Membrane associated cancer-oocyte neoantigen SAS1B/ovastacin is a candidate immunotherapeutic target for uterine tumors

// Eusebio S. Pires 1 , Ryan S. D’Souza 1 , Marisa A. Needham 1 , Austin K. Herr 1 , Amir A. Jazaeri 3 , Hui Li 2 , Mark H. Stoler 2 , Kiley L. Anderson-Knapp 2 , Theodore Thomas 3 , Arabinda Mandal 1 , Alain Gougeon 5 , Charles J. Flickinger 1 , David E. Bruns 2 , Brian A. Pollok 4 , John C. Herr 1 1 Department of Cell Biology at the School of Medicine, University of Virginia, Charlottesville, Virginia, USA 2 Department of Pathology at the School of Medicine, University of Virginia, Charlottesville, Virginia, USA 3 Department of Obstetrics and Gynecology at the School of Medicine, University of Virginia, Charlottesville, Virginia, USA 4 Neoantigenics, Inc, Charlottesville, Virginia, USA 5 CRCL, UMR Inserm-1052, CNRS-5286, Faculte de Medecine Laennec, Lyon, France Correspondence to: John C. Herr, e-mail: jch7k@virginia.edu Keywords: ASTL/SAS1B/ovastacin, cancer surface neoantigen, oocyte-specific protein, uterine tumor biomarkers, immunotherapy target Received: May 18, 2015      Accepted: August 07, 2015      Published: August 18, 2015 ABSTRACT The metalloproteinase SAS1B [ovastacin, ASTL, astacin-like] was immunolocalized on the oolemma of ovulated human oocytes and in normal ovaries within the pool of growing oocytes where SAS1B protein was restricted to follicular stages spanning the primary-secondary follicle transition through ovulation. Gene-specific PCR and immunohistochemical studies revealed ASTL messages and SAS1B protein in both endometrioid [74%] and malignant mixed Mullerian tumors (MMMT) [87%] of the uterus. A MMMT-derived cell line, SNU539, expressed cell surface SAS1B that, after binding polyclonal antibodies, internalized into EEA1/LAMP1-positive early and late endosomes. Treatment of SNU539 cells with anti-SAS1B polyclonal antibodies caused growth arrest in the presence of active complement. A saporin-immunotoxin directed to SAS1B induced growth arrest and cell death. The oocyte restricted expression pattern of SAS1B among adult organs, cell-surface accessibility, internalization into the endocytic pathway, and tumor cell growth arrest induced by antibody-toxin conjugates suggest therapeutic approaches that would selectively target tumors while limiting adverse drug effects in healthy cells. The SAS1B metalloproteinase is proposed as a prototype cancer-oocyte tumor surface neoantigen for development of targeted immunotherapeutics with limited on-target/off tumor effects predicted to be restricted to the population of growing oocytes.