The PRL family and other phosphatases as novel drug targets in pediatric cancer

Phosphatases are increasingly recognized as potential oncogenes in a variety of cancer types. Although phosphatases were largely thought to counteract the effect of kinases, they are increasingly recognized for their ability to modulate the strength and duration of signaling, and even activate signaling cascades. As enzymes, phosphatases represent a novel and useful class of drug targets in cancer, but to date, there are no FDA‐approved phosphatase inhibitors, and only a few in clinical trials. I have developed a 2‐pronged approach to identifying phosphatases that may serve as drug targets in cancer, specifically examining the protein tyrosine phosphatase 4A3 (PTP4A3 or PRL3). The tyrosine phosphatase PRL3 is a well‐known oncogenic phosphatase, and we have found that it plays an important role in pediatric Acute Lymphoblastic Leukemia (ALL) progression and survival. However, the oncogenic effects of other members of the PRL family, as well as other protein tyrosine phosphatases, have not been defined. Currently, I am in the process of further exploring the role of PRLs and other phosphatases in childhood cancer using a CRISPR/Cas9 screen that has been designed to knock‐out 280 known phosphatases in the human genome in T‐cell and B‐cell ALL using the Invitrogen™ Lentiarray™ Human CRISPR Library. Viability of the phosphatase knock‐outs will be examined to determine whether the phosphatase is an oncogenic drug target (contributes to tumor growth, so knock‐out will kill tumor cells), tumor suppressive (knock‐out will accelerate tumor growth) or neutral. Those phosphatases that affect tumor growth will be validated in comparison to normal lymphocytes. Identified phosphatases will be overexpressed in a zebrafish MYC induced ALL model that our lab has generated. Zebrafish will be examined for increases in penetrance and tumor progression, compared to cancerous zebrafish without overexpression of these phosphatases. PRL3 overexpression in this model results in enhanced leukemia onset and early circulation in the blood, compared to those animals with sole MYC overexpression. I am currently in the process of performing the same experiments with PRL1 and PRL2, and future studies will determine the mechanism of action through which these proteins promote leukemia progression. Finally, I am using an approach to screen FDA‐approved compounds for off‐label activity against all members of the PRL family to identify compounds that can be repurposed to target phosphatase activity. Preliminary results indicate that Pentamidine, an FDA‐approved drug used as an anti‐fungal, can inhibit PRL3 activity and prevent leukemia growth both in vitro in purified proteins and human cells lines and in vivo in our zebrafish leukemia model, as well as synergize with standard chemotherapies. Pre‐clinical testing of the effects of Pentamindine on patient derived xenografts is ongoing. In total, this project is not only identifying new therapies that may be useful in treating pediatric cancers, but will also point us in the direction towards determining the specific role of phosphatases in these cancers. Support or Funding Information St. Baldrick's Foundation Research Grant: 3048113685 NIH R37: 3200001810 This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .