Abstract 1017: Impact of microsatellite instability status and immune scores on tumor after neoadjuvant chemoradiation therapy in locally advanced rectal cancer
Authors: Bae Sung Uk , Kim Shin , Byun Sang Jun , Lee Hye Won 
Published: June 2022

Abstract
Purpose: Neoadjuvant chemoradiation therapy (CRT) is a widely and effectively used preoperative treatment strategy in locally advanced rectal cancers. Tumors demonstrating microsatellite instability (MSI) are known to be infiltrated by higher numbers of lymphocytes and to be characterized by a more favorable prognosis as compared with tumors with intact repair systems, and MSI is the only potential biomarker to predict the outcome of immune checkpoint inhibitors. However, the relationship between MSI and response to neoadjuvant CRT and changes in MSI status after CRT in rectal cancer are not well understood. The aim of the present study was to investigate the MSI status before and after CRT and its association with CRT response in rectal cancer.
Materials and methods: Formalin-fixed paraffin-embedded tissue specimens from pre-operative biopsy via sigmoidoscopy and surgical resection of the primary tumor were obtained from 84 rectal adenocarcinoma patients who underwent neoadjuvant CRT between March 2010 and May 2018. MSI assays were performed on DNA extracted from FFPE and matched normal tissues the extracted DNA was amplified by PCR with fluorescent dye-labeled primers targeting five microsatellite loci: BAT25, BAT26, D5S346, D2S123 and D17S250. Three immune biomarker scores were calculated from the 19 patients who were able to perform RNA sequencing analysis to compare the immune activities of the specimens between responder (n = 8) and non-responder (n = 11). The gene lists included in each score calculation were as follows: CXCL9, CXCL10, IDO1, IFNG, HLA-DRA, and STAT1; GZMA and PRF1; and CD247, CD2, CD3E, GZMH, NKG7, PRF1 and GZMK for the interferon-γ (IFN-γ) signature,1 the cytolytic activity2 and the immune signature,3 respectively.
Results: Patients were classified, according to Mandard tumor regression grade (TRG), as responders (grade 1 or 2; 25 patients, 29.8%) and non-responders (grade 3, 4 or 5; 59 patients, 70.2%). The TRG of 84 patients was significantly different according to the pre-CRT MSI status (P = 0.024), but was not different according to the post-CRT MSI status among 60 patients who were eligible for the paired MSI test (P = 0.788). Changes in MSI status were observed in 11 of the 60 patients; MSS to MSI-low in 9 patients, MSS to MSI-high in 1 patient, and MSI-low to MSS in 1 patient. All three immune biomarker scores (the IFN-γ signature, the cytolytic activity and the immune signature) were higher in the responder group without significant difference (p = 0.536, p = 0.152, and p = 0.272, respectively).
Conclusion: Our findings demonstrated that neoadjuvant CRT modulated the MSI status of rectal cancer and pre-CRT MSI status was significantly associated with response of CRT. Future studies on various immuno-profiles that can influence the CRT response using more samples are needed.
Citation Format: Sung Uk Bae, Shin Kim, Sang Jun Byun, Hye Won Lee. Impact of microsatellite instability status and immune scores on tumor after neoadjuvant chemoradiation therapy in locally advanced rectal cancer [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 1017.