Acetylation of intragenic histones on HPV16 correlates with enhanced HPV16 gene expression

Advances in cervical cancer: current insights and future directions

In alignment with the World Health Organization's strategy to eliminate cervical cancer, substantial progress has been made in the treatment of this malignancy. Cervical cancer, largely driven by human papillomavirus (HPV) infection, is considered preventable and manageable because of its well-established etiology. Advancements in precision screening technologies, such as DNA methylation triage, HPV integration detection, liquid biopsies, and artificial intelligence-assisted diagnostics, have augmented traditional screening methods such as HPV nucleic acid testing and cytology. Therapeutic strategies aimed at eradicating HPV and reversing precancerous lesions have been refined as pivotal measures for disease prevention. The controversy surrounding surgery for early-stage cervical cancer revolves around identifying optimal candidates for minimally invasive and conservative procedures without compromising oncological outcomes. Recent clinical trials have yielded promising results for the development of systemic therapies for advanced cervical cancer. Immunotherapies, such as immune checkpoint inhibitors (ICIs), antibody-drug conjugates (ADCs), and targeted therapy have demonstrated significant effectiveness, marking a substantial advancement in cervical cancer management. Various combination therapies have been validated, and ongoing trials aim to enhance outcomes through the development of novel drugs and optimized combination regimens. The prospect of eradicating cervical cancer as the first malignancy to be eliminated is now within reach. In this review, we provide a comprehensive overview of the latest scientific insights, with a particular focus on precision managements for various stages of cervical disease, and explore future research directions in cervical cancer.

The role of epigenetics in women's reproductive health: the impact of environmental factors

This paper explores the significant role of epigenetics in women's reproductive health, focusing on the impact of environmental factors. It highlights the crucial link between epigenetic modifications-such as DNA methylation and histones post-translational modifications-and reproductive health issues, including infertility and pregnancy complications. The paper reviews the influence of pollutants like PM2.5, heavy metals, and endocrine disruptors on gene expression through epigenetic mechanisms, emphasizing the need for understanding how dietary, lifestyle choices, and exposure to chemicals affect gene expression and reproductive health. Future research directions include deeper investigation into epigenetics in female reproductive health and leveraging gene editing to mitigate epigenetic changes for improving IVF success rates and managing reproductive disorders.

Comparative Review on Cancer Pathology from Aberrant Histone Chaperone Activity

Histone chaperones are integral to chromatin dynamics, facilitating the assembly and disassembly of nucleosomes, thereby playing a crucial role in regulating gene expression and maintaining genomic stability. Moreover, they prevent aberrant histone interactions prior to chromatin assembly. Disruption in histone chaperone function may result in genomic instability, which is implicated in pathogenesis. This review aims to elucidate the role of histone chaperones in cancer pathologies and explore their potential as therapeutic targets. Histone chaperones have been found to be dysregulated in various cancers, with alterations in expression levels, mutations, or aberrant interactions leading to tumorigenesis and cancer progression. In addition, this review intends to highlight the molecular mechanisms of interactions between histone chaperones and oncogenic factors, underscoring their roles in cancer cell survival and proliferation. The dysregulation of histone chaperones is significantly correlated with cancer development, establishing them as active contributors to cancer pathology and viable targets for therapeutic intervention. This review advocates for continued research into histone chaperone-targeted therapies, which hold potential for precision medicine in oncology. Future advancements in understanding chaperone functions and interactions are anticipated to lead to novel cancer treatments, enhancing patient care and outcomes.

Activation of AMPK inhibits cervical cancer growth by hyperacetylation of H3K9 through PCAF

BACKGROUND

Dysregulation in histone acetylation, a significant epigenetic alteration closely associated with major pathologies including cancer, promotes tumorigenesis, inactivating tumor-suppressor genes and activating oncogenic pathways. AMP-activated protein kinase (AMPK) is a cellular energy sensor that regulates a multitude of biological processes. Although a number of studies have identified the mechanisms by which AMPK regulates cancer growth, the underlying epigenetic mechanisms remain unknown.

METHODS

The impact of metformin, an AMPK activator, on cervical cancer was evaluated through assessments of cell viability, tumor xenograft model, pan-acetylation analysis, and the role of the AMPK-PCAF-H3K9ac signaling pathway. Using label-free quantitative acetylproteomics and chromatin immunoprecipitation-sequencing (ChIP) technology, the activation of AMPK-induced H3K9 acetylation was further investigated.

RESULTS

In this study, we found that metformin, acting as an AMPK agonist, activates AMPK, thereby inhibiting the proliferation of cervical cancer both in vitro and in vivo. Mechanistically, AMPK activation induces H3K9 acetylation at epigenetic level, leading to chromatin remodeling in cervical cancer. This also enhances the binding of H3K9ac to the promoter regions of multiple tumor suppressor genes, thereby promoting their transcriptional activation. Furthermore, the absence of PCAF renders AMPK activation incapable of inducing H3K9 acetylation.

CONCLUSIONS

In conclusion, our findings demonstrate that AMPK mediates the inhibition of cervical cancer growth through PCAF-dependent H3K9 acetylation. This discovery not only facilitates the clinical application of metformin but also underscores the essential role of PCAF in AMPK activation-induced H3K9 hyperacetylation.

RNA elements that control human papillomavirus mRNA splicing-targets for therapy?

Human papillomaviruses (HPVs) cause more than 4.5% of all cancer in the world and more than half of these cases are attributed to human papillomavirus type 16 (HPV16). Prophylactic vaccines are available but antiviral drugs are not. Novel targets for therapy are urgently needed. Alternative RNA splicing is extensively used by HPVs to express all their genes and HPV16 is no exception. This process must function to perfection since mis-splicing could perturb the HPV gene expression program by altering mRNA levels or by generating dysfunctional mRNAs. Cis-acting RNA elements on the viral mRNAs and their cognate cellular trans-acting factors control papillomavirus RNA splicing. The precise but delicate nature of the splicing process renders splicing sensitive to interference. As such, papillomavirus RNA splicing is a potential target for therapy. Here we summarize our current understanding of cis-acting HPV16 RNA elements that control HPV16 mRNA splicing via cellular proteins and discuss how they may be exploited as targets for therapy to papillomavirus infections and cancer.

The human papillomavirus late life cycle and links to keratinocyte differentiation

Regulation of human papillomavirus (HPV) gene expression is tightly linked to differentiation of the keratinocytes the virus infects. HPV late gene expression is confined to the cells in the upper layers of the epithelium where the virus capsid proteins are synthesized. As these proteins are highly immunogenic, and the upper epithelium is an immune-privileged site, this spatial restriction aids immune evasion. Many decades of work have contributed to the current understanding of how this restriction occurs at a molecular level. This review will examine what is known about late gene expression in HPV-infected lesions and will dissect the intricacies of late gene regulation. Future directions for novel antiviral approaches will be highlighted.

Advances in epigenetic modifications and cervical cancer research

Cervical cancer (CC) is an important public health problem for women, and perspectives and information regarding its prevention and treatment are quickly evolving. Human papilloma virus (HPV) has been recognized as a major contributor to CC development; however, HPV infection is not the only cause of CC. Epigenetics refers to changes in gene expression levels caused by non-gene sequence changes. Growing evidence suggests that the disruption of gene expression patterns which were governed by epigenetic modifications can result in cancer, autoimmune diseases, and various other maladies. This article mainly reviews the current research status of epigenetic modifications in CC based on four aspects, respectively DNA methylation, histone modification, noncoding RNA regulation and chromatin regulation, and we also discuss their functions and molecular mechanisms in the occurrence and progression of CC. This review provides new ideas for early screening, risk assessment, molecular targeted therapy and prognostic prediction of CC.

The role of RNA-binding proteins in the processing of mRNAs produced by carcinogenic papillomaviruses

Human papillomaviruses (HPV) are epitheliotropic DNA tumor viruses that are prevalent in the human population. A subset of the HPVs termed high-risk HPVs (HR-HPVs) are causative agents of anogenital cancers and head-and-neck cancers. Cancer is the result of persistent high-risk HPV infections that have not been cleared by the immune system of the host. These infections are characterized by dysregulated HPV gene expression, in particular constitutive high expression of the HPV E6 and E7 oncogenes and absence of the highly immunogenic viral L1 and L2 capsid proteins. HPVs make extensive use of alternative mRNA splicing to express its genes and are therefore highly dependent on cellular RNA-binding proteins for proper gene expression. Levels of RNA-binding proteins are altered in HPV-containing premalignant cervical lesions and in cervical cancer. Here we review our current knowledge of RNA-binding proteins that control HPV gene expression. We focus on RNA-binding proteins that control expression of the E6 and E7 oncogenes since they initiate and drive development of cancer and on the immunogenic L1 and L2 proteins as there silencing may contribute to immune evasion during carcinogenesis. Furthermore, cellular RNA-binding proteins are essential for HPV gene expression and as such may be targets for therapy to HPV infections and HPV-driven cancers.

hnRNP G/RBMX enhances HPV16 E2 mRNA splicing through a novel splicing enhancer and inhibits production of spliced E7 oncogene mRNAs

Human papillomavirus type 16 (HPV16) E2 is an essential HPV16 protein. We have investigated how HPV16 E2 expression is regulated and have identifed a splicing enhancer that is required for production of HPV16 E2 mRNAs. This uridine-less splicing enhancer sequence (ACGAGGACGAGGACAAGGA) contains 84% adenosine and guanosine and 16% cytosine and consists of three ‘AC(A/G)AGG’-repeats. Mutational inactivation of the splicing enhancer reduced splicing to E2-mRNA specific splice site SA2709 and resulted in increased levels of unspliced E1-encoding mRNAs. The splicing enhancer sequence interacted with cellular RNA binding protein hnRNP G that promoted splicing to SA2709 and enhanced E2 mRNA production. The splicing-enhancing function of hnRNP G mapped to amino acids 236–286 of hnRNP G that were also shown to interact with splicing factor U2AF65. The interactions between hnRNP G and HPV16 E2 mRNAs and U2AF65 increased in response to keratinocyte differentiation as well as by the induction of the DNA damage response (DDR). The DDR reduced sumoylation of hnRNP G and pharmacological inhibition of sumoylation enhanced HPV16 E2 mRNA splicing and interactions between hnRNP G and E2 mRNAs and U2AF65. Intriguingly, hnRNP G also promoted intron retention of the HPV16 E6 coding region thereby inhibiting production of spliced E7 oncogene mRNAs.

HnRNP D activates production of HPV16 E1 and E6 mRNAs by promoting intron retention

Human papillomavirus type 16 (HPV16) E1 and E6 proteins are produced from mRNAs with retained introns, but it has been unclear how these mRNAs are generated. Here, we report that hnRNP D act as a splicing inhibitor of HPV16 E1/E2- and E6/E7-mRNAs thereby generating intron-containing E1- and E6-mRNAs, respectively. N- and C-termini of hnRNP D contributed to HPV16 mRNA splicing control differently. HnRNP D interacted with the components of splicing machinery and with HPV16 RNA to exert its inhibitory function. As a result, the cytoplasmic levels of intron-retained HPV16 mRNAs were increased in the presence of hnRNP D. Association of hnRNP D with HPV16 mRNAs in the cytoplasm was observed, and this may correlate with unexpected inhibition of HPV16 E1- and E6-mRNA translation. Notably, hnRNP D40 interacted with HPV16 mRNAs in an HPV16-driven tonsillar cancer cell line and in HPV16-immortalized human keratinocytes. Furthermore, knockdown of hnRNP D in HPV16-driven cervical cancer cells enhanced production of the HPV16 E7 oncoprotein. Our results suggest that hnRNP D plays significant roles in the regulation of HPV gene expression and HPV-associated cancer development.

The role of HPV-induced epigenetic changes in cervical carcinogenesis (Review)

Cervical cancer is associated with infection by certain types of human papillomaviruses (HPVs), and this affects women worldwide. Despite the improvements in prevention and cure of HPV-induced cervical cancer, it remains the second most common type of cancer in women in the least developed regions of the world. Epigenetic modifications are stable long-term changes that occur in the DNA, and are part of a natural evolutionary process of necessary adaptations to the environment. They do not result in changes in the DNA sequence, but do affect gene expression and genomic stability. Epigenetic changes are important in several biological processes. The effects of the environment on gene expression can contribute to the development of numerous diseases. Epigenetic modifications may serve a critical role in cancer cells, by silencing tumor suppressor genes, activating oncogenes, and exacerbating defects in DNA repair mechanisms. Although cervical cancer is directly related to a persistent high-risk HPV infection, several epigenetic changes have been identified in both the viral DNA and the genome of the infected cells: DNA methylation, histone modification and gene silencing by non-coding RNAs, which initiate and sustain epigenetic changes. In the present review, recent advances in the role of epigenetic changes in cervical cancer are summarized.

Histone Deacetylase Inhibitors as Therapeutic Interventions on Cervical Cancer Induced by Human Papillomavirus

The role of epigenetic modifications on the carcinogenesis process has received a lot of attention in the last years. Among those, histone acetylation is a process regulated by histone deacetylases (HDAC) and histone acetyltransferases (HAT), and it plays an important role in epigenetic regulation, allowing the control of the gene expression. HDAC inhibitors (HDACi) induce cancer cell cycle arrest, differentiation, and cell death and reduce angiogenesis and other cellular events. Human papillomaviruses (HPVs) are small, non-enveloped double-stranded DNA viruses. They are major human carcinogens, being intricately linked to the development of cancer in 4.5% of the patients diagnosed with cancer worldwide. Long-term infection of high-risk (HR) HPV types, mainly HPV16 and HPV18, is one of the major risk factors responsible for promoting cervical cancer development. In vitro and in vivo assays have demonstrated that HDACi could be a promising therapy to HPV-related cervical cancer. Regardless of some controversial studies, the therapy with HDACi could target several cellular targets which HR-HPV oncoproteins could be able to deregulate. This review article describes the role of HDACi as a possible intervention in cervical cancer treatment induced by HPV, highlighting the main advances reached in the last years and providing insights for further investigations regarding those agents against cervical cancer.

Role of Viral Ribonucleoproteins in Human Papillomavirus Type 16 Gene Expression

Human papillomaviruses (HPVs) depend on the cellular RNA-processing machineries including alternative RNA splicing and polyadenylation to coordinate HPV gene expression. HPV RNA processing is controlled by cis-regulatory RNA elements and trans-regulatory factors since the HPV splice sites are suboptimal. The definition of HPV exons and introns may differ between individual HPV mRNA species and is complicated by the fact that many HPV protein-coding sequences overlap. The formation of HPV ribonucleoproteins consisting of HPV pre-mRNAs and multiple cellular RNA-binding proteins may result in the different outcomes of HPV gene expression, which contributes to the HPV life cycle progression and HPV-associated cancer development. In this review, we summarize the regulation of HPV16 gene expression at the level of RNA processing with focus on the interactions between HPV16 pre-mRNAs and cellular RNA-binding factors.

Tip60 and p300 function antagonistically in the epigenetic regulation of HPV18 E6/E7 genes in cervical cancer HeLa cells

BACKGROUND

High-risk HPV is a causative factor of cervical cancer. HPV DNA fragments integrate into host genome resulting in the constitutive expression of HPV genes E6 and E7 under the regulation of transcription factors, such as p300 and Tip60. Interestingly, Tip60, a factor with HAT (histone acetyl transferase) activity, represses HPV18 E6/E7 genes while another HAT p300 activates the transcription of HPV18 E6/E7.

OBJECTIVE

To explore the mechanism for the opposite roles of Tip60 and p300 in the virus gene regulation, and the influence of Tip60 and p300 in histone modifications in the regulatory sequence of HPV18 genes.

METHODS

Tip60 or p300 was either knocked down or overexpressed in HeLa cells. The effects on HPV E6E7 expression were determined with RT-qPCR. The association of RNA polymerase II and the enrichment of acetylated or methylated histones in HPV promoter region were measured by ChIP assays with specific antibodies.

RESULTS

ChIP results showed that Tip60 and p300 differently affected the modifications of histone H3K9 and the deposition of nucleosomes in HPV18 long control region (LCR). HPV18 LCR in HeLa cells is bivalent chromatin carrying both the active histone H3K9 acetylation mark and the repressive histone H3K9 trimethylation mark, the balance is maintained by Tip60 and p300.

CONCLUSION(S)

Based on the roles of Tip60 and p300 in HPV gene regulation, chemical compounds targeting Tip60 or p300 are potential anti-cervical cancer drugs.

Short half-life of HPV16 E6 and E7 mRNAs sensitizes HPV16-positive tonsillar cancer cell line HN26 to DNA-damaging drugs

Here we show that treatment of the HPV16-positive tonsillar cancer cell line HN26 with DNA alkylating cancer drug melphalan-induced p53 and activated apoptosis. Melphalan reduced the levels of RNA polymerase II and cellular transcription factor Sp1 that were associated with HPV16 DNA. The resulting inhibition of transcription caused a rapid loss of the HPV16 early mRNAs encoding E6 and E7 as a result of their inherent instability. As a consequence of HPV16 E6 and E7 down-regulation, the DNA damage inflicted on the cells by melphalan caused induction of p53 and activation of apoptosis in the HN26 cells. The BARD1-negative phenotype of the HN26 cells may have contributed to the failure to repair DNA damage caused by melphalan, as well as to the efficient apoptosis induction. Finally, nude mice carrying the HPV16 positive tonsillar cancer cells responded better to melphalan than to cisplatin, the chemotherapeutic drug of choice for tonsillar cancer. We concluded that the short half-life of the HPV16 E6 and E7 mRNAs renders HPV16-driven tonsillar cancer cells particularly sensitive to DNA damaging agents such as melphalan since melphalan both inhibits transcription and causes DNA damage.

SETD2-dependent H3K36me3 plays a critical role in epigenetic regulation of the HPV31 life cycle

The life cycle of HPV is tied to the differentiation status of its host cell, with productive replication, late gene expression and virion production restricted to the uppermost layers of the stratified epithelium. HPV DNA is histone-associated, exhibiting a chromatin structure similar to that of the host chromosome. Although HPV chromatin is subject to histone post-translational modifications, how the viral life cycle is epigenetically regulated is not well understood. SETD2 is a histone methyltransferase that places the trimethyl mark on H3K36 (H3K36me3), a mark of active transcription. Here, we define a role for SETD2 and H3K36me3 in the viral life cycle. We have found that HPV positive cells exhibit increased levels of SETD2, with SETD2 depletion leading to defects in productive viral replication and splicing of late viral RNAs. Reducing H3K36me3 by overexpression of KDM4A, an H3K36me3 demethylase, or an H3.3K36M transgene also blocks productive viral replication, indicating a significant role for this histone modification in facilitating viral processes. H3K36me3 is enriched on the 3' end of the early region of the high-risk HPV31 genome in a SETD2-dependent manner, suggesting that SETD2 may regulate the viral life cycle through the recruitment of H3K36me3 readers to viral DNA. Intriguingly, we have found that activation of the ATM DNA damage kinase, which is required for productive viral replication, is necessary for the maintenance of H3K36me3 on viral chromatin and for processing of late viral RNAs. Additionally, we have found that the HPV31 E7 protein maintains the increased SETD2 levels in infected cells through an extension of protein half-life. Collectively, our findings highlight the importance of epigenetic modifications in driving the viral life cycle and identify a novel role for E7 as well as the DNA damage response in the regulation of viral processes through epigenetic modifications.

Role of the DNA Damage Response in Human Papillomavirus RNA Splicing and Polyadenylation

Human papillomaviruses (HPVs) have evolved to use the DNA repair machinery to replicate its DNA genome in differentiated cells. HPV activates the DNA damage response (DDR) in infected cells. Cellular DDR factors are recruited to the HPV DNA genome and position the cellular DNA polymerase on the HPV DNA and progeny genomes are synthesized. Following HPV DNA replication, HPV late gene expression is activated. Recent research has shown that the DDR factors also interact with RNA binding proteins and affects RNA processing. DDR factors activated by DNA damage and that associate with HPV DNA can recruit splicing factors and RNA binding proteins to the HPV DNA and induce HPV late gene expression. This induction is the result of altered alternative polyadenylation and splicing of HPV messenger RNA (mRNA). HPV uses the DDR machinery to replicate its DNA genome and to activate HPV late gene expression at the level of RNA processing.

Adenosine causes read-through into the late region of the HPV16 genome in a guanosine-dependent manner

Adenosine plays an important role in cell death and differentiation as well as in tumorigenesis and the intra- and extra-cellular levels range from nanomolar to millimolar levels under various physiological or pathophysiological conditions. Here we report that adenosine can activate HPV16 late gene expression in a dose- and time-dependent manner, but only in the presence of guanosine. This activation occurred within hours after addition of the nucleosides and was primarily dependent on the ENT1 nucleoside transporter protein. Induction of HPV16 late gene expression was mainly the result of increased read-through at the early HPV16 polyadenylation signal into the late region of the HPV16 genome, thereby producing HPV16 late L2 mRNAs. The effect of guanosine and adenosine on HPV16 late gene expression was mediated by the increased binding to HPV16 mRNAs and nuclear export of the cellular HuR protein. Our results demonstrate that nucleosides can affect HPV16 gene expression.

The DNA damage response activates HPV16 late gene expression at the level of RNA processing

We show that the alkylating cancer drug melphalan activated the DNA damage response and induced human papillomavirus type 16 (HPV16) late gene expression in an ATM- and Chk1/2-dependent manner. Activation of HPV16 late gene expression included inhibition of the HPV16 early polyadenylation signal that resulted in read-through into the late region of HPV16. This was followed by activation of the exclusively late, HPV16 splice sites SD3632 and SA5639 and production of spliced late L1 mRNAs. Altered HPV16 mRNA processing was paralleled by increased association of phosphorylated BRCA1, BARD1, BCLAF1 and TRAP150 with HPV16 DNA, and increased association of RNA processing factors U2AF65 and hnRNP C with HPV16 mRNAs. These RNA processing factors inhibited HPV16 early polyadenylation and enhanced HPV16 late mRNA splicing, thereby activating HPV16 late gene expression.

hnRNP L controls HPV16 RNA polyadenylation and splicing in an Akt kinase-dependent manner

Inhibition of the Akt kinase activates HPV16 late gene expression by reducing HPV16 early polyadenylation and by activating HPV16 late L1 mRNA splicing. We identified ‘hot spots’ for RNA binding proteins at the early polyA signal and at splice sites on HPV16 late mRNAs. We observed that hnRNP L was associated with sequences at all HPV16 late splice sites and at the early polyA signal. Akt kinase inhibition resulted in hnRNP L dephosphorylation and reduced association of hnRNP L with HPV16 mRNAs. This was accompanied by an increased binding of U2AF65 and Sam68 to HPV16 mRNAs. Furthermore, siRNA knock-down of hnRNP L or Akt induced HPV16 gene expression. Treatment of HPV16 immortalized keratinocytes with Akt kinase inhibitor reduced hnRNP L binding to HPV16 mRNAs and induced HPV16 L1 mRNA production. Finally, deletion of the hnRNP L binding sites in HPV16 subgenomic expression plasmids resulted in activation of HPV16 late gene expression. In conclusion, the Akt kinase inhibits HPV16 late gene expression at the level of RNA processing by controlling the RNA-binding protein hnRNP L. We speculate that Akt kinase activity upholds an intracellular milieu that favours HPV16 early gene expression and suppresses HPV16 late gene expression.

Splicing and Polyadenylation of Human Papillomavirus Type 16 mRNAs

The human papillomavirus type 16 (HPV16) life cycle can be divided into an early stage in which the HPV16 genomic DNA is replicated, and a late stage in which the HPV16 structural proteins are synthesized and virions are produced. A strong coupling between the viral life cycle and the differentiation state of the infected cell is highly characteristic of all HPVs. The switch from the HPV16 early gene expression program to the late requires a promoter switch, a polyadenylation signal switch and a shift in alternative splicing. A number of cis-acting RNA elements on the HPV16 mRNAs and cellular and viral factors interacting with these elements are involved in the control of HPV16 gene expression. This review summarizes our knowledge of HPV16 cis-acting RNA elements and cellular and viral trans-acting factors that regulate HPV16 gene expression at the level of splicing and polyadenylation.

Genome-wide association studies and epigenome-wide association studies go together in cancer control

Completion of the human genome a decade ago laid the foundation for: using genetic information in assessing risk to identify individuals and populations that are likely to develop cancer, and designing treatments based on a person's genetic profiling (precision medicine). Genome-wide association studies (GWAS) completed during the past few years have identified risk-associated single nucleotide polymorphisms that can be used as screening tools in epidemiologic studies of a variety of tumor types. This led to the conduct of epigenome-wide association studies (EWAS). This article discusses the current status, challenges and research opportunities in GWAS and EWAS. Information gained from GWAS and EWAS has potential applications in cancer control and treatment.

HPV16 oncogene expression levels during early cervical carcinogenesis are determined by the balance of epigenetic chromatin modifications at the integrated virus genome

In cervical squamous cell carcinomas, high-risk human papillomavirus (HRHPV) DNA is usually integrated into host chromosomes. Multiple integration events are thought to be present within the cells of a polyclonal premalignant lesion and the features that underpin clonal selection of one particular integrant remain poorly understood. We previously used the W12 model system to generate a panel of cervical keratinocyte clones, derived from cells of a low-grade premalignant lesion naturally infected with the major HRHPV type, HPV16. The cells were isolated regardless of their selective advantage and differed only by the site of HPV16 integration into the host genome. We used this resource to test the hypothesis that levels of HPV16 E6/E7 oncogene expression in premalignant cells are regulated epigenetically. We performed a comprehensive analysis of the epigenetic landscape of the integrated HPV16 DNA in selected clones, in which levels of virus oncogene expression per DNA template varied ~6.6-fold. Across the cells examined, higher levels of virus expression per template were associated with more open chromatin at the HPV16 long control region, together with greater loading of chromatin remodelling enzymes and lower nucleosome occupancy. There were higher levels of histone post-translational modification hallmarks of transcriptionally active chromatin and lower levels of repressive hallmarks. There was greater abundance of the active/elongating form of the RNA polymerase-II enzyme (RNAPII-Ser2P), together with CDK9, the component of positive transcription elongation factor b complex responsible for Ser2 phosphorylation. The changes observed were functionally significant, as cells with higher HPV16 expression per template showed greater sensitivity to depletion and/or inhibition of histone acetyltransferases and CDK9 and less sensitivity to histone deacetylase inhibition. We conclude that virus gene expression per template following HPV16 integration is determined through multiple layers of epigenetic regulation, which are likely to contribute to selection of individual cells during cervical carcinogenesis.

RNA Binding Proteins that Control Human Papillomavirus Gene Expression

The human papillomavirus (HPV) life cycle is strictly linked to the differentiation program of the infected mucosal epithelial cell. In the basal and lower levels of the epithelium, early genes coding for pro-mitotic proteins and viral replication factors are expressed, while terminal cell differentiation is required for activation of late gene expression and production of viral particles at the very top of the epithelium. Such productive infections are normally cleared within 18–24 months. In rare cases, the HPV infection is stuck in the early stage of the infection. Such infections may give rise to cervical lesions that can progress to cancer, primarily cancer of the uterine cervix. Since cancer progression is strictly linked to HPV gene expression, it is of interest to understand how HPV gene expression is regulated. Cis-acting HPV RNA elements and cellular RNA-binding proteins control HPV mRNA splicing and polyadenylation. These interactions are believed to play a particularly important role in the switch from early to late gene expression, thereby contributing to the pathogenesis of HPV. Indeed, it has been shown that the levels of various RNA binding proteins change in response to differentiation and in response to HPV induced cervical lesions and cancer. Here we have compiled published data on RNA binding proteins involved in the regulation of HPV gene expression.