Characterization of Molecular Subtypes of Rat Mammary Cancer and Their Association With Environmental Exposures.

BACKGROUND/AIM: Breast cancer is a heterogeneous disease with many subtypes, and the association between these subtypes and exposure to environmental factors such as radiation remains controversial. Although the rat is used widely for research into human breast cancer, the heterogeneity and subtype definitions are unclear. Here, we leveraged an archive of rat mammary cancer samples and gene expression microarray data to classify tumors and examine their association with exposures. MATERIALS AND METHODS: Eighty-four mammary cancer and 12 normal mammary tissue samples were obtained from previous experiments in which rats were exposed to different types of radiation, chemical carcinogens, and diets. Tumors were then subjected to immunohistochemical (IHC) analysis of conventional biomarkers, as well as gene expression profiling; they were then classified by three approaches based on IHC results, the PAM50 classifier algorithm, and unsupervised clustering of gene expression profiles. RESULTS: IHC identified four subtypes (luminal A-like, luminal B-like 1, luminal B-like 2, and triple-negative), while PAM50 identified six (luminal A, luminal B, basal-like, HER2-enriched, normal-like, and claudin-low). Unsupervised clustering divided the tumors into three large, statistically significant, groups (named "luminal A", "luminal B", and "non-luminal" clusters). The results of the three approaches were significantly associated with each other. Exposure to radiation and chemical carcinogens during post-pubertal development was significantly associated with an increased risk of developing luminal A tumors, whereas exposure to a high corn-oil diet was associated with a higher likelihood of luminal B tumors. CONCLUSION: Rat mammary cancer subtypes resemble those in humans and are related to environmental factors.

Intravital microscopic thermometry of rat mammary epithelium by fluorescent nanodiamond.

Quantum sensing using the fluorescent nanodiamond (FND) nitrogen-vacancy center enables physical/chemical measurements of the microenvironment, although application of such measurements in living mammals poses significant challenges due to the unknown biodistribution and toxicity of FNDs, the limited penetration of visible light for quantum state manipulation/measurement, and interference from physiological motion. Here, we describe a microenvironmental thermometry technique using FNDs in rat mammary epithelium, an important model for mammary gland biology and breast cancer research. FNDs were injected directly into the mammary gland. Microscopic observation of mammary tissue sections showed that most FNDs remained in the mammary epithelium for at least 8 weeks. Pathological examination indicated no obvious change in tissue morphology, suggesting negligible toxicity. Optical excitation and detection were performed through a skin incision. Periodic movements due to respiration and heartbeat were mitigated by frequency filtering of the signal. Based on these methods, we successfully detected temperature elevation in the mammary epithelium associated with lipopolysaccharide-induced inflammation, demonstrating the sensitivity and relevance of the technique in biological contexts. This study lays the groundwork for expanding the applicability of quantum sensing in biomedical research, providing a tool for real-time monitoring of physiological and pathological processes.

Influence of Age on Leukemia Mortality Associated with Exposure to γ rays and 2-MeV Fast Neutrons in Male C3H Mice.

The relative biological effectiveness (RBE) of densely ionizing radiation can depend on the biological context. From a radiological perspective, age is an important factor affecting health risks of radiation exposure, but little is known about the modifying impact of age on the effects of densely ionizing radiation. Herein, we addressed the influence of age on leukemogenesis induced by accelerator-generated fast neutrons (mean energy, ∼2 MeV). Male C3H/HeNrs mice were exposed to 137Cs γ rays (0.2-3.0 Gy) or neutrons (0.0485-0.97 Gy, γ ray contamination 0.0105-0.21 Gy) at 1, 3, 8, or 35 weeks of age and observed over their lifetimes under specific pathogen-free conditions. Leukemia and lymphoma were diagnosed pathologically. Hazard ratio (HR) and RBE for myeloid leukemia mortality as well as the age dependence of these two parameters were modeled and analyzed using Cox regression. Neutron exposure increased HR concordant with a linear dose response. The increase of HR per dose depended on age at exposure, with no significant dose dependence at age 1 or 3 weeks but a significant increase in HR of 5.5 per Gy (γ rays) and 16 per Gy (neutrons) at 8 weeks and 5.8 per Gy (γ rays) and 9 per Gy (neutrons) at 35 weeks. The RBE of neutrons was 2.1 (95% confidence interval, 1.1-3.7), with no dependence on age. The development of lymphoid neoplasms was not related to radiation exposure. The observed increasing trend of radiation-associated mortality of myeloid leukemia with age at exposure supports previous epidemiological and experimental findings. The results also suggest that exposure at the susceptible age of 8 or 35 weeks does not significantly influence the RBE value for neutrons for induction of leukemia, unlike what has been documented for breast and brain tumors.

Relative Biological Effectiveness of Carbon Ion Beams for Induction of Medulloblastoma with Radiation-specific Chromosome 13 Deletion in Ptch1+/- Mice.

Carbon ion radiotherapy (CIRT) for pediatric cancer is currently limited because of the unknown risk of induction of secondary cancers. Medulloblastoma of Ptch1+/- mice offers a unique experimental system for radiation-induced carcinogenesis, in which tumors are classified into spontaneous and radiation-induced subtypes based on their features of loss of heterozygosity (LOH) that affect the wild-type Ptch1 allele. The present study aims to investigate in young Ptch1+/- mice the carcinogenic effect, and its age dependence, of the low-linear energy transfer (LET, ∼13 keV/µm) carbon ions, to which normal tissues in front of the tumor are exposed during therapy. We irradiated Ptch1+/- mice at postnatal day (P) 1, 4, or 10 with 290 MeV/u carbon ions (0.05-0.5 Gy; LET, 13 keV/µm) and monitored them for medulloblastoma development. Loss of heterozygosity of seven genetic markers on chromosome 13 (where Ptch1 resides) was studied to classify the tumors. Carbon ion exposure induced medulloblastoma most effectively at P1. The LOH patterns of tumors were either telomeric or interstitial, the latter occurring almost exclusively in the irradiated groups, allowing the use of interstitial LOH as a biomarker of radiation-induced tumors. Radiation-induced tumors developed during a narrow age window (most strongly at P1 and only moderately at P4, with suppressed tumorigenesis at P10). Calculated using previous results using 137Cs gamma rays, the values for relative biological effectiveness (RBE) regarding radiation-induced tumors were 4.1 (3.4, 4.8) and 4.3 (3.3, 5.2) (mean and 95% confidence interval) for exposure at P1 and 4, respectively. Thus, the RBE of carbon ions for medulloblastoma induction in Ptch1+/- mice was higher than the generally recognized RBE of 1-2 for cell killing, chromosome aberrations, and skin reactions.

Combined effects of radiation and simulated microgravity on intestinal tumorigenesis in C3B6F1 ApcMin/+ mice.

Explorations of the Moon and Mars are planned as future manned space missions, during which humans will be exposed to both radiation and microgravity. We do not, however, know the health effects for such combined exposures. In a ground-based experiment, we evaluated the combined effects of radiation and simulated microgravity on tumorigenesis by performing X-irradiation and tail suspension in C3B6F1 ApcMin/+ mice, a well-established model for intestinal tumorigenesis. Mice were irradiated at 2 weeks of age and underwent tail suspension for 3 or 11 weeks using a special device that avoids damage to the tail. The tail suspension treatment significantly reduced the thymus weight after 3 weeks but not 11 weeks, suggesting a transient stress response. The combination of irradiation and tail suspension significantly increased the number of small intestinal tumors less than 2 mm in diameter as compared with either treatment alone. The combined treatment also increased the fraction of malignant tumors among all small intestinal tumors as compared with the radiation-only treatment. Thus, the C3B6F1 ApcMin/+ mouse is a useful model for assessing cancer risk in a simulated space environment, in which simulated microgravity accelerates tumor progression when combined with radiation exposure.

Ionizing radiation promotes, whereas calorie restriction suppresses, NASH and hepatocellular carcinoma in mice.

The pathological conditions of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis (NASH) are the major risk factors for hepatocellular carcinoma (HCC). Exposure to DNA-damaging agents such as ionizing radiation is another risk factor for HCC; calorie restriction (CR), however, effectively delays the onset of radiation-induced HCC. We investigated whether NASH is relevant to radiation-induced HCC and the cancer-preventing effect of CR. Eight-day-old male B6C3F1 mice were irradiated with 3.8 Gy of X-rays and then fed a standard diet or 30% CR diet from 49 days of age until necropsy, which was performed from 56 to 600 days with ~100-day intervals to assess both pathological changes and gene expression levels. We found that early-life exposure to radiation accelerated lipid accumulation and NASH-like histopathological changes in the liver, accompanied by accelerated development of HCC. CR ameliorated the changes in lipid metabolism in the liver and reversed the NASH-like pathology, which effectively delayed HCC development. Gene-expression profiling revealed the radiation-related activation and CR-related suppression of the peroxisome proliferator-activated receptor gamma/Cd36 pathway of transmembrane fatty-acid translocation before development of the NASH-like state. Thus, early-life exposure to radiation affects lipid metabolism and induces a steatoinflammatory microenvironment that favors HCC development. Therefore, targeting this pathway by CR (or measures that mimic CR) may be a promising strategy for preventing HCC caused by either radiation or other DNA-damaging agents.

Azithromycin induces read-through of the nonsense Apc allele and prevents intestinal tumorigenesis in C3B6F1 ApcMin/+ mice.

Therapeutic strategies that promote read-through of a mutant gene have proved effective for certain non-neoplastic diseases. However, the efficacy of this approach is unproven regarding neoplastic diseases with germline nonsense mutations, including familial adenomatous polyposis. Here we examined the cancer-preventive efficacy of the macrolide antibiotic azithromycin, with a reported read-through effect, on intestinal tumorigenesis in C3B6F1 ApcMin/+ mice harboring a nonsense Apc mutation resulting in a truncated Apc protein. Mice were given drinking water lacking azithromycin or containing 0.0125-0.2 mg/mL azithromycin from 3 weeks of age. The small intestine and cecum were analyzed for pathological changes and alterations of intestinal flora. Azithromycin suppressed the number of tumors and the proportion of adenocarcinomas, with the most effective drinking-water concentration being 0.0125 mg/mL. Furthermore, azithromycin recovered the cellular level of full-length Apc, resulting in downregulation of ß-catenin and cyclin D1. Conversely, the effect of azithromycin on the diversity of the intestinal microbiota depended on the drinking-water concentration. These results suggest that the balance between azithromycin-mediate read-through of mutant Apc mRNA and antibacterial effects influences intestinal tumorigenesis. Thus, azithromycin is a potential anticancer agent for familial adenomatous polyposis patients harboring nonsense mutations.

Interstitial deletion of the Apc locus in ß-catenin-overexpressing cells is a signature of radiation-induced intestinal tumors in C3B6F1 ApcMin/+ mice†.

Recent studies have identified interstitial deletions in the cancer genome as a radiation-related mutational signature, although most of them do not fall on cancer driver genes. Pioneering studies in the field have indicated the presence of loss of heterozygosity (LOH) spanning Apc in a subset of sporadic and radiation-induced intestinal tumors of ApcMin/+ mice, albeit with a substantial subset in which LOH was not detected; whether copy number losses accompany such LOH has also been unclear. Herein, we analyzed intestinal tumors of C3B6F1 ApcMin/+ mice that were either left untreated or irradiated with 2 Gy of γ-rays. We observed intratumor mosaicism with respect to the nuclear/cytoplasmic accumulation of immunohistochemically detectable ß-catenin, which is a hallmark of Apc+ allele loss. An immunoguided laser microdissection approach enabled the detection of LOH involving the Apc+ allele in ß-catenin-overexpressing cells; in contrast, the LOH was not observed in the non-overexpressing cells. With this improvement, LOH involving Apc+ was detected in all 22 tumors analyzed, in contrast to what has been reported previously. The use of a formalin-free fixative facilitated the LOH and microarray-based DNA copy number analyses, enabling the classification of the aberrations as nondisjunction/mitotic recombination type or interstitial deletion type. Of note, the latter was observed only in radiation-induced tumors (nonirradiated, 0 of 8; irradiated, 11 of 14). Thus, an analysis considering intratumor heterogeneity identifies interstitial deletion involving the Apc+ allele as a causative radiation-related event in intestinal tumors of ApcMin/+ mice, providing an accurate approach for attributing individual tumors to radiation exposure.

Lung-Cancer Risk in Mice after Exposure to Gamma Rays, Carbon Ions or Neutrons: Egfr Pathway Activation and Frequent Nuclear Abnormality.

Lung is one of the high-risk organs for radiation-induced carcinogenesis, but the risk of secondary lung-cancer development after particle-beam therapy and the underlying mechanism(s) remain to be elucidated. To investigate the effects of particle-beam radiation on adjacent normal tissues during cancer therapy, 7-week-old male and female B6C3F1 mice were irradiated with 0.2-4 Gy of gamma rays (for comparison), carbon ions (290 MeV/u, linear energy transfer 13 keV/µm), or fast neutrons (0.05-1 Gy, mean energy, ∼2 MeV), and lung-tumor development was assessed by histopathology. Mice irradiated with ≥2 Gy of carbon ions or ≥0.2 Gy of neutrons developed lung adenocarcinoma (AC) significantly sooner than did non-irradiated mice. The relative biological effectiveness values for carbon ions for lung AC development were 1.07 for male mice and 2.59 for females, and the corresponding values for neutrons were 4.63 and 4.57. Genomic analysis of lung ACs revealed alterations in genes involved in Egfr signaling. Hyperphosphorylation of Erk and a frequent nuclear abnormality (i.e., nuclear groove) were observed in lung ACs of mice irradiated with carbon ions or neutrons compared with ACs from non-irradiated or gamma-ray-irradiated groups. Our data indicate that the induction of lung AC by carbon ions occurred at a rate similar to that for gamma rays in males and approximately 2-to 3-fold greater than that for gamma rays in females. In contrast, the effect of neutrons on lung AC development was approximately 4- to 5-fold greater than that of gamma rays. Our results provide valuable information concerning risk assessment of radiation-induced lung tumors after particle-beam therapy and increase our understanding of the molecular basis of tumor development.

DOSE-RATE EFFECT OF RADIATION ON RAT MAMMARY CARCINOGENESIS AND AN EMERGING ROLE FOR STEM CELL BIOLOGY.

The uncertain cancer risk of protracted radiation exposure at low dose rates is an important issue in radiological protection. Tissue stem/progenitor cells are a supposed origin of cancer and may contribute to the dose-rate effect on carcinogenesis. The authors have shown that female rats subjected to continuous whole body γ irradiation as juveniles or young adults have a notably reduced incidence of mammary cancer as compared with those irradiated acutely. Experiments using the mammosphere formation assay suggested the presence of radioresistant progenitor cells. Cell sorting indicated that basal progenitor cells in rat mammary gland were more resistant than luminal progenitors to killing by acute radiation, especially at high doses. Thus, the evidence indicates a cell-type-dependent inactivation of mammary cells that manifests only at high acute doses, implying a link to the observed dose-rate effect on carcinogenesis.

Impacts of psychological stress on high dose-rate radiation acute effects in a mouse experimental model.

Psychological stress affects health. Radiation workers in the medical field or astronauts living in space have possible risks of exposure to radiation, and psychological stress is considered to be easily induced in them due to activities performed in small areas or stress conditions. The impact of psychological stress on the effects of radiation was evaluated in senescence-accelerated mouse prone 10 (SAMP10) mice and ddY mice using a confrontational housing model, which makes dominant and subordinate mice in a cage live together without severe quarrel. Mice of ddY and SAMP10 have been previously demonstrated to be influenced in terms of acute and late effects, respectively, under psychological stress by this model. In SAMP10 mice, irradiation with 4 Gy induced the death of irradiated mice under psychological stress. In ddY mice, irradiation with 5 Gy X-rays alone had almost no effect on the mouse survival, but irradiation in conditions of psychological stress promoted acute death of irradiated mice. In addition, hypocellular bone marrow was also observed histopathologically in irradiated ddY mice under stress. Psychological stress may promote damage caused by radiation through modulation of radio-sensitivity in bone marrow in mice. This model would be useful for evaluation of modulation of radiation-induced various effects by psychological stress.

Genomic profile of radiation-induced early-onset mouse B-cell lymphoma recapitulates features of Philadelphia chromosome-like acute lymphoblastic leukemia in humans.

Epidemiological studies have revealed a radiation-related increase in the risk of developing acute lymphoblastic leukemia (ALL). Our recent study revealed early induction and increased risk of precursor B-cell (pB) lymphomas in mice after radiation exposure. However, the genomic landscape of radiation-induced B-cell lymphomas remains unclear. To identify the relevant genetic alterations in mice, whole-exome sequencing was performed on both early-onset and late-onset B-cell lymphomas that developed spontaneously or after gamma-irradiation. In addition to multiple driver mutations, the data revealed that interstitial deletion of chromosome 4, including Pax5, and missense mutations in Jak3 are unique genomic alterations in radiation-induced, early-onset B-cell lymphomas. RNA sequencing revealed a pB-cell-type gene-expression profile with no involvement of known fusion genes for human ALLs in the early-onset B-cell lymphomas. Activation of Jak3/Stat5 signaling in early-onset B-cell lymphomas was validated using western capillary electrophoresis. Those features were similar to those of Philadelphia chromosome-like ALL. Our data suggest a critical role for Pax5 loss-of-function mutations in initiating B-cell leukemogenesis coupled with activation of Jak3/Stat5 signaling as a basis for the rapid development of radiation-induced pB-ALL. These molecular signatures for radiation-induced cancers will inform both risk assessment and potential targeted therapies for pB-ALL.

Post-Irradiation Thymic Regeneration in B6C3F1 Mice Is Age Dependent and Modulated by Activation of the PI3K-AKT-mTOR Pathway.

The risk of radiation-induced carcinogenesis depends on age at exposure. We previously reported principal causative genes in lymphomas arising after infant or adult exposure to 4-fractionated irradiation as Pten or Ikzf1, respectively, suggesting that cells with mutation in these genes might be the origin of lymphomas arising after irradiation depending on age at exposure. Here, we clarified the age-dependent differences in thymus-cell dynamics in mice during the initial post-irradiation period. The thymocyte number initially decreased, followed by two regeneration phases. During the first regeneration, the proportion of phosphorylated-AKT-positive (p-AKT+) cells in cell-cycle phases S+G2/M of immature CD4-CD8- and CD4+CD8+ thymocytes and in phases G0/G1 of mature CD4+CD8- and CD4-CD8+ thymocytes was significantly greater in irradiated infants than in irradiated adults. During the second regeneration, the proportion of p-AKT+ thymocytes in phases G0/G1 increased in each of the three populations other than CD4-CD8- thymocytes more so than during the first regeneration. Finally, PI3K-AKT-mTOR signaling in infants contributed, at least in part, to biphasic thymic regeneration through the modification of cell proliferation and survival after irradiation, which may be associated with the risk of Pten mutation-associated thymic lymphoma.

Environmental Enrichment Increases Radiation-induced Apoptosis Not Spontaneous Apoptosis in Mouse Intestinal Crypt Cells.

BACKGROUND/AIM: An enriched environment (EE) modifies apoptotic cell death and promotes cell proliferation in the central nervous system (CNS) in mice. However, few studies have examined the effects of an EE on apoptosis in non-CNS organs in model orgamisms. In addition, the intestinal tract is one of organs at high-risk of carcinogenesis after radiation exposure. Herein we evaluated the effects of an EE on spontaneous and radiation-induced apoptosis in intestinal crypt cells of mice. MATERIALS AND METHODS: Juvenile (3-week-old) and adult (11-week-old) male B6C3F1 mice were housed in a standard environment or EE for 8 weeks and then were whole-body irradiated with 2 Gy X-rays. Apoptosis in the small intestine and colon was analyzed with antibody against cleaved caspase 3. RESULTS: The EE significantly reduced body weight; adipose tissue weight; and serum levels of total cholesterol, triglyceride, leptin, and insulin. Although EE did not change the spontaneous apoptotic index without irradiation, it significantly increased the index after irradiation in the colonic crypt. The apoptotic index in the small intestinal crypt showed similar patterns. CONCLUSION: An EE enhances radiation-induced apoptosis of stem/progenitor cells in the small intestine and colon without affecting spontaneous apoptosis. An EE may thus reduce the risk of cancer in the intestinal tract after radiation exposure such as radiotherapy.

An Enriched Environment Alters DNA Repair and Inflammatory Responses After Radiation Exposure.

After the Fukushima Daiichi Nuclear Power Plant accident, there is growing concern about radiation-induced carcinogenesis. In addition, living in a long-term shelter or temporary housing due to disasters might cause unpleasant stress, which adversely affects physical and mental health. It's been experimentally demonstrated that "eustress", which is rich and comfortable, has beneficial effects for health using mouse models. In a previous study, mice raised in the enriched environment (EE) has shown effects such as suppression of tumor growth and enhancement of drug sensitivity during cancer treatment. However, it's not yet been evaluated whether EE affects radiation-induced carcinogenesis. Therefore, to evaluate whether EE suppresses a radiation-induced carcinogenesis after radiation exposure, in this study, we assessed the serum leptin levels, radiation-induced DNA damage response and inflammatory response using the mouse model. In brief, serum and tissues were collected and analyzed over time in irradiated mice after manipulating the raising environment during the juvenile or adult stage. To assess the radiation-induced DNA damage response, we performed immunostaining for phosphorylated H2AX which is a marker of DNA double-strand break. Focusing on the polarization of macrophages in the inflammatory reaction that has an important role in carcinogenesis, we performed analysis using tissue immunofluorescence staining and RT-qPCR. Our data confirmed that EE breeding before radiation exposure improved the responsiveness to radiation-induced DNA damage and basal immunity, further suppressing the chronic inflammatory response, and that might lead to a reduction of the risk of radiation-induced carcinogenesis.

Non-contact measurement of internal body temperature using subcutaneously implanted diamond microparticles.

We constructed a highly sensitive fluorescence wide-field imaging system with a microwave source, implanted fluorescent diamond microparticles ("microdiamonds") subcutaneously into the dorsal skin of a mouse after sacrifice, and demonstrated the feasibility of using optically detected magnetic resonance (ODMR) to measure internal body temperature in a mammal.

Calorie Restriction Suppresses the Progression of Radiation-Induced Intestinal Tumours in C3B6F1 Apc Min/+ Mice.

BACKGROUND/AIM: Progress in cancer treatment and diagnosis has made second cancer after medical radiation exposure a particular concern among childhood cancer survivors. Calorie restriction (CR) is a broadly effective cancer prevention strategy, although its effects on radiation-induced intestinal tumours are unclear. Here we examined the cancer-preventative efficacy of a CR diet at different starting ages on radiation induction of intestinal tumours in mice. MATERIALS AND METHODS: Male C3B6F1 ApcMin/+ mice were irradiated with 0 or 2 Gy of X-rays at 2 weeks of age. After an interval of 2, 8 or 18 weeks, mice were fed with a non-CR (95 kcal/week/mouse) or CR (65 kcal/week/mouse) diet. Intestinal tumours were evaluated for number, size distribution and malignancy. RESULTS: CR suppressed the size and progression of both spontaneous and radiation-induced intestinal tumours depending on age at starting of CR. CR diets were effective even administered to adult mice. CONCLUSION: CR was effective for suppression of tumour progression, which was accelerated by radiation exposure. Use of CR might be a useful cancer-prevention strategy for radiation-induced tumours of the intestinal tract.

Exome of Radiation-induced Rat Mammary Carcinoma Shows Copy-number Losses and Mutations in Human-relevant Cancer Genes.

BACKGROUND/AIM: Our understanding of cancer risk from neutron exposure is limited. We aimed to reveal the characteristics of mammary carcinomas induced by neutrons. MATERIALS AND METHODS: Mammary carcinomas obtained from female Sprague-Dawley rats irradiated at 7 weeks of age with 0.97 Gy neutrons or 4 Gy γ-rays and from non-irradiated rats were classified into luminal and non-luminal subtypes by immunohistochemistry. Their mutational landscapes were determined by whole-exome sequencing. RESULTS: Neutrons significantly raised the incidence of luminal mammary carcinomas over the non-luminal subtype. Somatic mutations were identified in cancer genes involved in several signalling pathways, including Keap1/Nrf2, Pi3k/Akt and Wnt/ß-catenin. Focal copy-number losses involving cancer genes were observed mainly in carcinomas from the irradiated rats. CONCLUSION: Neutrons increase the incidence of luminal mammary carcinomas, probably through gene mutations similar to those found in human breast cancers, and focal copy-number losses including cancer genes that are characteristics of radiation-induced mammary carcinomas.

Early induction and increased risk of precursor B-cell neoplasms after exposure of infant or young-adult mice to ionizing radiation.

Epidemiological studies of atomic-bomb survivors have revealed an increased risk of lymphoid neoplasm (i.e. acute lymphoblastic leukemia) associated with radiation exposure. In particular, children are more susceptible to radiation-induced precursor lymphoid neoplasm than adults. Although ~75% of human lymphoid tumors are B-cell neoplasms, the carcinogenic risk associated with each stage of differentiation of B-cells after radiation exposure is poorly understood. Therefore, we irradiated mice at infancy or in young adulthood to investigate the effect of age at exposure on the risk of developing B-cell neoplasms. Histopathology was used to confirm the presence of lymphoid neoplasms, and the population of B-cell neoplasms was classified into the precursor B-cell (pro-B and pre-B cell) type and mature B-cell type, according to immunophenotype. The data revealed that precursor B-cell neoplasms were induced soon after radiation exposure in infancy or young adulthood, resulting in a greater risk of developing the neoplasms. This was particularly the case for the pro-B cell type after young adult exposure. Our findings suggest that exposure to radiation at young age increases the risk of developing precursor B-cell neoplasms in humans.