Which of the following genetic alterations is the earliest and most frequent initiating event in the adenoma-carcinoma sequence of colorectal cancer?

Explanation

## Adenoma-Carcinoma Sequence and Genetic Progression **Key Point:** The adenoma-carcinoma sequence is a well-established model of colorectal tumorigenesis involving sequential accumulation of genetic mutations. ### APC Gene — The Initiating Event **High-Yield:** APC (Adenomatous Polyposis Coli) mutations occur in approximately 80% of sporadic colorectal adenomas and are considered the earliest and most frequent initiating event in the adenoma-carcinoma sequence [cite:Robbins 10e Ch 17]. - Loss of APC function leads to constitutive Wnt/β-catenin signaling - Results in increased cell proliferation and adenoma formation - Occurs in the normal epithelium → early adenoma transition ### Sequential Mutation Pattern | Mutation | Frequency | Stage | Function | |----------|-----------|-------|----------| | **APC** | ~80% | Early adenoma | Tumor suppressor (Wnt pathway) | | **KRAS** | ~40% | Intermediate adenoma | Oncogene (growth signaling) | | **TP53** | ~50% | Late adenoma/carcinoma | Tumor suppressor (apoptosis, cell cycle) | | **SMAD4** | ~30% | Late carcinoma | Tumor suppressor (TGF-β pathway) | **Clinical Pearl:** Familial adenomatous polyposis (FAP) results from germline APC mutations, demonstrating the critical role of APC in adenoma initiation — virtually 100% of FAP patients develop colorectal cancer by age 40 if untreated. ### Why APC is the Initiating Event 1. Present in the earliest microscopic adenomas 2. Occurs before KRAS activation 3. Necessary but not sufficient for malignant transformation 4. Loss of APC alone drives adenoma formation; additional mutations (KRAS, TP53) drive progression to carcinoma