Prevent IgM MGUS Progression by Targeting the Driver Mutation Project Summary Patients with monoclonal gammopathy of undetermined significance of the IgM class (IgM MGUS) are at increased risk for Waldenstrom macroglobulinemia (WM), non-Hodgkin lymphoma (NHL), chronic lymphocytic leukemia (CLL), or primary light-chain (AL) amyloidosis. Myeloid differentiation factor 88 (MYD88) was discovered in the 1990s as a primary differentiation response gene in myeloid precursors. A missense mutation (L265P) changing leucine at position 265 to proline in MYD88 is found in ~90% of WM. This driver mutation also occurs in >50% of primary extranodal lymphomas and ~29% of activated B-cell diffuse large B- cell lymphomas (ABC-DLBCL), as well as in ~52% of IgM MGUS. No precancer-cancer pairs other than IgM MGUS-WM have a single amino acid substitution like MYD88 L265P occurring in most precancer and ~90% of cancer, making MYD88 L265P paradigmatic for the study of a single causative mutation in cancer prevention and interception. We found that the RING finger protein 138 (RNF138) attaches lysine 63 (K63)-linked polyubiquitin chains to MYD88 L265P, yet strikingly, it has little activity on WT MYD88. This posttranslational modification on MYD88 L265P is essential to its oncogenic action. We have used a deep learning artificial intelligence (AI) technology based on a neural network to virtually screen about ten million compounds. We identified scores of primary hit compounds targeting a binding site near L265P in MYD88. We validated primary hits from AI screening and evaluated their inhibition of MYD88 L265P ubiquitination and xenograft tumorigenesis. One compound attenuated lymphoma growth from NHL cells with MYD88 L265P but not that with WT MYD88. The E3 ligase RNF138 is primarily expressed in the testis and in immune cells, and its whole- body knockout in mice does not affect physiological functions other than that in the testis. RNF138 deletion attenuates tumorigenesis from WM and DLBCL cells with MYD88 L265P but not from cells with WT MYD88. We hypothesize that targeting the MYD88 L265P-RNF138 interaction prevents IgM MGUS progression. In this application, we propose two specific aims to identify and validate chemo- and immune-prevention agents to target MYD88 L265P-RNF138 and prevent IgM MGUS progression into WM and NHL. In Aim 1, we will use the AI-developed MYD88 L265P-targeting compound to prevent IgM MGUS progression. In Aim 2, we will use DNA vaccines against RNF138 to prevent IgM MGUS progression. We expect to generate effective chemical and immunological agents without overt toxicities for cancer prevention and interception against IgM MGUS progression.