Abstract Inflammatory cytokine responsive APOBEC3 cytidine deaminases have been studied extensively with regard to innate immunity and more recently during cancer evolution. However, the mechanisms by which the APOBEC3 enzymes promote cancer initiation and progression in the malignant microenvironment remains to be investigated, especially in hematopoietic malignancies. Through whole genome and whole transcriptome sequencing analyses of MPN patient samples FACS sorted into stem and progenitor populations, we have found a cell type and context specific nature of these enzymes, notably the upregulation of APOBEC3C (A3C) in the high-risk Myelofibrosis (MF) stem cell population as compared to normal aged counterparts. Through lentiviral overexpression of each APOBEC3 enzyme, we can now study the effects of changes in APOBEC3 transcript level in relation to the known changes in expression seen in many cancers, focusing on the upregulation of A3C. Using these techniques, we have identified novel RNA and DNA editing targets, as well as differential gene expression patterns of each APOBEC3 in normal CD34+ cord blood and aged normal bone marrow. Gene set enrichment analysis (GSEA) performed on this dataset has exposed numerous deregulated pathways brought on by exaggerated levels of APOBEC3, including changes in splicing pathways. In addition, novel identification of the relationship between A3C and ADAR1, another innate immune deaminase, has important implications in initiation and prognosis of MPNs. Both A3C and ADAR1 transcript levels are elevated in high risk MF stem cells, and co-immunoprecipitation studies reveal a direct binding of the enzymes. Furthermore, we are able to study the detailed effects of editing by both A3C and ADAR1 using editase-deficient mutant constructs, allowing for a mechanistic look into the role of these deaminases and their deregulation in vitro and in vivo using patient samples and humanized mouse models. This novel connection, as well as the role of A3C in initiation and progression of hematopoietic malignancies will continued to be studied using this system to elucidate effects on proliferation, differentiation, self-renewal, and changes to the cell cycle, in hopes of creating both a marker of early detection and potentially a druggable target. Citation Format: Jane Marie Isquith, Adam Mark, Jessica Pham, Mary Donohoe, Luisa Ladel, Catriona Jamieson. Effects of innate immune deaminase deregulation on initiation and progression of myeloproliferative neoplasms abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 899.