MCAT topics → Cell Division, Differentiation, and Specialization → Cancer — Oncogenes, Tumor Suppressors, Metastasis

Cancer — Oncogenes, Tumor Suppressors, Metastasis

MCAT trap: Applies the two-hit hypothesis to oncogenes rather than recognizing their dominant gain-of-function nature. Oncogenes act in a dominant gain-of-function manner — a single mutated allele is sufficient to promote uncontrolled proliferation.

Cancer biology on the MCAT comes down to understanding how normal cell division controls break down — and the molecular details matter. The core framework is two classes of genes: oncogenes, which are like stuck accelerators, and tumor suppressors, which are broken brakes. The exam tests this at multiple levels: pure recall (what does p53 do?), conceptual application (why does a single oncogene mutation matter but tumor suppressors need both alleles hit?), and passage-based inference (given a novel mutation in a regulatory gene, predict whether it drives or suppresses tumor growth). You need to be comfortable moving between all three modes in the same question block.

The trickiest part of this topic is keeping the dominance logic straight. Oncogenes are dominant gain-of-function mutations — one bad copy is enough to push proliferation. Tumor suppressors follow a loss-of-function logic, and Knudson's two-hit hypothesis explains why both copies must be inactivated. Students frequently mix these up, applying two-hit logic to oncogenes or treating tumor suppressors as if one mutation is sufficient. The MCAT will absolutely exploit this confusion, especially in passage questions that describe hereditary cancer syndromes where the first hit is germline.

Know your canonical examples cold: p53 and RB are tumor suppressors; RAS and MYC are oncogenes. The mistake students make with p53 is inferring oncogene status from the fact that it's mutated in most cancers — that's exactly backwards. And RAS is a GTPase that loses its off-switch when mutated, which is mechanistically different from simply overexpressing a receptor. These nuances show up in both standalone and passage questions.

Common misconceptions

Common mistake
Wrong: Oncogenes require both alleles to be mutated (like tumor suppressors) to drive cancer.
Right: Oncogenes act in a dominant gain-of-function manner — a single mutated allele is sufficient to promote uncontrolled proliferation.
The two-hit hypothesis applies specifically to tumor suppressors, not oncogenes. Oncogenes arise from proto-oncogenes via gain-of-function mutations — the mutant protein is hyperactive or constitutively active, and even one copy producing that aberrant signal is enough to drive proliferation. Think of it like one stuck gas pedal: it doesn't matter that the other copy is normal. Applying two-hit logic to oncogenes is one of the most reliable wrong-answer traps on this topic.
Common mistake
Wrong: p53 is an oncogene because its mutation is found in most human cancers.
Right: p53 is a tumor suppressor; its loss-of-function mutation is common in cancers because it normally halts the cell cycle and promotes apoptosis in response to DNA damage.
The logic here runs backward. p53 is mutated in most cancers precisely because it is a critical tumor suppressor — when it's working, it senses DNA damage and either halts the cell cycle for repair or triggers apoptosis. Losing p53 removes this checkpoint, which is why cancers accumulate mutations so readily. High mutation frequency in cancer = commonly lost brake, not a driver. That's the hallmark of a tumor suppressor, not an oncogene.
Common mistake
Wrong: Mutant RAS causes cancer by overexpressing a normal growth factor receptor.
Right: Mutant RAS is constitutively active (GTPase activity lost) and continuously signals proliferation regardless of upstream growth factor input.
Normal RAS is a GTPase signal relay: it turns on when bound to GTP and turns off when it hydrolyzes GTP to GDP. The oncogenic RAS mutation impairs GTPase activity, so the protein is stuck in the GTP-bound 'on' state and continuously signals cell proliferation regardless of whether a growth factor is present upstream. This is mechanistically distinct from overexpressing a receptor — the problem is a broken off-switch inside the cell, not more input signal from outside.
Common mistake
Wrong: The two-hit hypothesis requires both mutations to be somatic (acquired) events.
Right: In hereditary cancer syndromes, the first hit is a germline mutation present in every cell; only one additional somatic mutation is needed to inactivate the second allele.
In hereditary cancer syndromes like familial retinoblastoma, individuals are born with one defective RB allele in every cell of their body — that's the germline first hit. Only a single additional somatic mutation in any susceptible cell is needed to knock out the remaining functional allele. This is why hereditary cancer syndromes produce tumors earlier and more frequently than sporadic cases, where both hits must occur by chance in the same somatic cell. The 'two hits' refer to two alleles being inactivated, not two somatic events.
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What the exam tests

  1. Distinguish oncogenes (dominant, gain-of-function, one mutated allele sufficient) from tumor suppressors (recessive, loss-of-function, both alleles must be inactivated) and correctly apply the two-hit hypothesis only to tumor suppressors.
  2. Recognize and apply the hallmarks of cancer: sustained proliferative signaling, evasion of growth suppressors, resistance to apoptosis, replicative immortality, angiogenesis, and invasion/metastasis.
  3. Know the canonical gene examples — p53 (cell cycle arrest and apoptosis in response to DNA damage), RB (G1 checkpoint), BRCA1/2 (DNA repair) as tumor suppressors; RAS (constitutively active GTPase) and MYC (transcription factor driving proliferation) as oncogenes — and understand what each gene normally does and how its mutation drives cancer.
  4. Given a passage describing a mutation in a cell signaling or regulatory gene, predict whether the result is oncogenic transformation or tumor suppression, based on whether the mutation creates a constitutively active signal or eliminates a negative regulator.

Can you avoid these mistakes?

A patient inherits one mutated copy of the RB gene from a parent. How many additional mutational events are required for a retinal cell to become cancerous, and why? How would your answer differ if the mutation were in the RAS gene instead?
A passage describes a novel protein that normally binds to and degrades a transcription factor required for S-phase entry. A point mutation abolishes this binding. Is the mutated gene behaving as an oncogene or a tumor suppressor? What hallmark of cancer does this most directly relate to?
Why is p53 classified as a tumor suppressor rather than an oncogene, even though mutations in p53 are found in over 50% of human cancers? What does p53 normally do, and what happens when both copies are lost?
Mutant RAS and overexpression of a growth factor receptor both result in increased downstream proliferative signaling. What is the key mechanistic difference between them, and why does that difference matter for understanding how the mutation is dominant?

Related topics

Cell Cycle (G1, S, G2, M) and Checkpoints Mutation Types (Silent, Missense, Nonsense, Frameshift) Mitosis (Phases, Spindle, Cytokinesis) Meiosis I and II Spermatogenesis and Oogenesis

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