APC Mutations Are Not Confined to Hotspot Regions in Early-Onset Colorectal Cancer

Abstract

While overall colorectal cancer (CRC) cases have been declining worldwide there has been an increase in the incidence of the disease among patients under 50 years of age. Mutation of the APC gene is a common early event in CRC but is reported at lower rates in early-onset colorectal cancer (EOCRC) than in older patients. Here we investigate the APC mutation status of a cohort of EOCRC patients in New Zealand using a novel sequencing approach targeting regions of the gene encompassing the vast majority of known APC mutations. Using this strategy we find a higher rate (72%) of APC mutation than previously reported in EOCRC with mutations being spread throughout the gene rather than clustered in hotspots as seen with sporadic mutations in older patients. The rate of mutations falling within hotspots was similar to those previously seen in EOCRC and as such our study has implications for sequencing strategies for EOCRC patients. Overall there were low rates of both loss of heterozygosity and microsatellite instability whereas a relatively high rate (40%) of APC promoter methylation was found, possibly reflecting increasing exposure of young people to pro-oncogenic lifestyle factors.

Keywords: APC; DNA methylation; colorectal cancer; diet; early-onset; sequencing.

Figure 1

Sanger sequencing of APC c.3949G>C mutation. The guanine base indicated by the red arrow in the wild type APC sequence is mutated to a cytosine in one allele in the normal tissue of the patient to give a heterozygous sequence. In the tumour tissue the wild type guanine allele is barely visible.

Figure 2

APC mutations found in EOCRC samples mapped to their position within the protein. Each lollipop represents the position of a mutation with the colours of the lollipop representing different types of mutation: black, nonsense mutations; green, missense mutation; pink, frameshift mutation; yellow, splice variant. While a high proportion of mutations occur within the mutation cluster region (MCR, black bar), mutations are found throughout the first 1600 codons of APC including some upstream of exon 15 (green bar). Protein domains: green, oligomerisation; red, armadillo repeats; blue, 15 amino acid repeats; yellow, 20 amino acid repeats; purple, SAMP motif. The plot was generated using Mutation Mapper (www.cbioportal.org) and manually curated.