Characterization of the number and site of APC mutations in sporadic colorectal cancer.

630 Background: Truncating mutations in the adenomatous polyposis coli ( APC) gene are well-described events in the carcinogenesis of colorectal carcinomas (CRC) and may impact one or both APC alleles. These aberrations often fall within the mutation cluster region (MCR) of the APC gene to preserve a “just right” number of beta-catenin binding sites in the resulting mutant APC protein. Further clinical and genotypic characterization of CRCs based on number and site of mutations in APC determined using modern next generation sequencing methods is needed. Methods: Next generation sequencing of 70 CRC tumors was performed at a single institution via UNCseq to determine mutations in a panel of 247 oncogenes and tumor suppressors, including APC. RNASeq, DNA sequencing, and clinical characteristics from 224 colon and rectal cancer samples in The Cancer Genome Atlas (TCGA) project were also obtained. Results: In the UNCseq cohort, 58 patients (83%) had at least one inactivating APC mutation, and 33 (47%) had two mutations. Of those with at least one mutation, 81% had a mutation in the MCR (residues 1281-1556), but only 5/33 (15%) with two mutations had both in the MCR. In the TCGA cohort, 162 (72%) had at least one inactivating APC mutation, and 52 (23%) had two mutations. Of those with at least one mutation, 59% had a mutation in the MCR, but only 3/52 (6%) with two mutations had both in the MCR. Gene expression of APC was higher in those with no APC mutations vs. 1-2 mutations (p = 0.015), but was not significantly different between those with one vs. two mutations (p = 0.29). The absence of APC mutations was associated with microsatellite instability (p < 0.001) and with right-sided primary tumors (p = 0.001 by chi-square). Conclusions: CRCs have high frequency of biallelic APC mutations, and the majority of tumors with APC mutations had a single mutation within the MCR region of the APC gene. These genotypic factors may impact tumor biology and clinical features.