Evaluation of nonthreshold leukemogenic response to methyl nitrosourea in p53-deficient C3H/He mice

Hematopoietic neoplastic diseases develop in C3H/He and C57BL/6 mice after benzene exposure: strain differences in bone marrow tissue responses observed using microarrays

In this study, Trp53-deficient and wild-type mice of both C57BL/6 and C3H/He strains were exposed to benzene (33, 100, and 300 ppm; 6h/day, 5 days/week for 26 weeks) and then observed for lifetime. As results, first, the incidence of nonthymic lymphomas in C57BL/6 mice and acute myeloid leukemias (AMLs) in C3H/He mice showed linear responses at the lower exposure level in Trp53-deficient mice; second, the incidence of thymic lymphomas in C57BL/6 mice and nonthymic lymphomas in C3H/He mice increased without a plateau-like ceiling; thus, the former equivocal induction of hematopoietic neoplasms (HPNs) in the case of low-dose benzene exposure was assumed to be based on the DNA repair potential in wild-type mice, and the latter limited increase in HPNs in the case of high-dose benzene exposure was considered to be due to excessive apoptosis in wild-type mice. Concerning the incidence of AMLs, though a dose of 300 ppm benzene inhalation induced 9% AMLs in wild-type C3H/He mice-AML-prone, it induced AMLs in 38% of Trp53-deficient C3H/He mice. Because AMLs were also observed in Trp53-deficient mice, including in the C57BL/6 mice, benzene exposure may also be a potent inducer of AMLs in mice with some strain differences. In the present study, to elucidate the hematopoietic stem cell-specific, aryl hydrocarbon-receptor-related low-dose adverse effect, global gene expression in the bone marrow was analyzed at 28 days after 2-week-intermittent exposure to 150 mg/kg b.w. benzene, by gavage, i.e., equivalent to the above inhalation protocol with 300 ppm. We observed two conceptually different gene expression profiles; "common gene profiles" (CGPs) shared among mice in each group, and "stochastic gene profiles" (SGPs), i.e., unique union genes from one individual mouse to another. The CGPs of the experimental group and the SGPs of each individual mouse were separately characterized by individual assay. Concerning the CGPs, reciprocal strain differences between C3H/He and C57BL/6 mice in expression gene profiles, both plausible for leukemogenesis, were identified; namely, dominant downmodulations of Sltm and Cryl1, related to suppression of apoptosis and genomic instability in C3H/He mice, respectively, and dominant downmodulations of Atrx/rad54 and Kdm2a, related to a decrease in DNA repair and genomic instability, respectively, in C57BL/6 mice. These findings imply that these reciprocal gene expression differences induced by benzene exposure may lead each strain to undergo different hematopoietic neoplastic pathways. In contrast, each individual mouse often shows a unique SGP. SGPs often include transcription factors, which regulate reciprocal signaling pathways including further SGPs. Among them, apoptosis-related genes expressed in C57BL/6 mice and those in C3H/He mice were attributable to different combinations of SGPs. Such stochastic case-by-case gene expression may be in good agreement with the individual and strain differences observed following benzene exposure. Because gene chip microarray techniques can elucidate stochastic changes in gene expression profiles, possible stochastic toxicology and its future role are discussed.

Benzene-induced hematopoietic neoplasms including myeloid leukemia in Trp53-deficient C57BL/6 and C3H/He mice

This research focused on three major questions regarding benzene-induced hematopoietic neoplasms (HPNs). First, why are HPNs induced equivocally and at only threshold level with low-dose benzene exposure despite the significant genotoxicity of benzene even at low doses both in experiments and in epidemiology? Second, why is there no linear increase in incidence at high-dose exposure despite a lower acute toxicity (LD(50) > 1000 mg/kg body weight; WHO, 2003, Benzene in drinking-water. Background document for development of WHO Guidelines for Drinking-Water Quality)? Third, why are particular acute myeloid leukemias (AMLs) not commonly observed in mice, although AMLs are frequently observed in human cases of occupational exposure to benzene? In this study, we hypothesized that the threshold-like equivocal induction of HPNs at low-dose benzene exposure is based on DNA repair potential in wild-type mice and that the limited increase in HPNs at a high-dose exposure is due to excessive apoptosis in wild-type mice. To determine whether Trp53 deficiency satisfies the above hypotheses by eliminating or reducing DNA repair and by allowing cells to escape apoptosis, we evaluated the incidence of benzene-induced HPNs in Trp53-deficient C57BL/6 mice with specific regard to AMLs. We also used C3H/He mice, AML prone, with Trp53 deficiency to explore whether a higher incidence of AMLs on benzene exposure might explain the above human-murine differences. As a result, heterozygous Trp53-deficient mice of both strains showed a nonthreshold response of the incidence of HPNs at the lower dose, whereas both strains showed an increasing HPN incidence up to 100% with increasing benzene exposure dose, including AMLs, that developed 38% of heterozygous Trp53-deficient C3H/He mice compared to only 9% of wild-type mice exposed to the high dose. The detection of AMLs in heterozygous Trp53-deficient mice, even in the C57BL/6 strain, implies that benzene may be a potent inducer of AMLs also in mice with some strain differences.

p53 (TRP53) Deficiency-Mediated Antiapoptosis Escape after 5 Gy X Irradiation Still Induces Stem Cell Leukemia in C3H/He Mice: Comparison between Whole-Body Assay and Bone Marrow Transplantation (BMT) Assay

Mice exposed to a lethal dose of radiation were repopulated with heterozygous p53(+/-) (TRP53(+/-)) bone marrow cells and then exposed to doses of 1, 3 and 5 Gy 1 month later. This resulted in the transplanted bone marrow-specific diseases other than competitively induced nonhematopoietic neoplasms. Interestingly, the present study showed a high frequency of stem cell leukemia, i.e., leukemias characterized by a lack of differentiation due also to p53 deficiency, even after 5 Gy irradiation. The frequencies of stem cell leukemias (and those of total hematopoietic malignancies) were 16% (24%) at 1 Gy and 45% (75%) at 3 Gy. Furthermore, markedly high incidences of stem cell leukemias were observed at 5 Gy in p53(+/-) mice, i.e., 87% (100%) in the transplantation assay and 60% (83.3%) in the whole-body assay, whereas a conventional whole-body assay induced only 14% in wild-type mice. The high incidence of stem cell leukemias observed in this study using heterozygous p53-deficient mice agrees with results of a previous study of homozygous p53-deficient mice and is consistent with the high frequency of loss of heterozygosity in the p53 wild-type allele observed in leukemias. This suggests that the target cells for radiation-induced stem cell leukemias may be p53-deficient hematopoietic stem cells.