ABSTRACT The terrorist attacks on the World Trade Center (WTC) created an unprecedented environmental exposure to WTC aerosolized dust and gases that contained known and suspected carcinogens including polycyclic aromatic hydrocarbons, polychlorinated biphenyls, furans, dioxins and asbestos. Studies from our group have reported an excess of cancer cases in the WTC-exposed Fire Department of the City of New York (FDNY) firefighters,(2, 3) including a trend towards higher incidence of leukemia and multiple myeloma. We continue to prospectively follow the entire FDNY cohort of firefighters for cancer incidence and have built a biorepository of samples consisting of fractionated blood components and serum from 2000+ WTC-exposed firefighters and non-WTC- exposed firefighter controls. Using this cohort, we have conducted preliminary studies in 781 WTC-exposed firefighters that demonstrate a significantly higher prevalence of monoclonal gammopathy of undetermined significance (MGUS), a precursor for multiple myeloma. (4) Furthermore, we have conducted deep targeted genome sequencing of a pilot cohort of WTC-exposed firefighters (N=481) and non-WTC-exposed age matched firefighter controls (N=52). We observed a high incidence of Clonal hematopoiesis (CH) and leukemia associated mutations (57/481; 12%) in WTC-exposed firefighters. The rate of mutations in controls was 2% and is comparable to age matched controls in published studies (5, 6). Clonal hematopoiesis (CH) is acquisition of leukemia associated somatic mutations that carry increase risk of hematologic cancer as well as all-cause mortality. The mutations seen in WTC-exposed firefighters affected DNMT3A, TET2, TP53, Splicing and other commonly recurrently mutated genes. These mutations suggest a high rate of CH, and CH in other cohorts has been shown to be associated with risk for development of myeloid neoplasms, inflammatory conditions and heart diseases. We now propose to comprehensively determine the prevalence of CH in a large FDNY WTC-exposed cohort and study the molecular and cellular mechanisms of damage caused by WTC particulate matter (WTC- PM). Aim 1 will determine the prevalence of clonal hematopoiesis mutations using deep targeted sequencing in a large cohort of WTC-exposed firefighter and 2 cohorts of control samples (non-WTC firefighters and non- firefighter age matched controls). Mutations will be correlated with clinical variables such as level of WTC exposure, age, serial blood counts, smoking history and other clinical characteristics. Aim 2 will determine the functional effects of WTC particulate matter on development of clonal hematopoiesis in vivo by using mouse models with commonly mutated TET2 and DNMT3A genes. Lastly, aim 3 will determine the effect of WTC-PM exposure on cellular changes at the single molecule level to assess the mechanisms and magnitude of DNA damage in exposed cells. These results will further our understanding on the mechanism via which environmental expo...