Cancer arises when a single cell acquires a somatic mutation that gives it a selective growth advantage, allowing it to outcompete its neighbors. Subsequent divisions of this founder cell produce a clone, which continues to accumulate additional mutations — a process called clonal evolution. Each new mutation that further increases growth or survival is positively selected. This Darwinian evolutionary process explains why cancers become progressively more aggressive and drug-resistant over time.
The mutational landscape of cancer reveals tens to hundreds of somatic mutations per cancer genome, but only a subset are driver mutations that confer selective advantage. The remainder are passenger mutations — neutral mutations that accumulated by chance. Identifying driver mutations is the focus of cancer genomics and the basis for targeted therapy.
The stepwise process by which a cancer cell lineage acquires successive mutations that confer selective growth advantages, analogous to Darwinian natural selection within a tissue.
Oncogenes are mutated or overexpressed versions of normal cellular genes called proto-oncogenes. Proto-oncogenes encode proteins that normally promote cell growth and division — including growth factors, growth factor receptors, signal transducers, and transcription factors. When proto-oncogenes are converted to oncogenes by gain-of-function mutations, chromosomal translocations, or gene amplification, they drive uncontrolled proliferation.
The best-studied example is Ras, a small GTPase that normally cycles between an active GTP-bound state and an inactive GDP-bound state. Oncogenic Ras mutations (most commonly at codons 12, 13, or 61) abolish GTPase activity, locking Ras in the active GTP-bound state and constitutively activating downstream proliferative pathways (Raf/MEK/ERK, PI3K/Akt). Ras mutations occur in approximately 30% of all human cancers.
A mutated proto-oncogene that promotes cell proliferation in a constitutive, growth-factor-independent manner via gain-of-function mutations, acting dominantly at the cellular level.
Src was the first oncogene identified (in Rous sarcoma virus). Growth factor receptors such as HER2/ErbB2 (amplified in ~20% of breast cancers) and EGFR (mutated in non-small-cell lung cancer) are also important oncogenes. The Philadelphia chromosome — a translocation between chromosomes 9 and 22 creating the BCR-ABL fusion kinase — drives chronic myelogenous leukemia (CML) and was the target for the first successful targeted cancer drug, imatinib.
Tumor suppressor genes normally restrain cell growth; loss-of-function mutations in both alleles are required to abrogate their function (the two-hit hypothesis of Knudson). In hereditary cancer syndromes, one allele is already mutant in the germline; a single somatic mutation in the remaining wild-type allele ("second hit") suffices to cause cancer.
The retinoblastoma protein (Rb) is the master brake of the cell cycle. In its hypophosphorylated state, Rb sequesters the E2F transcription factor, preventing entry into S phase. Mitogens activate cyclin-dependent kinases (CDK4/6-cyclin D) that phosphorylate and inactivate Rb, releasing E2F to drive S-phase gene expression. Rb is lost in retinoblastoma, osteosarcoma, and many other cancers.
p53 is the most frequently mutated gene in human cancer, altered in approximately 50% of all cases. p53 is a transcription factor that is activated by DNA damage, oncogene activation, and hypoxia. Activated p53 induces cell-cycle arrest (via p21/CDKN1A) and apoptosis (via Bax, PUMA, Noxa). Loss of p53 allows cells with damaged DNA to continue dividing, accelerating tumor progression. MDM2, an E3 ubiquitin ligase, normally targets p53 for degradation; MDM2 is itself a p53 target gene, creating a negative feedback loop.
Knudson's model stating that both alleles of a tumor suppressor gene must be inactivated for loss of function — in hereditary cancers, one allele is mutant in the germline and the second is lost somatically.
1. Which term describes the subset of somatic mutations in a cancer genome that actually confer a growth advantage?
2. In what approximate percentage of human cancers are Ras mutations found?
3. What molecular mechanism makes oncogenic Ras constitutively active?
4. The BCR-ABL fusion kinase is caused by a translocation between which two chromosomes?
5. According to Knudson's two-hit hypothesis, how many alleles of a tumor suppressor must be inactivated for cancer to develop?
6. Rb prevents cell-cycle entry into S phase by sequestering which transcription factor?
7. In approximately what percentage of all human cancers is p53 mutated?
8. MDM2 normally limits p53 activity by which mechanism?
9. Which type of mutation typically activates an oncogene versus inactivating a tumor suppressor gene?
10. p53 induces cell-cycle arrest partly by transcriptionally activating which CDK inhibitor?
Part 1 complete!
Having established the molecular basis of oncogenes and tumor suppressors, we now examine how cancers exploit these mutations to achieve the broader constellation of traits — the hallmarks of cancer — that enable tumor progression, metastasis, and evasion of therapy.
Hanahan and Weinberg described six core hallmarks of cancer, since expanded to ten. The original six are: (1) sustained proliferative signaling (oncogene-driven growth factor independence); (2) evasion of growth suppressors (loss of Rb, p53 pathways); (3) resistance to cell death (upregulation of Bcl-2, loss of pro-apoptotic signals); (4) replicative immortality (telomerase reactivation); (5) angiogenesis induction (VEGF upregulation); (6) invasion and metastasis (EMT, MMP upregulation).
Later additions include: reprogramming of energy metabolism (Warburg effect — aerobic glycolysis), evading immune destruction, tumor-promoting inflammation, and genome instability/mutation. These hallmarks are interrelated — genome instability accelerates the accumulation of mutations enabling the other hallmarks.
Normal somatic cells have a finite replicative lifespan due to telomere shortening — telomeres lose 50–100 bp per division. When telomeres reach a critical length, cells enter replicative senescence (p53/p21-mediated) or crisis (telomere fusions, chromosome instability). Telomerase is an RNA-dependent DNA polymerase that extends telomeres using its integral RNA component (hTERC) as a template and its catalytic reverse transcriptase subunit (hTERT). Telomerase is active in embryonic cells and stem cells but silenced in most adult somatic cells. Cancer cells reactivate telomerase (via hTERT promoter mutations or amplification) in ~85–90% of cancers, enabling unlimited replication.
An RNA-protein complex (reverse transcriptase + hTERC RNA template) that adds TTAGGG repeats to chromosome ends, counteracting telomere shortening and enabling replicative immortality.
Genome instability is an enabling characteristic that accelerates mutation acquisition. Microsatellite instability (MSI) results from defective mismatch repair (MMR); chromosomal instability (CIN) results from defects in chromosome segregation machinery. Tumors with high mutational burden (TMB) due to MSI or carcinogen exposure are particularly responsive to immune checkpoint immunotherapy.
Modern cancer therapy targets the hallmarks directly: kinase inhibitors (imatinib for BCR-ABL, vemurafenib for BRAF V600E) block oncogenic signaling; CDK4/6 inhibitors restore Rb-mediated cell-cycle control; PARP inhibitors exploit homologous recombination defects in BRCA1/2-mutant cancers (synthetic lethality). Immunotherapy with PD-1/PD-L1 or CTLA-4 checkpoint inhibitors releases cytotoxic T cells from tumor-induced suppression, achieving durable responses in multiple cancer types.
1. Which hallmark of cancer is directly enabled by telomerase reactivation?
2. The Warburg effect in cancer cells refers to which metabolic phenomenon?
3. PARP inhibitors are selectively lethal to BRCA1/2-mutant cancer cells via which principle?
4. Which growth factor is most important for the angiogenesis hallmark of cancer?
5. Imatinib (Gleevec) was designed to treat CML by targeting which molecular abnormality?
6. Microsatellite instability (MSI) results from defects in which DNA repair pathway?
7. PD-1/PD-L1 checkpoint inhibitors restore anti-tumor immunity by which mechanism?
8. In what percentage of cancers is telomerase reactivated?
9. Which of the following is classified as an "enabling characteristic" rather than a core hallmark of cancer?
10. CDK4/6 inhibitors (e.g., palbociclib) block cancer cell proliferation by which mechanism?
Part 2 complete!
Type your answer in each box, then click Check answer for feedback.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20