Evaluating Mismatch Repair Deficiency in Pancreatic Adenocarcinoma: Challenges and Recommendations

Purpose: Immune checkpoint inhibition has been shown to generate profound and durable responses in MMR-D solid tumors and has elicited interest in detection tools and strategies to guide therapeutic decision-making. Herein we address questions on the appropriate screening, detection methods, patient selection, and initiation of therapy for MMR-D PDAC and assess the utility of NGS in providing additional prognostic and predictive information for MMR-D PDAC. Experimental Design: Archival and prospectively acquired samples and matched normal DNA from N= 833 PDAC cases were analyzed using a hybridization capture based, NGS assay designed to perform targeted deep sequencing of all exons and selected introns of 341- 468 cancer-associated genes. A computational program using NGS data derived the MSI status from the tumor-normal paired genome sequencing data. Review of available germline testing, IHC, and MSI PCR results were performed to assess and confirm MMR-D and MSI status. Results: MMR-D in PDAC is a rare event among PDAC patients (7/833) occurring at a frequency of 0.8%. Loss of MMR protein expression by IHC, high mutational load and elevated MSIsensor scores were correlated with MMR-D PDAC. All 7 MMR-D PDAC patients in the study were found to have Lynch Syndrome. Four (57%) of the MMR-D patients treated with immune checkpoint blockade had treatment benefit (1 complete response; 2 partial responses; 1 stable disease). Conclusions: An integrated approach of germline testing and somatic analyses of tumor tissues in advanced PDAC using NGS may help guide future development of immune and molecularly directed therapies in PDAC patients.

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