Hypermutation in the E2 gene of human papillomavirus type 16 in cervical intraepithelial neoplasia

Carcinogenesis and epidemiology of cervical cancer: The hallmark of human papillomavirus-associated cancer

Cervical cancer affects women worldwide and is the most common human papillomavirus (HPV)-associated cancer. Carcinogenesis caused by HPV results in specific cancer behavior because of the underlying viral infection. The mechanism and timing of the transformation from viral infection to cancer cells have been elucidated in detail. Treatments for this cancer are based on its characteristics and are being implemented. Moreover, HPV infection is widespread worldwide and is transmitted through sexual activity. Although the HPV vaccination is the most effective strategy of preventing cervical cancer, it is not feasible to vaccinate the entire human population especially in low- and middle-income countries. In order to consider the next step for HPV vaccination, we need to understand the characteristics of HPV carcinogenesis and cervical cancer. Additionally, treatment aimed at preservation of reproductive function in patients with cervical cancer is often required, as the cervix is a reproductive organ and because the disease is more prevalent in the adolescent and young adult generation. Thus, there are still many challenges in the diagnosis, treatment, and prevention of cervical cancer.

Anelloviruses versus human immunity: how do we control these viruses?

One continuous companion and one of the major players in the human blood virome are members of the Anelloviridae family. Anelloviruses are probably found in all humans, infection occurs early in life and the composition (anellome) is thought to remain stable and personal during adulthood. The stable anellome implies a great balance between the host immune system and the virus. However, the lack of a robust culturing system hampers direct investigation of interactions between virus and host cells. Other techniques, however, including next generation sequencing, AnelloScan-antibody tests, evolution selection pressure analysis, and virus protein structures, do provide new insights into the interactions between anelloviruses and the host immune system. This review aims at providing an overview of the current knowledge on the immune mechanisms acting on anelloviruses and the countering viral mechanisms allowing immune evasion.

The Impact of First-Time SARS-CoV-2 Infection on Human Anelloviruses

Members of the Anelloviridae family dominate the blood virome, emerging early in life. The anellome, representing the variety of anelloviruses within an individual, stabilizes by adulthood. Despite their supposedly commensal nature, elevated anellovirus concentrations under immunosuppressive treatment indicate an equilibrium controlled by immunity. Here, we investigated whether anelloviruses are sensitive to the immune activation that accompanies a secondary infection. As a model, we investigated 19 health care workers (HCWs) with initial SARS-CoV-2 infection, with blood sampling performed pre and post infection every 4 weeks in a 3-month-follow-up during the early 2020 COVID-19 pandemic. A concurrently followed control group (n = 27) remained SARS-CoV-2-negative. Serum anellovirus loads were measured using qPCR. A significant decrease in anellovirus load was found in the first weeks after SARS-CoV-2 infection, whereas anellovirus concentrations remained stable in the uninfected control group. A restored anellovirus load was seen approximately 10 weeks after SARS-CoV-2 infection. For five subjects, an in-time anellome analysis via Illumina sequencing could be performed. In three of the five HCWs, the anellome visibly changed during SARS-CoV-2 infection and returned to baseline in two of these cases. In conclusion, anellovirus loads in blood can temporarily decrease upon an acute secondary infection.

Control of Human Anelloviruses by Cytosine to Uracil Genome Editing

Anelloviruses are the most common viruses infecting humans. Every human carries a nonpathogenic personal anellovirus virome (anellome), yet it is unknown which mechanisms contribute to its stability. Here, we assessed the dynamics and impact of a host antiviral defense mechanism-cytidine deaminase activity leading to C to U editing in anelloviruses-on the stability of the anellome. We investigated anellome sequence data obtained from serum samples collected every 6 months from two healthy subjects followed for more than 30 years. The subjects were infected by a total of 64 anellovirus lineages. Minus-stranded C to U editing was observed in lineages belonging to the Alpha-, Beta-, and Gammatorquevirus genera. The edited genomes were present within virus particles, therefore editing must have occurred at the late stages of the virus life cycle. Editing was favored by 5'-TC contexts in the virus genome, indicating that apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like, catalytic subunit 3 or A3 (APOBEC3) proteins are involved. Within a lineage, mutational dynamics varied over time and few fixations of mutations were detected, indicating that C to U editing is a dead end for a virus genome. We detected an editing coldspot in the GC-rich regions, suggesting that the GC-rich region is crucial for genome packaging, since only packaged virus particles were included in the analysis. Finally, we noticed a lineage-specific reduced concentration after an editing event, yet no clearance. In conclusion, cytidine deaminase activity does not clear anelloviruses, nor does it play a major role in virus evolution, but it does contribute to the stability of the anellome. IMPORTANCE Despite significant attention on anellovirus research, the interaction between the anellovirus virome and the human host remains unknown. We show the dynamics of APOBEC3-mediated cytidine deaminase activity on anelloviruses during a 30-year period of chronic infection and postulate that this antiviral mechanism controls anelloviruses. These results expand our knowledge of anellovirus-host interactions, which may be important for the design of gene therapies.

APOBEC3: Friend or Foe in Human Papillomavirus Infection and Oncogenesis?

Human papillomavirus (HPV) infection is a causative agent of multiple human cancers, including cervical and head and neck cancers. In these HPV-positive tumors, somatic mutations are caused by aberrant activation of DNA mutators such as members of the apolipoprotein B messenger RNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) family of cytidine deaminases. APOBEC3 proteins are most notable for their restriction of various viruses, including anti-HPV activity. However, the potential role of APOBEC3 proteins in HPV-induced cancer progression has recently garnered significant attention. Ongoing research stems from the observations that elevated APOBEC3 expression is driven by HPV oncogene expression and that APOBEC3 activity is likely a significant contributor to somatic mutagenesis in HPV-positive cancers. This review focuses on recent advances in the study of APOBEC3 proteins and their roles in HPV infection and HPV-driven oncogenesis. Further, we discuss critical gaps and unanswered questions in our understanding of APOBEC3 in virus-associated cancers.

HPV16 and HPV18 Genome Structure, Expression, and Post-Transcriptional Regulation

Human papillomaviruses (HPV) are a group of small non-enveloped DNA viruses whose infection causes benign tumors or cancers. HPV16 and HPV18, the two most common high-risk HPVs, are responsible for ~70% of all HPV-related cervical cancers and head and neck cancers. The expression of the HPV genome is highly dependent on cell differentiation and is strictly regulated at the transcriptional and post-transcriptional levels. Both HPV early and late transcripts differentially expressed in the infected cells are intron-containing bicistronic or polycistronic RNAs bearing more than one open reading frame (ORF), because of usage of alternative viral promoters and two alternative viral RNA polyadenylation signals. Papillomaviruses proficiently engage alternative RNA splicing to express individual ORFs from the bicistronic or polycistronic RNA transcripts. In this review, we discuss the genome structures and the updated transcription maps of HPV16 and HPV18, and the latest research advances in understanding RNA cis-elements, intron branch point sequences, and RNA-binding proteins in the regulation of viral RNA processing. Moreover, we briefly discuss the epigenetic modifications, including DNA methylation and possible APOBEC-mediated genome editing in HPV infections and carcinogenesis.

Vitamin D in gynecological diseases

BACKGROUND

Most reproductive system studies suggest the protective effects of vitamin D, but vitamin D deficiency and insufficiency are growing global health issues. The present study investigates the association between vitamin D deficiency/insufficiency and gynecologic diseases to identify illness risks at different serum vitamin D levels in Taiwan.

METHODS

A total of 7699 female adults aged ≥20 years with results for both serum vitamin D and gynecologic-associated diseases were drawn from the Taiwan MJ cohort. We analyzed the correlation between serum vitamin D levels and results from reproductive system evaluations, including history of dysmenorrhea, results of Pap smear, high-risk human papillomavirus (HPV) infection of the cervix, mammography, and ultrasound of breast and pelvis.

RESULTS

Over 80% of participants showed vitamin D deficiency/insufficiency. Participants with abnormal Pap smear results, high-risk HPV infection, and history of dysmenorrhea showed significantly lower levels of serum vitamin D (p < 0.001-0.05). Serum vitamin D deficiency was significantly associated with positive high-risk HPV infection of the cervix (p < 0.05) and dysmenorrhea (p < 0.001). After controlling for age as a confounding variable for each gynecologic disease, level of serum vitamin D was significantly associated with abnormal breast ultrasound (odds ratio = 0.724) and uterus ultrasound (odds ratio = 0.673 - 0.8), and dysmenorrhea (odds ratio = 0.829).

CONCLUSION

Associations were found between vitamin D deficiency and endometriosis, uterine myoma, dysmenorrhea, abnormal Pap smear results, and high-risk HPV infection of the cervix. Therefore, vitamin D supplements may present a cost-effective benefit for the prevention and treatment of gynecologic diseases, and thus reduction of healthcare expenditures.

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

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

HPV Meets APOBEC: New Players in Head and Neck Cancer

Besides smoking and alcohol, human papillomavirus (HPV) is a factor promoting head and neck squamous cell carcinoma (HNSCC). In some human tumors, including HNSCC, a number of mutations are caused by aberrantly activated DNA-modifying enzymes, such as the apolipoprotein B mRNA editing enzyme catalytic polypeptide-like (APOBEC) family of cytidine deaminases. As the enzymatic activity of APOBEC proteins contributes to the innate immune response to viruses, including HPV, the role of APOBEC proteins in HPV-driven head and neck carcinogenesis has recently gained increasing attention. Ongoing research efforts take the cue from two key observations: (1) APOBEC expression depends on HPV infection status in HNSCC; and (2) APOBEC activity plays a major role in HPV-positive HNSCC mutagenesis. This review focuses on recent advances on the role of APOBEC proteins in HPV-positive vs. HPV-negative HNSCC.

Mutations in the HPV16 genome induced by APOBEC3 are associated with viral clearance

HPV16 causes half of cervical cancers worldwide; for unknown reasons, most infections resolve within two years. Here, we analyze the viral genomes of 5,328 HPV16-positive case-control samples to investigate mutational signatures and the role of human APOBEC3-induced mutations in viral clearance and cervical carcinogenesis. We identify four de novo mutational signatures, one of which matches the COSMIC APOBEC-associated signature 2. The viral genomes of the precancer/cancer cases are less likely to contain within-host somatic HPV16 APOBEC3-induced mutations (Fisher's exact test, P = 6.2 x 10-14), and have a 30% lower nonsynonymous APOBEC3 mutation burden compared to controls. We replicate the low prevalence of HPV16 APOBEC3-induced mutations in 1,749 additional cases. APOBEC3 mutations also historically contribute to the evolution of HPV16 lineages. We demonstrate that cervical infections with a greater burden of somatic HPV16 APOBEC3-induced mutations are more likely to be benign or subsequently clear, suggesting they may reduce persistence, and thus progression, within the host.

Different antiviral activities of natural APOBEC3C, APOBEC3G, and APOBEC3H variants against hepatitis B virus

Some APOBEC3 family members have antiviral activity against retroviruses and DNA viruses. Hepatitis B virus (HBV) is a DNA virus that is the major causative factor of severe liver diseases such as cirrhosis and hepatocellular carcinoma. To determine whether APOBEC3 variants in humans have different anti-HBV activities, we evaluated natural variants of APOBEC3C, APOBEC3G, and APOBEC3H using an HBV-replicating cell culture model. Our data demonstrate that the APOBEC3C variant S188I had increased restriction activity and hypermutation frequency against HBV DNA. In contrast, the APOBEC3G variant H186R did not alter the anti-HBV and hypermutation activities. Among APOBEC3H polymorphisms (hap I-VII) and splicing variants (SV-200, SV-183, SV-182, and SV-154), hap II SV-183 showed the strongest restriction activity. These data suggest that the genetic variations in APOBEC3 genes may affect the efficiency of HBV elimination in humans.

The APOBEC3 genes and their role in cancer: insights from human papillomavirus

The interaction between human papillomaviruses (HPV) and the apolipoprotein-B mRNA-editing catalytic polypeptide-like (APOBEC)3 (A3) genes has garnered increasing attention in recent years, with considerable efforts focused on understanding their apparent roles in both viral editing and in HPV-driven carcinogenesis. Here, we review these developments and highlight several outstanding questions in the field. We consider whether editing of the virus and mutagenesis of the host are linked or whether both are essentially separate events, coincidentally mediated by a common or distinct A3 enzymes. We discuss the viral mechanisms and cellular signalling pathways implicated in A3 induction in virally infected cells and examine which of the A3 enzymes might play the major role in HPV-associated carcinogenesis and in the development of therapeutic resistance. We consider the parallels between A3 induction in HPV-infected cells and what might be causing aberrant A3 activity in HPV-independent cancers such as those arising in the bladder, lung and breast. Finally, we discuss the implications of ongoing A3 activity in tumours under treatment and the therapeutic opportunities that this may present.

TaME-seq: An efficient sequencing approach for characterisation of HPV genomic variability and chromosomal integration

HPV genomic variability and chromosomal integration are important in the HPV-induced carcinogenic process. To uncover these genomic events in an HPV infection, we have developed an innovative and cost-effective sequencing approach named TaME-seq (tagmentation-assisted multiplex PCR enrichment sequencing). TaME-seq combines tagmentation and multiplex PCR enrichment for simultaneous analysis of HPV variation and chromosomal integration, and it can also be adapted to other viruses. For method validation, cell lines (n = 4), plasmids (n = 3), and HPV16, 18, 31, 33 and 45 positive clinical samples (n = 21) were analysed. Our results showed deep HPV genome-wide sequencing coverage. Chromosomal integration breakpoints and large deletions were identified in HPV positive cell lines and in one clinical sample. HPV genomic variability was observed in all samples allowing identification of low frequency variants. In contrast to other approaches, TaME-seq proved to be highly efficient in HPV target enrichment, leading to reduced sequencing costs. Comprehensive studies on HPV intra-host variability generated during a persistent infection will improve our understanding of viral carcinogenesis. Efficient identification of both HPV variability and integration sites will be important for the study of HPV evolution and adaptability and may be an important tool for use in cervical cancer diagnostics.

Keratinocyte differentiation induces APOBEC3A, 3B, and mitochondrial DNA hypermutation

Mitochondrial DNA (mtDNA) mutations are found in many types of cancers and suspected to be involved in carcinogenesis, although the mechanism has not been elucidated. In this study, we report that consecutive C-to-T mutations (hypermutations), a unique feature of mutations induced by APOBECs, are found in mtDNA from cervical dysplasia and oropharyngeal cancers. In vitro, we found that APOBEC3A (A3A) and 3B (A3B) expression, as well as mtDNA hypermutation, were induced in a cervical dysplastic cell line W12 when cultured in a differentiating condition. The ectopic expression of A3A or A3B was sufficient to hypermutate mtDNA. Fractionation of W12 cell lysates and immunocytochemical analysis revealed that A3A and A3B could be contained in mitochondrion. These results suggest that mtDNA hypermutation is induced upon keratinocyte differentiation, and shed light on its molecular mechanism, which involves A3s. The possible involvement of mtDNA hypermutations in carcinogenesis is also discussed.

Intra-host sequence variability in human papillomavirus

Human papillomaviruses (HPVs) co-evolve slowly with the human host and each HPV genotype displays epithelial tropisms. We assessed the evolution of intra HPV genotype variants within samples, and their association to anogenital site, cervical cytology and HIV status. Variability in the L1 gene of 35 HPV genotypes was characterized phylogenetically using maximum likelihood, and portrayed by phenotype. Up to a thousand unique variants were identified within individual samples. In-depth analyses of the most prevalent genotypes, HPV16, HPV18 and HPV52, revealed that the high diversity was dominated by a few abundant variants. This suggests high intra-host mutation rates. Clades of HPV16, HPV18 and HPV52 were associated to anatomical site and HIV co-infection. Particularly, we observed that one HPV16 clade was specific to vaginal cells and one HPV52 clade was specific to anal cells. One major HPV52 clade, present in several samples, was strongly associated with cervical neoplasia. Overall, our data suggest that tissue tropism and HIV immunosuppression are strong shapers of HPV evolution.

Within-Host Variations of Human Papillomavirus Reveal APOBEC Signature Mutagenesis in the Viral Genome

Persistent infection with oncogenic human papillomaviruses (HPVs) causes cervical cancer, accompanied by the accumulation of somatic mutations into the host genome. There are concomitant genetic changes in the HPV genome during viral infection; however, their relevance to cervical carcinogenesis is poorly understood. Here, we explored within-host genetic diversity of HPV by performing deep-sequencing analyses of viral whole-genome sequences in clinical specimens. The whole genomes of HPV types 16, 52, and 58 were amplified by type-specific PCR from total cellular DNA of cervical exfoliated cells collected from patients with cervical intraepithelial neoplasia (CIN) and invasive cervical cancer (ICC) and were deep sequenced. After constructing a reference viral genome sequence for each specimen, nucleotide positions showing changes with >0.5% frequencies compared to the reference sequence were determined for individual samples. In total, 1,052 positions of nucleotide variations were detected in HPV genomes from 151 samples (CIN1, n = 56; CIN2/3, n = 68; ICC, n = 27), with various numbers per sample. Overall, C-to-T and C-to-A substitutions were the dominant changes observed across all histological grades. While C-to-T transitions were predominantly detected in CIN1, their prevalence was decreased in CIN2/3 and fell below that of C-to-A transversions in ICC. Analysis of the trinucleotide context encompassing substituted bases revealed that TpCpN, a preferred target sequence for cellular APOBEC cytosine deaminases, was a primary site for C-to-T substitutions in the HPV genome. These results strongly imply that the APOBEC proteins are drivers of HPV genome mutation, particularly in CIN1 lesions.IMPORTANCE HPVs exhibit surprisingly high levels of genetic diversity, including a large repertoire of minor genomic variants in each viral genotype. Here, by conducting deep-sequencing analyses, we show for the first time a comprehensive snapshot of the within-host genetic diversity of high-risk HPVs during cervical carcinogenesis. Quasispecies harboring minor nucleotide variations in viral whole-genome sequences were extensively observed across different grades of CIN and cervical cancer. Among the within-host variations, C-to-T transitions, a characteristic change mediated by cellular APOBEC cytosine deaminases, were predominantly detected throughout the whole viral genome, most strikingly in low-grade CIN lesions. The results strongly suggest that within-host variations of the HPV genome are primarily generated through the interaction with host cell DNA-editing enzymes and that such within-host variability is an evolutionary source of the genetic diversity of HPVs.

Utility of high-throughput DNA sequencing in the study of the human papillomaviruses

The Papillomaviridae family is probably the most diverse group of viruses that affect vertebrates. The study of the relationship between infection by certain types of human papillomavirus (HPV) and the development of neoplastic epithelial lesions is of particular interest because of the high prevalence of HPV-related carcinomas in populations of developing countries. To understand the mechanisms of infection and their association with different clinical manifestations, molecular tools play an important role in the description of new types of HPV, the characterization of effector properties of the viral factors, the specific diagnosis and monitoring of HPV types, and the alteration patterns at genetic level in the host. Technological advances in the field of DNA sequencing have led to the development of different next-generation sequencing systems, allowing obtaining a large amount of data and broadening the applications to study viral diseases. In this review, we summarize the main approaches and their perspectives where the use of massively parallel sequencing has been proved as a useful tool in the research of the HPV infection.

HPV11 E6 mutation by overexpression of APOBEC3A and effects of interferon-ω on APOBEC3s and HPV11 E6 expression in HPV11.HaCaT cells

BACKGROUND

Condyloma acuminatum, infected by low-risk human papillomaviruses (e.g., HPV6 and HPV11), is one of the most widespread sexually transmitted diseases. Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 proteins (APOBEC3s, A3s) are cellular cytidine deaminases acting as antiviral factors through hypermutation of viral genome. However, it remains unknown whether A3s results in HPV11 gene mutations and interferon-ω (IFN-ω) exhibits antiviral activities through the A3s system. Here we investigated whether enhanced APOBEC3A (A3A) resulted in the E6 gene mutations and explore the effects of recombinant human interferon-ω (rhIFN-ω) on A3s/E6 expression in HaCaT keratinocytes containing the genome of HPV 11 (HPV11.HaCaT cells).

METHODS

A3A-overexpressed HPV11.HaCaT (A3A-HPV11.HaCaT) cells were established by lentiviral infection and verified by immunofluorescence and western-blotting. Cell cycle, E6 gene mutations, APOBEC3s/E6 gene expression and subcellular localization were detected by FACS, 3D-PCR and sequencing, qRT-PCR and immunofluorescence respectively.

RESULTS

The results suggested that A3A-HPV11.HaCaT cells were successfully established. Enhanced A3A induced S-phase arrest, G > A/C > T mutations and obvious reduction of E6 mRNA expression. A3A/A3B mRNA expression was up-regulated at 6 h and 12 h and obvious A3A staining existed throughout HPV11.HaCaT cells after rhIFN-ω treatment. RhIFN-ω could also inhibit mRNA expression of HPV11 E6 significantly.

CONCLUSIONS

Enhanced A3A repressed HPV11 E6 expression through gene hypermutation, and rhIFN-ω might be an effective agent against HPV11 infection by up-regulation of A3A.

Roles of APOBEC3A and APOBEC3B in Human Papillomavirus Infection and Disease Progression

The apolipoprotein B messenger RNA-editing, enzyme-catalytic, polypeptide-like 3 (APOBEC3) family of cytidine deaminases plays an important role in the innate immune response to viral infections by editing viral genomes. However, the cytidine deaminase activity of APOBEC3 enzymes also induces somatic mutations in host genomes, which may drive cancer progression. Recent studies of human papillomavirus (HPV) infection and disease outcome highlight this duality. HPV infection is potently inhibited by one family member, APOBEC3A. Expression of APOBEC3A and APOBEC3B is highly elevated by the HPV oncoproteins E6 and E7 during persistent virus infection and disease progression. Furthermore, there is a high prevalence of APOBEC3A and APOBEC3B mutation signatures in HPV-associated cancers. These findings suggest that induction of an APOBEC3-mediated antiviral response during HPV infection may inadvertently contribute to cancer mutagenesis and virus evolution. Here, we discuss current understanding of APOBEC3A and APOBEC3B biology in HPV restriction, evolution, and associated cancer mutagenesis.

APOBEC-mediated genomic alterations link immunity and viral infection during human papillomavirus-driven cervical carcinogenesis

Cervical cancer is one of the most frequently diagnosed cancers and is a major cause of death from gynecologic cancers worldwide; the cancer burden from cervical cancer is especially heavy in less developed countries. Most cases of cervical cancer are caused by persistent infection with carcinogenic human papillomavirus (HPV) genotypes 16 and 18. Non-resolving inflammation caused by HPV infection provides a microenvironment that facilitates cancer development. Molecular alterations during the process of HPV-induced carcinogenesis are characterized by DNA methylation within the HPV genome, promoter hypermethylation of tumor suppressor genes in the host genome, as well as genomic instability caused by viral DNA integrating into the host genome. Catalytic polypeptide-like apolipoprotein B mRNA editing enzymes (APOBECs) normally function as part of the innate immune system. APOBEC expression is stimulated upon viral infection and plays an important role in HPV-induced cervical cancer. APOBECs catalyze the deamination of cytosine bases in nucleic acids, which leads to a conversion of target cytosine (C) to uracil (U) and consequently a change in the single-stranded DNA/RNA sequence. APOBEC proteins mediate the complex interactions between HPV and the host genome and link immunity and viral infection during HPV-driven carcinogenesis. Understanding the effects of APOBECs in HPV-induced cervical carcinogenesis will enable the development of better tools for HPV infection control and personalized prevention and treatment strategies.

APOBEC3G is increasingly expressed on the human uterine cervical intraepithelial neoplasia along with disease progression

PROBLEM

APOBEC3G (A3G) is a cytidine deaminase that exhibits antiviral activity by introducing C-to-T hypermutation in viral DNA. We recently observed the distinct presence of C-to-T hypermutation of human papillomavirus DNA in uterine cervical intraepithelial neoplasia (CIN), suggesting the possible involvement of A3G in the mutation-inducing process. Consequently, we investigated the association of A3G expression with CIN progression in this study.

METHOD OF STUDY

Patients who had undergone cervical conization due to CIN1 (n=11), CIN2 (n=9), CIN3 (n=12), and micro-invasive squamous cell carcinoma (n=2) were included. The expression profiles of A3G and p16 proteins in cervical lesions and A3G-positive immune cells around the lesions were examined by immunohistochemistry.

RESULTS

Immunoreactive A3G protein was detected in the CIN and squamous cell carcinoma lesions. Its expression intensity and positive areas were increased and spread in accordance with the progression of CIN, respectively. The co-expression of p16 was observed on the A3G-positive atypical cells. The numbers of A3G-positive immune cells in CIN3 lesions were significantly higher than those of CIN1-2 lesions.

CONCLUSION

These findings indicate that A3G is associated with CIN, suggesting its important roles in human papillomavirus-induced pathophysiological processes such as CIN progression and viral elimination.

Heat Increases the Editing Efficiency of Human Papillomavirus E2 Gene by Inducing Upregulation of APOBEC3A and 3G

Apolipoprotein B mRNA-editing catalytic polypeptide (APOBEC) 3 proteins have been identified as potent viral DNA mutators and have broad antiviral activity. In this study, we demonstrated that apolipoprotein B mRNA-editing catalytic polypeptide 3A (A3A) and A3G expression levels were significantly upregulated in human papillomavirus (HPV)-infected cell lines and tissues. Heat treatment resulted in elevated expression of A3A and A3G in a temperature-dependent manner in HPV-infected cells. Correspondingly, HPV-infected cells heat-treated at 44 °C showed accumulated G-to-A or C-to-T mutation in HPV E2 gene. Knockdown of A3A or A3G could promote cell viability, along with the lower frequency of A/T in HPV E2 gene. In addition, regressing genital viral warts also harbored high G-to-A or C-to-T mutation in HPV E2 gene. Taken together, we demonstrate that apolipoprotein B mRNA-editing catalytic polypeptide 3 expression and editing function was heat sensitive to a certain degree, partly explaining the mechanism of action of local hyperthermia to treat viral warts.

Oncogenic Role of Tumor Viruses in Humans

Viruses are the intracellular pathogens that reproduce only in the living cell and manipulate the cellular machinery to produce more viruses. Viral replications can affect cellular genes of the host in multiple cancerous ways. Approximately, 20% of all human oncogenesis is caused by cancer-causing viruses known as oncoviruses. Viral infection causes chronic inflammation leading to cell death, uncontrollable proliferation, and modulated expression of some of the regulatory proteins. Oncogenesis is a multistep phenomenon in which normal host cells are transformed into cancerous cells on the basis of host genetic variability. Oncogenic viruses encode genes that cause viral replication and transformation of the host cells to produce viral proteins and protein complexes. The phenomenon from basic viral infection to tumorigenesis is lengthy due to the involvement of factors like immunity complications, cellular mutations, and exposure to other cancerous agents. The viruses that are involved in human cancer development are Hepatitis B virus (HBV), Hepatitis C virus (HCV), Epstein-Barr virus (EBV), Human papilloma virus (HPV), Kaposi's sarcoma herpes virus (KSHV), and Human T lymphotrophic virus 1 (HTLV-1). This review article summarizes advanced knowledge related to human oncogenic viruses and the molecular mechanisms that lead to tumorigenesis in humans.

Co-infection of Epstein-Barr virus and human papillomavirus in human tumorigenesis

Viral infections contribute to approximately 12% of cancers worldwide, with the vast majority occurring in developing countries and areas. Two DNA viruses, Epstein-Barr virus (EBV) and human papillomavirus (HPV), are associated with 38% of all virus-associated cancers. The probability of one patient infected with these two distinct types of viruses is increasing. Here, we summarize the co-infection of EBV and HPV in human malignancies and address the possible mechanisms for the co-infection of EBV and HPV during tumorigenesis.

Detection of hypermutated human papillomavirus type 16 genome by Next-Generation Sequencing

Human papillomavirus type 16 (HPV16) is a major cause of cervical cancer. We previously demonstrated that C-to-T and G-to-A hypermutations accumulated in the HPV16 genome by APOBEC3 expression in vitro. To investigate in vivo characteristics of hypermutation, differential DNA denaturation-PCR (3D-PCR) was performed using three clinical specimens obtained from HPV16-positive cervical dysplasia, and detected hypermutation from two out of three specimens. One sample accumulating hypermutations in both E2 and the long control region (LCR) was further subjected to Next-Generation Sequencing, revealing that hypermutations spread across the LCR and all early genes. Notably, hypermutation was more frequently observed in the LCR, which contains a viral replication origin and the early promoter. APOBEC3 expressed abundantly in an HPV16-positive cervix, suggesting that single-stranded DNA exposed during viral replication and transcription may be efficient targets for deamination. The results further strengthen a role of APOBEC3 in introducing HPV16 hypermutation in vivo.