Cervical neoplasia and HPV
Carcinogens and risk factors for genital tract cancers; HPV and cervical neoplasia (CIN and screening)
Cancer arises when DNA damage (initiation) is followed by clonal growth (promotion). In the cervix the key agent is persistent high-risk human papillomavirus, whose E6 and E7 proteins disable p53 and pRB in the transformation zone, producing a long, detectable pre-invasive phase (CIN) that screening can find. CPSP asks carcinogen classes, HPV molecular biology, CIN grading, co-factors and the basis of screening and vaccination.
Chemical carcinogenesis
- Initiation: a single exposure causes a permanent, irreversible DNA mutation.
- Promotion: repeated exposure makes initiated cells proliferate without mutating DNA; early promotion is reversible, and a promoter given before the initiator causes no tumour (phorbol esters, croton oil). Unopposed oestrogen acts as a promoter on endometrium.
- Direct-acting carcinogens need no activation (alkylating agents such as melphalan). Indirect carcinogens need cytochrome P450 oxidation to reactive epoxides that form DNA adducts (benzo[a]pyrene, aflatoxin, aromatic amines, nitrosamines); cyclophosphamide is also a prodrug.
| Carcinogen | Cancer |
|---|---|
| Polycyclic hydrocarbons, nitrosamines (tobacco) | Lung, bladder, cervix (concentrated in cervical mucus) |
| Aromatic amines (β-naphthylamine, benzidine; dye, rubber industries) | Bladder urothelial carcinoma |
| Asbestos | Mesothelioma, lung |
| Nickel, chromium | Nasal sinus, lung |
| Vinyl chloride; aflatoxin | Liver angiosarcoma; hepatocellular carcinoma |
| Benzene | Leukaemia |
| Arsenic | Skin, lung, liver |
| Diethylstilboestrol in utero | Clear cell adenocarcinoma of the vagina |
| UV light | Skin: pyrimidine dimers (ionising radiation causes strand breaks) |
- Oncogenic viruses: HPV 16 and 18 (cervix, vulva, vagina, anus, penis, oropharynx), EBV, hepatitis B and C, HHV-8 (Kaposi), HTLV-1.
HPV biology
- A small, non-enveloped DNA virus. It enters through microabrasions to infect basal keratinocytes and completes its life cycle only as cells differentiate, shedding virions from the surface. It causes no viraemia or cell lysis and little inflammation, and E6/E7 dampen interferon, so it escapes immunity.
- Low-risk types 6 and 11: genital warts (condylomata acuminata), laryngeal papillomas, CIN 1. High-risk types 16, 18 (also 31, 33, 45): HPV 16 is commonest overall and in squamous carcinoma; HPV 18 is relatively more frequent in adenocarcinoma.
| Viral protein | Target | Effect |
|---|---|---|
| E6 | p53, tagged by E6-AP ubiquitin ligase for proteasomal breakdown | No G1 arrest or apoptosis; TP53 gene itself usually normal |
| E6 | Telomerase (TERT) switched on | Immortal cells |
| E7 | pRB, releasing E2F | Cells pass the G1/S checkpoint unchecked |
| E2 | Normally represses E6/E7 | Lost when the viral genome integrates into host DNA in cancer (episomal in low-grade lesions) |
- Loss of pRB feedback makes p16 accumulate: strong block-like p16 staining marks high-grade HPV lesions in cervix, vulva and oropharynx.
- Koilocytes (enlarged wrinkled nuclei with a perinuclear halo, upper layers) indicate productive infection.
- About 90% of infections clear within 1–2 years; persistence of high-risk HPV drives CIN 3 and cancer over years.
The transformation zone
- Everted columnar epithelium is replaced by squamous metaplasia, most actively in adolescence, pregnancy and with high parity. Immature dividing metaplastic cells are easily infected, so almost all CIN arises in the transformation zone.
- Co-factors (only in women with persistent high-risk HPV): early marriage and first intercourse, many partners, smoking (fewer Langerhans cells), high parity, long-term combined pill, immunosuppression (HIV, where cervical cancer is AIDS-defining; transplant), other STIs.
- HPV infects the whole lower genital tract, so VIN, VAIN and anal lesions can follow CIN.
CIN grading and cytology
| Grade | Extent of atypia | Cytology term | Behaviour |
|---|---|---|---|
| CIN 1 | Lower third | LSIL (mild dyskaryosis) | Productive infection; usually regresses |
| CIN 2 | Lower two-thirds | HSIL | May regress or progress |
| CIN 3 (carcinoma in situ) | More than two-thirds or full thickness | HSIL | Transforming infection; precursor of invasion |
- Dysplasia: high nuclear–cytoplasmic ratio, pleomorphism, hyperchromasia, mitoses, loss of polarity. In CIN the basement membrane is intact; once breached the lesion is invasive and can metastasise.
- Cervical cancer spreads into the parametrium, where the ureter lies 1.5–2 cm lateral to the cervix; bilateral ureteric obstruction causes uraemia, the commonest cause of death. Lymph spread is to parametrial, obturator, internal and external iliac, then common iliac and para-aortic nodes.
Screening, colposcopy and vaccination
- Screening is secondary prevention: early detection and treatment of pre-invasive disease. A smear must sample the transformation zone; only mature superficial cells suggest it was missed.
- Primary HPV testing is more sensitive than cytology for CIN 2 or worse with a very high negative predictive value, but less specific, so positives are triaged by cytology.
- Colposcopy: 5% acetic acid coagulates nuclear protein, so dense-nucleated CIN turns acetowhite (visual inspection with acetic acid in low-resource settings); Lugol's iodine stains glycogen-rich normal epithelium brown. Punctation (capillaries end-on) and mosaicism suggest CIN; atypical vessels suggest invasion.
- LLETZ excises the zone and provides histology (ablation gives no specimen); small rise in preterm birth.
- Vaccines contain recombinant L1 virus-like particles with no DNA; neutralising IgG blocks entry. They do not treat established CIN, because infected cells express E6/E7, not L1; vaccinated women still need screening.
Remember E6 destroys p53, E7 inactivates pRB: '6 with p53, 7 with RB'.
Exam trap An initiator mutates; a promoter only makes mutated cells divide, and must come after the initiator.
Exam trap CIN grade depends on how much of the epithelial thickness is atypical, not on the number of koilocytes.
Test yourself on this topic
5 of the 50 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Five years after a hysterectomy for CIN 3 with clear margins, a 48-year-old woman has a vault smear showing high-grade dyskaryosis, and biopsy confirms vaginal intraepithelial neoplasia. What best explains this?
Answer: HPV infects the whole lower genital tract
HPV infects the squamous epithelium of the whole lower genital tract, including the cervix, vagina, vulva and perianal skin. Removing the cervix does not remove infected vaginal epithelium, so new intraepithelial neoplasia can arise elsewhere in the same field. This is why women treated for CIN, especially by hysterectomy, need follow-up vault cytology. Vaginal and vulval neoplasia are more common in women with previous CIN.
Field effect of HPV
- HPV infects the whole lower genital tract
- Cervix, vagina, vulva, perianal skin
- New VAIN or VIN can arise later
- Vault smears after hysterectomy for CIN
Remember Remove the cervix, but the field remains.
Exam trap Clear cervical margins do not end follow-up after treatment for CIN.
Why the other options are wrong
- Cervical cells were implanted in the vault Implantation is not the usual mechanism.
- CIN 3 always spreads to the vagina Not inevitable; this is a field effect.
- The hysterectomy caused metaplasia Surgery does not cause neoplasia.
- A new low-risk HPV infection Low-risk types do not cause high-grade lesions.
Among women with persistent high-risk HPV infection in a village survey, those with seven or more full-term pregnancies had more invasive cervical cancer than those with one or two. What is thought to explain the effect of high parity?
Answer: Transformation zone stays everted longer
High parity is a recognised co-factor for cervical cancer in HPV-positive women. Repeated pregnancies keep the transformation zone everted on the ectocervix for longer, exposing more actively metaplastic cells to HPV and other agents. Hormonal changes and cervical trauma at delivery may add to this. It does not cause cancer without HPV infection.
High parity as a co-factor
- HPV is the cause
- Parity is a co-factor
- Everted transformation zone
- Hormones and birth trauma may add
Remember Many pregnancies keep the vulnerable zone exposed.
Exam trap Co-factors matter only in women who already have persistent high-risk HPV.
Why the other options are wrong
- Pregnancy destroys HPV antibodies Not a recognised effect.
- Labour introduces new HPV types Labour does not create new infections.
- High parity reduces cervical blood flow Not the mechanism.
- Breastfeeding raises cervical oestrogen Breastfeeding lowers, not raises, oestrogen.
A mother asks whether the HPV vaccine offered to her 11-year-old daughter could give her HPV infection. The vaccine contains particles that look like the virus but contain no DNA. What are these particles made of?
Answer: Virus-like particles of L1 protein
HPV vaccines are made from the major capsid protein L1, produced by recombinant technology, which self-assembles into virus-like particles. Because they contain no viral genome, they cannot cause infection or cancer. They induce high titres of neutralising IgG, which reaches the genital tract and blocks infection of basal cells. The vaccine is most effective given before sexual exposure.
HPV vaccine
- Recombinant L1 protein
- Forms virus-like particles
- No DNA: cannot infect
- Neutralising IgG blocks entry
Remember An empty shell of L1 protein.
Exam trap Vaccinated women still need screening; the vaccine does not cover every high-risk type.
Why the other options are wrong
- Live attenuated HPV 16 No live HPV vaccine exists.
- Inactivated whole HPV virions Whole virions are not used.
- Recombinant E6 and E7 proteins Used experimentally as therapeutic vaccines, not in prophylactic vaccines.
- HPV DNA plasmids DNA vaccines are not the licensed vaccines.
In low-grade CIN the HPV 16 genome is usually episomal, but in invasive carcinoma it is often integrated into host DNA. Integration typically breaks the viral genome at one gene, increasing E6 and E7 expression. Which viral gene is disrupted?
Answer: E2
The viral E2 protein normally represses transcription of the E6 and E7 genes. When the circular HPV genome integrates into a host chromosome, the break commonly falls within E2, so this control is lost. E6 and E7 are then expressed at high levels, degrading p53 and inactivating pRB, which promotes genomic instability and progression to cancer. L1 and L2 code for capsid proteins, and E4 is involved in virion release.
HPV integration
- Episomal in low-grade lesions
- Integrated in cancer
- Break disrupts E2
- E6/E7 overexpressed
Remember Lose E2, unleash E6 and E7.
Exam trap The capsid genes L1 and L2 are the vaccine targets, not the cancer drivers.
Why the other options are wrong
- L1 A capsid gene; its loss does not raise E6/E7.
- L2 The minor capsid protein gene.
- E4 Involved in virus release from superficial cells.
- E1 only E1 may be affected, but loss of E2 is what derepresses E6/E7.
A pharmacology lecture compares carcinogens used in or met during gynaecological practice. Which of the following damages DNA directly, without needing metabolic activation?
Answer: Melphalan, an alkylating agent
Direct-acting carcinogens are electrophiles that react with DNA without any metabolic step. Alkylating anticancer drugs such as melphalan and chlorambucil are examples, which is why survivors of ovarian cancer treated with them have a raised risk of later leukaemia. Most environmental carcinogens are procarcinogens: polycyclic hydrocarbons, aflatoxin, aromatic amines and vinyl chloride all need oxidation, mainly by cytochrome P450 enzymes, to become reactive. Cyclophosphamide is an exception among alkylating agents, needing hepatic activation.
Direct vs indirect carcinogens
- Direct: alkylating agents (melphalan)
- Indirect: need P450 activation
- Hydrocarbons, aflatoxin, aromatic amines
- Cyclophosphamide needs liver activation
Remember Alkylating drugs attack DNA as they are.
Exam trap Not every alkylating agent is direct-acting: cyclophosphamide is a prodrug.
Why the other options are wrong
- Benzo[a]pyrene from tobacco Needs P450 activation to an epoxide.
- Aflatoxin B1 from stored grain Activated by P450 to an epoxide in the liver.
- 2-naphthylamine from dye works Needs hepatic activation and release in urine.
- Vinyl chloride from plastics Activated by P450 to a reactive epoxide.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Neoplasia of the female genital tract, Carcinogens and risk factors for genital tract cancers; HPV and cervical neoplasia (CIN, screening)