Questions: Carcinogenesis and the Multi-Hit Hypothesis

Question 1

A cell acquires an activating KRAS mutation, enabling it to proliferate without external growth signals. Why does this cell not immediately become a cancer?

• A. KRAS is a passenger mutation and therefore has no functional effect on proliferation
• B. The cell still retains functional p53, Rb, apoptosis pathways, and other independent safeguards that can restrain or eliminate a rogue proliferating cell
• C. KRAS activation causes cells to terminally differentiate, opposing the drive to proliferate
• D. A single mutation is sufficient for a benign tumor, but a second hit to KRAS is required for malignancy

Answer: (B)

This is the core logic of the multi-hit model. KRAS mutation disables one layer of control (growth factor dependence), but cells have multiple independent safeguards — p53 monitors DNA damage, Rb gates entry into S phase, apoptosis eliminates damaged cells, and immune surveillance eliminates abnormal cells. A single driver mutation is insufficient because all the remaining checkpoints still function. Cancer requires sequential mutations that progressively disable each independent safeguard, which is why most cancers require 4–7 driver mutations accumulated over years or decades.

Question 2

In colorectal cancer, the sequence APC loss → KRAS activation → TP53 loss → 18q deletion progresses over roughly 10–15 years. What does this slow timeline most directly reflect?

• A. A rare single high-impact mutation that requires decades to occur spontaneously
• B. The accumulation of multiple driver mutations through clonal evolution, each providing a growth advantage that expands the clone before the next mutation arises
• C. The immune system's ability to hold colorectal cancer cells dormant for long periods
• D. The fact that colorectal mutations are mostly passenger mutations with individually weak effects

Answer: (B)

The multi-hit model predicts exactly this slow progression. Each driver mutation gives the clone a selective growth advantage, expanding the population and increasing the chance that a subsequent mutation will arise. The next hit then must occur in a cell already carrying all previous hits. Each step involves clonal expansion followed by a waiting time for the next rare event. The 10–15-year timeline is not a coincidence — it is a direct consequence of needing multiple sequential rare events. This slow progression is also why colonoscopy screening works: catching pre-cancerous polyps before they acquire later hits enables removal before malignancy.

Question 3

Most mutations found in a sequenced solid tumor are driver mutations that directly contribute to cancer progression.

• T. True
• F. False

Answer: False

False. Most mutations in solid tumors are passenger mutations — byproducts of the genomic instability that accumulates as cells divide rapidly and DNA repair becomes impaired. They are carried along in the expanding clone but do not themselves confer growth advantage. Only a small subset — typically the 4–7 driver mutations in oncogenes and tumor suppressor genes — directly promote proliferation, survival, or invasion. This distinction is clinically critical: targeted therapies like imatinib (BCR-ABL) and vemurafenib (BRAF V600E) work because they target specific driver mutations, not the passenger noise.

Question 4

The multi-hit model predicts that cancer incidence should increase dramatically with age, because older individuals have had more time to accumulate the required number of independent driver mutations.

• T. True
• F. False

Answer: True

True, and this prediction matches the observed epidemiology. If cancer requires 4–7 independent driver mutations, each occurring at some low rate per cell division, then the probability of accumulating all necessary hits in a single lineage rises steeply with age. The log-log plot of cancer incidence versus age shows approximately power-law behavior, consistent with the multi-hit model. The colorectal progression sequence — taking 10–15 years from first mutation to invasive carcinoma — illustrates how even after the first hit, substantial time is needed for subsequent hits to accumulate.

Question 5

Why is the distinction between driver mutations and passenger mutations clinically important for cancer treatment?

Model answer: Driver mutations are the functionally significant changes that directly confer growth advantage — they activate oncogenes (e.g., KRAS, BCR-ABL) or disable tumor suppressors (e.g., TP53, APC, BRCA1) in ways that drive proliferation, survival, or invasion. Passenger mutations are neutral bystanders accumulated through genomic instability; they are carried along but do not contribute to the cancer phenotype. Clinically, targeted therapies are designed to inhibit specific driver mutations: imatinib blocks the BCR-ABL kinase in CML, vemurafenib targets BRAF V600E in melanoma. Targeting passenger mutations would have no therapeutic effect. Identifying true drivers also guides prognosis, hereditary risk assessment (germline BRCA1/2 mutations), and selection of patients likely to respond to specific treatments.

The driver/passenger distinction also matters for understanding tumor evolution: when targeted therapies eliminate cells dependent on one driver, resistance can arise through additional driver mutations (e.g., KRAS mutations bypassing EGFR inhibitors). Passenger mutations are not under selection pressure in the same way and are less likely to be the source of resistance. This dynamic view of clonal evolution under selective pressure — including therapeutic pressure — is central to modern oncology.