Assessment Of Microsatellite Instability (Msi) In Cell Free Dna (Cfdna) Of Colorectal Cancers (Crc) Patients (Pts).

672Background: While there is accumulating evidence that tumor specific mutations can be accurately detected in cfDNA, the determination of MSI status in cfDNA has not been investigated, perhaps because the complexity of detecting tumor instability embedded within stutter amplification patterns generated from normal cfDNA was considered too challenging to allow the detection of altered microsatellite profiles. We explored the possibility of detecting MSI in cfDNA. Methods: Circulating cfDNA was isolated from Streck tubes from 13 patients with CRC; 6 pts had serial collections. Isolation and testing of cfDNA for MSI was performed using kits purchased from Promega. Microsatellite alterations were scored by comparing the amplicon stutter profiles of 5 mononucleotide markers generated from cfDNA to those present in genomic DNA extracted from buffy coats from the same pts. MSI was diagnosed based on a shift in the size of a predominant stutter peak together with a commensurate shift in the stutter profile resu...