Abstract 611: Assessment of tumor mutation burden and microsatellite instability by single-cell circulating tumor cell genomic profiling
Authors: Cani Andi K. , Dolce Emily M. , Liu Chia-Jen , Rupp Brittany , Darga Elizabeth P. , Paoletti Costanza , Thomas Dafydd G. , Wu Yi-Mi , Robinson Dan R. , Nagrath Sunitha , Chinnaiyan Arul M. , Tomlins Scott A. , Udager Aaron M. , Carethers John M. , Cobain Erin F. , Hayes Daniel F. 
Published: June 2022

Abstract
The success of immune checkpoint inhibitors rests on biomarkers such as tumor mutation burden (TMB) and microsatellite instability (MSI), both FDA-approved predictors of anti PD-1/L1 therapy benefit. Tissue biopsies often collected once in the metastatic setting through an invasive procedure, or archived primary tumor tissue often collected much prior to treatment consideration, are the specimen types of choice for biomarker identification. The tissue sample originates from a limited region of one disease site, which may limit its usefulness given intra-patient tumor heterogeneity. TMB and MSI measurement by liquid biopsy, including proteins, circulating tumor cells (CTC), and cell-free circulating tumor DNA (ctDNA), is an attractive, minimally-invasive way to obtain a real-time picture of the entire disease. While TMB and MSI assessment from ctDNA have been reported, their measurement can be limited by low ctDNA tumor fraction. Single-cell next generation sequencing of CTC, on the other hand, is a particularly well-suited, but largely unexplored method of measuring TMB and MSI to complement tissue and ctDNA for better overall specificity of detection. In this proof-of-concept study, we show the ability to detect single-cell TMB and MSI. We analyzed 14 CTC and 4 ctDNA samples from 6 metastatic breast cancer patients, as well as 3 single cells and 1 cell pellet sample each from HCT-116 (MSI-High) and WiDr (MSI-Low) cell lines. CTC and cell line cells were enriched with the CellSearch® system and/or isolated with the DEPArray™ system. Whole genome amplified single-cell DNA was sequenced with the Oncomine Comprehensive Assay covering ~500 genes and 1.1Mb of genomic space. TMB and MSI scores obtained in CTC and ctDNA were compared to those measured in matched clinical tissue biopsies. Single-cell TMB scores and MSI status were assessable in all CTC tested. CTC TMB scores were highly concordant with the matched tissue samples (r=1.00), as were ctDNA TMB scores (r=0.98) in patients with assessable TMB scores in both biospecimen types compared. Importantly, TMB was detectable in CTC from one patient whose tissue sample was inadequate for clinical sequencing, and from another patient with inadequate, low tumor fraction ctDNA. Intriguingly, one patient harbored 3 TMB-high and 2 TMB-low CTC, potentially indicating intra-patient TMB heterogeneity. The known MSI-low status from clinical tumor tissue sequencing was correctly detected in CTC and ctDNA from all patients. MSI status and scores from single cells of HCT-116 and WiDr cell lines purified with the DEPArray™ system (mimicking CTC isolation), perfectly matched that of the corresponding cell pellet samples (r=1.00). Taken together, these data suggest the potential validity and continued interrogation of potential utility of CTC TMB and MSI detection to complement tissue and ctDNA in guiding checkpoint inhibitor immunotherapy.
Citation Format: Andi K. Cani, Emily M. Dolce, Chia-Jen Liu, Brittany Rupp, Elizabeth P. Darga, Costanza Paoletti, Dafydd G. Thomas, Yi-Mi Wu, Dan R. Robinson, Sunitha Nagrath, Arul M. Chinnaiyan, Scott A. Tomlins, Aaron M. Udager, John M. Carethers, Erin F. Cobain, Daniel F. Hayes. Assessment of tumor mutation burden and microsatellite instability by single-cell circulating tumor cell genomic profiling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 611.