AbstractCr(VI) is a human carcinogen of significant public health concern and a substantial exposure in a number ofoccupational settings. Cr(VI) induces mutations changes in gene copy number and exposure has beenassociated with humans cancers in exposed populations and animal models. Our novel preliminary datademonstrate that Cr(VI) exposure causes amplification in ribosomal DNA (rDNA) copy number and changes inthe nucleolus (the crudely understood nuclear organelle that is the site of ribosomal RNA (rRNA) transcriptionand integration of myriad cellular functions). A crucial element of nucleolar function is rDNA copy number(rDNA CN). rDNA CN modulates (i) epigenetic states across the genome (ii) DNA damage responses (iii) cellcycle progression (iv) chromosome segregation and (v) global genetic stability. Furthermore disruption ofrDNA arrays ribosome biogenesis and the nucleolus are central to carcinogenesis. Our central medicalhypothesis is that Cr-induced changes in rDNA CN are responsible for Cr-induced carcinogenesis. Our centralbasic hypothesis is that rDNA arrays are not fixed but rather a genetically dynamic component of the nucleargenome with copy number that is modulated by Cr exposure. Our proposal examines rDNA changes uponCr(VI) exposure to reveal a novel pathway of Cr toxicity with medical and basic relevance. Key elements are acareful investigation of the toxicology of Cr-induced-rDNA-amplification (Cr-i-rDNA-a) hypotheses-drivenfunctional genomic analysis the rDNA and the nucleolus upon Cr(VI) exposure and extensive genetic analysesof Cr-i-rDNA-a using a powerful model organism. Our first aim will investigate the toxicology of Chromium-induced-rDNA-amplification (Cr-i-rDNA-a) in a human lung epithelial cell model. We will determine dose-responses of Cr-i-rDNA-a map amplification boundaries in Cr-i-rDNA-a examine temporal profiles andrecovery from Cr(VI) exposure and examine whether Chromium-induced CN changes are responsible for Cr-induced carcinogenesis. Our second aim investigates the functional genomics of Cr-induced nucleolar stressand Cr induced transformation in a human lung epithelial cell model. Examining genome-wide responses to Crexposure is critical to understand how Cr induces rDNA amplification nuclelar stress and carcinogenesis. Ourthird aim addresses the genetic determinants of Chromium-rDNA interactions. We will examine Cr-i-rDNA-a inspecific cells quantify the extent of copy number change isolate the affected tissues and use high-throughputtechniques to characterize the changes. Our efforts will shed light on Cr-rDNA interactions with research thatis directly relevant to the human health mission of the NIH. The manifold effects of rDNA CN indicate thatperturbing this central regulator with Cr will have profound consequences to cellular function. We anticipatethat determinants of complex human diseases with strong environmental components such as cancer willultimately be traced to environmentally triggered variation in rDNA segments of the genome.