Analyzing microsatellite instability and gene mutation in circulating cell-free DNA to monitor colorectal cancer progression

BACKGROUND: Colorectal cancer (CRC) is the second leading cause of cancer-related deaths worldwide. Detection of microsatellite instability (MSI) status and gene mutations may be useful for molecular targeted therapy. The liquid biopsy is a newly developed, non-invasive method for tumor diagnosis and monitoring. In this study, we evaluated the possible clinical value of liquid biopsy by analyzing MSI and gene mutation. METHODS: Next-generation sequencing (NGS) was used to analyze MSI and gene mutation in circulating cell-free DNA (cfDNA) and tissue DNA extracted from 6 CRC patients’ plasma and matched primary tumor tissue (MPTT) samples, respectively. RESULTS: A total of 6 patients (4 male, 2 female) were included for analysis, whose stage ranges from stage I through stage III. NGS-based panel of 5 quasi-monomorphic microsatellite markers (MSI-NGS) BAT-25, BAT-26, NR21, NR24 as well as NR27, and 4 mismatch repair (MMR) genes (MSH2, MSH6, PMS2, MLH1) expressions assessed by immunohistochemistry (MMR-IHC) and NGS (MMR-NGS) were used to determine MSI status synergistically. Comprehensive analysis of NGS and IHC results showed that the overall incidences of MSI in plasma and MPTT samples from these patients were 1/6 and 2/6, respectively. 4 patients were defined as microsatellite stable (MSS) in both plasma and MPTT. In the above 6 patients, MSI-NGS detection in cfDNA accurately identified 1/2 of tissue high-level microsatellite instability (MSI-H) and 4/4 of tissue MSS for an overall accuracy of 5/6. Gene mutational profiles in these CRC patients’ plasma and MPTT samples were analyzed by NGS. Tumor-specific gene mutations were detected in 2/6 of plasma and 4/4 of MPTT samples. The two mutation-positive plasma samples were from CRC patients at stage IIb and stage IIIc. CONCLUSIONS: Analyzing MSI and gene mutation might be a non-invasive supplementary way to reveal the molecular characteristics of CRC.