SUMMARY Multiple Myeloma (MM) is a malignancy of bone marrow plasma cells preceded by a series of premalignant and transitional stages. Cells at all stages exhibit significant genomic aberrations, the sources of which are not well understood. This has left a major gap in our understanding of the mechanisms that drive MM initiation and progression, a gap that this proposal is designed to fill. Recent evidence suggests that cytidine deaminases—enzymes that convert cytosine to uracil in DNA—are important culprits in genomic instability in MM. One important example is the activation induced deaminase, AID, which initiates somatic hypermutation and class switch recombination but which also mutates many regions of the B cell genome and causes translocations similar to those found in MM. The related APOBEC3 family of cytidine deaminases can also mutate DNA, and numerous findings link AID and APOBEC3 enzymes to genome instability and mutations in MM. Our preliminary data demonstrate that MM cells express APOBEC3B, C, D, F and G, with APOBEC3B being expressed particularly strongly. Interestingly, APOBEC3B has recently been implicated in genome instability in breast cancer. We further find that expression of AID and certain APOBEC3 enzymes increases levels of DNA strand breaks in MM cells. Based on this, we hypothesize that AID and APOBEC3 family enzymes are a major cause of genomic aberrations and disease progression in MM. Recent findings provide a strong link between lipid disregulation, immune activation, and MM, with as much as a third of the clonal gammopathies found in MM patients reacting to lipids. Hence, an important overall guiding hypothesis of our work is that chronic B cell activation arising from elevated levels of inflammatory lipids contributes to increased activity of AID/APOBEC3 and MM disease progression. We systematically test these hypotheses in three Aims. In Aim 1, we determine the extent to which AID/APOBEC3 enzymes, alone and in concert, contribute to biochemical and molecular measures of nuclear deaminase activity and genomic damage. In Aim 2, we determine how these deaminases are regulated, revealing those regions of the MM genome that are susceptible to mutation, DNA breaks, and translocations due to their action. Finally, in Aim 3 we use established and novel mouse model systems and MM and its premalignant stage propagated in humanized mice to assess the in vivo effects of inflammatory lipids on the expression/activity of AID/APOBEC3 factors, DNA damage and mutation of the MM genome, and clonal evolution of MM. Together, the proposed experiments will provide a comprehensive picture of the activity, targeting, and outcome of cytidine deaminase action in MM, with broad implications for disease pathogenesis.