Intratumoral heterogeneity for inactivating frameshift mutation of CUX1 and SIRT1 genes in gastric and colorectal cancers

Both CUX1 and SIRT1 are considered tumor suppressor genes (TSGs), but it is not known whether CUX1 and SIRT1 alterations are different between high microsatellite instability (MSI-H) and microsatellite stable MSI (MSS) cancers. We identified frameshift mutations of CUX1 in 4 cases of colorectal cancer (CRC) and of SIRT1 in 1 case of gastric cancer (GC) and 3 cases of CRC. All of them were found in GC or CRC with MSI-H (3.5% of MSI-H for each gene), but neither in GC nor CRC with MSS. In addition, we analyzed intratumoral heterogeneity (ITH) of the CUX1 frameshift mutation and found that two CRCs (12.5%) harbored regional ITH of the frameshift mutation. Our data indicate that there exist frameshift mutations of CUX1 and SIRT1 genes as well as ITH of CUX1 frameshift mutation in MSI-H cancers, which together might play a role in tumorigenesis of GC and CRC with MSI-H.