Familial Adenomatous Polyposis APC 5q Autosomal Dominant

A 22-year-old man presents with rectal bleeding and iron-deficiency anemia. Colonoscopy reveals hundreds of sessile and pedunculated adenomas carpeting the entire colon and rectum, with two villous polyps in the sigmoid showing high-grade dysplasia. His father underwent prophylactic proctocolectomy at age 30 for the same condition. Physical examination reveals congenital hypertrophy of the retinal pigment epithelium (CHRPE) and multiple osteomas of the mandible. Genetic testing identifies a pathogenic truncating mutation in the APC gene located at the locus marked **A** in the diagram. Which of the following best describes the molecular consequence of this APC mutation in the pathogenesis of adenoma formation?