AG, a 32-year-old woman with known family history of hereditary nonpolyposis colon cancer (HNPCC) syndrome, is returning for her annual screening colonoscopy. One of the biopsy samples reveals changes...

AG, a 32-year-old woman with known family history of hereditary nonpolyposis colon cancer (HNPCC) syndrome, is returning for her annual screening colonoscopy. One of the biopsy samples reveals changes concerning for malignancy. HNPCC results from defects in genes involved in DNA mismatch repair (i.e., repair of mismatched base pairs during replication, e.g., C pairing to A). What step in the pathogenesis of this carcinoma is NOT likely to have occurred?

A. AG inherited one defective DNA mismatch repair gene.

B. One cell among all AG’s colonic epithelial cells acquired a mutation that made the other copy of the repair gene nonfunctional.

C. A chance error during replication led to a faulty pairing (e.g., G to T), which remained unrepaired.

D. The defect in DNA mismatch repair transformed this cell into a malignant cell capable of unregulated proliferation.

E. Accumulation of errors led to the dysfunction of several gene products involved in growth and cell cycle regulation.