Biology • Year 12 • Module 8 • Lesson 10
Cancer — Cell Cycle, Oncogenes, Tumour Suppressors and Metastasis
Build HSC Band 5–6 extended-response technique: synthesise data, molecular mechanisms, and real-world public health examples to evaluate claims about cancer causation and prevention.
1. Stimulus-based extended response — Australia, HPV and the near-elimination of cervical cancer (Band 5–6)
8 marks Band 5–6
Stimulus. Australia introduced the world’s first national HPV vaccination program (Gardasil, targeting HPV 6, 11, 16, 18) for school-aged girls in 2007, extended to boys in 2013. Combined with a national cervical screening program, Australia is projected to become the first country to eliminate cervical cancer as a public health problem — defined as fewer than 4 cases per 100,000 women per year — around 2028. Before the vaccination program, Australia recorded approximately 1,000 new cervical cancer diagnoses per year. By 2022, the annual incidence had fallen to under 700, with the rate continuing to decline. HPV strains 16 and 18 account for approximately 70% of cervical cancers worldwide.
The table below summarises the effect of the national HPV vaccination program on HPV-related disease incidence in Australian adolescents (girls, 12–17 years), adapted from Brotherton et al. (2020) Vaccine.
| HPV-related outcome | 2007 (pre-vaccine baseline) | 2019 (post-vaccine) | Reduction |
|---|---|---|---|
| High-grade cervical lesions (CIN 2+) per 100,000 | 397 | 29 | −93% |
| Genital wart diagnoses per 100,000 | 322 | 14 | −96% |
| HPV 16/18 prevalence in vaccinated cohort | 23% | 1.1% | −95% |
Source: Adapted from Brotherton et al. (2020) Vaccine 38(31): 4866–4875. Values are approximate.
Q1. Analyse and evaluate, using lesson content, why HPV 16/18 are so effective at initiating cervical cancer, why Australia’s three-pronged public health strategy (vaccination + screening + treatment) is necessary, and assess whether vaccination alone would be sufficient to eliminate cervical cancer by 2028.
In your answer you must:
- Explain the molecular mechanisms by which HPV E6 and E7 proteins initiate cancer, naming the specific tumour suppressors affected and the consequences of their loss.
- Explain why persistent HPV infection alone is insufficient for cervical cancer — linking to the multi-hit model of carcinogenesis.
- Use the data table to quantify the impact of vaccination on HPV-related disease in Australia.
- Evaluate all three components of Australia’s strategy against each other and reach a justified conclusion about whether vaccination alone could achieve elimination.
2. Evaluate this claim — the oncogene misconception (Band 5–6)
7 marks Band 5–6
“Cancer is caused by oncogenes that are switched on by carcinogens. The oncogene produces too much of its protein, flooding the cell with growth signals. Cancer could therefore be prevented simply by inhibiting the overproduction of these proteins. Tumour suppressor genes are irrelevant to this problem because they don’t affect how much protein the oncogene makes — they work in completely separate pathways.”
Source: invented student explanation.
Q2. Evaluate this claim. Identify which elements are biologically defensible, which are incorrect or oversimplified, and reformulate the claim into a scientifically accurate statement that properly integrates both oncogenes and tumour suppressors in the development of cancer.
Q1 — Sample Band 6 response (8 marks), annotated
HPV 16/18 initiate cervical cancer through a molecular mechanism that targets two of the most critical brakes on the cell cycle simultaneously. The HPV E6 protein binds to p53 (the “guardian of the genome”) and targets it for ubiquitin-mediated degradation, eliminating the G1/S checkpoint that would normally detect DNA damage and trigger either repair or apoptosis. The HPV E7 protein binds to RB1 (retinoblastoma protein) and prevents it from blocking entry into S phase. With both p53 and RB1 inactivated, infected cells divide without growth-signal restraint and cannot halt division in response to accumulating damage. [1 — E6/E7 mechanism; named tumour suppressors and consequences]
However, persistent HPV infection alone is not sufficient to cause cervical cancer. Cancer requires the accumulation of 4–8 driver mutations in a single cell lineage (the multi-hit model). HPV provides two critical hits (p53 and RB1 inactivation) but the cell must accumulate further somatic mutations in other genes before becoming fully malignant. Moreover, most HPV infections are cleared by the immune system within 1–2 years — only persistently infected cells have sufficient time to acquire the additional required mutations. [1 — multi-hit model and immune clearance explanation]
The data from Brotherton et al. (2020) demonstrate the dramatic impact of Australia’s vaccination program. In vaccinated girls aged 12–17, the prevalence of HPV 16/18 fell from 23% to 1.1% (−95%), high-grade cervical lesions (CIN 2+) fell from 397 to 29 per 100,000 (−93%), and genital warts fell from 322 to 14 per 100,000 (−96%). These reductions confirm that removing the biological carcinogen (HPV 16/18) is effective at preventing the precancerous changes that precede cervical cancer. [1 — quantified use of data]
However, vaccination alone is not sufficient to eliminate cervical cancer for three reasons. First, HPV 16/18 cause approximately 70% of cervical cancers — the remaining 30% are caused by other HPV strains not covered by the original quadrivalent vaccine (though the newer nonavalent Gardasil-9 covers additional strains). Second, there are large cohorts of women who were not vaccinated before first HPV exposure — screening (Pap smears and HPV testing) is the only tool to detect and remove precancerous lesions in this group. Third, even in vaccinated women, cervical screening provides a safety net for the small fraction in whom vaccination was incomplete or who carry strains not in the vaccine. [1 — reasons vaccination alone is insufficient]
Screening (now using primary HPV DNA testing in Australia) detects persistent HPV infection and high-grade lesions (CIN 2+) before they progress to invasive cancer. This directly intercepts the multi-hit pathway — removing pre-cancerous tissue before additional driver mutations can accumulate. Treatment (colposcopy, LLETZ, cryotherapy) removes confirmed lesions. Without these interventions, even in a highly vaccinated population, women infected with vaccine-escaped strains or diagnosed with pre-existing lesions would still progress to cervical cancer. [1 — role of screening and treatment in the pipeline]
On balance, Australia’s three-pronged strategy is necessary because each component targets a different stage of the carcinogenic pathway: vaccination removes the primary biological carcinogen before exposure (primary prevention); screening detects precancerous change before malignancy (secondary prevention); treatment eliminates confirmed lesions before invasion (tertiary prevention). Vaccination alone could approach but not fully achieve elimination by 2028, because an estimated 20–30% of future cervical cancers will occur in women not fully protected by the current vaccine formulation or who were exposed before vaccination. The combination of all three strategies is what places Australia on track for elimination. [1 — evaluative judgement: vaccination alone insufficient, three-pronged strategy necessary + justified conclusion]
Marking criteria.
- 1 mark — Correctly explains HPV E6 mechanism (E6 degrades p53; consequence: G1/S checkpoint lost, cells with damaged DNA cannot arrest or undergo apoptosis) and E7 mechanism (E7 inactivates RB1; consequence: cells enter S phase without restriction), with both tumour suppressors named.
- 1 mark — Applies the multi-hit model: HPV provides two hits (p53 + RB1 loss) but cancer requires 4–8 total driver mutations; immune clearance prevents most infections from progressing; only persistent infection + additional somatic mutations = cervical cancer.
- 1 mark — Uses at least two specific figures from the data table (e.g. CIN 2+ −93%, HPV 16/18 prevalence −95% or genital warts −96%) to quantify the vaccine’s impact.
- 1 mark — Identifies at least two reasons vaccination alone is insufficient (non-covered strains; unvaccinated cohorts; need for safety net) with specific reasoning.
- 1 mark — Explains the mechanistic role of screening (detects persistent HPV / precancerous lesions before invasion; intercepts the multi-hit pathway) and treatment (removes lesions before additional mutations accumulate).
- 1 mark — Evaluates all three components explicitly against each other (primary/secondary/tertiary prevention framework or equivalent) rather than simply listing them.
- 1 mark — Reaches a justified, evidence-based conclusion: vaccination alone could not fully achieve elimination by 2028; the three-pronged strategy is necessary and sufficient together, with specific reasoning (non-covered strains, unvaccinated cohorts, pre-existing infections).
- 1 mark — Uses precise lesson terminology throughout: biological carcinogen, tumour suppressor, p53, RB1, multi-hit model, metastasis (or its absence in pre-cancerous stage), apoptosis, carcinogen.
Q2 — Sample Band 6 response (7 marks), annotated
The claim is partially correct in one narrow respect but substantially incorrect in its mechanism, its scope, and its conclusion. [1 — clear evaluative opening]
What is defensible: The claim is correct that carcinogens (including chemicals, radiation, and biological agents such as viruses) can activate oncogenes by introducing mutations. Oncogenes do drive excessive cell division by generating growth signals. And targeted drugs do exist that block specific oncogene products — BRAF inhibitors (vemurafenib) and HER2-targeted antibodies (trastuzumab) are real examples of this approach. [1 — identifies the defensible element]
Error 1 — “too much protein”: The claim that oncogene mutations cause cancer by “producing too much protein” confuses quantity with regulation. In the most common oncogene, RAS, a single amino acid substitution locks the RAS protein in its GTP-bound active state regardless of growth factor signalling. The protein is not necessarily overproduced; it is constitutively active — unable to switch off. The problem is unregulated activity, not quantity. Gene amplification (e.g. HER2 in some breast cancers) does involve extra copies of the gene, but the paradigmatic RAS/BRAF oncogene mechanisms do not. [1 — corrects the overproduction error with precision]
Error 2 — “preventing cancer by inhibiting overproduction alone”: Cancer requires 4–8 driver mutations in a single cell lineage. Oncogene activation is typically only one or two of these. Tumour suppressor loss (recessive, requiring both alleles to be inactivated) is equally important — TP53 is mutated in ∼50% of all cancers and RB1 in many others. Inhibiting only the oncogene product leaves the lost tumour suppressor brakes unreplaced. Clinical experience with BRAF inhibitors confirms this: dramatic initial responses are followed by relapse as tumour cells acquire alternative driver mutations. Cancer treatment requires addressing multiple pathways simultaneously. [1 — refutes “inhibit overproduction alone would prevent cancer”]
Error 3 — “tumour suppressors work in completely separate pathways”: This is incorrect. p53 and RB1 directly interact with oncogene-driven cell cycle progression. p53 is activated in response to oncogene-induced hyperproliferative signals via the ARF/MDM2 pathway — the cell’s emergency brake when the accelerator is pressed too hard. Loss of both the accelerator regulation (oncogene) AND the emergency brake (tumour suppressor) is what enables fully malignant cancer. The HPV example from the lesson perfectly illustrates this: HPV simultaneously activates oncogene-like growth signalling AND inactivates both p53 and RB1 tumour suppressors, because neither alone is sufficient. [1 — refutes “separate pathways” claim with mechanistic detail]
Defensible reformulation: “Cancer arises from the accumulation of multiple driver mutations that simultaneously activate oncogenes (converting growth-promoting proto-oncogenes into constitutively active forms) and inactivate tumour suppressor genes (removing the cell cycle checkpoints and apoptosis signals that would normally limit uncontrolled division). Oncogene products are typically constitutively active rather than overproduced. Cancer prevention and treatment must address both oncogene activation and tumour suppressor loss, and must account for the multiple independent mutations required for fully malignant disease.” [1 — defensible reformulation using precise lesson terminology]
Marking criteria.
- 1 mark — States a clear overall evaluative judgement (partially correct but substantially incorrect; or equivalent).
- 1 mark — Correctly identifies the defensible element (carcinogens do activate oncogenes; oncogenes do drive cell division; targeted inhibitors do exist).
- 1 mark — Correctly refutes “too much protein” — specifies constitutive activity vs overproduction, with a named example (RAS, BRAF or HER2 amplification as the exception).
- 1 mark — Correctly refutes “inhibit overproduction would prevent cancer” — cancer requires multiple hits; oncogene inhibition alone leaves tumour suppressor loss unaddressed; clinical relapse data supports this.
- 1 mark — Correctly refutes “tumour suppressors work in completely separate pathways” — explains the mechanistic connection (p53 responds to oncogene-driven stress; tumour suppressor loss enables sustained oncogene-driven division; HPV as dual mechanism).
- 1 mark — Reformulates into a biologically defensible statement that integrates oncogene activation (constitutive activity) AND tumour suppressor loss (both alleles, recessive) as complementary requirements for malignant cancer, using precise lesson terminology.
- 1 mark — Response uses precise terminology throughout: proto-oncogene, constitutively active, gain-of-function/loss-of-function, two-hit hypothesis, p53/RB1, multi-hit model, apoptosis.