1 934 111 CHRONIC LOW DOSE IRRADIATION ALTERS HEPATIC TRANSCRIPTIONAL PROFILES, BUT NOT GLOBAL DNA METHYLATION IN MEDAKA (ORYZIAS LATIPES). IONIZING RADIATION (IR) RESULTING FROM BOTH NATURAL AND ANTHROPOGENIC SOURCES IS UBIQUITOUS THROUGHOUT THE ENVIRONMENT. HISTORICALLY, STUDIES ON THE BIOLOGICAL IMPACTS OF RADIATION PRIMARILY FOCUSED ON RESPONSES TO ACUTE DOSES OF RADIATION, WITH LITTLE ADVANCEMENT IN OUR UNDERSTANDING OF ENVIRONMENTALLY RELEVANT EXPOSURES. EPIGENETIC MECHANISMS ARE CAPABLE OF MEDIATING ORGANISMAL RESPONSES TO ENVIRONMENTAL STRESSORS AND DNA METHYLATION PLAYS IMPORTANT ROLES IN GENE REGULATION AND PROMOTING CHROMOSOMAL STABILITY. HERE, WE ASSESS BROAD-SCALE TRANSCRIPTIONAL AND EPIGENETIC VARIATION RESULTING FROM CHRONIC EXPOSURE TO LOW DOSES OF IONIZING RADIATION (LDIR; 5.78, 53.76, OR 520.23 MGY/DAY) USING JAPANESE MEDAKA FISH (ORYZIAS LATIPES) IN A REPLICATED MESOCOSM DESIGN. WE OBSERVED SIGNIFICANT CHANGES TO THE HEPATIC TRANSCRIPTOME INDUCED BY A 3-MONTH CHRONIC EXPOSURE TO IR, WHEREAS GLOBAL DNA METHYLATION APPEARED LARGELY UNAFFECTED. OUR FINDINGS REVEAL A SET OF GENES, INCLUDING THOSE INVOLVED IN IMMUNE FUNCTION, RESPONDING TO ENVIRONMENTALLY RELEVANT IR EXPOSURES, WHICH DO NOT APPEAR TO BE MEDIATED BY A SYSTEMIC GLOBAL SHIFT IN DNA METHYLATION. 2020 2 2741 35 EXPOSURE TO IONIZING RADIATION DISRUPTS NORMAL EPIGENETIC AGING IN JAPANESE MEDAKA. ALTERATIONS TO THE EPIGENOME ARE A HALLMARK OF BIOLOGICAL AGING AND AGE-DEPENDENT PATTERNING OF THE DNA METHYLOME ("EPIGENETIC AGING") CAN BE MODELED TO PRODUCE EPIGENETIC AGE PREDICTORS. RATES OF EPIGENETIC AGING VARY AMONGST INDIVIDUALS AND CORRELATE TO THE ONSET OF AGE-RELATED DISEASE AND ALL-CAUSE MORTALITY. YET, THE ORIGINS OF EPIGENETIC-TO-CHRONOLOGICAL AGE DISCORDANCE ARE NOT EMPIRICALLY RESOLVED. HERE, WE INVESTIGATE THE RELATIONSHIP BETWEEN AGING, DNA METHYLATION, AND ENVIRONMENTAL EXPOSURES IN JAPANESE MEDAKA (ORYZIAS LATIPES). WE FIND AGE-ASSOCIATED DNA METHYLATION PATTERNING ENRICHED IN GENOMIC REGIONS OF LOW CPG DENSITY AND THAT, SIMILAR TO MAMMALS, MOST AGE-RELATED CHANGES OCCUR DURING EARLY LIFE. WE CONSTRUCT AN EPIGENETIC CLOCK CAPABLE OF PREDICTING CHRONOLOGICAL AGE WITH A MEAN ERROR OF 61.1 DAYS (~8.4% OF AVERAGE LIFESPAN). TO TEST THE ROLE OF ENVIRONMENTAL FACTORS IN DRIVING EPIGENETIC AGE VARIATION, WE EXPOSED MEDAKA TO CHRONIC, ENVIRONMENTALLY RELEVANT DOSES OF IONIZING RADIATION. BECAUSE MOST ORGANISMS SHARE AN EVOLUTIONARY HISTORY WITH IONIZING RADIATION, WE HYPOTHESIZED THAT EXPOSURE WOULD REVEAL FUNDAMENTAL INSIGHTS INTO ENVIRONMENT-BY-EPIGENETIC AGING INTERACTIONS. RADIATION EXPOSURE DISRUPTED EPIGENETIC AGING BY ACCELERATING AND DECELERATING NORMAL AGE-ASSOCIATED PATTERNING AND WAS MOST PRONOUNCED IN CYTOSINES THAT WERE MODERATELY ASSOCIATED WITH AGE. THESE FINDINGS EMPIRICALLY DEMONSTRATE THE ROLE OF DNA METHYLATION IN INTEGRATING ENVIRONMENTAL FACTORS INTO AGING TRAJECTORIES. 2021 3 4019 28 LOW-DOSE OR LOW-DOSE-RATE IONIZING RADIATION-INDUCED BIOEFFECTS IN ANIMAL MODELS. ANIMAL EXPERIMENTAL STUDIES INDICATE THAT ACUTE OR CHRONIC LOW-DOSE IONIZING RADIATION (LDIR) (