Radiation-induced lymphomas in MSM, (BALB/cHeA x MSM) F1 and (BALB/cHeA x STS/A) F1 hybrid mice

Development of mammary cancer in γ-irradiated F1 hybrids of susceptible Sprague-Dawley and resistant Copenhagen rats, with copy-number losses that pinpoint potential tumor suppressors

Copenhagen rats are highly resistant to mammary carcinogenesis, even after treatment with chemical carcinogens and hormones; most studies indicate that this is a dominant genetic trait. To test whether this trait is also dominant after radiation exposure, we characterized the susceptibility of irradiated Copenhagen rats to mammary carcinogenesis, as well as its inheritance, and identified tumor-suppressor genes that, when inactivated or mutated, may contribute to carcinogenesis. To this end, mammary cancer-susceptible Sprague-Dawley rats, resistant Copenhagen rats, and their F1 hybrids were irradiated with 4 Gy of γ-rays, and tumor development was monitored. Copy-number variations and allelic imbalances of genomic DNA were studied using microarrays and PCR analysis of polymorphic markers. Gene expression was assessed by quantitative PCR in normal tissues and induced mammary cancers of F1 rats. Irradiated Copenhagen rats exhibited a very low incidence of mammary cancer. Unexpectedly, this resistance trait did not show dominant inheritance in F1 rats; rather, they exhibited intermediate susceptibility levels (i.e., between those of their parent strains). The susceptibility of irradiated F1 rats to the development of benign mammary tumors (i.e., fibroadenoma and adenoma) was also intermediate. Copy-number losses were frequently observed in chromosome regions 1q52-54 (24%), 2q12-15 (33%), and 3q31-42 (24%), as were focal (38%) and whole (29%) losses of chromosome 5. Some of these chromosomal regions exhibited allelic imbalances. Many cancer-related genes within these regions were downregulated in mammary tumors as compared with normal mammary tissue. Some of the chromosomal losses identified have not been reported previously in chemically induced models, implying a novel mechanism inherent to the irradiated model. Based on these findings, Sprague-Dawley × Copenhagen F1 rats offer a useful model for exploring genes responsible for radiation-induced mammary cancer, which apparently are mainly located in specific regions of chromosomes 1, 2, 3 and 5.

A wild-derived inbred mouse strain, MSM/Ms, provides insights into novel skin tumor susceptibility genes

Cancer is one of the most catastrophic human genetic diseases. Experimental animal cancer models are essential for gaining insights into the complex interactions of different cells and genes in tumor initiation, promotion, and progression. Mouse models have been extensively used to analyze the genetic basis of cancer susceptibility. They have led to the identification of multiple loci that confer, either alone or in specific combinations, an increased susceptibility to cancer, some of which have direct translatability to human cancer. Additionally, wild-derived inbred mouse strains are an advantageous reservoir of novel genetic polymorphisms of cancer susceptibility genes, because of the evolutionary divergence between wild and classical inbred strains. Here, we review mapped Stmm (skintumor modifier of MSM) loci using a Japanese wild-derived inbred mouse strain, MSM/Ms, and describe recent advances in our knowledge of the genes responsible for Stmm loci in the 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) two-stage skin carcinogenesis model.

The Japanese Wild-Derived Inbred Mouse Strain, MSM/Ms in Cancer Research

MSM/Ms is a unique inbred mouse strain derived from the Japanese wild mouse, Mus musculus molossinus, which has been approximately 1 million years genetically distant from standard inbred mouse strains mainly derived from M. m. domesticus. Due to its genetic divergence, MSM/Ms has been broadly used in linkage studies. A bacterial artificial chromosome (BAC) library was constructed for the MSM/Ms genome, and sequence analysis of the MSM/Ms genome showed approximately 1% of nucleotides differed from those in the commonly used inbred mouse strain, C57BL/6J. Therefore, MSM/Ms mice are thought to be useful for functional genome studies. MSM/Ms mice show unique characteristics of phenotypes, including its smaller body size, resistance to high-fat-diet-induced diabetes, high locomotive activity, and resistance to age-onset hearing loss, inflammation, and tumorigenesis, which are distinct from those of common inbred mouse strains. Furthermore, ES (Embryonic Stem) cell lines established from MSM/Ms allow the MSM/Ms genome to be genetically manipulated. Therefore, genomic and phenotypic analyses of MSM/Ms reveal novel insights into gene functions that were previously not obtained from research on common laboratory strains. Tumorigenesis-related MSM/Ms-specific genetic traits have been intensively investigated in Japan. Furthermore, radiation-induced thymic lymphomas and chemically-induced skin tumors have been extensively examined using MSM/Ms.

Genetic susceptibility: radiation effects relevant to space travel

Genetic variation in the capacity to repair radiation damage is an important factor influencing both cellular and tissue radiosensitivity variation among individuals as well as dose rate effects associated with such damage. This paper consists of two parts. The first part reviews some of the available data relating to genetic components governing such variability among individuals in susceptibility to radiation damage relevant for radiation protection and discusses the possibility and extent to which these may also apply for space radiations. The second part focuses on the importance of dose rate effects and genetic-based variations that influence them. Very few dose rate effect studies have been carried out for the kinds of radiations encountered in space. The authors present here new data on the production of chromosomal aberrations in noncycling low passage human ATM+/+ or ATM+/- cells following irradiations with protons (50 MeV or 1 GeV), 1 GeV(-1) n iron ions and gamma rays, where doses were delivered at a high dose rate of 700 mGy(-1) min, or a lower dose rate of 5 mGy min(-1). Dose responses were essentially linear over the dose ranges tested and not significantly different for the two cell strains. Values of the dose rate effectiveness factor (DREF) were expressed as the ratio of the slopes of the dose-response curves for the high versus the lower (5 mGy min(-1)) dose rate exposures. The authors refer to this as the DREF5. For the gamma ray standard, DREF5 values of approximately two were observed. Similar dose rate effects were seen for both energies of protons (DREF5 ≈ 2.2 in both cases). For 1 GeV(-1) n iron ions [linear energy transfer (LET) ≈ 150 keV μ(-1)], the DREF5 was not 1 as might have been expected on the basis of LET alone but was approximately 1.3. From these results and conditions, the authors estimate that the relative biological effectiveness for 1 GeV(-1) n iron ions for high and low dose rates, respectively, were about 10 and 15 rather than around 20 for low dose rates, as has been assumed by most recommendations from radiation protection organizations for charged particles of this LET. The authors suggest that similar studies using appropriate animal models of carcinogenesis would be valuable.

Independent genetic control of early and late stages of chemically induced skin tumors in a cross of a Japanese wild-derived inbred mouse strain, MSM/Ms

MSM/Ms is an inbred mouse strain derived from a Japanese wild mouse, Mus musculus molossinus. In this study, we showed that MSM/Ms mice exhibit dominant resistance when crossed with susceptible FVB/N mice and subjected to the two-stage skin carcinogenesis protocol using 7,12-dimethylbenz(a)anthracene (DMBA)/ 12-O-tetradecanoylphorbol-13-acetate (TPA). A series of F1 backcross mice were generated by crossing p53(+/+) or p53(+/-) F1 (FVB/N × MSM/Ms) males with FVB/N female mice. These generated 228 backcross animals, approximately half of which were p53(+/-), enabling us to search for p53-dependent skin tumor modifier genes. Highly significant linkage for papilloma multiplicity was found on chromosomes 6 and 7 and suggestive linkage was found on chromosomes 3, 5 and 12. Furthermore, in order to identify stage-dependent linkage loci we classified tumors into three categories (<2mm, 2-6mm and >6mm), and did linkage analysis. The same locus on chromosome 7 showed strong linkage in groups with <2mm or 2-6mm papillomas. No linkage was detected on chromosome 7 to papillomas >6mm, but a different locus on chromosome 4 showed strong linkage both to papillomas >6mm and to carcinomas. This locus, which maps near the Cdkn2a/p19(Arf) gene, was entirely p53-dependent, and was not seen in p53 (+/-) backcross animals. Suggestive linkage conferring susceptibility to carcinoma was also found on chromosome 5. These results clearly suggest distinct loci regulate each stage of tumorigenesis, some of which are p53-dependent.

Two loci controlling susceptibility to radiation-induced lymphomagenesis on mouse chromosome 4: cdkn2a, a candidate for one locus, and a novel locus distinct from cdkn2a

BALB/c mice are sensitive to radiation-induced lymphomagenesis, while STS mice are resistant. Using 219 [(BALB/c x STS)F(1) x BALB/c] (N2C) and 197 [(BALB/c x STS)F(1) x STS] (N2S) animals, we performed a genome-wide search for loci controlling susceptibility to lymphomagenesis induced by radiation. Association of markers with the survival of animals was analyzed by the log rank test. For N2C mice, a significant correlation was detected, with four markers on the proximal to mid portion of chromosome 4: D4Mit302 and D4Mit255, P = 0.0075; D4Mit17, P = 0.034; and D4Mit86, P = 0.048. On the other hand, no significant linkage was detected in N2S mice. We analyzed BALB/c mice congenic for the STS allele in different regions of chromosome 4 and identified a locus with a conspicuous effect on survival located within a 7-Mb region between D4Mit302 and D4Mit144, where BALB/c mice harbor hypomorphic variant alleles of the tumor suppressor gene Cdkn2a, which encodes the cyclin-dependent kinase inhibitor protein p16INK4a. Using pooled F(2) intercrosses between the BALB/c and congenic lines carrying the STS allele near D4Mit17, but not in the range from D4Mit302 to D4Mit144, we assigned the second locus to an 11.4-Mb region in the vicinity of D4Mit17. Although Cdkn2a is a likely candidate for the locus controlling susceptibility to lymphomagenesis on chromosome 4, a novel tumor susceptibility gene different from Cdkn2a exists near the primary locus.

Variations in radiosensitivity among individuals: a potential impact on risk assessment?

To have an impact on risk assessment for purposes of radiation protection recommendations, significantly broad variations in carcinogenic radiosensitivity would have to exist in significant proportions in the human population. Even if we knew all the genes where mutations would have major effects, individual genome sequencing does not seem useful, since we do not know all these genes, nor can we be certain of the phenotypic effect of polymorphisms discovered. Further, sequencing would not reveal epigenetic changes in gene expression. Another approach to develop phenotypic biomarkers for cells or tissues for which variations in radiation response may reflect the variations in carcinogenic sensitivity. To be useful, experimental evidence for such a correlation would be crucial, and it is also evident that correlations may be tissue or tumor specific. Some cellular markers are discussed that have shown promise in this regard. They include chromosome aberration induction and DNA repair assays that are sufficiently sensitive to measure after modest or low doses or dose rates. To this end we summarize here some of these assays and review the results of a number of experiments from our laboratory that show clear differences in DNA repair capacity reflected by gamma-H2AX foci formation in cells from a high proportion (perhaps 1/3) of apparently normal individuals. A low dose-rate assay was used to amplify such differences. Another promising assay combines G(2) chromosomal radiosensitivity with the above gamma-H2AX foci on mitotic chromosomes. There are other potentially useful assays as well.

Establishment of germline-competent embryonic stem cell lines from the MSM/Ms strain

MSM/Ms is an inbred mouse strain established from the Japanese wild mouse, Mus musculus molossinus, which has been phylogenetically distinct from common laboratory mouse strains for about 1 million years. The nucleotide substitution rate between MSM/Ms and C57BL/6 is estimated to be 0.96%. MSM/Ms mice display unique characteristics not observed in the commonly used laboratory strains, including an extremely low incidence of tumor development, high locomotor activity, and resistance to high-fat-diet-induced diabetes. Thus, functional genomic analyses using MSM/Ms should provide a powerful tool for the identification of novel phenotypes and gene functions. We report here the derivation of germline-competent embryonic stem (ES) cell lines from MSM/Ms blastocysts, allowing genetic manipulation of the M. m. molossinus genome. Fifteen blastocysts were cultured in ES cell medium and three ES lines, Mol/MSM-1, -2, and -3, were established. They were tested for germline competency by aggregation with ICR morulae and germline chimeras were obtained from all three lines. We also injected Mol/MSM-1 ES cells into blastocysts of ICR or C57BL/6 x BDF1 mice and found that blastocyst injection resulted in a higher production rate of chimeric mice than did aggregation. Furthermore, Mol/MSM-1 subclones electroporated with a gene trap vector were also highly efficient at producing germline chimeras using C57BL/6 x BDF1 blastocyst injection. This Mol/MSM-1 ES line should provide an excellent new tool allowing the genetic manipulation of the MSM/Ms genome.

Allelic loss analysis of lymphomas induced in Fas-heterozygous deficient mice

Mutations of Fas (CD95/Apo-1) gene have been reported in various malignancies and therefore the Fas gene has been considered to be a tumor suppressor gene. To examine an involvement of Fas gene as a tumor suppressor gene in radiation lymphomagenesis, we examined the loss of heterozygosity (LOH) in lymphomas from (MSM/Ms x MRL-MpJ/Fas (lpr)) F(1) and (BALB/cHeA x MRL-MpJ/Fas (lpr)) F(1) hybrid mice. Lymphoma development by X-irradiation was efficiently observed in both F(1) hybrids. Frequent LOH was found on chromosomes 12 and 4 in the tumors from both F(1) mice, but no allelic loss on chromosome 19 containing Fas locus was found, and no wild-type allele of the Fas gene was lost in 51 lymphomas. Therefore, the putative tumor-suppressor gene regions responsible for lymphomagenesis might not considerably differ due to the Fas gene status.

Variations in Prkdc encoding the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) and susceptibility to radiation-induced apoptosis and lymphomagenesis

DNA double-strand breaks (DSBs) induced by ionizing radiation enforce cells to die, if unrepaired; while if misrepaired, DSBs may cause malignant transformation. The DSB repair system predominant in mammals requires DNA-dependent protein kinase (DNA-PK). Previously, we identified the apoptosis susceptibility gene Radiation-induced apoptosis 1 (Rapop1) on mouse chromosome 16. The STS/A (STS) allele at Rapop1 leads to decreased sensitivity to apoptosis in the BALB/cHeA (BALB/c) background. In the present study, we established Rapop1 congenic strains C.S-R1 and C.S-R1L, which contain the STS genome in a 0.45 cM interval critical for Rapop1 in common in the BALB/c background. Within the segment critical for Rapop1, Prkdc encoding the catalytic subunit of DNA-PK (DNA-PKcs) was assigned. Two variations T6,418C and G11,530A, which induce amino acid substitutions C2,140R downstream from the putative leucine zipper motif and V3,844M near the kinase domain, respectively, were found between BALB/c and STS for Prkdc. The majority of inbred strains such as C57BL/6J carried the STS allele at Prkdc; a few strains including 129/SvJ and C.B17 carried the BALB/c allele. DNA-PK activity as well as DNA-PKcs expression was profoundly diminished in BALB/c and 129/SvJ mice as compared with C57BL/6 and C.S-R1 mice. In the crosses (C.S-R1 x BALB/c)F(1) x 129/SvJ and (C.S-R1 x BALB/c)F(1) x C.B17, enhanced apoptosis occurred in the absence of the wild-type allele at Prkdc. C.S-R1 and C.S-R1L were both less sensitive to radiation lymphomagenesis than BALB/c. Our study provides strong evidence for Prkdc as a candidate for Rapop1 and a susceptibility gene for radiation lymphomagenesis as well.

A susceptibility locus for radiation lymphomagenesis on mouse chromosome 16

BALB/cHeA (BALB/c) mice are sensitive to radiation lymphomagenesis, while STS/A (STS) mice are resistant. We have selected a recombinant mouse, R1, with a STS-derived D16Mit165-D16Mit34 segment in the vicinity of the centromere of chromosome 16 among progeny from a (CcS-7/Dem x BALB/c)F1 x BALB/c backcross. To test the susceptibility to radiation lymphomagenesis, we generated a genetic cross by mating the male and female R1 progeny obtained by 4-6 backcrosses of R1 to BALB/c. The mice were subjected to 4 x 1.7 Gy of X-irradiation. Of 120 mice analyzed, 94 developed lymphomas (91, of thymic type; 3, of nonthymic type) within 315 days of observation. The analysis indicated a link between the susceptibility to lymphomagenesis and the marker D16Mit34 on chromosome 16. The mice heterozygous for the BALB/c and STS alleles at D16Mit34 were less sensitive to lymphomagenesis than the mice homozygous at this locus. There was no significant difference in latency among the genotypes. Our study showed the existence of a susceptibility locus for radiation lymphomagenesis on chromosome 16 and revealed aspects of the genetics of lymphoma susceptibility.

Analysis of highly frequent allelic loss region on distal chromosome 12 in murine radiation-induced lymphomas

Recent genetic studies of tumorigenesis have strongly suggested an existence of tumor suppressor gene(s) on murine chromosome 12 and human chromosome 14q32. We previously described that putative tumor suppressor gene(s) might reside between D12Mit53 and D12Mit233. We analyzed three genes, Tcl1, Yy1 and Tnfalphaip2, which had been mapped around the region, as the candidates in radiation lymphomagenesis of (BALB/c x MSM/Ms)F1 hybrid mice. The locus order and distances of the three genes and microsatellite loci were estimated as follows: [centromere] - Tcl1-(> or =0.085 cM)-D12Mit50-(0.085 cM)D12Mit132-(1.96 cM)D12Mit122-(0.085 cM)D12Mit53-(1.37 cM)-[D12Mit233,D12Mit279,Yy1]-(0.085 cM)-D12Mit181-(> or =0.17 cM)-Tnfalphaip2 - [telomere]. Allele losses at Tcl1, Yy1 genes and D12Mit233 were observed in 94(45%), 143(68%) and 147(70%) of 210 lymphomas, respectively. In semi-quantitative analysis of Yy1 mRNA levels by RT-PCR, kinetics of the yield of the Yy1-cDNA-specific PCR products showed almost the same profiles among thymic lymphomas with allelic loss at Yy1, lymphomas with both alleles retained and normal thymus. These results suggest that Tcl1, Yy1 and Tnfalphaip2 genes are not predominantly involved in radiation lymphomagenesis of mice. In further analysis of the common allelic loss region, we found new loci, Y152pR1 and Y184pR2, from YACs which located in the hot region between D12Mit53 and D12Mit233, and the highest frequency of allelic loss (71%) was observed at the Y184pR2 locus. The LOH patterns of individual lymphomas suggest that putative tumor suppressor gene(s) lies between Y152pR1 and Y184pR2.

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.

Frequent loss of heterozygosity on chromosomes 4, 12 and 19 in radiation-induced lymphomas in mice

We found frequent loss of heterozygosity (LOH) on chromosomes 4, 12 and 19 in radiation-induced lymphomas from (BALB/cHeA x STS/A) F1 hybrid mice by allelotype analysis at polymorphic microsatellite loci. The incidences of LOH were 27% (20 of 74 lymphomas), 57% (42 of 74 lymphomas) and 50% (37 of 74 lymphomas) on chromosomes 4 (at D4Mit31), 12 (at D12Mit17) and 19 (at D19Mit11), respectively. These frequent LOH regions are homologous to human chromosomes 9p and 1p, chromosome 12q32.1 and chromosome 10q, respectively. Strain-specific preferential allele loss was observed only on chromosome 4. However, no bias in the frequency of loss between alleles of maternal and paternal origin was observed, indicating that genomic imprinting may not be predominantly involved in these lymphomas. The results suggest that these three regions might harbor tumor suppressor genes responsible for this lymphomagenesis.