Project Summary – Project 4 Dana-Farber Cancer Institute A prominent feature of multiple myeloma (MM) and other malignancies is significant genomic instability leading to clonal evolution and disease progression. We have previously described the mutational landscape in multiple myeloma (MM) and shown that the number of mutations correlates with overall and event free survival in MM. In the current funding period, using deep WGS, we investigated 183 newly diagnosed patients with MM and demonstrated a significant variability in mutational load in MM. Moreover, all MM subgroups had activated mutational signatures indicative of homologous recombination (HR) and NER dysfunction as a prominent late mutational process, whereas APOBEC signature is activated in the intermediate phase of disease progression in high-risk MM. Importantly, a subgroup with low DNA damage had a superior outcome. Our recent studies also demonstrate the acquisition of significantly higher number of mutations at relapse following high-dose therapy compared to RVD (a 3-drug standard-dose combination) and identify HR dysfunction as a prominent underlying mechanism. These observations are also consistent with our previous data which show that HR is dysregulated and significantly contributes to genomic instability and development of drug resistance in MM. Investigating the mechanisms underlying genomic evolution, we have also identified a kinase gene signature impacting DNA repair (especially HR) and genome stability in cancer cells including MM. We further demonstrate that PDZ Binding Kinase (PBK), a member of this signature, impacts DNA repair and genome stability in MM cells through direct phosphorylation of DNA repair genes and that of a transcription factor FOXM1, which regulates major DNA repair pathways. Based on these and other data, we hypothesize that MM genome is defined by complex clonal architecture that drives the disease at diagnosis and evolves further at relapse and it is mediated by dysregulated DNA repair and related genes. Towards this goal, we will investigate myeloma cell clonal complexity at diagnosis and relapse and their impact on outcome (Specific Aim 1); functionally validate PDZ Binding Kinase (PBK) for its impact on MM cell clonal evolution, growth and survival (Specific Aim 2); and evaluate PBK inhibitors, alone and in combination with existing MM drugs, in vitro and in murine models of MM (Specific Aim 3). This study will improve our understanding of mechanisms which contribute to genetic instability, clonal evolution and progression in MM and will identify novel targets and therapeutic strategies to inhibit growth and prevent/delay evolution of the disease.