ONCOPATHOLOGY - MCQs WITH EXPLANATIONS

## ONCOPATHOLOGY - MCQs WITH EXPLANATIONS ## 1. Cell Cycle Regulation **Q1.** A cancer cell line shows hyperphosphorylated RB protein throughout the cell cycle, even in the absence of growth factors. Which of the following is the MOST likely underlying mechanism? A) Loss of p53 function B) Overexpression of cyclin D-CDK4 complex C) Mutation in E2F transcription factor D) Defective DNA damage checkpoints E) Loss of p21 expression **Answer: B) Overexpression of cyclin D-CDK4 complex** **Explanation:** Hyperphosphorylated RB releases E2F, allowing S-phase entry. The cyclin D-CDK4 complex is specifically responsible for initiating RB phosphorylation. Overexpression of this complex would lead to constitutive RB phosphorylation regardless of external growth signals, making cells growth factor-independent. --- **Q2.** In normal cells, what happens immediately after RB protein becomes hyperphosphorylated? A) Cell cycle arrest at G1/S checkpoint B) Activation of p53-mediated apoptosis C) Release of E2F transcription factor D) Degradation of cyclin D E) Activation of DNA repair mechanisms **Answer: C) Release of E2F transcription factor** **Explanation:** Hyperphosphorylated RB dissociates from the RB-E2F complex, releasing the E2F transcription factor. Free E2F then transcribes genes necessary for S-phase progression, including cyclins E and A, and DNA polymerases. --- **Q3.** A patient's tumor shows complete loss of p16INK4a expression due to promoter hypermethylation. Which cell cycle transition would be MOST directly affected? A) G0 to G1 transition B) G1 to S transition C) S to G2 transition D) G2 to M transition E) M to G1 transition **Answer: B) G1 to S transition** **Explanation:** p16INK4a normally acts as a CDK inhibitor by competing with cyclin D for CDK4/6 binding, thereby preventing RB phosphorylation. Loss of p16 allows uncontrolled cyclin D-CDK4/6 activity, leading to premature RB phosphorylation and unregulated G1/S transition. --- **Q4.** Which of the following statements about the G1/S restriction point is MOST accurate? A) It is primarily controlled by p53 activation B) Once cells pass this point, they are committed to complete the cell cycle C) It occurs after DNA replication has begun D) Cyclin B-CDK1 is the primary regulator at this checkpoint E) It can be bypassed without affecting cell viability **Answer: B) Once cells pass this point, they are committed to complete the cell cycle** **Explanation:** The G1/S restriction point is a critical regulatory point primarily controlled by the sequential activity of cyclin D-CDK4/6 and cyclin E-CDK2 complexes, which together phosphorylate RB. Once cells pass this checkpoint, they are committed to completing S, G2, and M phases regardless of external growth signals, hence it's often called the "point of no return." --- **Q5.** A tumor sample shows high levels of cyclin D1 but normal levels of CDK4. The cells still show uncontrolled proliferation. What is the MOST likely explanation? A) The CDK4 has gained kinase activity B) There is concurrent loss of p16INK4a C) The RB protein is mutated D) p21 levels are elevated E) E2F is constitutively active **Answer: B) There is concurrent loss of p16INK4a** **Explanation:** Even with normal CDK4 levels, high cyclin D1 can drive proliferation if p16INK4a (which normally competes with cyclin D for CDK4 binding) is lost. This dual hit – cyclin D1 amplification/overexpression plus p16 loss – is a common mechanism in many cancers to bypass G1/S control. --- **Q6.** In cells exposed to DNA damage, which sequence of events occurs FIRST? A) p21 transcription → CDK inhibition → cell cycle arrest B) p53 phosphorylation → p53 stabilization → p21 activation C) Apoptosis activation → caspase cascade → cell death D) RB dephosphorylation → E2F sequestration → S-phase block E) DNA repair protein recruitment → damage resolution **Answer: B) p53 phosphorylation → p53 stabilization → p21 activation** **Explanation:** DNA damage triggers immediate p53 phosphorylation and stabilization (preventing its MDM2-mediated degradation). Stabilized p53 then acts as a transcription factor to upregulate p21, which subsequently inhibits CDKs and arrests the cell cycle. ## 2. Cyclins and CDK Inhibitors **Q7.** A cancer cell line has a deletion in the gene encoding p27. Which of the following would be the PRIMARY consequence? A) Inability to exit from G0 phase B) Defective G2/M checkpoint C) Accelerated G1/S transition D) Impaired DNA repair E) Enhanced apoptosis sensitivity **Answer: C) Accelerated G1/S transition** **Explanation:** p27 (CDKN1B) is a CDK inhibitor that normally restrains cyclin E-CDK2 and cyclin A-CDK2 complexes, particularly in G1. Loss of p27 removes this brake, allowing accelerated progression through G1/S transition even in suboptimal growth conditions. --- **Q8.** Which cyclin-CDK complex is responsible for the breakdown of the nuclear membrane during mitosis? A) Cyclin D-CDK4 B) Cyclin E