Mutations in APC, Kirsten-ras, and p53--alternative genetic pathways to colorectal cancer

Abstract

Colorectal cancer is one of the most significant causes of cancer death. A genetic model for colorectal cancer has been proposed in which the sequential accumulation of mutations in specific genes, including adenomatous polyposis coli (APC), Kirsten-ras (K-ras), and p53, drives the transition from healthy colonic epithelia through increasingly dysplastic adenoma to colorectal cancer. We have characterized tumor mutation spectra in a large cohort of colorectal cancer patients. In marked contrast to the predictions of the sequential model of mutation accumulation, only 6.6% of tumors were found to contain mutations in APC, K-ras, and p53, with 38.7% of tumors containing mutations in only one of these genes. The most common combination of mutations was p53 and APC (27.1%), whereas mutations in both p53 and K-ras were extremely rare. Statistical analysis (two-sided Fisher's exact test) confirmed that mutations in K-ras and p53 co-occurred less frequently than expected by chance (P < 0.01, Fisher's exact test). This finding suggests that these mutations lie on alternate pathways of colorectal tumor development. The heterogeneous pattern of tumor mutations in our patient cohort suggests that multiple alternative genetic pathways to colorectal cancer exist and that the widely accepted genetic model of cancer development is not representative of the majority of colorectal tumors.

Figure 1

A model of the genetic changes required for progression from adenoma to carcinoma in the development of colorectal cancer (6). The proposed order of mutations in APC, K-ras, p53, and the DNA MMR genes is illustrated.

Figure 2

Spectrum of p53 mutations in colorectal cancer. (A) p53 mutations in Dundee patients with colorectal cancer and (B) p53 mutations in all white colorectal solid tumors in the IARC p53 mutation database (18). The amino acid positions of the most frequently mutated hotspot codons are highlighted. (C) The position of each of the hotspot codons relative to the conserved regions and functional domains of p53 is illustrated.

Figure 3

Distribution of APC, K-ras, and p53 mutations in patients with colorectal cancer. The presence of mutations in APC, K-ras, and p53 was determined in a cohort of patients with colorectal cancer (n = 106) by a combination of WAVE denaturing HPLC analysis and direct sequencing, as described in Materials and Methods. The percentage of tumors with each combination of mutations is illustrated.

Figure 4

Distribution of APC, K-ras, and p53 mutations according to Dukes' stages. The presence of mutations in APC, K-ras, and p53 was determined in a cohort of patients with colorectal cancer (n = 106) by a combination of WAVE denaturing HPLC analysis and direct sequencing, as described in Materials and Methods. (A) Tumors were categorized by the number of mutations and further subdivided according to Dukes' stages. (B) Tumors were categorized according to Dukes' stages and further subdivided by the presence of mutations in APC, K-ras, and p53. *, P = 0.032, comparing Dukes' B and Dukes' C.