Cancer Biology and Loss of Cell Cycle Control (2C) Practice Test

A lab studied the Rb–E2F pathway in a cancer cell line. In normal cells, hypophosphorylated Rb binds E2F and limits transcription of S-phase genes; phosphorylation of Rb by cyclin D–CDK4/6 releases E2F. The cancer line expresses an Rb truncation that cannot bind E2F. When treated with a CDK4/6 inhibitor, the cancer cells show little change in S-phase gene expression, while a control line with intact Rb shows decreased S-phase gene expression and reduced proliferation. Which outcome is most consistent with these findings?

Researchers compared a healthy epithelial cell line to a tumor-derived line. Both were exposed to ionizing radiation (IR) to induce DNA double-strand breaks. In healthy cells, IR caused a strong accumulation of p53 protein and a marked decrease in S-phase entry 12 hours later. In the tumor-derived line, IR did not increase p53 levels, and S-phase entry remained high. Sequencing revealed a missense mutation in the tumor line’s p53 DNA-binding domain. Based on these results, which conclusion is most consistent with the data regarding cell cycle regulation in cancer?