Development of a cellular assay system to study the genome replication of high- and low-risk mucosal and cutaneous human papillomaviruses

The POU-HD TFs impede the replication efficiency of several human papillomavirus genomes

Human papillomavirus (HPV) is a double-stranded DNA virus that infects cutaneous and mucosal epithelial cells. HPV replication initiates at the origin (ori), located within a noncoding region near the major early promoter. Only two viral proteins, E1 and E2, are essential for replication, with the host cell contributing other necessary factors. However, the role of host cell proteins in regulating HPV replication remains poorly understood. While several binding sites for cellular transcription factors (TFs), such as POU-HD proteins, have been mapped in the regulatory region, their functional importance is unclear. Some POU-HD TFs have been shown to influence replication in a system where E1 and E2 are provided exogenously. In this study, we investigated the impact of several POU-HD TFs on the replication of the HPV5, HPV11, and HPV18 genomes in U2OS cells and human primary keratinocytes. We demonstrated that OCT1, OCT6, BRN5A, and SKN1A are expressed in HPV host cells and that their overexpression inhibits HPV genome replication, whereas knocking down OCT1 had a positive effect. Using the replication-deficient HPV18-E1- genome, we demonstrated that OCT1-mediated inhibition of HPV replication involves modulation of HPV early promoters controlling E1 and E2 expression. Moreover, using Oct6 mutants deficient either in DNA binding or transcriptional regulation, we showed that the inhibition of HPV18 replication is solely dependent on Oct6's DNA binding activity. Our study highlights the complex regulatory roles of POU-HD factors in the HPV replication.

Characterization and comparative analysis of phosphorylation patterns in HPV18 and HPV11 E1 helicases: Implications for viral genome replication

The genome of human papillomaviruses (HPVs) encodes the E1 replication factor, whose biological activities are regulated by cellular protein kinases. Here, the phosphorylation pattern of the E1 helicase of oncogenic mucosotropic HPV18 was investigated both in vitro and in vivo. Four serine residues located in a short peptide within a localization regulatory region were found to be phosphorylated in both experimental settings. We demonstrate that this peptide is targeted in vitro by various protein kinases, including CK2, PKA, and CKD2/cyclin A/B/E complexes. Through point mutagenesis, we show that phosphorylation of this region is essential for E1 subcellular localization, the interaction of E1 with the E2 protein, and replication of the HPV18 genome. Furthermore, we demonstrate the functional conservation of this phosphorylation across the E1 proteins of the low-risk mucosotropic HPV11 and high-risk cutaneotropic HPV5. These findings provide deeper insights into the phosphorylation-mediated regulation of biological activities of the E1 protein.

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.

Transcription Properties of Beta-HPV8 and HPV38 Genomes in Human Keratinocytes

Persistent infections with high-risk human papillomaviruses (HR-HPV) from the genus alpha are established risk factors for the development of anogenital and oropharyngeal cancers. In contrast, HPV from the genus beta have been implicated in the development of cutaneous squamous cell cancer (cSCC) in epidermodysplasia verruciformis (EV) patients and organ transplant recipients. Keratinocytes are the in vivo target cells for HPV, but keratinocyte models to investigate the replication and oncogenic activities of beta-HPV genomes have not been established. A recent study revealed, that beta-HPV49 immortalizes normal human keratinocytes (NHK) only, when the viral E8^E2 repressor (E8^-) is inactivated (T. M. Rehm, E. Straub, T. Iftner, and F. Stubenrauch, Proc Natl Acad Sci U S A 119:e2118930119, 2022, https://doi.org/10.1073/pnas.2118930119). We now demonstrate that beta-HPV8 and HPV38 wild-type or E8^- genomes are unable to immortalize NHK. Nevertheless, HPV8 and HPV38 express E6 and E7 oncogenes and other transcripts in transfected NHK. Inactivation of the conserved E1 and E2 replication genes reduces viral transcription, whereas E8^- genomes display enhanced viral transcription, suggesting that beta-HPV genomes replicate in NHK. Furthermore, growth of HPV8- or HPV38-transfected NHK in organotypic cultures, which are routinely used to analyze the productive replication cycle of HR-HPV, induces transcripts encoding the L1 capsid gene, suggesting that the productive cycle is initiated. In addition, transcription patterns in HPV8 organotypic cultures and in an HPV8-positive lesion from an EV patient show similarities. Taken together, these data indicate that NHK are a suitable system to analyze beta-HPV8 and HPV38 replication. IMPORTANCE High-risk HPV, from the genus alpha, can cause anogenital or oropharyngeal malignancies. The oncogenic properties of high-risk HPV are important for their differentiation-dependent replication in human keratinocytes, the natural target cell for HPV. HPV from the genus beta have been implicated in the development of cutaneous squamous cell cancer in epidermodysplasia verruciformis (EV) patients and organ transplant recipients. Currently, the replication cycle of beta-HPV has not been studied in human keratinocytes. We now provide evidence that beta-HPV8 and 38 are transcriptionally active in human keratinocytes. Inactivation of the viral E8^E2 repressor protein greatly increases genome replication and transcription of the E6 and E7 oncogenes, but surprisingly, this does not result in immortalization of keratinocytes. Differentiation of HPV8- or HPV38-transfected keratinocytes in organotypic cultures induces transcripts encoding the L1 capsid gene, suggesting that productive replication is initiated. This indicates that human keratinocytes are suited as a model to investigate beta-HPV replication.

Human Beta Papillomavirus Type 8 E1 and E2 Proteins Suppress the Activation of the RIG-I-like Receptor MDA5

Persistent infections of the skin with the human papillomavirus of genus beta (β-HPV) in immunocompetent individuals are asymptomatic, but in immunosuppressed patients, β-HPV infections exhibit much higher viral loads on the skin and are associated with an increased risk of skin cancer. Unlike with HPV16, a high-risk α-HPV, the impact of β-HPV early genes on the innate immune sensing of viral nucleic acids has not been studied. Here, we used primary skin keratinocytes and U2OS cells expressing HPV8 or distinct HPV8 early genes and well-defined ligands of the nucleic-acid-sensing receptors RIG-I, MDA5, TLR3, and STING to analyze a potential functional interaction. We found that primary skin keratinocytes and U2OS cells expressed RIG-I, MDA5, TLR3, and STING, but not TLR7, TLR8, or TLR9. While HPV16-E6 downregulated the expression of RIG-I, MDA5, TLR3, and STING and, in conjunction with HPV16-E7, effectively suppressed type I IFN in response to MDA5 activation, the presence of HPV8 early genes showed little effect on the expression of these immune receptors, except for HPV8-E2, which was associated with an elevated expression of TLR3. Nevertheless, whole HPV8 genome expression, as well as the selective expression of HPV8-E1 or HPV8-E2, was found to suppress MDA5-induced type I IFN and the proinflammatory cytokine IL-6. Furthermore, RNA isolated from HPV8-E2 expressing primary human keratinocytes, but not control cells, stimulated a type I IFN response in peripheral blood mononuclear cells, indicating that the expression of HPV8-E2 in keratinocytes leads to the formation of stimulatory RNA ligands that require the active suppression of immune recognition. These results identify HPV8-E1 and HPV8-E2 as viral proteins that are responsible for the immune escape of β-HPV from the innate recognition of viral nucleic acids, a mechanism that may be necessary for establishing persistent β-HPV infections.

Restriction of viral gene expression and replication prevents immortalization of human keratinocytes by a beta-human papillomavirus

High-risk (HR) human papillomaviruses (HPV) from the genus alpha cause anogenital and oropharyngeal cancers, whereas the contribution of HPV from the genus beta to the development of cutaneous squamous cell cancer is still under debate. HR-HPV genomes display potent immortalizing activity in human keratinocytes, the natural target cell for HPV. This paper shows that immortalization of keratinocytes by the beta-HPV49 genome requires the inactivation of the viral E8^E2 repressor protein and the presence of the E6 and E7 oncoproteins but also of the E1 and E2 replication proteins. This reveals important differences in the carcinogenic properties of HR-HPV and beta-HPV but also warrants further investigations on the distribution and mutation frequencies of beta-HPV in human cancers.

Beta-human papillomaviruses (HPV) have been implicated in the development of cutaneous squamous cell cancer (cSCC) in epidermodysplasia verruciformis (EV) patients and organ transplant recipients. In contrast to high-risk (HR) HPV, which cause anogenital and oropharyngeal cancers, immortalizing activity of complete beta-HPV genomes in normal human keratinocytes (NHK), the natural target cells for HPV, has not been reported. We now demonstrate that the beta-HPV49 wild-type genome is transcriptionally active in NHK but lacks immortalizing activity unless the E8 gene, which encodes the E8^E2 repressor, is inactivated. HPV49 E8− immortalized keratinocytes maintain high levels of viral gene expression and very high copy numbers of extrachromosomal viral genomes during long-term cultivation. Not only disruption of the viral E6 and E7 oncogenes but also of the E1 or E2 replication genes renders E8− genomes incapable of immortalization. E8−/E1− and E8−/E2− genomes display greatly reduced E6 and E7 RNA levels in short-term assays. This strongly suggests that high-level expression of E6 and E7 from extrachromosomal templates is necessary for immortalization. The requirement for an inactivation of E8 while maintaining E1 and E2 expression highlights important differences in the carcinogenic properties of HR-HPV and beta-HPV. These findings strengthen the notion that beta-HPV have carcinogenic potential that warrants further investigations into the distribution of beta-HPV in human cancers.

Analysis of the Replication Mechanisms of the Human Papillomavirus Genomes

The life-cycle of human papillomaviruses (HPVs) includes three distinct phases of the viral genome replication. First, the viral genome is amplified in the infected cells, and this amplification is often accompanied by the oligomerization of the viral genomes. Second stage includes the replication of viral genomes in concert with the host cell genome. The viral genome is further amplified during the third stage of the viral-life cycle, which takes place only in the differentiated keratinocytes. We have previously shown that the HPV18 genomes utilize at least two distinct replication mechanisms during the initial amplification. One of these mechanisms is a well-described bidirectional replication via theta type of replication intermediates. The nature of another replication mechanism utilized by HPV18 involves most likely recombination-dependent replication. In this paper, we show that the usage of different replication mechanisms is a property shared also by other HPV types, namely HPV11 and HPV5. We further show that the emergence of the recombination dependent replication coincides with the oligomerization of the viral genomes and is dependent on the replicative DNA polymerases. We also show that the oligomeric genomes of HPV18 replicate almost exclusively using recombination dependent mechanism, whereas monomeric HPV31 genomes replicate bi-directionally during the maintenance phase of the viral life-cycle.

Cyclic AMP-Dependent Protein Kinase Exhibits Antagonistic Effects on the Replication Efficiency of Different Human Papillomavirus Types

Several types of widespread human papillomaviruses (HPVs) may induce the transformation of infected cells, provoking the development of neoplasms. Two main genera of HPVs are classified as mucosatropic alphapapillomaviruses and cutaneotropic betapapillomaviruses (α- and β-HPVs, respectively), and they both include high-risk cancer-associated species. The absence of antiviral drugs has driven investigations into the details of the molecular mechanisms of the HPV life cycle. HPV replication depends on the viral helicase E1 and the transcription factor E2. Their biological activities are controlled by numerous cellular proteins, including protein kinases. Here, we report that ubiquitously expressed cyclic AMP-dependent protein kinase A (PKA) differentially regulates the replication of α-HPV11, α-HPV18, and β-HPV5. PKA stimulates the replication of both α-HPVs studied but has a more profound effect on the replication of high-risk α-HPV18. However, the replication of β-HPV5 is inhibited by activated PKA in human primary keratinocytes and U2OS cells. We show that the activation of PKA signaling by different pharmacological agents induces the rapid proteasomal degradation of the HPV5 E2 protein, which in turn leads to the downregulation of E2-dependent transcription. In contrast, PKA-stimulated induction of HPV18 replication is the result of the downregulation of the E8^E2 transcript encoding a potent viral transcriptional inhibitor together with the rapid upregulation of E1 and E2 protein levels. IMPORTANCE Several types of human papillomaviruses (HPVs) are causative agents of various types of epithelial cancers. Here, we report that ubiquitously expressed cyclic AMP-dependent protein kinase A (PKA) differentially regulates the replication of various types of HPVs during the initial amplification and maintenance phases of the viral life cycle. The replication of the skin cancer-related pathogen HPV5 is suppressed, whereas the replication of the cervical cancer-associated pathogen HPV18 is activated, in response to elevated PKA activity. To inhibit HPV5 replication, PKA targets the viral transcriptional activator E2, inducing its rapid proteasomal degradation. PKA-dependent stimulation of HPV18 replication relies on the downregulation of another E2 gene product, E8^E2, which encodes a potent transcriptional repressor. Our findings highlight, for the first time, protein kinase-related mechanistic differences in the regulation of the replication of mucosal and cutaneous HPV types.

Measuring the serum level of retinol-binding protein can enhance the treatment of recalcitrant warts

BACKGROUND

Warts are common benign (60%-65%) self-limited tumors of the epidermis caused by human papillomaviruses (HPVs). However, some warts fail to resolve despite of different treatments and become recalcitrant. Vitamin A has antiproliferative and antikeratinizing properties by which the disruption of HPV replication can be occurred. Concentrations of retinol-binding protein (RBP) and retinol in the circulation highly correlate with each others.

AIM

To assess the serum level of RBP in patients with resistant warts to evaluate the possible role of retinol in the disease pathogenesis.

PATIENTS

This case-control study included 30 patients with resistant cutaneous warts (defined as failure of cure after conventional treatment as 12 weeks of salicylic acid application, 4 or more cycles of cryotherapy or electrocautery and/or other physical treatment modalities) and 30 age- and sex-matched healthy controls. RBP level in the serum was measured by ELISA.

RESULTS

There was a significant difference between cases and controls regarding the level of serum RBP (P = .001). However, serum RBP level did not differ significantly regarding sociodemographic or clinical data (P > .05 each). RBP is a good biomarker for significant early detection and discrimination between cases and controls (P = .001) at a cutoff point < 563.3 mg/l with sensitivity (93%) and specificity (80%).

CONCLUSION

Low serum RBP level in our studied patients may suggest an important role of retinol in the resistant warts pathogenesis. Thus measuring serum RBP will help to identify patients who are going to have resistant warts in the future.

Uncovering the Role of the E1 Protein in Different Stages of Human Papillomavirus 18 Genome Replication

The life cycle of human papillomaviruses (HPVs) comprises three distinct phases of DNA replication: initial amplification, maintenance of the genome copy number at a constant level, and vegetative amplification. The viral helicase E1 is one of the factors required for the initiation of HPV genome replication. However, the functions of the E1 protein during other phases of the viral life cycle are largely uncharacterized. Here, we studied the role of the HPV18 E1 helicase in three phases of viral genome replication by downregulating E1 expression using RNA interference or inducing degradation of the E1 protein via inhibition of casein kinase 2α expression or catalytic activity. We generated a novel modified HPV18 genome expressing Nanoluc and tagged E1 and E2 proteins and created several stable HPV18-positive cell lines. We showed that, in contrast to initial amplification of the HPV18 genome, other phases of viral genome replication involve also an E1-independent mechanism. We characterize two distinct populations of HPV18 replicons existing during the maintenance and vegetative amplification phases. We show that a subset of these replicons, including viral genome monomers, replicate in an E1-dependent manner, while some oligomeric forms of the HPV18 genome replicate independently of E1 function.IMPORTANCE Human papillomavirus (HPV) infections pose serious medical problem. To date, there are no HPV-specific antivirals available due to poor understanding of the molecular mechanisms of virus infection cycle. The infection cycle of HPV involves initial amplification of the viral genomes and maintenance of the viral genomes with a constant copy number, followed by another round of viral genome amplification and new viral particle formation. The viral protein E1 is critical for the initial amplification of the viral genome. However, E1 involvement in other phases of the viral life cycle has remained controversial. In the present study, we show that at least two different replication modes of the HPV18 genome are undertaken simultaneously during the maintenance and vegetative amplification phases, i.e., replication of the majority of the HPV18 genome proceeds under the control of the host cell replication machinery without E1 function, whereas a minority of the genome replicates in an E1-dependent manner.

Differential phosphorylation determines the repressor and activator potencies of GLI1 proteins and their efficiency in modulating the HPV life cycle

The Sonic Hedgehog (Shh) signalling pathway plays multiple roles during embryonic development and under pathological conditions. Although the core components of the Shh pathway are conserved, the regulation of signal transduction varies significantly among species and cell types. Protein kinases Ulk3 and Pka are involved in the Shh pathway as modulators of the activities of Gli transcription factors, which are the nuclear mediators of the signal. Here, we investigate the regulation and activities of two GLI1 isoforms, full-length GLI1 (GLI1FL) and GLI1ΔN. The latter protein lacks the first 128 amino acids including the conserved phosphorylation cluster and the binding motif for SUFU, the key regulator of GLI activity. Both GLI1 isoforms are co-expressed in all human cell lines analysed and possess similar DNA binding activity. ULK3 potentiates the transcriptional activity of both GLI1 proteins, whereas PKA inhibits the activity of GLI1ΔN, but not GLI1FL. In addition to its well-established role as a transcriptional activator, GLI1FL acts as a repressor by inhibiting transcription from the early promoters of human papillomavirus type 18 (HPV18). Additionally, compared to GLI1ΔN, GLI1FL is a more potent suppressor of replication of several HPV types. Altogether, our data show that the N-terminal part of GLI1FL is crucial for the realization of its full potential as a transcriptional regulator.

E2 protein is the major determinant of specificity at the human papillomavirus origin of replication

The replication of human papillomavirus (HPV) genomes requires E1 and E2 proteins as the viral trans-factors and the replication origin, located in the URR, as a cis-element. The minimal requirements for an HPV replication origin vary among different virus types but always include one or more binding sites for the E2 protein. The requirements for an E1 binding site seem to vary among different HPV genera, with alpha-HPV11 and -18 minimal origins able to replicate without E1 binding site in contrast to beta-HPV8. In the present article, we analysed the sequence requirements for the beta-HPV5 minimal origin of replication. We show that the HPV5 URR is able to replicate in U2OS cells without the sequence proposed as an E1 binding site, albeit at lower levels than wt URR, given that three E2 binding sites are intact and both viral replication proteins are present. The lack of an absolute requirement of the E1 binding site for the origin of replication of HPV5 led us to analyse whether the viral E1 and E2 proteins from other HPV types are competent to support replication from this origin. Surprisingly, the E1 and E2 proteins from beta-HPV types support replication from the origin in contrast to proteins from alpha-HPV types 11, -16, or -18. Furthermore, the replication proteins E1 and E2 of these alpha-HPV types are unable to support the replication of HPV5 URR, even if the E1 binding site is intact. In light of these results, we performed a detailed analysis of the ability of different combinations of E1 and E2 proteins from various alpha- and beta-HPV types to support the replication of URR sequences from the respective HPV types in the U2OS cell line.

Activity of CK2α protein kinase is required for efficient replication of some HPV types

Inhibition of human papillomavirus (HPV) replication is a promising therapeutic approach for intervening with HPV-related pathologies. Primary targets for interference are two viral proteins, E1 and E2, which are required for HPV replication. Both E1 and E2 are phosphoproteins; thus, the protein kinases that phosphorylate them might represent secondary targets to achieve inhibition of HPV replication. In the present study, we show that CX4945, an ATP-competitive small molecule inhibitor of casein kinase 2 (CK2) catalytic activity, suppresses replication of different HPV types, including novel HPV5NLuc, HPV11NLuc and HPV18NLuc marker genomes, but enhances the replication of HPV16 and HPV31. We further corroborate our findings using short interfering RNA (siRNA)-mediated knockdown of CK2 α and α' subunits in U2OS and CIN612 cells; we show that while both subunits are expressed in these cell lines, CK2α is required for HPV replication, but CK2α' is not. Furthermore, we demonstrate that CK2α acts in a kinase activity-dependent manner and regulates the stability and nuclear retention of endogenous E1 proteins of HPV11 and HPV18. This unique feature of CK2α makes it an attractive target for developing antiviral agents.

Characterization of an HPV33 natural variant with enhanced transcriptional activity suggests a role for C/EBPβ in the regulation of the viral early promoter

The Long Control Region (LCR) of the human papillomavirus (HPV) genome encompasses the early promoter (EP) that drives expression of the viral oncogenes in infected cells and HPV-associated cancers. Here, we report on a natural variant of HPV33 that displays higher EP activity than the prototype in transfected C33A and HeLa cervical carcinoma cells, and in the osteosarcoma U2OS cell line which supports replication of HPV episomes. This increased promoter activity was ascribed to a single nucleotide variation in the LCR, T7791C, in a putative binding site for the transcription factor C/EBPβ. T7791C abrogated binding of recombinant C/EBPβ to this site in vitro and stimulated the EP in vivo, suggesting that it abrogates a negatively-acting regulatory element. A second C/EBPβ binding site was identified in vitro that activated the EP in vivo and whose function and location in the epithelial-specific enhancer is shown to be conserved in the highly prevalent HPV18. These results suggest that C/EBPβ is both an activator and a repressor of the HPV33 EP, acting via two distinct binding sites. Prediction of C/EBPβ sites in the LCR of 186 HPV types suggests that C/EBPβ regulation of the EP is common among high‐risk viruses from the α genus.

Interaction of the Human Papillomavirus E1 Helicase with UAF1-USP1 Promotes Unidirectional Theta Replication of Viral Genomes

Human papillomaviruses (HPVs) are important pathogens that replicate their double-stranded circular DNA genome in the nucleus of infected cells. HPV genomes replicate in infected cells via bidirectional theta replication and a poorly understood unidirectional mechanism, and the onset of viral replication requires the engagement of cellular DNA damage response pathways. In this study, we showed that the previously described interaction between the viral E1 helicase and the cellular UAF1-USP1 complex is necessary for the completion of bidirectional replication and the subsequent initiation of the unidirectional replication mechanism. Our results suggest HPVs may use the cellular Fanconi anemia DNA damage pathway to achieve the separation of daughter molecules generated by bidirectional theta replication. Additionally, our results indicate that the unidirectional replication of the HPV genome is initiated from restarted bidirectional theta replication forks.

Human papillomaviruses (HPVs) are important pathogens with a significant medical burden. HPV genomes replicate in infected cells via bidirectional theta replication and a poorly understood unidirectional mechanism. In this report, we provide evidence that the previously described interaction between the viral E1 helicase and the cellular UAF1-USP1 deubiquitinating enzyme complex, a member of the Fanconi anemia DNA damage response pathway, is required for the completion of the bidirectional theta replication of the HPV11 genome and the subsequent initiation of the unidirectional replication. We show that unidirectional replication proceeds via theta structures and is supported by the cellular Bloom helicase, which interacts directly with E1 and whose engagement in HPV11 replication requires UAF1-USP1 activity. We propose that the unidirectional replication of the HPV11 genome initiates from replication fork restart events. These findings suggest a new role for the Fanconi anemia pathway in HPV replication.

The molecular biology and HPV drug responsiveness of cynomolgus macaque papillomaviruses support their use in the development of a relevant in vivo model for antiviral drug testing

Due to the extreme tissue and species restriction of the papillomaviruses (PVs), there is a great need for animal models that accurately mimic PV infection in humans for testing therapeutic strategies against human papillomaviruses (HPVs). In this study, we present data that demonstrate that in terms of gene expression during initial viral DNA amplification, Macaca fascicularis PV (MfPV) types 5 and 8 appear to be similar to mucosal oncogenic HPVs, while MfPV1 (isolated from skin) resembles most high-risk cutaneous beta HPVs (HPV5). Similarities were also observed in replication properties during the initial amplification phase of the MfPV genomes. We demonstrate that high-risk mucosal HPV-specific inhibitors target the transient replication of the MfPV8 genomes, which indicates that similar pathways are used by the high-risk HPVs and MfPVs during their genome replication. Taking all into account, we propose that Macaca fascicularis may serve as a highly relevant model for preclinical tests designed to evaluate therapeutic strategies against HPV-associated lesions.

Nuclear myosin 1 associates with papillomavirus E2 regulatory protein and influences viral replication

Nuclear myosin 1c (NM1) associates with RNA polymerases and is a partner in the chromatin remodeling complex B-WICH. This complex, which also contains WSTF and SNF2h proteins, is involved in transcriptional regulation. We report herein that papillomavirus protein E2 binds to NM1 and co-precipitates with the WSTF and SNF2h proteins. Our data suggest that E2 associates with the cellular B-WICH complex through binding to NM1. E2 and NM1 associate via their N-terminal domains and this interaction is ATP dependent. The cellular multifunctional protein Brd4 and beta-actin are also present in the NM1-E2 complex. NM1 downregulation by siRNA increases the replication of the BPV1 and HPV5 genomes but does not affect HPV18 genome replication. These results suggest that the B-WICH complex may play a role in the papillomavirus life cycle through NM1 and E2 protein interaction.

Identification of several high-risk HPV inhibitors and drug targets with a novel high-throughput screening assay

Human papillomaviruses (HPVs) are oncogenic viruses that cause numerous different cancers as well as benign lesions in the epithelia. To date, there is no effective cure for an ongoing HPV infection. Here, we describe the generation process of a platform for the development of anti-HPV drugs. This system consists of engineered full-length HPV genomes that express reporter genes for evaluation of the viral copy number in all three HPV replication stages. We demonstrate the usefulness of this system by conducting high-throughput screens to identify novel high-risk HPV-specific inhibitors. At least five of the inhibitors block the function of Tdp1 and PARP1, which have been identified as essential cellular proteins for HPV replication and promising candidates for the development of antivirals against HPV and possibly against HPV-related cancers.

Human papillomaviruses are causative agents of many different cancers; they are most commonly associated with cervical cancer which leads to about quarter of a million deaths each year. Regardless of extensive studies for decades there is no specific cure against HPV infection. During this research, we have engineered modified HPV marker genomes that express Renilla luciferase reporter gene which expression level correlates directly with viral genome copy number. We have used such modified HPV genome in high-throughput screening of NCI Diversity Set IV chemical library and have identified a number of novel high-risk HPV-specific chemical compounds and drug targets. Such Renilla-expressing marker genomes could be used in various cell systems suitable for HPV replication studies to conduct high-throughput screens and quantify viral genome copy number quickly and effectively.

Phosphorylation of the Bovine Papillomavirus E2 Protein on Tyrosine Regulates Its Transcription and Replication Functions

Papillomaviruses are small, double-stranded DNA viruses that encode the E2 protein, which controls transcription, replication, and genome maintenance in infected cells. Posttranslational modifications (PTMs) affecting E2 function and stability have been demonstrated for multiple types of papillomaviruses. Here we describe the first phosphorylation event involving a conserved tyrosine (Y) in the bovine papillomavirus 1 (BPV-1) E2 protein at amino acid 102. While its phosphodeficient phenylalanine (F) mutant activated both transcription and replication in luciferase reporter assays, a mutant that may act as a phosphomimetic, with a Y102-to-glutamate (E) mutation, lost both activities. The E2 Y102F protein interacted with cellular E2-binding factors and the viral helicase E1; however, in contrast, the Y102E mutant associated with only a subset and was unable to bind to E1. While the Y102F mutant fully supported transient viral DNA replication, BPV genomes encoding this mutation as well as Y102E were not maintained as stable episomes in murine C127 cells. These data imply that phosphorylation at Y102 disrupts the helical fold of the N-terminal region of E2 and its interaction with key cellular and viral proteins. We hypothesize that the resulting inhibition of viral transcription and replication in basal epithelial cells prevents the development of a lytic infection.

IMPORTANCE

Papillomaviruses (PVs) are small, double-stranded DNA viruses that are responsible for cervical, oropharyngeal, and various genitourinary cancers. Although vaccines against the major oncogenic human PVs are available, there is no effective treatment for existing infections. One approach to better understand the viral replicative cycle, and potential therapies to target it, is to examine the posttranslational modification of viral proteins and its effect on function. Here we have discovered that the bovine papillomavirus 1 (BPV-1) transcription and replication regulator E2 is phosphorylated at residue Y102. While a phosphodeficient mutant at this site was fully functional, a phosphomimetic mutant displayed impaired transcription and replication activity as well as a lack of an association with certain E2-binding proteins. This study highlights the influence of posttranslational modifications on viral protein function and provides additional insight into the complex interplay between papillomaviruses and their hosts.

Initial amplification of the HPV18 genome proceeds via two distinct replication mechanisms

Determining the mechanism of HPV18 replication is paramount for identifying possible drug targets against HPV infection. We used two-dimensional and three-dimensional gel electrophoresis techniques to identify replication intermediates arising during the initial amplification of HPV18 episomal genomes. We determined that the first rounds of HPV18 replication proceed via bidirectional theta structures; however, a notable accumulation of almost fully replicated HPV18 genomes indicates difficulties with the completion of theta replication. We also observed intermediates that were created by a second replication mechanism during the initial amplification of HPV18 genomes. The second replication mechanism does not utilize specific initiation or termination sequences and proceeds via a unidirectional replication fork. We suggest a significant role for the second replication mechanism during the initial replication of the HPV18 genome and propose that the second replication mechanism is recombination-dependent replication.

Human Papillomavirus Type 18 cis-Elements Crucial for Segregation and Latency

Stable maintenance replication is characteristic of the latency phase of HPV infection, during which the viral genomes are actively maintained as extrachromosomal genetic elements in infected proliferating basal keratinocytes. Active replication in the S-phase and segregation of the genome into daughter cells in mitosis are required for stable maintenance replication. Most of our knowledge about papillomavirus genome segregation has come from studies of bovine papillomavirus type 1 (BPV-1), which have demonstrated that the E2 protein cooperates with cellular trans-factors and that E2 binding sites act as cis-regulatory elements in the viral genome that are essential for the segregation process. However, the genomic organization of the regulatory region in HPVs, and the properties of the viral proteins are different from those of their BPV-1 counterparts. We have designed a segregation assay for HPV-18 and used it to demonstrate that the E2 protein performs segregation in combination with at least two E2 binding sites. The cooperative binding of the E2 protein to two E2 binding sites is a major determinant of HPV-18 genome segregation, as demonstrated by the change in spacing between adjacent binding sites #1 and #2 in the HPV-18 Upstream Regulatory Region (URR). Duplication or triplication of the natural 4 bp 5’-CGGG-3’ spacer between the E2 binding sites increased the cooperative binding of the E2 molecules as well as E2-dependent segregation. Removal of any spacing between these sites eliminated cooperative binding of the E2 protein and disabled segregation of the URR and HPV-18 genome. Transfer of these configurations of the E2 binding sites into viral genomes confirmed the role of the E2 protein and binding sites #1 and #2 in the segregation process. Additional analysis demonstrated that these sites also play an important role in the transcriptional regulation of viral gene expression from different HPV-18 promoters.

DAXX modulates human papillomavirus early gene expression and genome replication in U2OS cells

Background

The human papillomavirus (HPV) genomes can replicate, and are maintained as autonomously replicating extrachromosomal plasmids in human U2OS cells. Previous studies have shown that HPV genomes are transcriptionally active in U2OS cells and can express the viral early proteins required for initiation and establishment of HPV replication. In the present work, we have examined the involvement of cellular DAXX protein in HPV replication in U2OS cells.

Methods

We have used indirect immunofluorescence and FISH analysis in order to study HPV replication compartments in U2OS cells. In addition, we have used siRNA knock-down for examining the effect of the DAXX protein on HPV replication and transcription in U2OS cells.

Results

We show that a portion of HPV replication foci are partially co-localized with components of ND10, cellular DAXX and PML proteins. In addition, we demonstrate that the knock-down of the cellular DAXX protein modulates the HPV genome replication and transcription in U2OS cells – papillomavirus replication is reduced in the absence of this component of ND10.

Conclusions

The DAXX protein modulates the early gene expression and the transient replication of HPV genomes in U2OS cells.

The Cell Cycle Timing of Human Papillomavirus DNA Replication

Viruses manipulate the cell cycle of the host cell to optimize conditions for more efficient viral genome replication. One strategy utilized by DNA viruses is to replicate their genomes non-concurrently with the host genome; in this case, the viral genome is amplified outside S phase. This phenomenon has also been described for human papillomavirus (HPV) vegetative genome replication, which occurs in G2-arrested cells; however, the precise timing of viral DNA replication during initial and stable replication phases has not been studied. We developed a new method to quantitate newly synthesized DNA levels and used this method in combination with cell cycle synchronization to show that viral DNA replication is initiated during S phase and is extended to G2 during initial amplification but follows the replication pattern of cellular DNA during S phase in the stable maintenance phase. E1 and E2 protein overexpression changes the replication time from S only to both the S and G2 phases in cells that stably maintain viral episomes. These data demonstrate that the active synthesis and replication of the HPV genome are extended into the G2 phase to amplify its copy number and the duration of HPV genome replication is controlled by the level of the viral replication proteins E1 and E2. Using the G2 phase for genome amplification may be an important adaptation that allows exploitation of changing cellular conditions during cell cycle progression. We also describe a new method to quantify newly synthesized viral DNA levels and discuss its benefits for HPV research.

The Nuclear DNA Sensor IFI16 Acts as a Restriction Factor for Human Papillomavirus Replication through Epigenetic Modifications of the Viral Promoters

The human interferon-inducible IFI16 protein, an innate immune sensor of intracellular DNA, was recently demonstrated to act as a restriction factor for human cytomegalovirus (HCMV) and herpes simplex virus 1 (HSV-1) infection by inhibiting both viral-DNA replication and transcription. Through the use of two distinct cellular models, this study provides strong evidence in support of the notion that IFI16 can also restrict human papillomavirus 18 (HPV18) replication. In the first model, an immortalized keratinocyte cell line (NIKS) was used, in which the IFI16 protein was knocked down through the use of small interfering RNA (siRNA) technology and overexpressed following transduction with the adenovirus IFI16 (AdVIFI16) vector. The second model consisted of U2OS cells transfected by electroporation with HPV18 minicircles. In differentiated IFI16-silenced NIKS-HPV18 cells, viral-load values were significantly increased compared with differentiated control cells. Consistent with this, IFI16 overexpression severely impaired HPV18 replication in both NIKS and U2OS cells, thus confirming its antiviral restriction activity. In addition to the inhibition of viral replication, IFI16 was also able to reduce viral transcription, as demonstrated by viral-gene expression analysis in U2OS cells carrying episomal HPV18 minicircles and HeLa cells. We also provide evidence that IFI16 promotes the addition of heterochromatin marks and the reduction of euchromatin marks on viral chromatin at both early and late promoters, thus reducing both viral replication and transcription. Altogether, these results argue that IFI16 restricts chromatinized HPV DNA through epigenetic modifications and plays a broad surveillance role against viral DNA in the nucleus that is not restricted to herpesviruses.

IMPORTANCE

Intrinsic immunity is mediated by cellular restriction factors that are constitutively expressed and active even before a pathogen enters the cell. The host nuclear factor IFI16 acts as a sensor of foreign DNA and an antiviral restriction factor, as recently demonstrated by our group for human cytomegalovirus (HCMV) and herpes simplex virus 1 (HSV-1). Here, we provide the first evidence that IFI16 inhibits HPV18 replication by repressing viral-gene expression and replication. This antiviral restriction activity was observed in immortalized keratinocytes transfected with the religated genomes and in U2OS cells transfected with HPV18 minicircles, suggesting that it is not cell type specific. We also show that IFI16 promotes the assembly of heterochromatin on HPV DNA. These changes in viral chromatin structure lead to the generation of a repressive state at both early and late HPV18 promoters, thus implicating the protein in the epigenetic regulation of HPV gene expression and replication.

Model systems to study the life cycle of human papillomaviruses and HPV-associated cancers

The prevalent human papillomaviruses (HPVs) infect either cutaneous or mucosal epithelium. Active Infections lead to epithelial hyperprolifeation and are usually cleared in healthy individuals within a year. Persistent infections in the anogenital tracts by certain high-risk genotypes such as HPV-16, HPV-18 and closely related types, can progress to high grade dysplasias and carcinomas in women and men, including cervical, vulva, penile and anal cancers. A significant fraction of the head and neck cancers are also caused by HPV-16. The viral oncogenes responsible for neoplastic conversion are E6 and E7 that disrupt the pathways controlled by the two major tumor suppressor genes, p53 and members of pRB family. Because HPV cannot be propagated in conventional submerged monolayer cell cultures, organotypic epithelial raft cultures that generate a stratified and differentiated epithelium have been used to study the viral life cycle. This article describes several systems to examine aspects of the viral productive phase, along with the advantages and limitations. Animal model systems of HPV carcinogenesis are also briefly described.

The transcription map of HPV11 in U2OS cells adequately reflects the initial and stable replication phases of the viral genome

BACKGROUND

Although prophylactic vaccines have been developed against HPV6, HPV11, HPV16 and HPV18 there is the clear unmet medical need in order to justify the development of drugs targeting human papillomavirus replication. The native host cells of HPVs are human primary keratinocytes which can be cultivated in raft cultures. However, this method is difficult to use in high-throughput screening assays and the need for a cost-effective cellular system for screening potential anti-HPV drug candidates during all stages of HPV genome replication remains.

METHODS

U2OS cells were transfected with HPV11 wt or E8- minicircle genomes and their gene expression was studied via 3' RACE, 5' RACE or via real time PCR methods. The DNA replication of these genomes was detected by Southern blot methods.

RESULTS

The analysis of HPV11 transcripts in U2OS cells showed that the patterns of promoter use, splice sites and polyadenylation cleavage sites are identical to those previously characterized in human HPV-related lesions, human squamous carcinoma cell lines (e.g., SSC-4) and laryngeal papillomas. Transcriptional initiation from the three previously described HPV11 promoters in the E6 and E7 ORFs (P90, P264, and P674-714) were functional, and these promoters were used together with two promoter regions in the E1 ORF (P1092 and P1372). Mutating the E8 ORF ATG start codon to ACG eliminated the translation of fusion proteins from the E8 ORF coupled to E1 and E2 proteins C-terminal sequences, leading to the de-repression of gene expression (particularly from the P1092 promoter) and to the activation of genome replication. These data suggested that the expression of the functional E8^E2 protein is used to control viral gene expression and copy number of the HPV11 genome. The analysis of HPV11 E1 expression plasmids showed that the E6/E7 region, together with the E1 coding region, is crucial for the production of functionally active E1 protein.

CONCLUSIONS

The data presented in this paper suggest that in human osteosarcoma cell line U2OS the gene expression pattern of the HPV11 truly reflect the expression profile of the replicating HPV genome and therefore this cellular system is suitable for drug development program targeting HPV replication.

The transcription map of human papillomavirus type 18 during genome replication in U2OS cells

The human osteosarcoma cell line U2OS is useful for studying genome replication of human papillomavirus (HPVs) subtypes that belong to different phylogenetic genera. In this study, we defined the HPV18 transcription map in U2OS cells during transient replication, stable maintenance and vegetative amplification by identifying viral promoter regions, transcription polyadenylation and splicing sites during HPV18 genome replication. Mapping of the HPV18 transcription start sites in U2OS cells revealed five distinct promoter regions (P₁₀₂, P₅₂₀, P₈₁₁, P₁₁₉₃ and P₃₀₀₀). With the exception of P₃₀₀₀, all of these regions have been previously identified during productive HPV18 infection. Collectively, the data suggest that U2OS cells are suitable for studying the replication and transcription properties of HPVs and to serve as a platform for conducting high-throughput drug screens to identify HPV replication inhibitors. In addition, we have identified mRNA species that are initiated from the promoter region P₃₀₀₀, which can encode two E2C regulator proteins that contain only the C-terminal hinge and DNA-binding and dimerization domains of E2. We show that these proteins regulate the initial amplification of HPV18 by modulating viral transcription. Moreover, we show that one of these proteins can act as a transcriptional activator of promoter P₁₀₂.

Reversible mitochondrial DNA accumulation in nuclei of pluripotent stem cells

According to the endosymbiotic hypothesis, the precursor of mitochondria invaded the precursor of eukaryotic cells, a process that began roughly 2 billion years ago. Since then, the majority of the genetic material translocated from the mitochondria to the nucleus, where now almost all mitochondrial proteins are expressed. Only a tiny amount of DNA remained in the mitochondria, known as mitochondrial DNA (mtDNA). In this study, we report that the transfer of mtDNA fragments to the nucleus of pluripotent stem cells is still ongoing. We show by in situ hybridization and agarose DNA two-dimensional gel technique that induced pluripotent stem (iPS) cells contain high levels of mtDNA in the nucleus. We found that a large proportion of the accumulated mtDNA sequences appear to be extrachromosomal. Accumulation of mtDNA in the nucleus is present not only in the iPS cells, but also in embryonic stem (ES) cells. However upon differentiation, the level of mtDNA in the nuclei of iPS and ES cells is substantially reduced. This reversible accumulation of mtDNA in the nucleus supports the notion that the nuclear copy number of mtDNA sequences may provide a novel mechanism by which chromosomal DNA is dynamically regulated in pluripotent stem cells.

Mapping of betapapillomavirus human papillomavirus 5 transcription and characterization of viral-genome replication function

Betapapillomavirus replication and transcription have not been studied in detail because of a lack of suitable cellular systems supporting human papillomavirus (HPV) genome replication. We have recently shown that the human osteosarcoma cell line U2OS provides a useful environment for the genome replication of many different HPVs, including the betapapillomaviruses HPV5 and HPV8. Using mutational analysis and complementation assay, we demonstrated herein that the viral early proteins E1 and E2 are viral transfactors that are necessary and sufficient for HPV5 genome replication. We also identified four HPV5 early promoter regions with transcription start sites (TSSs) at nucleotides (nt) 184/191, 460, 840, and 1254, respectively, and the HPV late promoter with a TSS at nt 7640. In addition, we mapped the HPV5 early polyadenylation cleavage sites via 3' rapid amplification of cDNA ends (3'RACE) to nt 4457 and 4475. In total, 14 different viral mRNA species, originating from the HPV5 genome, were mapped in U2OS cells during transient and stable replication. The main splicing donor and acceptor sites identified herein are consistent with the data previously obtained in HPV5-positive skin lesions. In addition, we identified novel E8 open reading frame (ORF)-containing transcripts (E8^E1C and E8^E2C) expressed from the HPV5 genome. Similar to several other papillomaviruses, the product of the E8^E2C mRNA acts as a repressor of viral genome replication.

Recombination-dependent oligomerization of human papillomavirus genomes upon transient DNA replication

We describe the extensive and progressive oligomerization of human papillomavirus (HPV) genomes after transfection into the U2OS cell line. The HPV genomic oligomers are extrachromosomal concatemeric molecules containing the viral genome in a head-to-tail orientation. The process of oligomerization does not depend on the topology of the input DNA, and it does not require any other viral factors besides replication proteins E1 and E2. We provide evidence that oligomerization of the HPV18 and HPV11 genomes involves homologous recombination. We also demonstrate oligomerization of the HPV18 and HPV11 genomes in SiHa, HeLa, and C-33 A cell lines and provide examples of oligomeric HPV genomes in clinical samples obtained from HPV-infected patients.

Engagement of the ATR-dependent DNA damage response at the human papillomavirus 18 replication centers during the initial amplification

We have previously demonstrated that the human papillomavirus (HPV) genome replicates effectively in U2OS cells after transfection using electroporation. The transient extrachromosomal replication, stable maintenance, and late amplification of the viral genome could be studied for high- and low-risk mucosal and cutaneous papillomaviruses. Recent findings indicate that the cellular DNA damage response (DDR) is activated during the HPV life cycle and that the viral replication protein E1 might play a role in this process. We used a U2OS cell-based system to study E1-dependent DDR activation and the involvement of these pathways in viral transient replication. We demonstrated that the E1 protein could cause double-strand DNA breaks in the host genome by directly interacting with DNA. This activity leads to the induction of an ATM-dependent signaling cascade and cell cycle arrest in the S and G(2) phases. However, the transient replication of HPV genomes in U2OS cells induces the ATR-dependent pathway, as shown by the accumulation of γH2AX, ATR-interacting protein (ATRIP), and topoisomerase IIβ-binding protein 1 (TopBP1) in viral replication centers. Viral oncogenes do not play a role in this activation, which is induced only through DNA replication or by replication proteins E1 and E2. The ATR pathway in viral replication centers is likely activated through DNA replication stress and might play an important role in engaging cellular DNA repair/recombination machinery for effective replication of the viral genome upon active amplification.

Phosphorylation regulates binding of the human papillomavirus type 8 E2 protein to host chromosomes

The papillomavirus E2 proteins are indispensable for the viral life cycle, and their functions are subject to tight regulation. The E2 proteins undergo posttranslational modifications that regulate their properties and roles in viral transcription, replication, and genome maintenance. During persistent infection, the E2 proteins from many papillomaviruses act as molecular bridges that tether the viral genomes to host chromosomes to retain them within the host nucleus and to partition them to daughter cells. The betapapillomavirus E2 proteins bind to pericentromeric regions of host mitotic chromosomes, including the ribosomal DNA loci. We recently reported that two residues (arginine 250 and serine 253) within the chromosome binding region of the human papillomavirus type 8 (HPV8) E2 protein are required for this binding. In this study, we show that serine 253 is phosphorylated, most likely by protein kinase A, and this modulates the interaction of the E2 protein with cellular chromatin. Furthermore, we show that this phosphorylation occurs in S phase, increases the half-life of the E2 protein, and promotes chromatin binding from S phase through mitosis.

Activation of cap-dependent translation by mucosal human papillomavirus E6 proteins is dependent on the integrity of the LXXLL binding motif

The human papillomavirus (HPV) type 16 (HPV16) E6 protein can stimulate mechanistic target of rapamycin complex 1 (mTORC1) signaling and cap-dependent translation through activation of the PDK1 and mTORC2 kinases. Here we report that HPV18 E6 also enhances cap-dependent translation. The integrity of LXXLL and PDZ protein binding domains is important for activation of cap-dependent translation by high-risk mucosal HPV E6 proteins. Consistent with this model, low-risk mucosal HPV6b and HPV11 E6 proteins, which do not contain a PDZ protein binding motif, also activate cap-dependent translation and mTORC1, albeit at a lower efficiency than high-risk HPV E6 proteins. In contrast, cutaneous HPV5 and HPV8 E6 proteins, which lack LXXLL and PDZ motif protein binding, do not enhance cap-dependent translation. Mutagenic analyses of low-risk HPV E6 proteins revealed that association with the LXXLL motif containing ubiquitin ligase E6AP (UBE3A) correlates with activation of cap-dependent translation. Hence, activation of mTORC1 and cap-dependent translation may be important for the viral life cycle in specific epithelial tissue types and contribute to cellular transformation in cooperation with other biological activities of high-risk HPV E6-containing proteins.

Topical vitamin A treatment of recalcitrant common warts

BACKGROUND

Common warts (verruca vulgaris) are benign epithelial proliferations associated with human papillomavirus (HPV) infection. Salicylic acid and cryotherapy are the most frequent treatments for common warts, but can be painful and cause scarring, and have high failure and recrudescence rates. Topical vitamin A has been shown to be a successful treatment of common warts in prior informal studies.

CASE

The subject is a healthy, physically-active 30 old female with a 9 year history of common warts on the back of the right hand. The warts resisted treatment with salicylic acid, apple cider vinegar and an over-the-counter blend of essential oils marketed for the treatment of warts. Daily topical application of natural vitamin A derived from fish liver oil (25,000 IU) led to replacement of all the warts with normal skin. Most of the smaller warts had been replaced by 70 days. A large wart on the middle knuckle required 6 months of vitamin A treatment to resolve completely.

CONCLUSION

Retinoids should be further investigated in controlled studies to determine their effectiveness in treating common warts and the broad range of other benign and cancerous lesions induced by HPVs.