ORIGINAL PROJECT SUMMARY There is a great need for the development of molecularly targeted therapies in Acute Lymphoblastic Leukemia (ALL). The lack of targeted therapies for this disease is an important problem because ALL is primarily a pediatric cancer, and intensive rounds of general cytotoxic chemotherapy have severe and long- term adverse effects in children. Additionally, adults and children whose ALL does not respond to cytotoxic chemotherapy, or who develop relapse, have a universally poor prognosis due to limited treatment options. The well-known drivers of ALL are mis-expressed transcription factors, such as MYC, which are not easily therapeutically targeted without undesirable side-effects on normal cells. Preliminary data, which built on a novel in vivo transplantation screen in a MYC-induced ALL model in zebrafish, demonstrated that the tyrosine phosphatase PRL3 collaborated with MYC to enhance leukemia progression. In vitro and in vivo studies using human cells showed that PRL3 is linked with MYC expression, and plays a critical role in ALL survival. Phosphatases like PRL3 are most well-known as tumor suppressors, and despite emerging evidence that some phosphatases have oncogenic abilities, there are currently no phosphatase inhibitors used in the clinic for cancer patients, and only one in clinical trial. The overall objective of this application is to clearly define the oncogenic role of the tyrosine phosphatase PRL3 in MYC-driven ALL and to establish that it is a novel molecular target in this disease. The central hypothesis is that PRL3 is a direct transcriptional target of MYC, and synergizes with MYC to promote a pro-survival phenotype in ALL cells. The rationale for this research is that PRL3 belongs to a unique family of phosphatases that can be selectively inhibited; defining the mechanisms through which PRL3 promotes ALL survival will provide a strong scientific rationale for the development of PRL and other phosphatase inhibitors for use as targeted therapy for cancer patients. This hypothesis will be tested by pursuing three specific aims: 1) Define a mechanism through which MYC regulates PRL3 expression by using enChIP and biochemical approaches to assess MYC interaction with a novel enhancer region near PRL3; 2) determine the pro-survival mechanisms that are controlled by PRL3 by utilizing an unbiased biotin ligase approach to identify PRL3 substrates in ALL cells; and 3) establish that small molecule inhibition of the PRL family is a potent therapeutic strategy in ALL by completing comprehensive pre-clinical testing of three new PRL inhibitors and a panel of FDA-approved tyrosine phosphatase inhibitors. This research is innovative because it departs from the status quo to use new methods to define an oncogenic role for a phosphatase in cancer. The proposed research is also significant because it is expected to vertically advance and expand understanding of the role that phosphatases play in cancer progression. Ulti...