Cancer — Cell Cycle, Oncogenes, Tumour Suppressors and Metastasis

Activity 1 — Classify Oncogene or Tumour Suppressor

1. RAS mutation — oncogene, dominant. The RAS gene is a proto-oncogene encoding a GTPase signalling protein that normally cycles between active (GTP-bound) and inactive (GDP-bound) states — it is active only transiently when growth factors stimulate it. The mutation locks RAS in its GTP-bound active state (constitutive activation) because the GTPase activity that normally hydrolyses GTP to GDP is abolished. One mutant allele is sufficient (dominant) because the permanently active RAS protein signals for cell division continuously regardless of whether the other allele produces normal RAS. The cell receives a constant growth signal even without growth factors, driving continuous division. This is a gain-of-function, dominant oncogene mutation.

2. BRCA1 — tumour suppressor, two-hit hypothesis. BRCA1 is a tumour suppressor gene encoding a protein involved in DNA double-strand break repair (homologous recombination). The inherited non-functional allele is the first hit — present in every cell since birth. The somatic mutation in the breast epithelial cell is the second hit — it inactivates the remaining functional allele in that one cell. Now that cell has no functional BRCA1 protein. Without BRCA1, DNA double-strand breaks cannot be repaired accurately → mutations accumulate in other cancer-related genes → cell cycle regulation fails → breast cancer develops. This is a loss-of-function, recessive tumour suppressor mutation requiring both alleles to be inactivated.

3. HPV — biological carcinogen targeting two tumour suppressors. HPV uses both tumour suppressor inactivation mechanisms simultaneously. E6 protein binds to p53 and recruits a ubiquitin ligase complex that marks p53 for proteasomal degradation — the cell loses the ability to halt the cell cycle at the G1/S checkpoint in response to DNA damage, and loses the ability to trigger apoptosis in damaged cells. E7 protein binds to and inactivates the RB1 protein (retinoblastoma protein) — normally RB1 sequesters the transcription factor E2F, blocking S-phase entry. When HPV E7 inactivates RB1, E2F is released and cells continuously enter S phase regardless of cell cycle signals. HPV is a biological carcinogen because it is a living organism that promotes cancer through its proteins rather than through direct chemical or physical DNA damage.

4. Retinoblastoma — two-hit hypothesis. Inherited form: every retinal cell (and all other cells) already carries one inactivated RB1 allele from birth (first hit inherited). Cancer develops as soon as any retinal cell acquires a second somatic mutation in the remaining RB1 allele (second hit). Since there are millions of retinal cells, each requiring only one more mutation, the probability of this happening in multiple cells across both eyes is very high — explaining multiple bilateral tumours and early childhood onset. Non-inherited (sporadic) form: both RB1 alleles start functional. Cancer requires two independent somatic mutations to occur in the same retinal cell — both mutations must arise independently in the same cell lineage. This double event is statistically much less likely in any single cell, explaining the rarity, unilateral occurrence (only one cell achieves both mutations), and later onset of sporadic retinoblastoma.

Activity 2 — Clinical Scenarios

1. Metastatic bowel cancer — why surgery alone is not curative. Before the primary bowel tumour was surgically removed, cells from that tumour had already completed the early steps of metastasis — they had acquired mutations enabling them to detach from the primary tumour mass (loss of E-cadherin and gain of matrix metalloproteinase activity), penetrate blood vessel or lymphatic walls (intravasation), survive in the circulation, and extravasate into liver and lung tissue. These cells were circulating or had already colonised secondary sites before the surgery. Removing the primary tumour does not eliminate cancer cells already established elsewhere. Molecular requirements for liver and lung metastases: the bowel cancer cells must have acquired mutations in (1) cell adhesion molecules (e.g. E-cadherin loss) enabling detachment; (2) extracellular matrix proteases (e.g. matrix metalloproteinases) enabling invasion; (3) anoikis-resistance pathways enabling survival without attachment; (4) genes enabling survival against immune attack in circulation; (5) angiogenic factors (e.g. VEGF) enabling new blood vessel formation at secondary sites. Metastatic cancer requires treatment targeting multiple sites simultaneously (chemotherapy, immunotherapy, targeted therapy) rather than localised surgical excision.

2. BRCA1 mutation — cause vs susceptibility. BRCA1 normally encodes a protein critical for DNA double-strand break repair via homologous recombination — it maintains genome stability by accurately repairing breaks that arise during normal DNA replication and from environmental damage. When BRCA1 is lost, DNA breaks are repaired inaccurately (via error-prone pathways), causing genomic instability and accelerated accumulation of further mutations in other cancer-related genes. Why BRCA1 mutation does not directly cause cancer: carrying an inherited BRCA1 mutation means every cell in the body has one non-functional allele, but cancer only develops when a second somatic mutation inactivates the remaining allele in a breast (or ovarian) cell. Most cells never acquire this second hit. The mutation predisposes to cancer — it does not guarantee it. Why risk increases dramatically: in a person without the inherited mutation, two independent BRCA1-inactivating mutations must occur in the same cell — a double event requiring considerable chance. In a person with the inherited mutation, every breast epithelial cell already has one allele inactivated. Only one additional mutation is needed in any breast cell — statistically far more likely across a lifetime of cell divisions. The distinction between cause and susceptibility: a cause is necessary and sufficient for the disease; susceptibility means the probability of developing the disease is dramatically increased but not certain. BRCA1 mutations increase susceptibility by lowering the mutational barrier to tumour suppressor loss, without being sufficient alone to cause cancer.

Multiple Choice

1. C — Oncogenes: gain-of-function, dominant (one mutant copy drives division). Tumour suppressors: loss-of-function, recessive (both copies must be lost). Option A reverses the functions. Option B is wrong — both types are found in normal cells (proto-oncogenes and tumour suppressors are normal genes). Option D is wrong — oncogenes can also arise as somatic mutations; tumour suppressors can be inherited.

2. B — Mutant RAS sends a continuous division signal (no external growth factors needed), and without p53 the cell cannot halt for DNA damage or trigger apoptosis — a highly cancer-prone state. Option A is wrong — the normal RAS allele does not counteract a gain-of-function oncogene. Option C is wrong — p53 does not activate RAS. Option D is wrong — tumour suppressors inhibit division, not permit it.

3. D — HPV acts as a biological carcinogen by disabling p53 (via E6) and RB1 (via E7), progressively enabling mutation accumulation; HPV infection alone is not sufficient and most infections are cleared. Option A is wrong — HPV is directly mechanistically linked to cervical cancer. Option B is wrong — transformation requires additional mutations over years. Option C is wrong — HPV does not act via free radicals.

4. A — The defining feature of malignancy is invasion and metastasis — the ability to spread. Benign tumours may divide rapidly (Option B is wrong), can be large (Option C is wrong), and both can involve oncogene or tumour suppressor mutations (Option D is wrong).

5. C — Prophylactic mastectomy removes the breast epithelial tissue where the second somatic BRCA1 hit would most likely occur. The inherited first-hit mutation remains in all cells, and the patient retains elevated ovarian/other cancer risks, but the most vulnerable cell population is removed. Option A is wrong — the surgery logically reduces risk by removing the target tissue. Option B is wrong — the surgery does not remove the mutation. Option D is wrong — BRCA1 increases risk of multiple cancer types including ovarian.

Short Answer Model Answers

Q6 (4 marks): Normal p53 function: p53 (encoded by TP53) is a tumour suppressor gene encoding a transcription factor that acts as the cell's primary guardian against DNA damage. p53 detects DNA damage signals (e.g. from double-strand breaks, nucleotide mismatches, or oncogene activation) and responds in one of two ways: (1) if damage is repairable, p53 halts the cell cycle at the G1/S checkpoint by activating transcription of p21, which inhibits cyclin-dependent kinases and prevents entry into S phase — allowing DNA repair to occur before replication; (2) if damage is irreparable, p53 activates pro-apoptotic genes (e.g. BAX), triggering programmed cell death to eliminate the damaged cell before it can propagate mutations [2 marks]. When both TP53 alleles are mutated: the cell loses the ability to detect DNA damage and halt the cell cycle. Cells with DNA damage that would normally be repaired or eliminated now continue dividing — replicating damaged DNA and passing mutations to daughter cells. Each subsequent division can accumulate additional mutations in other cell cycle regulatory genes (proto-oncogenes, other tumour suppressors). Without p53-mediated apoptosis, heavily mutated cells that would normally be eliminated survive and continue contributing to the malignant cell population. The accumulation of mutations in multiple cell cycle regulatory genes is what progressively drives a cell toward fully malignant, metastatic cancer [2 marks — 4 marks total].

Q7 (5 marks): PAH carcinogens (tobacco smoke): (a) PAHs are metabolically activated in lung epithelial cells to reactive PAH diol epoxides, which form covalent adducts with guanine bases in DNA — specifically at G residues in TP53 codons 157, 248, and 273. (b) These adducts cause G→T transversion mutations during replication, directly mutating the TP53 gene and producing non-functional p53 protein [1 mark]. HPV (E6 and E7): (a) E6 viral oncoprotein binds to p53 and recruits ubiquitin ligase E6AP, tagging p53 for proteasomal degradation — inactivating it post-translationally without mutating the gene. E7 viral oncoprotein binds RB1, the retinoblastoma protein, preventing it from sequestering the transcription factor E2F — allowing continuous S-phase entry regardless of normal cell cycle signals. (b) p53 and RB1 are both inactivated simultaneously [1.5 marks]. Why combined effect is greater than either alone: p53 is disabled by BOTH mechanisms simultaneously in a smoker with HPV — direct mutation of TP53 by PAH carcinogens AND degradation of p53 protein by HPV E6. The combined loss is absolute and robust against any single compensatory mechanism. Additionally: PAH exposure generates ongoing DNA damage requiring p53 for detection and repair. With both p53 disabled AND RB1 inactivated, cells with PAH-induced DNA damage cannot halt the cell cycle, cannot repair the damage, and cannot undergo apoptosis. E7-released E2F drives cells directly into DNA synthesis while unrepaired PAH damage is copied. The combined disruption of multiple checkpoints by two independent mechanisms produces a multiplicative increase in mutation accumulation rate and cancer risk [1.5 marks]. p53 is the critical node connecting both insults — its absence is the pivotal event enabling both to produce maximal cancer risk simultaneously [1 mark — 5 marks total].

Q8 (6 marks): Specific mutations in melanoma progression: UV radiation (UVB) causes thymine dimers → C→T mutations in melanocyte DNA. The earliest and most common driver mutation is BRAF V600E — a C→T change at a CC dinucleotide in the BRAF proto-oncogene that produces a constitutively active kinase. BRAF V600E signals continuously for melanocyte proliferation without growth factor input. This alone is insufficient for malignancy — benign naevi (moles) frequently carry BRAF V600E. Additional mutations must accumulate: CDKN2A deletion inactivates p16 (RB1 pathway brake at G1/S), PTEN deletion enables pro-survival PI3K signalling, TP53 mutation eliminates apoptosis — and further mutations in cell adhesion molecules and proteases enable invasion [2 marks]. Why melanoma illustrates multi-hit model: each individual mutation provides a growth advantage but is not sufficient alone for malignant melanoma — BRAF V600E produces benign moles; losing CDKN2A alone may cause dysplastic naevi; only accumulation of 4–8+ driver mutations across multiple cell cycle regulatory systems produces a fully malignant, invasive, metastatic cell. This multi-hit requirement explains why melanoma takes years to develop despite constant UV exposure throughout life, and why melanoma predominantly arises in older individuals [2 marks]. Why early detection is critical given metastasis: before metastasis, melanoma is confined to the epidermis or superficial dermis — surgical excision with clear margins is curative in >99% of cases (stage I melanoma ~98% 5-year survival). Metastasis requires cells to acquire additional mutations enabling detachment (E-cadherin loss), extracellular matrix invasion (matrix metalloproteinases), intravasation, survival in circulation, extravasation, and secondary tumour establishment. Once these steps are complete and secondary tumours are established in lymph nodes, liver, lung, and brain, treatment must address multiple sites simultaneously. Five-year survival for stage IV (metastatic) melanoma remains approximately 30–50% despite recent immunotherapy advances. Early detection intercepts the disease before cells have acquired the full metastatic mutation set — when it remains a localised, surgically addressable problem rather than a systemic disease [2 marks — 6 marks total].