the expression levels of the Pten, Top3b, and Ikaros genes were downregulated in both lymphoma groups, but the expression level of Ercc4 was downregulated only in the MNU group. This study also examined the expression levels of Sparc, Cxcl1, and Myc (synonym: c-Myc), which are located in the copy number gained chromosomes. Sparc was upregulated specifically in the radiation group, and Cxcl1 in the MNU group. c-Myc was upregulated in both groups. There was limited correlation between the DNA copy number profiles and the expression of the cancer-related genes in mouse lymphomagenesis. The chromosome aberrations and novel expression profiles of the cancer-related genes within the altered regions may provide important clues to the genetic mechanism for the development of lymphoma.

Comparison of properties of spontaneous and radiation-induced mouse thymic lymphomas: role of Trp53 and radiation

Mouse thymic lymphomas are readily induced by radiation and also arise without irradiation when the mice are null in Trp53 functions. In the present study, spontaneous thymic lymphomas in Trp53-/- mice were compared to those arising in irradiated Trp53+/- mice, revealing three features characteristic of the spontaneous lymphomas. (1) Mp53D2, a Trp53 modifier that affects the latent period of radiogenic thymic lymphomas in Trp53+/- mice, had no effect on the development of spontaneous lymphomas. (2) A sex difference in the latency was found. (3) A marked difference was noted in the frequency of allelic loss at the Ikaros gene on chromosome 11, encoding a transcription factor required for normal lymphocyte development and differentiation; 2% in the lymphomas of Trp53-/- mice and 78% in the radiogenic lymphomas of Trp53+/- mice, suggesting that loss of Trp53 may reduce the requirement for the loss of Ikaros for lymphomagenesis. Furthermore, allelic loss analysis on chromosome 19 localized a region that may harbor an unknown tumor suppressor gene. These results suggest intricate steps of lymphomagenesis influenced by the presence or absence of Trp53.

The D4Mit12 locus on mouse chromosome 4 provides susceptibility to both gamma-ray-induced and N-methyl-N-nitrosourea-induced thymic lymphomas

Low-penetrance genes control different susceptibilities to gamma-ray-induced thymic lymphomas in mouse strains. Our previous genetic analyses with backcross mice between BALB/c and MSM strains and congenic lines localized one such gene near the D4Mit12 locus on chromosome 4. N-Methyl-N-nitrosourea (MNU) is a guanine base-alkylating agent and differs from gamma-radiation in its mechanism of mutagenic action. Accordingly, in this study, we examined whether or not the locus also provides susceptibility to MNU-induced thymic lymphomas using 84 offsprings derived from congenic mice for D4Mit12. Association analysis provided a suggestive linkage at D4Mit12 (P = 0.0075) and the linkage was sustained by the peak of likelihood ratio statistical values being at the same position as that for the gamma-ray-induced lymphomas. The results strongly suggest that the BALB/c allele near D4Mit12 is associated with susceptibility to lymphomas induced by two carcinogenic agents having different mechanisms of mutagenic action.

Allelic losses in mouse skin tumors induced by gamma-irradiation of p53 heterozygotes

Skin tumors were induced by gamma-irradiation in F(1) mice between C3H/He or BALB/c and MSM carrying a p53-deficient allele. The incidence was 39.1% (34/87) in p53(KO/+) mice of the C3H/MSM genetic background and 14.3% (19/133) in those of the BALB/MSM background. Interestingly, most of the tumors (82%) lost the wild-type p53 allele and no skin tumor was found in p53(+ / +) F(1) mice. This suggests a requirement of p53 loss for the skin cancer development. Genome scan localized a chromosomal locus showing frequent allelic losses near D12Mit2, which may harbor a tumor suppressor gene. In addition, 23 loci distributed on 13 chromosomes exhibited allelic losses at frequencies of more than 20%. The genome-wide occurrence of allelic losses suggests that genomic instability of the skin tumors may be implicated in radiation-induced carcinogenesis. The present study is the first to report a mouse model system useful for the analysis of radiation induction of skin cancer in man.