Intratumoral Heterogeneity of Frameshift Mutations in MECOM Gene is Frequent in Colorectal Cancers with High Microsatellite Instability

MECOM gene, also known as EVI, encodes a transcriptional regulator involved in hematopoiesis, apoptosis, development and proliferation. In blood system, MECOM is considered an oncogene, but in solid tumors it has both oncogenic and tumor suppressor activities. Low frequent somatic mutations of MECOM have been detected in many cancers including colorectal cancers (CRC), but the mutation status with respect to the microsatellite instability (MSI) has not been studied. There is an A7 mononucleotide repeat in MECOM coding sequences that could be a mutation target in the cancers with MSI. We analyzed the A7 of MECOM in 79 CRCs with high MSI (MSI-H) and 65 microsatellite stable/low MSI (MSS/MSI-L) CRCs by single-strand conformation polymorphism analysis and DNA sequencing. Overall, we found MECOM frameshift mutations in 6 (7.6 %) CRCs with MSI-H, but not in MSS/MSI-L cancers (0/65) (p < 0.025). We also analyzed intratumoral heterogeneity (ITH) of the MECOM frameshift mutation in 16 CRCs and found that four CRCs (25.0 %) harbored regional ITH of the frameshift mutations. Our data indicate that MECOM gene harbors both somatic frameshift mutations and mutational ITH, which together may be features of CRC with MSI-H.