Adhesion GPCR regulation of acute myeloid leukemia stem cells - Resubmission - 1

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Despite advances in our understanding of the genetic origins of AML, treatment options have remained essentially unchanged for 30 years, and clinical outcomes remain poor. Leukemia stem cells (LSCs) represent the population of blasts that are resistant to chemotherapy and re-initiate AML after therapy; thus, this subset of blasts must be eradicated to cure disease. Unfortunately, therapies developed specifically to target LSCs have yet to be validated in the clinic. We and others recently identified a novel AML antigen, CD97, that is expressed in the vast majority of human AMLs. Our recently published studies have revealed several features of CD97 that suggest that it may be an excellent therapeutic target in AML: 1) CD97 is one of the most commonly expressed AML antigens; 2) CD97 regulates blast growth, survival, and differentiation; 3) CD97 regulates LSC function, as demonstrated in serial transplantation experiments of primary AML; and, 4) CD97 is not required for HSC function, suggesting low toxicity of CD97-targeting therapeutics. Highlighting its clinical importance, CD97 mRNA expression is an independent predictor of disease-free and overall survival in AML. CD97 is an adhesion class G-protein coupled receptor (aGPCR) characterized by a long, extracellular ligand- binding domain and a GPCR-Autoproteolysis-INducing (GAIN) domain that can induce signals that may or may not require extracellular domain shedding. Isoforms of CD97 produced by alternative splicing differ in the composition of the ligand-binding domain, but at present, it is unclear if the various CD97 isoforms mediate unique or overlapping roles in AML. Our overall hypothesis is that the various CD97 isoforms play distinct roles in leukemogenesis and LSC self-renewal by virtue of their unique ligand binding and/or signaling properties. Our specific goals are to determine the role of CD97 isoforms in leukemic transformation and LSC function, to identify the molecular and structural requirements for CD97 activity, and to utilize novel human synthetic antibodies (sAbs) against CD97 with different epitope specificities to evaluate the function of CD97 as well as test their anti-leukemic activity. We will determine the roles of the various structural subdomains of CD97 required for LSC function utilizing our novel CD97 Abs, CD97 constructs expressing multiple naturally occurring and engineered structural variants of CD97, and complementary in vitro and in vivo models of mouse and human AML. Given our team's complementary expertise in LSC biology, antibody engineering, and aGPCR biology, we are uniquely positioned to investigate the mechanisms of CD97 signaling and function in LSCs. Collectively, these studies will dramatically increase our understanding of the molecular mechanisms that regulate LSC self- renewal and help expedite translation of CD97 antibody therapies to the clinic. Finally, these studies may have broader consequences since CD97 plays disease-modifying roles in other human cancers.