Abstract 3034: Proteomic profiling identifies PTK7 as a novel immunotherapeutic candidate for neuroblastoma
Authors: Sadanand Arhanti , Maximov Victor , Lee Jasmine , Suttapitugsakul Suttipong , Shim Jenny , Doering Chris , Wu Ronghu , Schnepp Robert , Spencer Trent , Goldsmith Kelly 
Published: July 2021

Abstract
Anti-GD2 antibody therapy significantly improved high-risk neuroblastoma (NB) patient survival yet is associated with toxicity due to GD2 expression on pain fibers. Also, GD2 may be downregulated in NB following chemotherapy and relapse. It is paramount to identify novel tumor-associated antigens that are upregulated on the cell surface throughout therapy with minimal normal tissue expression. We utilized a novel cell surface capturing technology to define the NB cell surfaceome before and after chemotherapy and nominate potential targets for cell-mediated immunotherapy. Two NB patient derived xenografts (PDXs) and one cell line xenograft (IMR5) were treated with topotecan/cyclophosphamide in vivo. Treated and untreated tumors were disaggregated and N-glycoproteins on intact cells were oxidized, selectively labeled with biocytin-hydrazide, lysed, proteins digested, and cell surface peptides enriched using NeutrAvidin beads, followed by LC-MS analysis. We prioritized cell surface proteins that were abundant on all NB models and did not change under the selective pressure of chemotherapy. Our top hit was confirmed for cell surface expression by flow, evaluated for primary tumor and normal cell expression using gene expression datasets, and evaluated for functional relevance using CRISPR/CAS9 and immunotherapy targeting via chimeric antigen receptor (CAR). In two replicates, 292 cell surface proteins were identified as abundant and unchanged following chemotherapy. 34/292 proteins had high abundance of ≥70 a.u. across all xenografts, including NB surface proteins ALK and NCAM1. One protein identified was PTK7, a member of the WNT signaling pathway that is overexpressed in adult tumors. PTK7 is highly expressed in primary NB tumors with very low normal tissue expression. PTK7 protein is present on the cell surface in 17/17 NB cell lines and PDXs. While high PTK7 expression in primary NB is associated with poor patient prognosis, CRISPR/CAS9 knockout of PTK7 shows no functional effect on cell survival, proliferation or chemotherapy response. We developed a codon-optimized, second-generation CAR construct specific to PTK7. Our CAR-engineered T cells specifically bound to recombinant human-PTK7 Fc chimera. PTK7 CAR Jurkat T cells became activated in the presence of PTK7-expressing KELLY, IMR-5, and NB-1643 NB cells, and activation was not seen in the presence of PTK7 negative CMK leukemia cells or NB-1643 PTK7 knockout cells, confirming activation is specific to the CAR recognizing PTK7. Primary T cell PTK7 CAR studies are ongoing to assess cytotoxicity against NB. Multiple surface proteins remain stable on NB tumors after chemotherapy treatment. PTK7 is a potential cell surface antigen that could be utilized for immunotherapy approaches in NB while minimizing damage to normal tissues. PTK7 is robustly expressed in other pediatric and adult cancers, suggesting broader clinical relevance.
Citation Format: Arhanti Sadanand, Victor Maximov, Jasmine Lee, Suttipong Suttapitugsakul, Jenny Shim, Chris Doering, Ronghu Wu, Robert Schnepp, Trent Spencer, Kelly Goldsmith. Proteomic profiling identifies PTK7 as a novel immunotherapeutic candidate for neuroblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3034.