Frameshift Mutations of AKAP9 Gene in Gastric and Colorectal Cancers with High Microsatellite Instability

A-kinase-anchoring protein 9 (AKAP9) coordinates the cellular location and function of protein kinase A. AKAP9 plays an important role in centrosome duplication, cell cycle progression and maintenance of cell membrane integrity, alterations of which contribute to tumorigenesis. Somatic mutations of AKAP9 gene have been detected in many cancers including gastric (GC) and colorectal cancers (CRC), but the mutation status with respect to microsatellite instability (MSI) has not been reported. In a public database, we found that AKAP9 gene had mononucleotide repeats in the coding sequences that might be mutation targets in the cancers with MSI. We analyzed the mutations in 79 GCs and 124 CRCs including high MSI (MSI-H) and microsatellite stable/low MSI (MSS/MSI-L) cases by single-strand conformation polymorphism analysis and DNA sequencing. Overall, we found AKAP9 frameshift mutations in 4 (11.7 %) GCs and 20 (17.7 %) CRCs with MSI-H (24/113), but not in MSS/MSI-L cancers (0/90) (p < 0.001). In addition, we analyzed intratumoral heterogeneity (ITH) of AKAP9 frameshift mutations in 16 CRCs and found that five CRCs (31.3 %) harbored regional ITH of the AKAP9 frameshift mutations. Our data indicate that AKAP9 gene harbors not only somatic frameshift mutations but also mutational ITH, which together may be features of GC and CRC with MSI-H. Our results also suggest that regional mutation analysis is needed for a better evaluation of mutation status in these tumors to overcome ITH.