Can microsatellite status of colorectal cancer (CRC) be reliably assessed after neoadjuvant therapy?

Abstract

1525 Background: Universal testing of resected CRC for Microsatellite Instability (MSI) status has been widely advocated. While PCR-based MSI analysis is the current gold standard test, immunohistochemistry (IHC) of mismatch repair (MMR) proteins is more commonly performed. At present, it remains un-established whether PCR-based MSI analysis remains reliable when performed on post-neoadjuvant treated specimens. Our goal was to examine the effect of neoadjuvant therapy on MSI analysis using both in vitro and clinical data. Methods: MSI analysis was performed using the standard panel of 7 markers. In vitro, MMR deficient (HCT116) and proficient (HCT116+Ch3) isogenic cell lines were exposed to 3 cycles of single agent 5-Fluorouracil, Oxaliplatin and Irinotecan and MSI analysis was tested after each cycle. To evaluate the induction of genomic instability, 3 additional coding microsatellite tracts in ATR, BLM and CHK1 were assessed by fragment analysis. In parallel, clinical data from 238 CRCs that were resected after neoadjuvant chemoradiation (n=191) or chemotherapy (n=47) between 9/2009 and 8/2011 were queried for MSI results. MSI analysis performed on paired pre-treatment and post-treatment resection tissues was compared. Results: No changes in MSI status were detected and there was no increase in genomic instability post-treatment in cell line models. In the clinical setting, pre-neoadjuvant MSI testing was limited by the availability of pre-treatment biopsy tissue. 3 patients found to be MMR proficient based on MSI analysis performed on pre-treatment biopsy remained microsatellite stable when analyzed on post-treatment resection tissue. 7 patients were MMR deficient based on MSI analysis performed on post-treatment resection tissue but most had strong clinical evidence to expect MMR deficiency. Conclusions: Based on both preclinical in vitro and clinical data, results of the MSI analysis does not appear to be affected by neoadjuvant therapy for both MMR proficient and deficient cases. In addition, preclinical results do not suggest that neoadjuvant chemoradiation induces an increase in genomic instability. Therefore, MSI analysis remains a reliable gold standard test on post-treatment specimens.