Lynch Syndrome MLH1 MSH2 MMR Autosomal Dominant

A 42-year-old man presents with iron-deficiency anemia and altered bowel habits. Colonoscopy reveals a poorly differentiated right-sided adenocarcinoma with mucinous features and prominent tumor-infiltrating lymphocytes. Family history is significant for colorectal cancer in his father (age 47), endometrial cancer in his paternal grandfather (age 55), and urothelial carcinoma in a paternal aunt. Immunohistochemistry shows loss of MLH1 and PMS2 expression with retained MSH2/MSH6. BRAF V600E is wild-type and MLH1 promoter is unmethylated. Panel testing confirms a pathogenic mutation in the locus marked **C** in the diagram. Which of the following best describes the molecular consequence of this germline mutation?