A previously healthy 66-year-old man has been feeling tired for 5 months. He goes to his physician, who performs a complete physical examination, including stool guaiac, which is positive. A colonoscopy is performed, and a large, sessile 4.5 cm mass with surface ulceration is found in the cecum. A biopsy of this mass microscopically shows irregular glands with crowded, tall columnar cells having marked nuclear hyperchromatism. Which of the following gene alterations is he most likely to have?
Acquired p53 gene mutation