Viral Oncogenesis — Questions

HPV-positive oropharyngeal cancer, driven mainly by HPV16, carries a markedly better prognosis than the tobacco-related HPV-negative cancers. It is rising in high-income countries, arises in the tonsil and tongue base, and uses p16 immunostaining as its surrogate marker.

It is not caused by the low-risk types 6 and 11, its incidence is increasing rather than falling, and p16 positivity is central to its diagnosis.

Human immunodeficiency virus (HIV) impairs the cell-mediated immunity that clears HPV, so people living with HIV clear the virus poorly, carry multiple high-risk types and progress to cancer faster. This is why they undergo more intensive cervical screening.

Clearance is slower, not faster, the natural history is clearly affected, infection is not confined to low-risk types, and there is no protective effect.

E2 normally represses the E6/E7 promoter; when integration of viral DNA disrupts the E2 gene, that repression is lost and E6 and E7 are expressed without restraint. This is the molecular switch from productive infection to transformation.

E6 and E7 then degrade p53 and inactivate the retinoblastoma protein, but E2 loss itself acts by derepression, not by degrading p53 directly or by any role in entry, capsid formation or virion assembly.

• E6 degrades p53: E6 recruits the cellular ubiquitin ligase E6AP to target p53 for proteasomal degradation, removing p53-dependent apoptosis and cell-cycle arrest.
• E7 inactivates Rb: E7 binds the retinoblastoma protein (Rb) and displaces it from the E2F transcription factors.
• E2F released: free E2F transcribes the S-phase genes, so the cell is driven into DNA synthesis.
• G1/S checkpoint overridden: the combined action pushes the cell across the G1/S restriction point without normal checkpoint control.
• Loss of both the Rb and p53 brakes together produces unchecked proliferation and the genomic instability that underlies transformation.

E6: recruits a cellular ubiquitin ligase to degrade p53, removing the G1 DNA-damage checkpoint and apoptosis, so damaged cells are not eliminated.

E7: binds and inactivates the retinoblastoma protein (Rb), releasing the E2F transcription factors and driving the cell from G1 into S phase unchecked.

Point in the cycle: both act at the G1/S transition, E6 removing the checkpoint brake while E7 forces entry.

Net effect: continuous, deregulated proliferation with genomic instability, and, where a high-risk type persists and integrates, progression to immortalisation and malignant transformation.

• High-risk HPV is very common and usually transient in young women: most infections clear spontaneously, so testing younger women returns many positives that never progress, driving overinvestigation and overtreatment of lesions destined to regress.
• Invasive cervical cancer is rare before age 25: the cancer takes years to develop from persistent infection, so screening earlier averts little cancer while causing harm.
• South Africa still starts at 25, below the WHO general-population floor of 30: the high HIV prevalence and the earlier, faster progression in women living with HIV justify beginning earlier than WHO advises for lower-prevalence settings.

• Classification: a small non-enveloped double-stranded DNA virus of the family Polyomaviridae, genus Alphapolyomavirus, with a circular genome of about 5 kilobases.
• Skin commensal: a near-universal component of the normal skin flora, acquired in childhood and carried lifelong without symptoms, with adult seroprevalence around 60 to 80%.
• Causal role in cancer: the established cause of about 80% of Merkel cell carcinomas, making it the only human polyomavirus with a proven oncogenic role; the remaining Merkel cell carcinomas are ultraviolet-driven and virus-negative.
• Oncogenic mechanism: in the tumour the viral genome is clonally integrated into host DNA and expresses a truncated large T antigen that retains the retinoblastoma protein (RB)-binding region but has lost the capacity to replicate the virus, so it drives proliferation, while the small t antigen supports transformation.
• Merkel cell carcinoma: an aggressive cutaneous neuroendocrine carcinoma presenting as a rapidly enlarging, painless red or violet nodule on sun-exposed skin of older or immunosuppressed people, identified histologically by a dot-like cytokeratin 20 pattern and treated in advanced disease with programmed death-1 (PD-1) or its ligand (PD-L1) checkpoint inhibitors.

A complete answer covers the taxonomy, how types are defined, and the clinical risk grouping.

Family and genera

HPV belongs to the family Papillomaviridae. More than 450 human types fall into five genera: Alphapapillomavirus (mucosal and some cutaneous, containing the oncogenic types), Betapapillomavirus (cutaneous, linked to skin cancer in epidermodysplasia verruciformis), Gammapapillomavirus, Mupapillomavirus and Nupapillomavirus (cutaneous).

How types are defined

A type differs by more than 10% in its L1 gene sequence from the nearest known type; smaller differences define subtypes and variants. Classification is genetic because the virus could not be cultured and raises little cross-reacting antibody.

High-risk versus low-risk

High-risk mucosal types (16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58 and 59) encode E6 and E7 proteins that inactivate p53 and the retinoblastoma protein and cause cancer; HPV16 and 18 cause most cervical cancer. Low-risk types (chiefly 6 and 11) cause anogenital warts and respiratory papillomatosis but not cancer.

A complete answer traces infection, persistence, integration, and the action of the E6 and E7 oncoproteins.

From infection to persistence

HPV infects the basal keratinocytes of the cervical transformation zone through a microabrasion. Most infections are productive and cleared within about two years; malignant risk arises only when a high-risk type persists.

Integration and loss of E2

Progression is associated with integration of viral DNA into the host genome, which typically disrupts the E2 gene. Because E2 represses the early promoter, its loss derepresses the E6 and E7 oncogenes, which are then overexpressed.

E6 and E7 drive transformation

E6 recruits the E6-AP ubiquitin ligase to degrade p53, disabling apoptosis and the DNA-damage response. E7 binds and inactivates the retinoblastoma protein (Rb), releasing E2F and forcing cell-cycle entry. Together they cause genomic instability and aneuploidy, and persistent expression drives graded cervical intraepithelial neoplasia (CIN) to invasive carcinoma over years to decades.

A complete answer follows the sequence from infection through persistence and integration to the loss of both tumour-suppressor brakes, and notes that the virus is necessary but not sufficient.

Infection and productive cycle

The virus reaches the basal keratinocytes through a microabrasion in the epithelium and establishes its genome as an episome. In a productive infection the viral life cycle is tied to keratinocyte differentiation, and most such infections are transient and cleared by cell-mediated immunity.

Persistence and integration

Transformation is the exception and requires a persistent infection with a high-risk type, chiefly HPV16 and HPV18. Progression is associated with integration of the viral genome into the host chromosome, which disrupts the E2 gene. Because E2 represses the viral oncogenes, its loss lets E6 and E7 be expressed without restraint.

Deregulated oncoprotein expression

E6 recruits the ubiquitin ligase E6AP to degrade p53, removing the apoptosis and arrest response, while E7 inactivates the retinoblastoma protein (Rb), releasing E2F and forcing the cell into S phase. The combined loss of both tumour suppressors drives continuous proliferation, genomic instability, and activation of telomerase.

Progression to cancer

These changes accumulate over years as the epithelium passes through progressively higher-grade intraepithelial neoplasia (CIN) to invasive carcinoma. Cofactors including persistence, immunosuppression (notably HIV co-infection) and smoking raise the risk, and only a small fraction of infected women develop cancer, which is why HPV is necessary but not sufficient.

A complete answer centres on clonal, cell-associated persistence.

Persistence and spread. After reverse transcription and integration, HTLV-1 makes little infectious free virus. It spreads between cells by direct contact at the virological synapse, and it persists chiefly by driving the infected T cell to divide, so the provirus is copied by the host’s own machinery and passed to both daughter cells (clonal, mitotic spread). Host replication, not the viral enzyme, therefore maintains the infection and keeps the genome stable.

Why reverse-transcriptase inhibitors do not help. Established infection is sustained by proliferation of already-infected clones, not by ongoing reverse transcription, so inhibitors of reverse transcriptase and integrase do not lower the proviral load or treat the associated diseases; their only role is post-exposure prophylaxis, before integration is established.

An other-high-risk result with normal triage cytology carries intermediate risk, so the woman repeats HPV testing in twelve months; only persistence or abnormal cytology triggers colposcopy.

Immediate colposcopy, large loop excision of the transformation zone (LLETZ), ten-year recall or hysterectomy all over- or under-treat an intermediate-risk result.

Adult T-cell leukaemia/lymphoma is marked by circulating multilobulated flower cells and by hypercalcaemia with lytic bone lesions.

Owl-eye inclusions point to cytomegalovirus, koilocytes to human papillomavirus, Reed-Sternberg cells to Hodgkin lymphoma, and heterophile antibodies to Epstein-Barr virus infectious mononucleosis.

Women living with HIV progress faster, so a negative high-risk HPV result is followed by five-yearly screening for life, not the ten-year interval used when HIV-negative.

Ten years is the HIV-negative recall; annual or one-off screening has no place in the primary HPV algorithm.

HPV18 sits in the highest-risk channel (with HPV16 and HPV45), which goes straight to screen-and-treat. With no visible lesion on visual inspection with acetic acid (VIA), the primary-care treatment is thermal ablation, with HPV follow-up at twelve months.

Recall, delayed repeat, cytology alone or no treatment all under-manage a highest-risk result.

The other-high-risk channel carries intermediate risk, so it is triaged with reflex cytology run on the same sample; only abnormal cytology leads to colposcopy and treatment.

Immediate large loop excision of the transformation zone (LLETZ), routine recall, a five-year repeat or colposcopy for everyone all mismanage an intermediate-risk result.

HPV16, with HPV18 and HPV45, is the highest-risk channel and goes straight to screen-and-treat: assessment under visual inspection with acetic acid (VIA), then thermal ablation if no lesion is seen or colposcopy and large loop excision of the transformation zone (LLETZ) if a lesion is present.

Recall, a twelve-month repeat, reflex cytology first or no action would all delay treatment of a highest-risk result.

A negative high-risk HPV result has a high negative predictive value, so an HIV-negative woman returns at roughly ten-year intervals.

The shorter intervals belong to women living with HIV (five years); annual, one-year or three-year recall is not used after a negative primary HPV test.

A failed cellular control means the sample may not have contained adequate cervical cells, so a negative HPV result is unreliable and the test is repeated. A positive HPV signal stays valid even if the cellular control is out of range.

Reporting the unreliable result either way, or referring straight for colposcopy, or ignoring the control all risk a false-negative screen.

Infectious agents account for roughly 13% of cancers worldwide, about 2.2 million new cases a year. Oncogenic viruses make up the majority of that figure, with the bacterium Helicobacter pylori contributing most of the remainder.

The share is higher in low- and middle-income countries, where most virus-attributable cancers arise.

South Africa starts primary HPV screening at age 25 for all women, regardless of HIV status, deliberately lower than the WHO general-population floor of 30 because of the high HIV prevalence and the early-onset disease that follows it.

The other ages fall outside the South African guideline.

The WHO elimination threshold is an age-standardised incidence below 4 per 100,000 women, the goal of the 90-70-90 strategy for 2030.

The other figures are either below the target or well above it.

HPV18 integrates more readily and is over-represented in cervical adenocarcinoma, whereas HPV16 is more replicative and dominates squamous cancers.

Genital warts are caused by low-risk types such as 6 and 11.

EBV-positive gastric cancer is a distinct subtype with amplification of the PD-L1 immune-checkpoint ligand (and JAK2), against a largely wild-type p53 background, marking the virus as a driver.

Ultraviolet and aflatoxin signatures and BCR-ABL belong to other cancers.

In the equatorial African belt, holoendemic Plasmodium falciparum malaria drives B-cell proliferation and weakens EBV-specific immunity, raising the chance of the c-myc translocation that defines Burkitt lymphoma.

Aflatoxin, ultraviolet light, tobacco and alcohol are cofactors for other cancers.

LMP1 behaves as a constitutively active CD40 receptor, signalling without ligand to drive nuclear factor kappa B (NF-kB) and JAK-STAT survival pathways.

LMP2A mimics the B-cell receptor and EBNA2 mimics Notch.

Integration commonly disrupts the viral E2 gene, and because E2 represses E6 and E7, its loss lets these oncoproteins be expressed without restraint.

HPV carries no captured host oncogene, and the cancer cells produce no virus.

LMP1 behaves as a constitutively active CD40 receptor, signalling without any ligand and without T-cell help, and so drives the survival and proliferation pathway NF-kB. It is the principal transforming protein of Epstein-Barr virus.

The B-cell-receptor mimic is LMP2A, not LMP1; LMP1 does not directly inhibit p53, amplify c-myc or mimic the interleukin-6 receptor.

The L1 capsid protein self-assembles into virus-like particles that elicit neutralising antibodies, preventing infection.

The vaccine is prophylactic, not therapeutic: it neither cures established infection, clears integrated DNA, nor kills tumour cells.

E7 binds and degrades the retinoblastoma protein (Rb), releasing the E2F transcription factors and forcing the cell into S phase.

p53 is the target of E6, not E7; the others are not the transforming target of E7.

HTLV-1 causes adult T-cell leukaemia/lymphoma (an aggressive CD4 T-cell malignancy) and HTLV-1-associated myelopathy / tropical spastic paraparesis (HAM/TSP), a chronic inflammatory disease of the spinal cord.

The other pairs belong to KSHV, the Kaposi sarcoma-associated herpesvirus (Kaposi sarcoma and primary effusion lymphoma), Epstein-Barr virus (Burkitt lymphoma and nasopharyngeal carcinoma), the hepatitis viruses (hepatocellular carcinoma) and human papillomavirus (cervical and anal cancer).

HTLV-1, the human tumour retrovirus, causes adult T-cell leukaemia/lymphoma after a latency of decades in a small fraction of carriers.

The other malignancies are driven by different viruses.

Restoring immunity is decisive: antiretroviral therapy is started immediately, whatever the CD4 count, and often causes regression, with liposomal doxorubicin reserved for advanced disease.

Splenectomy, rituximab monotherapy and aciclovir have no role, and withholding treatment worsens outcomes.

In endemic areas KSHV spreads chiefly through saliva, horizontally and within families from early childhood.

In low-prevalence Western settings it spreads mainly sexually, efficiently among men who have sex with men; transfusion, faecal-oral and vector routes are not the endemic pattern.

Integration selects a truncated large T that keeps the Rb-binding region but loses the replication function, so the virus drives proliferation precisely because it can no longer complete its life cycle.

The antigen is retained in the tumour cells, not absent, secreted, or fused to MYCL.

The middle target is that 70% of women are screened with a high-performance test by age 35 and again by 45.

The two 90s flank it: 90% of girls fully HPV-vaccinated by 15, and 90% of women with cervical disease treated. Cure rates and cytology coverage are not among the targets.

KSHV is present in essentially every Kaposi sarcoma lesion and is its necessary cause. The lesions behave as a virus-driven angioproliferative process rather than a classic monoclonal tumour.

The lower proportions understate what is effectively a complete association.

KICS is a severe systemic inflammatory illness with high interleukin-6 and interleukin-10 and a high KSHV blood viral load, often with Kaposi sarcoma but without the lymph-node pathology of multicentric Castleman disease.

It is neither a discrete tumour nor a congenital, inherited or vaccine-related condition.

Epstein-Barr virus, a lymphocryptovirus, is the only other human gammaherpesvirus; like KSHV it establishes latency in lymphocytes and can cause B-cell lymphoma.

Cytomegalovirus and human herpesvirus 6 are betaherpesviruses; herpes simplex virus and varicella-zoster virus are alphaherpesviruses.

Because cell-free virus is scarce, HTLV-1 is detected and quantified by PCR of integrated proviral DNA, not by RNA. Serological screening by enzyme immunoassay is confirmed by Western blot or line immunoassay, which also distinguishes HTLV-1 from HTLV-2.

Merkel cell polyomavirus is a non-enveloped double-stranded DNA polyomavirus (genus Alphapolyomavirus) and the only human polyomavirus with a proven causal role in a human cancer, Merkel cell carcinoma.

It is not an RNA virus, a herpesvirus, a retrovirus or a papillomavirus.

Both host interleukin-6 and the KSHV-encoded viral homologue drive the polyclonal lymphoproliferation of multicentric Castleman disease, which is the rationale for rituximab and for anti-interleukin-6 agents such as siltuximab.

The other cytokines listed are not the principal driver of this disorder.

Nasopharyngeal carcinoma is over 95% Epstein-Barr virus (EBV)-positive, and it clusters in East and Southeast Asia against host genetic and dietary cofactors.

The other viruses drive different tumours and are not found in nasopharyngeal carcinoma.

Primary effusion lymphoma is a KSHV-positive lymphoma arising from monoclonal plasmablastic B cells, often co-infected with Epstein-Barr virus, that presents as malignant effusions in body cavities without a solid mass.

It is usually HIV-positive, is not human-papillomavirus-driven, forms no solid nodal mass, and is of B-cell rather than T-cell origin.

A virus supplies only some of the steps to malignancy; host mutations and cofactors accumulated over time supply the rest, so the virus is necessary but not sufficient.

The long latency and low penetrance exclude a purely inherited, single-step, or virus-independent process.

The lifetime risk of adult T-cell leukaemia/lymphoma is about 5%, and of HAM/TSP about 2%; most carriers remain asymptomatic for life. The leukaemia is strongly linked to infection acquired in infancy through breastfeeding.

Infant hepatitis B vaccination reduces hepatocellular carcinoma, the first demonstration that a vaccine can prevent a human cancer and strong proof of the causal link.

It is a prophylactic subunit vaccine, not therapeutic, live, mRNA, or active against hepatitis C.

Gardasil 9 covers HPV 6, 11, 16, 18, 31, 33, 45, 52 and 58; its seven oncogenic types account for about 90% of cervical cancers.

The bivalent and quadrivalent vaccines (types 16 and 18) cover close to 70%, so the nonavalent gain is real but not total.

The proviral load, reflecting the number of infected clones, reaches a stable set point that varies more than a thousandfold between people and is the best predictor of progression to both the leukaemia and the myelopathy.

All forms of Burkitt lymphoma share a translocation that places the c-myc oncogene under the control of an immunoglobulin gene, classically t(8;14), which drives constitutive proliferation. It arises as an off-target accident of the antibody-gene machinery during the germinal-centre reaction.

t(14;18) defines follicular lymphoma, t(9;22) chronic myeloid leukaemia, t(11;14) mantle-cell lymphoma; EBNA2 amplification is not the lesion, though EBV contributes in the endemic form.

All Burkitt lymphomas carry a translocation, classically t(8;14), that places c-myc under an immunoglobulin promoter. EBV contributes by blocking the apoptosis that Myc overexpression would otherwise trigger.

BCR-ABL, BCL2 translocation, EGFR amplification and TERT-promoter mutations define other tumours.

Epstein-Barr virus growth-transforms resting B lymphocytes, driving them to proliferate indefinitely as lymphoblastoid cell lines; this immortalising capacity is the basis of its oncogenic potential. In the host it is restrained by EBV-specific T cells, which is why transformation manifests as disease where that control fails.

It does not lyse B cells on contact, does not integrate, is not confined to epithelium, and does not need malaria to grow (malaria is a cofactor for endemic Burkitt lymphoma specifically).

HTLV-1 spreads by cell-to-cell contact, and breastfeeding is the principal route of mother-to-child transmission; avoiding it reduces vertical spread.

Sexual and parenteral routes transmit between adults but are not the main vertical route.

A single HPV test detects around 90% of high-grade precancer and a negative result is highly reassuring, giving higher sensitivity and negative predictive value (NPV) than cytology and letting screening intervals lengthen safely. Its trade-off is lower specificity, which genotyping and triage address.

HPV testing is not more specific, cheaper or more subjective, and it detects viral DNA rather than cellular changes.

Direct-acting antivirals cure over 98% of infections, and cure measurably lowers the subsequent risk of hepatocellular carcinoma, the cleanest demonstration that removing a virus prevents cancer.

There is no licensed hepatitis C vaccine, and the other options do not by themselves establish causation.

WHO recommends starting primary HPV screening at age 30 in the general population, prioritising ages 30 to 49.

South Africa deliberately lowers the start to 25; the younger and older ages fall outside both recommendations.

Essentially every cervical cancer carries high-risk HPV, making it the strongest virus-cancer association known.

The lower figures understate a relationship that is, for practical purposes, complete.

Integration drives insertional activation of cell-cycle and immortalisation genes and yields a truncated HBx protein, a direct route that can cause cancer without the cirrhotic background the inflammatory route needs.

Inflammation, high viral load, hepatitis D co-infection and aflatoxin all raise risk but act through the cirrhotic pathway.

In direct oncogenesis the viral genome is present in essentially all tumour cells, and a continuously expressed oncoprotein maintains transformation.

Chronic inflammation and a dependence on immunosuppression describe the indirect routes.

Nasopharyngeal carcinoma is EBV-positive in essentially every tumour cell regardless of geography, and runs the latency II programme (Epstein-Barr nuclear antigen 1 (EBNA1), LMP1, LMP2 and the EBV-encoded small RNAs (EBERs)). It clusters in southern China and in middle-aged men, and plasma EBV DNA and a rising immunoglobulin A (IgA) antibody to the viral capsid antigen track and even precede the tumour.

Latency 0 expresses no proteins and is the resting reservoir; the heterophile test belongs to acute mononucleosis; the c-myc translocation defines Burkitt lymphoma, not nasopharyngeal carcinoma.

The epidemic, AIDS-associated form became the commonest cancer of the HIV epidemic and is frequently aggressive and visceral.

The classic, endemic and iatrogenic forms arise in other settings, and primary effusion lymphoma is a separate KSHV-associated malignancy, not a form of Kaposi sarcoma.

E6 recruits the cellular ubiquitin ligase E6AP to degrade p53, removing the apoptosis and arrest response.

E7 targets the retinoblastoma protein, E5 hyperactivates the epidermal growth factor receptor, L1 is the capsid protein, and E2 represses E6 and E7.

HBZ, encoded on the antisense 3’ long terminal repeat, is expressed continuously and is the one viral protein always present in leukaemic cells.

Tax initiates transformation but is often silenced later; Env, Gag and Rex are structural or regulatory proteins, not clonal maintainers.

Immunohistochemistry for the latency-associated nuclear antigen (LANA) shows a characteristic stippled nuclear pattern and is the confirmatory test across all KSHV diseases.

p16 marks high-risk human papillomavirus disease, CD20 is a pan-B-cell marker, Ki-67 measures proliferation, and no glycoprotein B stain is used for this purpose.

RTA, encoded by ORF50, is sufficient on its own to initiate lytic replication; in latency the latency-associated nuclear antigen (LANA) represses its promoter to keep the virus quiescent.

LANA, viral cyclin and viral FLIP are latent proteins, and K8.1 is a lytic glycoprotein rather than the switch itself.

Human immunodeficiency virus carries no oncogene and transforms no cell itself. It raises cancer risk only indirectly, by depleting CD4 T cells and removing the immune surveillance that holds the other oncogenic viruses in check.

The other four are direct carcinogens that express viral oncoproteins within the tumour cell: HPV (E6 and E7), Epstein-Barr virus (LMP1 and EBNA), Merkel cell polyomavirus (truncated large T) and Kaposi sarcoma-associated herpesvirus (its pirated cell-cycle and survival genes).

Hepatitis C has no DNA stage, does not integrate, and encodes no oncogene, and its proteins are not found in all tumour cells; it causes liver cancer through decades of inflammation and cirrhosis.

HPV, HBV, EBV and Merkel cell polyomavirus all express oncoproteins or integrate.

The cell-mediated immunodeficiency of HTLV-1 infection predisposes to Strongyloides stercoralis hyperinfection (and to tuberculosis); disseminated strongyloidiasis is a classic association and worsens the prognosis of adult T-cell leukaemia/lymphoma.

The other infections listed are the opportunists of HIV-related immunosuppression rather than the characteristic HTLV-1 association.

A high-risk HPV DNA test has replaced cytology as the primary screen, because it detects the cause of disease before cellular changes appear and is more sensitive.

Cytology is now a triage test for HPV-positive samples; visual inspection, colposcopy and serology are not primary screens.

Human papillomavirus causes roughly half of all virus-attributable cancers, driven above all by cervical cancer.

Hepatitis B is second, then hepatitis C and Epstein-Barr virus; KSHV and HTLV-1 together account for only a small share.

Tax is highly immunogenic, so cytotoxic T cells select against cells that express it. Silencing the 5’ long terminal repeat shuts Tax off and lets the infected cell persist, while HBZ on the 3’ repeat continues.

Silencing lowers rather than raises virion production and is unrelated to integration or a lytic cycle.

Once established, HTLV-1 is maintained by the mitotic proliferation of infected T-cell clones rather than by reverse transcription of new virus, so inhibitors of reverse transcriptase and integrase do not reduce the proviral load. They have a role only in post-exposure prophylaxis, before the provirus integrates.

HTLV-1 is an RNA retrovirus rather than a DNA virus, the failure is intrinsic to clonal persistence rather than resistance or dosing, and the virus is lymphotropic, not sequestered in neurons.

HPV45 is the next most oncogenic type after 16 and 18 and is strongly linked to cervical adenocarcinoma, which cytology screens poorly. Reporting it individually lets the algorithm send it straight to treatment.

It is neither low-risk nor non-oncogenic, is detected by the assay, and its individual reporting is unrelated to vaccine cross-reactivity.

A minority of cells in a lesion express lytic and paracrine genes whose secreted angiogenic factors (viral G-protein-coupled-receptor-driven VEGF, viral interleukin-6) drive proliferation in neighbouring cells, so the lesion is polyclonal and regresses when immune control returns.

Kaposi sarcoma is therefore not a monoclonal, single-oncogene or integration-driven malignancy, and it is unequivocally KSHV-associated.

• Higher sensitivity: HPV testing detects the cause of disease before cellular changes appear and finds around 90% of high-grade precancer, more than a single cytology slide.
• High negative predictive value: because nearly all cervical cancer is HPV-driven, a negative result is highly reassuring and lets screening intervals be safely lengthened.
• Objective and reproducible: it is a defined laboratory result rather than a subjective slide read.
• Scalable: it runs on automated high-throughput platforms.
• Better access: it can be performed on a self-collected sample, reaching women who do not attend for a speculum examination. Genotyping the highest-risk types then offsets the lower specificity of testing for a frequently transient infection, and cytology is retained as a triage test on HPV-positive samples.

KSHV underlies all three; they separate cleanly on cell of origin, clonality and clinical behaviour.

Kaposi sarcoma occurs in four epidemiological forms (classic, endemic African, iatrogenic, AIDS-associated). Primary effusion lymphoma is universally KSHV-positive and usually co-infected with Epstein-Barr virus. Multicentric Castleman disease is KSHV-associated in essentially all HIV-positive cases, with the blood KSHV viral load tracking activity.

A complete answer traces the route into the memory B-cell reservoir, then maps the programmes onto the tumours.

Establishing latency. After entry into a B cell (gp350/220 binding CD21), the virus runs its full growth programme (latency III) to drive the cell to proliferate. The infected cell passes through the germinal centre and settles as a resting memory B cell, which expresses essentially no viral genes (latency 0) and so is invisible to the immune system, expressing EBNA1 only when it divides (latency I). This memory B-cell pool is the lifelong reservoir; reactivation to the lytic cycle occurs when such a cell differentiates into a plasma cell.

Mapping programmes to tumours. The programme a tumour runs reflects its origin and its visibility to T cells:

• Latency I (EBNA1 only): Burkitt lymphoma and gastric carcinoma.
• Latency II (adds LMP1 and LMP2): nasopharyngeal carcinoma and Hodgkin lymphoma.
• Latency III (the full growth programme): the lymphomas of immunosuppression, including post-transplant and HIV-associated disease, which can only persist where T-cell control has failed.

A complete answer links seroprevalence and transmission to the four clinical forms.

Seroprevalence and transmission. KSHV seroprevalence is low in the West (a few %), intermediate around the Mediterranean, and very high in sub-Saharan Africa (approaching half of adults in places). It spreads mainly by saliva in childhood in endemic areas and sexually (notably among men who have sex with men) in low-prevalence settings, with inefficient transplant and transfusion transmission.

The four forms of Kaposi sarcoma. Classic (indolent, elderly Mediterranean men); endemic African (adults and children, pre-AIDS, sometimes aggressive); iatrogenic (transplant immunosuppression, regresses when it is reduced); and epidemic, AIDS-associated (advanced HIV, aggressive and often visceral, the defining cancer of the epidemic).

A complete answer groups the strategies rather than listing viruses, and ends with the point that a virus is necessary but not sufficient.

Inactivating the tumour-suppressor pathways

The direct DNA tumour viruses force a resting cell into S phase by disabling the two master tumour suppressors together: the retinoblastoma protein (Rb), which restrains cell-cycle entry, and p53, which triggers arrest or apoptosis in response to inappropriate proliferation. High-risk HPV does this with E7 against Rb and E6 against p53; the polyomavirus large T antigen engages both; adenovirus E1A and E1B define the paradigm.

Driving constitutive proliferative and survival signalling

Rather than degrade tumour suppressors, the herpesviral and retroviral oncoproteins mimic the cell’s own growth signals: Epstein-Barr virus LMP1 acts as a ligand-independent CD40 receptor driving survival signalling, KSHV deploys pirated cytokine and cell-cycle genes, and HTLV-1 Tax switches on host proliferation pathways.

Insertional mutagenesis

The retroviruses integrate a DNA copy of their genome into the host chromosome. Each integration is in effect a mutation, and a provirus landing beside a host proto-oncogene can activate it, producing a clonal tumour.

Indirect mechanisms

Where no oncoprotein is supplied, transformation proceeds through chronic inflammation and regeneration, which accumulate host mutations (hepatitis B and C), or through loss of immune surveillance over latently infected cells.

Necessary but not sufficient

In every case the virus supplies only some of the steps to malignancy; host mutations and cofactors are also required, which is why cancer is clonal, late, and develops in only a small fraction of those infected.

A complete answer explains the surveillance mechanism, names the virus-tumour pairs, and notes the effect of restoring immunity.

Loss of surveillance. The latent oncogenic viruses persist for life in most adults but are normally held in check by T-cell surveillance, which removes infected and transformed cells. When that surveillance fails, in advanced HIV infection, after transplant immunosuppression, or in primary immunodeficiency, the latently infected cells proliferate unchecked and the associated cancers emerge.

The virus-tumour pairs. The characteristic pairings are Kaposi sarcoma-associated herpesvirus with Kaposi sarcoma and primary effusion lymphoma; Epstein-Barr virus with the B-cell lymphomas, including post-transplant lymphoproliferative disorder and the central nervous system lymphoma of advanced HIV; and high-risk human papillomavirus with cervical and other anogenital cancers, which are more common and more aggressive in women living with HIV.

HIV’s role and the effect of treatment. HIV itself encodes no oncogene and transforms no cell; its entire oncogenic contribution is the depletion of CD4 T cells and the resulting loss of control over the other viruses. Restoring immune function with antiretroviral therapy reduces the incidence of these cancers, though it does not abolish it.

A complete answer names the seven recognised tumour viruses, pairs each with its principal cancer, and applies the direct or indirect distinction consistently.

Human immunodeficiency virus is the eighth recognised carcinogen but is not a tumour virus: it encodes no oncogene and raises cancer risk only indirectly, through the immunosuppression that unmasks the viruses above. A direct carcinogen expresses an oncoprotein within the tumour cell, with the viral genome present in essentially all tumour cells; an indirect carcinogen promotes cancer through chronic inflammation or loss of immune surveillance.

A complete answer covers the test, who is screened and when, and the genotype-directed actions.

Test and population. Primary screening is a high-risk HPV DNA test for all women from age 25, regardless of HIV status. A clinician-collected sample is taken into liquid medium so reflex cytology can be run from the same vial, and self-sampling is offered to widen coverage.

Interval. HPV-negative women return at routine intervals: about ten years if HIV-negative and five years if living with HIV, for life.

Genotype-directed action on a positive result. A result positive for HPV 16, 18 or 45 goes to screen-and-treat: assessment under visual inspection with acetic acid (VIA), then thermal ablation if no lesion is seen or colposcopy and large loop excision of the transformation zone (LLETZ) if a lesion is present, with HPV follow-up at twelve months. A result positive for other high-risk types is triaged by reflex cytology: normal cytology leads to a repeat HPV test in twelve months, low-grade change to assessment and ablation, and high-grade change to colposcopy and LLETZ. In an HIV-negative woman aged 40 or older, an other-high-risk result may be treated directly without waiting for cytology.

A complete answer explains both the persistence and the oncogenic roles.

Latency programme. In the host KSHV is overwhelmingly latent. The genome circularises into a multicopy episome, is copied once per cell cycle by host machinery, and expresses only a small set of latent genes, with no virion production but full lytic potential retained.

LANA. LANA tethers the episome to host mitotic chromosomes so it segregates to daughter cells, maintaining the infection. It also drives transformation: it inactivates p53 and the retinoblastoma protein, stabilises beta-catenin and Myc, and represses the lytic switch RTA to keep the virus latent.

A complete answer links the virus, the malaria cofactor and the genetic lesion, and gives the clinical picture.

The virus. Endemic Burkitt lymphoma is almost universally EBV-positive, the tumour cells showing the most restricted (latency I) programme, with Epstein-Barr nuclear antigen 1 (EBNA1) only.

The malaria cofactor. It maps to the equatorial-African belt of intense Plasmodium falciparum malaria. Chronic malaria drives polyclonal B-cell proliferation, raises the activity of the antibody-gene-editing enzymes, and weakens EBV-specific T-cell control, all of which expand the pool of infected, dividing B cells in which a transforming accident can occur.

The lesion and clinical picture. That accident is the c-myc translocation, classically t(8;14). The result is the commonest childhood cancer of the region, presenting characteristically as a rapidly growing jaw or orbital tumour.

A complete answer defines the principle, gives the mechanism of tumour selectivity, names a licensed example, and notes the limitations.

Principle

Oncolytic viruses are the therapeutic counterpart to viral oncogenesis: viruses that selectively infect and lyse tumour cells while sparing normal tissue. They act in two ways, by direct lysis of infected cancer cells and by releasing tumour antigens that provoke an anti-tumour immune response, so they function partly as in-situ vaccines.

Tumour selectivity

Selectivity exploits the defective antiviral defences of cancer cells. Transformed cells often have impaired interferon signalling and disordered proliferation, which favour viral replication; engineered viruses add further targeting by deleting genes the virus needs only in normal cells.

Example

The best-established agent is talimogene laherparepvec, a modified herpes simplex virus type 1 expressing granulocyte-macrophage colony-stimulating factor, licensed for advanced melanoma.

Limitations

Antiviral immunity can clear the virus before it acts, delivery to deep tumours is difficult, and efficacy as a single agent is modest, so oncolytic viruses are mainly used in combination with immune checkpoint inhibitors.