Impact of microsatellite instability (MSI) on tumor clonal evolution in metastatic colorectal cancer (mCRC)

616 Background: For mCRC, the contribution of clinical and pathologic factors to concordance between formalin-fixed, paraffin-embedded (FFPE) tissue-based and ctDNA-based mutation profiling remains unclear. MSI is hypothesized to confer a higher rate of somatic alteration resulting in clonal evolution over time compared to microsatellite stable (MSS) patients, but this has not been previously confirmed. Methods: All mCRC patients were consented for a prospective genomic matching protocol (Assessment of Targeted Therapies Against Colorectal Cancer [ATTACC]) using CLIA-certified platforms. Archived tumor DNA from primary or metastatic tissue was sequenced on a 46- or 50-gene panel (Ion Torrent). Paired ctDNA samples were isolated from blood and sequenced with an ultra high-sensitivity assay (Guardant360). Mutation data were normalized by excluding genomic regions not covered on both platforms and were filtered to remove germline or synonymous variants. Results: An initial cohort of 139 patients was included in our analyses, with a median of two lines of intervening chemotherapy between FFPE and plasma DNA collection; 6 (4.3%) of the patients had MSI (MSI-H) mCRC. We detected 472 total mutations in either tissue or ctDNA, with a global concordance rate of 34.5%. Global concordance was not associated with tissue source, nor treatment with specific standard cytotoxic and/or biologic agents. Mutations detected in MSI-H CRC showed significantly greater discordance compared to MSS CRC (OR = 2.5, p = 0.025). This finding was validated using an independent cohort of 17 MSI-H mCRC patients (OR = 2.78, p = 0.00015). Among recurrently altered genes, we found that MSI-H cases were significantly more likely to have gained new TP53 mutations (OR = 8.17, p = 0.001) and to have lost PIK3CA mutations (OR = 8.04, p = 0.036) in ctDNA compared to MSS cases. Conclusions: MSI correlates with discordance between tissue DNA and ctDNA-based mutation profiling, which suggests that MSI-H CRC undergoes distinct patterns of clonal evolution including acquisition of new TP53 mutations. This may have implications for targeted and immunologic therapies in this unique population, and suggests a utility for repeated molecular testing.