P53 represses human papillomavirus type 16 DNA replication via the viral E2 protein

Senataxin mediates R-loop resolution on HPV episomes

Three-stranded DNA-RNA structures known as R-loops that form during papillomavirus transcription can cause transcription-replication conflicts and lead to DNA damage. We found that R-loops accumulated at the viral early promoter in human papillomavirus (HPV) episomal cells but were greatly reduced in cells with integrated HPV genomes. RNA-DNA helicases unwind R-loops and allow for transcription and replication to proceed. Depletion of the RNA-DNA helicase senataxin (SETX) using siRNAs increased the presence of R-loops at the viral early promoter in HPV-31 (CIN612) and HPV-16 (W12) episomal HPV cell lines. Depletion of SETX reduced viral transcripts in episomal HPV cell lines. The viral E2 protein, which binds with high affinity to specific palindromes near the promoter and origin, complexes with SETX, and both SETX and E2 are present at the viral p97 promoter in CIN612 and W12 cells. SETX overexpression increased E2 transcription activity on the p97 promoter. SETX depletion also significantly increased integration of viral genomes in CIN612 cells. Our results demonstrate that SETX resolves viral R-loops to proceed with HPV transcription and prevent genome integration.IMPORTANCEPapillomaviruses contain small circular genomes of approximately 8 kilobase pairs and undergo unidirectional transcription from the sense strand of the viral genome. Co-transcriptional R-loops were recently reported to be present at high levels in cells that maintain episomal HPV and were also detected at the early viral promoter. R-loops can inhibit transcription and DNA replication. The process that removes R-loops from the PV genome and the requisite enzymes are unknown. We propose a model in which the host RNA-DNA helicase senataxin assembles on the HPV genome to resolve R-loops in order to maintain the episomal status of the viral genome.

A human papillomavirus 16 E2-TopBP1 dependent SIRT1-p300 acetylation switch regulates mitotic viral and human protein levels and activates the DNA damage response

An interaction between human papillomavirus 16 (HPV16) E2 and the cellular proteins TopBP1 and BRD4 is required for E2 plasmid segregation function. The E2-TopBP1 interaction promotes increased mitotic E2 protein levels in U2OS and N/Tert-1 cells, as well as in human foreskin keratinocytes immortalized by HPV16 (HFK + HPV16). SIRT1 deacetylation reduces E2 protein stability and here we demonstrate that increased E2 acetylation occurs during mitosis in a TopBP1 interacting-dependent manner, promoting E2 mitotic stabilization. p300 mediates E2 acetylation and acetylation is increased due to E2 switching off SIRT1 function during mitosis in a TopBP1 interacting-dependent manner, confirmed by increased p53 stability and acetylation on lysine 382, a known target for SIRT1 deacetylation. SIRT1 can complex with E2 in growing cells but is unable to do so during mitosis due to the E2-TopBP1 interaction; SIRT1 is also unable to complex with p53 in mitotic E2 wild-type cells but can complex with p53 outside of mitosis. E2 lysines 111 and 112 are highly conserved residues across all E2 proteins and we demonstrate that K111 hyper-acetylation occurs during mitosis, promoting E2 interaction with Topoisomerase 1 (Top1). We demonstrate that K112 ubiquitination promotes E2 proteasomal degradation during mitosis. E2-TopBP1 interaction promotes mitotic acetylation of CHK2, promoting phosphorylation and activation of the DNA damage response (DDR). The results present a new model in which the E2-TopBP1 complex inactivates SIRT1 during mitosis, and activates the DDR. This is a novel mechanism of HPV16 activation of the DDR, a requirement for the viral life cycle.

IMPORTANCE

Human papillomaviruses (HPVs) are causative agents in around 5% of all human cancers. While there are prophylactic vaccines that will significantly alleviate HPV disease burden on future generations, there are currently no anti-viral strategies available for the treatment of HPV cancers. To generate such reagents, we must understand more about the HPV life cycle, and in particular about viral-host interactions. Here, we describe a novel mitotic complex generated by the HPV16 E2 protein interacting with the host protein TopBP1 that controls the function of the deacetylase SIRT1. The E2-TopBP1 interaction disrupts SIRT1 function during mitosis in order to enhance acetylation and stability of viral and host proteins. We also demonstrate that the E2-TopBP1 interaction activates the DDR. This novel complex is essential for the HPV16 life cycle and represents a novel anti-viral therapeutic target.

A human papillomavirus 16 E2-TopBP1 dependent SIRT1-p300 acetylation switch regulates mitotic viral and human protein levels

An interaction between human papillomavirus 16 (HPV16) E2 and the cellular proteins TopBP1 and BRD4 is required for E2 plasmid segregation function. The E2-TopBP1 interaction promotes increased mitotic E2 protein levels in U2OS and N/Tert-1 cells, as well as in human foreskin keratinocytes immortalized by HPV16 (HFK+HPV16). SIRT1 deacetylation reduces E2 protein stability and here we demonstrate that increased E2 acetylation occurs during mitosis in a TopBP1 interacting dependent manner, promoting E2 mitotic stabilization. p300 mediates E2 acetylation and acetylation is increased due to E2 switching off SIRT1 function during mitosis in a TopBP1 interacting dependent manner, confirmed by increased p53 stability and acetylation on lysine 382, a known target for SIRT1 deacetylation. SIRT1 can complex with E2 in growing cells but is unable to do so during mitosis due to the E2-TopBP1 interaction; SIRT1 is also unable to complex with p53 in mitotic E2 wild type cells but can complex with p53 outside of mitosis. E2 lysines 111 and 112 are highly conserved residues across all E2 proteins and we demonstrate that K111 hyper-acetylation occurs during mitosis, promoting E2 interaction with Topoisomerase 1 (Top1). We also demonstrate that K112 ubiquitination promotes E2 proteasomal degradation during mitosis. The results present a model in which the E2-TopBP1 complex inactivates SIRT1 during mitosis and E2 acetylation on K111 by p300 increases, promoting interaction with Top1 that protects K112 from ubiquitination and therefore E2 proteasomal degradation.

Importance

Human papillomaviruses are causative agents in around 5% of all human cancers. While there are prophylactic vaccines that will significantly alleviate HPV disease burden on future generations, there are currently no anti-viral strategies available for the treatment of HPV cancers. To generate such reagents, we must understand more about the HPV life cycle, and in particular about viral-host interactions. Here we describe a novel mitotic complex generated by the HPV16 E2 protein interacting with the host protein TopBP1 that controls the function of the deacetylase SIRT1. The E2-TopBP1 interaction disrupts SIRT1 function during mitosis in order to enhance acetylation and stability of viral and host proteins. This novel complex is essential for the HPV16 life cycle and represents a novel anti-viral therapeutic target.

Biomolecular Condensation of the Human Papillomavirus E2 Master Regulator with p53: Implications in Viral Replication

p53 exerts its tumour suppressor activity by modulating hundreds of genes and it can also repress viral replication. Such is the case of human papillomavirus (HPV) through targeting the E2 master regulator, but the biochemical mechanism is not known. We show that the C-terminal DNA binding domain of HPV16 E2 protein (E2C) triggers heterotypic condensation with p53 at a precise 2/1 E2C/p53 stoichiometry at the onset for demixing, yielding large regular spherical droplets that increase in size with E2C concentration. Interestingly, transfection experiments show that E2 co-localizes with p53 in the nucleus with a grainy pattern, and recruits p53 to chromatin-associated foci, a function independent of the DNA binding capacity of p53 as judged by a DNA binding impaired mutant. Depending on the length, DNA can either completely dissolve or reshape heterotypic droplets into irregular condensates containing p53, E2C, and DNA, and reminiscent of that observed linked to chromatin. We propose that p53 is a scaffold for condensation in line with its structural and functional features, in particular as a promiscuous hub that binds multiple cellular proteins. E2 appears as both client and modulator, likely based on its homodimeric DNA binding nature. Our results, in line with the known role of condensation in eukaryotic gene enhancement and silencing, point at biomolecular condensation of E2 with p53 as a means to modulate HPV gene function, strictly dependent on host cell replication and transcription machinery.

Experimental Support for Human Papillomavirus Genome Amplification Early after Infectious Delivery

Even though replication and transcription of human papillomavirus type 16 (HPV16) has been intensively studied, little is known about immediate-early events of the viral life cycle due to the lack of an efficient infection model allowing genetic dissection of viral factors. We employed the recently developed infection model (Bienkowska-Haba M, Luszczek W, Myers JE, Keiffer TR, et al. 2018. PLoS Pathog 14:e1006846) to investigate genome amplification and transcription immediately after infectious delivery of viral genome to nuclei of primary keratinocytes. Using 5-ethynyl-2'-deoxyuridine (EdU) pulse-labeling and highly sensitive fluorescence in situ hybridization, we observed that the HPV16 genome is replicated and amplified in an E1- and E2-dependent manner. Knockout of E1 resulted in failure of the viral genome to replicate and amplify. In contrast, knockout of the E8^E2 repressor led to increased viral genome copy number, confirming previous reports. Genome copy control by E8^E2 was confirmed for differentiation-induced genome amplification. Lack of functional E1 had no effect on transcription from the early promoter, suggesting that viral genome replication is not required for p97 promoter activity. However, infection with an HPV16 mutant virus defective for E2 transcriptional function revealed a requirement of E2 for efficient transcription from the early promoter. In the absence of the E8^E2 protein, early transcript levels are unaltered and even decreased when normalized to genome copy number. Surprisingly, a lack of functional E8^E2 repressor did not affect E8^E2 transcript levels when normalized to genome copy number. These data suggest that the main function of E8^E2 in the viral life cycle is to control genome copy number. IMPORTANCE It is being assumed that human papillomavirus (HPV) utilizes three different modes of replication during its life cycle: initial amplification during the establishment phase, genome maintenance, and differentiation-induced amplification. However, HPV16 initial amplification was never formally proven due to a lack of an infection model. Using our recently established infection model (Bienkowska-Haba M, Luszczek W, Myers JE, Keiffer TR, et al. 2018. PLoS Pathog 14:e1006846), we demonstrate herein that viral genome is indeed amplified in an E1- and E2-dependent manner. Furthermore, we find that the main function of the viral repressor E8^E2 is to control viral genome copy number. We did not find evidence that it regulates its own promoter in a negative feedback loop. Our data also suggest that the E2 transactivator function is required for stimulation of early promoter activity, which has been debated in the literature. Overall, this report confirms the usefulness of the infection model for studying early events of the HPV life cycle using mutational approaches.

A Critical Role for p53 during the HPV16 Life Cycle

Human papillomaviruses (HPV) are causative agents in ano-genital and oral cancers; HPV16 is the most prevalent type detected in human cancers. The HPV16 E6 protein targets p53 for proteasomal degradation to facilitate proliferation of the HPV16 infected cell. However, in HPV16 immortalized cells E6 is predominantly spliced (E6*) and unable to degrade p53. Here, we demonstrate that human foreskin keratinocytes immortalized by HPV16 (HFK+HPV16), and HPV16 positive oropharyngeal cancers, retain significant expression of p53. In addition, p53 levels increase in HPV16+ head and neck cancer cell lines following treatment with cisplatin. Introduction of full-length E6 into HFK+HPV16 resulted in attenuation of cellular growth (in hTERT immortalized HFK, E6 expression promoted enhanced proliferation). An understudied interaction is that between E2 and p53 and we investigated whether this was important for the viral life cycle. We generated mutant genomes with E2 unable to interact with p53 resulting in profound phenotypes in primary HFK. The mutant induced hyper-proliferation, but an ultimate arrest of cell growth; β-galactosidase staining demonstrated increased senescence, and COMET assays showed increased DNA damage compared with HFK+HPV16 wild-type cells. There was failure of the viral life cycle in organotypic rafts with the mutant HFK resulting in premature differentiation and reduced proliferation. The results demonstrate that p53 expression is critical during the HPV16 life cycle, and that this may be due to a functional interaction between E2 and p53. Disruption of this interaction has antiviral potential. IMPORTANCE Human papillomaviruses are causative agents in around 5% of all cancers. There are currently no antivirals available to combat these infections and cancers, therefore it remains a priority to enhance our understanding of the HPV life cycle. Here, we demonstrate that an interaction between the viral replication/transcription/segregation factor E2 and the tumor suppressor p53 is critical for the HPV16 life cycle. HPV16 immortalized cells retain significant expression of p53, and the critical role for the E2-p53 interaction demonstrates why this is the case. If the E2-p53 interaction is disrupted then HPV16 immortalized cells fail to proliferate, have enhanced DNA damage and senescence, and there is premature differentiation during the viral life cycle. Results suggest that targeting the E2-p53 interaction would have therapeutic benefits, potentially attenuating the spread of HPV16.

APOBEC Mutagenesis Is Concordant between Tumor and Viral Genomes in HPV-Positive Head and Neck Squamous Cell Carcinoma

APOBEC is a mutagenic source in human papillomavirus (HPV)-mediated malignancies, including HPV+ oropharyngeal squamous cell carcinoma (HPV + OPSCC), and in HPV genomes. It is unknown why APOBEC mutations predominate in HPV + OPSCC, or if the APOBEC-induced mutations observed in both human cancers and HPV genomes are directly linked. We performed sequencing of host somatic exomes, transcriptomes, and HPV16 genomes from 79 HPV + OPSCC samples, quantifying APOBEC mutational burden and activity in both host and virus. APOBEC was the dominant mutational signature in somatic exomes. In viral genomes, there was a mean of five (range 0-29) mutations per genome. The mean of APOBEC mutations in viral genomes was one (range 0-5). Viral APOBEC mutations, compared to non-APOBEC mutations, were more likely to be low-variant allele fraction mutations, suggesting that APOBEC mutagenesis actively occurrs in viral genomes during infection. HPV16 APOBEC-induced mutation patterns in OPSCC were similar to those previously observed in cervical samples. Paired host and viral analyses revealed that APOBEC-enriched tumor samples had higher viral APOBEC mutation rates (p = 0.028), and APOBEC-associated RNA editing (p = 0.008), supporting the concept that APOBEC mutagenesis in host and viral genomes is directly linked and occurrs during infection. Using paired sequencing of host somatic exomes, transcriptomes, and viral genomes, we demonstrated for the first-time definitive evidence of concordance between tumor and viral APOBEC mutagenesis. This finding provides a missing link connecting APOBEC mutagenesis in host and virus and supports a common mechanism driving APOBEC dysregulation.

The role of infectious agents in cancer of the ocular region

The purpose of the study was to investigate the association between infectious agents and the development of cancer in the ocular adnexa. A comprehensive literary study was carried out, reviewing and summarizing previous reports on the topic. A broad range of malignancies of the ocular adnexa are associated with infectious agents. A strong association and possible causal relationship between the infectious agent and the development of ocular adnexal cancer are seen in Merkel cell carcinoma (Merkel cell polyomavirus), Burkitt lymphoma (Epstein-Barr virus) and Kaposi sarcoma (human herpesvirus 8). Infection with Chlamydia psittaci has been associated with the development of extranodal marginal zone B-cell lymphoma in Italy. Human papillomavirus infection has been associated with the development of squamous cell carcinomas of the ocular adnexa, although with a highly variable reported prevalence. By exploring the role of infectious agents in the ocular adnexa and the mechanism by which they contribute to oncogenesis, the diagnostics, management and prevention of these malignancies may also improve. Antibiotic treatment and vaccines against infectious agents may be valuable in future treatment. Additionally, the presence of infectious agents within the tumours may have a prognostic or predictive value.

Detection of precancerous lesions in the cervix and HPV infection in women in the region of Maniapure, Bolivar State

Human papillomavirus (HPV) is the causative agent of cervical cancer (CC), the second most common cause of cancer deaths in Venezuela. Early detection and prompt treatment of precancerous lesions prevent up to 80% of CC cases. In Venezuela, difficult access to CC screening means that the disease is detected at advanced stages, especially in more vulnerable indigenous populations. The aim of the study was to detect precancerous cervical lesions and HPV infection in 60 women who attended the gynaecology service at the Maniapure Outpatient Clinic in Bolivar State, Venezuela. The study was carried out to detect precancerous cervical lesions using visual inspection with acetic acid (VIA), the Schiller test and conventional cytology (Pap testing). HPV detection and typing were carried out using the polymerase chain reaction. 58.3% of the women in the study belonged to the Eñepa indigenous community and 41.7% were white Creole women. The Schiller test showed irregularities in the staining of the exocervical epithelium in 8.33% of the patients, suggesting HPV infection. VIA was positive for 10.0% of the women. In the cytopathology report, 81.67% tested negative for intraepithelial lesions. The overall frequency of HPV detection was 35.0%. HPV infection was detected in 45.71% of the Eñepa women and 20.0% of the Creole women. 71.43% of the women had a high-risk single HPV infection. The percentage of viral infection was lower in the Creole patients than in the indigenous population; therefore, CC screening programmes in the latter population need to be improved.

Mechanisms by which HPV Induces a Replication Competent Environment in Differentiating Keratinocytes

Human papillomaviruses (HPV) are the causative agents of cervical cancer and are also associated with other genital malignancies, as well as an increasing number of head and neck cancers. HPVs have evolved their life cycle to contend with the different cell states found in the stratified epithelium. Initial infection and viral genome maintenance occurs in the proliferating basal cells of the stratified epithelium, where cellular replication machinery is abundant. However, the productive phase of the viral life cycle, including productive replication, late gene expression and virion production, occurs upon epithelial differentiation, in cells that normally exit the cell cycle. This review outlines how HPV interfaces with specific cellular signaling pathways and factors to provide a replication-competent environment in differentiating cells.

Host cell restriction factors that limit transcription and replication of human papillomavirus

The life cycle of human papillomaviruses (HPV) is tightly regulated by the differentiation state of mucosal and cutaneous keratinocytes. To counteract viral infection, constitutively expressed cellular factors, which are defined herein as restriction factors, directly mitigate viral gene expression and replication. In turn, some HPV gene products target these restriction factors and abrogate their anti-viral effects to establish efficient gene expression and replication programs. Ironically, in certain circumstances, this delicate counterbalance between viral gene products and restriction factors facilitates persistent infection by HPVs. This review serves to recapitulate the current knowledge of nuclear restriction factors that directly affect the HPV infectious cycle.

p53 down-regulates SARS coronavirus replication and is targeted by the SARS-unique domain and PLpro via E3 ubiquitin ligase RCHY1

Highly pathogenic severe acute respiratory syndrome coronavirus (SARS-CoV) has developed strategies to inhibit host immune recognition. We identify cellular E3 ubiquitin ligase ring-finger and CHY zinc-finger domain-containing 1 (RCHY1) as an interacting partner of the viral SARS-unique domain (SUD) and papain-like protease (PL(pro)), and, as a consequence, the involvement of cellular p53 as antagonist of coronaviral replication. Residues 95-144 of RCHY1 and 389-652 of SUD (SUD-NM) subdomains are crucial for interaction. Association with SUD increases the stability of RCHY1 and augments RCHY1-mediated ubiquitination as well as degradation of p53. The calcium/calmodulin-dependent protein kinase II delta (CAMK2D), which normally influences RCHY1 stability by phosphorylation, also binds to SUD. In vivo phosphorylation shows that SUD does not regulate phosphorylation of RCHY1 via CAMK2D. Similarly to SUD, the PL(pro)s from SARS-CoV, MERS-CoV, and HCoV-NL63 physically interact with and stabilize RCHY1, and thus trigger degradation of endogenous p53. The SARS-CoV papain-like protease is encoded next to SUD within nonstructural protein 3. A SUD-PL(pro) fusion interacts with RCHY1 more intensively and causes stronger p53 degradation than SARS-CoV PL(pro) alone. We show that p53 inhibits replication of infectious SARS-CoV as well as of replicons and human coronavirus NL63. Hence, human coronaviruses antagonize the viral inhibitor p53 via stabilizing RCHY1 and promoting RCHY1-mediated p53 degradation. SUD functions as an enhancer to strengthen interaction between RCHY1 and nonstructural protein 3, leading to a further increase in in p53 degradation. The significance of these findings is that down-regulation of p53 as a major player in antiviral innate immunity provides a long-sought explanation for delayed activities of respective genes.

The Role of the DNA Damage Response throughout the Papillomavirus Life Cycle

The DNA damage response (DDR) maintains genomic integrity through an elaborate network of signaling pathways that sense DNA damage and recruit effector factors to repair damaged DNA. DDR signaling pathways are usurped and manipulated by the replication programs of many viruses. Here, we review the papillomavirus (PV) life cycle, highlighting current knowledge of how PVs recruit and engage the DDR to facilitate productive infection.

[Regulation of human papillomavirus (HPV) genome replication in the viral life cycle and its association with the viral persistence and cancer development]

High-risk human papillomavirus (HR-HPV) infections account for more than 5% of all cancers (11% in women) such as cervical cancer worldwide. HPVs infect to basal cells of the stratified squamous epithelium and establish persistent infection within the basal compartment. HR-HPV infections can persist more than a decade, leading to development of cancers. The life cycle of HPVs is tightly associated with the differentiation processes of the stratified squamous epithelium; the replication of the viral genome and the expression of the viral genes are strictly regulated depending on differentiation of the host keratinocytes. The viral genome is transiently amplified immediately following infection and then maintained at constant copy numbers in the basal cells. In terminally differentiating keratinocytes, the viral genome is drastically amplified. However, molecular mechanisms underlying switching these three stages of viral genome replication in the viral life cycle are poorly understood. Recently, it has become evident that DNA damage response pathways are involved in the regulation of HPV genome replication. In this review, we would like to introduce recent findings describing the associations of DNA damage response with HPV genome replication.

Epidemiology of ocular surface squamous neoplasia in Africa

OBJECTIVES

To describe the epidemiology and an aetiological model of ocular surface squamous neoplasia (OSSN) in Africa.

METHODS

Systematic and non-systematic review methods were used. Incidence was obtained from the International Agency for Research on Cancer. We searched PubMed, EMBASE, Web of Science and the reference lists of articles retrieved. Meta-analyses were conducted using a fixed-effects model for HIV and cigarette smoking and random effects for human papilloma virus (HPV).

RESULTS

The incidence of OSSN is highest in the Southern Hemisphere (16° South), with the highest age-standardised rate (ASR) reported from Zimbabwe (3.4 and 3.0 cases/year/100 000 population for males and females, respectively). The mean ASR worldwide is 0.18 and 0.08 cases/year/100 000 among males and females, respectively. The risk increases with exposure to direct daylight (2-4 h, OR = 1.7, 95% CI: 1.2-2.4 and ≥5 h OR = 1.8, 95% CI: 1.1-3.1) and outdoor occupations (OR = 1.7, 95% CI: 1.1-2.6). Meta-analysis also shows a strong association with HIV (6 studies: OR = 6.17, 95% CI: 4.83-7.89) and HPV (7 studies: OR = 2.64, 95% CI: 1.27-5.49) but not cigarette smoking (2 studies: OR = 1.40, 95% CI: 0.94-2.09). The effect of atopy, xeroderma pigmentosa and vitamin A deficiency is unclear.

CONCLUSIONS

Africa has the highest incidence of OSSN in the world, where males and females are equally affected, unlike other continents where male disease predominates. African women probably have increased risk due to their higher prevalence of HIV and HPV infections. As the survival of HIV-infected people increases, and given no evidence that anti-retroviral therapy (ART) reduces the risk of OSSN, the incidence of OSSN may increase in coming years.

Inactivation of p53 rescues the maintenance of high risk HPV DNA genomes deficient in expression of E6

The human papillomavirus DNA genome undergoes three distinct stages of replication: establishment, maintenance and amplification. We show that the HPV16 E6 protein is required for the maintenance of the HPV16 DNA genome as an extrachromosomal, nuclear plasmid in its natural host cell, the human keratinocyte. Based upon mutational analyses, inactivation of p53 by E6, but not necessarily E6-mediated degradation of p53, was found to correlate with the ability of E6 to support maintenance of the HPV16 genome as a nuclear plasmid. Inactivation of p53 with dominant negative p53 rescued the ability of HPV16 E6STOP and E6SAT mutant genomes to replicate as extrachromosomal genomes, though not to the same degree as observed for the HPV16 E6 wild-type (WT) genome. Inactivation of p53 also rescued the ability of HPV18 and HPV31 E6-deficient genomes to be maintained at copy numbers comparable to that of HPV18 and HPV31 E6WT genomes at early passages, though upon further passaging copy numbers for the HPV18 and 31 E6-deficient genomes lessened compared to that of the WT genomes. We conclude that inactivation of p53 is necessary for maintenance of HPV16 and for HPV18 and 31 to replicate at WT copy number, but that additional functions of E6 independent of inactivating p53 must also contribute to the maintenance of these genomes. Together these results suggest that re-activation of p53 may be a possible means for eradicating extrachromosomal HPV16, 18 or 31 genomes in the context of persistent infections.

Human papillomaviruses (HPVs) infect epithelial tissues. HPVs that infect mucosal epithelia cause infectious lesions in the anogenital tract and oral cavity. HPV infections are normally cleared by the immune system; however, in rare cases, infections can persist for years. Persistent infections by certain HPVs place one at a high risk of developing carcinomas of the cervix, other anogenital tissues, and the head/neck region. These HPVs are responsible for over 5% of all human cancers. For an HPV infection to persist, the viral circular genome must be maintained, i.e. replicated and inherited during cell division. In this study we define the mechanism by which the viral gene E6 contributes to the maintenance of the HPV genome. We demonstrate that E6 must inactivate the cellular factor, p53, for the viral genome to be maintained. Significantly, p53, is inactivated in many types of human cancers and because much research has been done on p53, promising new drugs have been identified that can re-activate p53. If such drugs can re-activate the p53 that has been inactivated by E6, then we hypothesize that these drugs could be used to cure patients with persistent HPV infections and thereby reduce their risk of developing HPV associated cancers.

The papillomavirus E2 proteins

The papillomavirus E2 proteins are pivotal to the viral life cycle and have well characterized functions in transcriptional regulation, initiation of DNA replication and partitioning the viral genome. The E2 proteins also function in vegetative DNA replication, post-transcriptional processes and possibly packaging. This review describes structural and functional aspects of the E2 proteins and their binding sites on the viral genome. It is intended to be a reference guide to this viral protein.

The p53-microRNA-34a axis regulates cellular entry receptors for tumor-associated human herpes viruses

A growing number of reports indicate the frequent presence of DNA sequences and gene products of human cytomegalovirus in various tumors as compared to adjacent normal tissues, the brain tumors being studied most intensely. The mechanisms underlying the tropism of human cytomegalovirus to the tumor cells or to the cells of tumor origin, as well as the role of the host's genetic background in virus-associated oncogenesis are not well understood. It is also not clear why cytomegalovirus can be detected in many but not in all tumor specimens. Our in silico prediction results indicate that microRNA-34a may be involved in replication of some human DNA viruses by targeting and downregulating the genes encoding a diverse group of proteins, such as platelet-derived growth factor receptor-alpha, complement component receptor 2, herpes simplex virus entry mediators A, B, and C, and CD46. Notably, while their functions vary, these surface molecules have one feature in common: they serve as cellular entry receptors for human DNA viruses (cytomegalovirus, Epstein-Barr virus, human herpes virus 6, herpes simplex viruses 1 and 2, and adenoviruses) that are either proven or suspected to be linked with malignancies. MicroRNA-34a is strictly dependent on its transcriptional activator tumor suppressor protein p53, and both p53 and microRNA-34a are frequently mutated or downregulated in various cancers. We hypothesize that p53-microRNA-34a axis may alter susceptibility of cells to infection with some viruses that are detected in tumors and either proven or suspected to be associated with tumor initiation and progression.

HPV-18 E6 mutants reveal p53 modulation of viral DNA amplification in organotypic cultures

Human papillomaviruses (HPVs) amplify in differentiated strata of a squamous epithelium. The HPV E7 protein destabilizes the p130/retinoblastoma susceptibility protein family of tumor suppressors and reactivates S-phase reentry, thereby facilitating viral DNA amplification. The high-risk HPV E6 protein destabilizes the p53 tumor suppressor and many other host proteins. However, the critical E6 targets relevant to viral DNA amplification have not been identified, because functionally significant E6 mutants are not stably maintained in transfected cells. Using Cre-loxP recombination, which efficiently generates HPV genomic plasmids in transfected primary human keratinocytes, we have recapitulated a highly productive infection of HPV-18 in organotypic epithelial cultures. By using this system, we now report the characterization of four HPV-18 E6 mutations. An E6 null mutant accumulated high levels of p53 and amplified very poorly. p53 siRNA or ectopic WT E6 partially restored amplification, whereas three missense E6 mutations that did not effectively destabilize p53 complemented the null mutant poorly. Unexpectedly, in cis, two of the missense mutants amplified, albeit to a lower extent than the WT and only in cells with undetectable p53. These observations and others implicate p53 and additional host proteins in regulating viral DNA amplification and also suggest an inhibitory effect of E6 overexpression. We show that high levels of viral DNA amplification are critical for late protein expression and report several previously undescribed viral RNAs, including bicistronic transcripts predicted to encode E5 and L2 or an alternative form of E1^E4 and L1.

Human papillomavirus early proteins and apoptosis

INTRODUCTION

The human papillomavirus (HPV) associated apoptosis can be primarily attributed to some early proteins, such as E2, E5, E6, E7, and so on. Though these proteins have a low molecular size, they are capable to interact with a series of host cellular regulation proteins to induce or inhibit apoptosis. The oncoproteins E6 can inhibit the apoptosis mainly through p53 pathway. The E5 protein can protect cells from tumor necrosis factor-related apoptosis. The protein E2 protein have regulatory functions in viral transcription and induction of apoptosis. The oncoprotein E7 plays the role in both apoptosis activation and inhibition. In addition, the HPV full-length E2 proteins involve in activating or repressing the transcription of E6/E7, so as to regulating the apoptosis caused by E6 and E7.

MATERIALS AND METHODS

We search major databases (such as Elsevier) with the following selection criteria: HPV, early protein, apoptosis.

CONCLUSIONS

In this review, we summary the literature related with E2, E5, E6, and E7 proteins, and describe the regulatory principles and specific mechanism by which HPV early proteins can interfere with apoptosis and trigger gynaecopathias for women.

Interference with p53 functions in human viral infections, a target for novel antiviral strategies?

Viral infections cause a major stress in host cells. The cellular responses to stress are mediated by p53, which by deregulation of cell cycle and apoptosis, may also be part of the host cell reaction to fight infections. Therefore, during evolutionary viral adaptation to host organisms, viruses have developed strategies to manipulate host cell p53 dependent pathways to facilitate their viral life cycles. Thus, interference with p53 function is an important component in viral pathogenesis. Many viruses have proteins that directly affect p53, whereas others alter the regulation of p53 in an indirect manner, mediated by Hdm2 or Akt, or induction of interferon. Rescue of p53 activity is becoming an area of therapeutic development in oncology. It might be feasible that manipulation of p53 mediated responses can become a therapeutic option to limit viral replication or dissemination. In this report, the mechanisms by which viral proteins manipulate p53 responses are reviewed, and it is proposed that a pharmacological rescue of p53 functions might help to control viral infections.

Regulation of human papillomavirus type 16 early gene expression in trophoblastic and cervical cells

We compared the outcome of different cellular and viral factors on the regulation of the HPV-16 early viral gene expression in trophoblastic and cervical cancer cells. A high variability of the long control (LCR) activity was observed, prompting us to evaluate the role of secreted factors in the control of the early gene expression in trophoblastic cell lines. Endogenous progesterone and exogenous dexamethasone were found to activate LCR driven transcriptional activity. Since host cells express HPV early proteins to regulate LCR activity, we investigated the effect of the combined HPV-16 early proteins on the LCR driven transcription and the possible involvement of E2. A physiological level of HPV-16 early proteins expression strongly induced the LCR driven reporter activity. According to mutational analysis, E1 and E2 proteins, indispensable for viral replication, were not involved in LCR extrachromosomal transcriptional regulation. This suggests that E5 and/or E6 and/or E7, consequently, activated viral transcription.

The natural history of human papillomavirus infections of the mucosal epithelia

Human papillomaviruses (HPVs), members of a very large family of small DNA viruses, cause both benign papillomas and malignant tumors. While most research on these viruses over the past 30 years has focused on their oncogenic properties in the genital tract, they also play an important role in diseases of the upper aerodigestive tract. Rapidly accelerating advances in knowledge have increased our understanding of the biology of these viruses and this knowledge, in turn, is being applied to new approaches to prevent, diagnose, and treat HPV-induced diseases. In this introductory article, we provide an overview of the structure and life cycle of the mucosal HPVs and their interactions with their target tissues and cells. Finally, we provide our thoughts about treatments for HPV-induced diseases, present and future.

Replication and partitioning of papillomavirus genomes

Papillomaviruses establish persistent infection in the dividing, basal epithelial cells of the host. The viral genome is maintained as a circular, double-stranded DNA, extrachromosomal element within these cells. Viral genome amplification occurs only when the epithelial cells differentiate and viral particles are shed in squames that are sloughed from the surface of the epithelium. There are three modes of replication in the papillomavirus life cycle. Upon entry, in the establishment phase, the viral genome is amplified to a low copy number. In the second maintenance phase, the genome replicates in dividing cells at a constant copy number, in synchrony with the cellular DNA. And finally, in the vegetative or productive phase, the viral DNA is amplified to a high copy number in differentiated cells and is destined to be packaged in viral capsids. This review discusses the cis elements and protein factors required for each stage of papillomavirus replication.

The E8--E2 gene product of human papillomavirus type 16 represses early transcription and replication but is dispensable for viral plasmid persistence in keratinocytes

A conserved E8(wedge)E2 spliced mRNA is detected in keratinocytes transfected with human papillomavirus type 16 (HPV-16) plasmid DNA. Expression of HPV-16 E8--E2 (16-E8--E2) is independent of the major early promoter, P97, and is modulated by both specific splicing events and conserved cis elements in the upstream regulatory region in a manner that differs from transcriptional regulation of other early viral genes. Mutations that disrupt the predicted 16-E8--E2 message also increase initial HPV-16 plasmid amplification 8- to 15-fold and major early gene (P97) transcription 4- to 5-fold over those of the wild type (wt). Expressing the 16-E8--E2 gene product from the cytomegalovirus (CMV) promoter represses HPV-16 early gene transcription from P97 in a dose-dependent manner, as detected by RNase protection assays. When expressed from the CMV promoter, 16-E8--E2 also inhibits the amplification of an HPV-16 plasmid and a heterologous simian virus 40 (SV40) ori plasmid that contains E2 binding sites in cis. In contrast, cotransfections with HPV-16 wt genomes that express physiologic levels of 16-E8--E2 are sufficient to repress HPV-16 plasmid amplification but are limiting and insufficient for the repression of SV40 amplification. 16-E8--E2-dependent repression of HPV-16 E1 expression is sufficient to account for this observed inhibition of initial HPV-16 plasmid amplification. Unlike with other papillomaviruses, primary human keratinocytes immortalized by the HPV-16 E8 mutant genome contain more than eightfold-higher levels of unintegrated plasmid than the wt, demonstrating that 16-E8(wedge)E2 limits the viral copy number but is not required for plasmid persistence and maintenance.