Human Papillomaviruses

Introduction

Papillomavirus is one of two genera in the Papovaviridae family. The viruses in this category, however, vary significantly from those in the other genus, Polyomavirus, in terms of genome size, organization, and pathogenesis. Papillomaviruses (Latin: papilla = ‘nipple’; oma = ‘tumour’) cause benign skin tumors (papillomas) in their hosts, which contain varying quantities of infectious virus.

The skin, mouth, anus, conjunctiva, and lower genital tracts of both males and females are infected by papillomaviruses. The majority of infections are asymptomatic and subclinical, regardless of where they occur. The most common sexually transmitted infection is genital human papillomavirus (HPV). Oncogenic or high-risk HPV forms have been shown to be the leading cause of cervical cancer in women, as well as vulvar, vaginal, penile, anal, and oropharyngeal cancers.

While a recurrent infection with a high-risk HPV type is needed for the development of cervical cancer, many women will spontaneously clear the infection and are therefore no longer at risk of developing cancer. HPV forms 16 and 18 are estimated to cause more than 70% of cervical cancer cases, and testing for HPV along with Pap smear testing is a commonly accepted method for cervical cancer screening.

Ever since, papillomaviruses have been linked to a number of squamous cell carcinomas (SCCs) of the mucosal and cutaneous epithelia. This has sparked interest in these viruses, leading to major advances in our knowledge of their evolutionary history and pathogenic processes.

Viral classification

The Papillomaviridae family includes a wide variety of ancient DNA viruses found in mammals, birds, reptiles, and fish. There are over 650 different animal and human papillomavirus (HPV) varieties, with 440 of them co-evolving to exist and persist in the human population. HPVs are classified into five major phylogenetic genera: Alpha (α)-, Beta (β)-, Gamma (γ)-, Mu (μ)-, and Nu (v)- papillomaviruses. Within the L1 gene, viruses from various genera share 60% similarity. Within a genus, HPVs are classified into species based on 60 to 70% similarity.

Each genus’ viruses have evolved to fit into specific ecological niches within their hosts. The cutaneous epithelium is invaded by viruses belonging to the Beta, Gamma, Mu, and Nu genera, while viruses belonging to the Alpha genus infect both the cutaneous and mucosal epithelia. Infections caused by HPVs from the Beta and Gamma genera are often asymptomatic. Based on their ability to cause cancer, mucosal Alpha HPV types are further categorized as low-risk HPVs (LR-HPVs) or high-risk HPVs (HR-HPVs). HPV16, 31, and 58 genotypes have distinct carcinogenic properties, according to studies.

Structure

The papillomavirus capsids are icosahedral in shape, with 72 capsomeres measuring 52–55 nm in diameter. A double-stranded DNA molecule with a molecular weight of 5 x 10⁶ Da can be contained within the HPV virion. The virion has a circular supercoiled form. The genome of papillomaviruses is about 8 kilobases long (ranging from 6953 to 8607 base pairs). The six nonstructural gene ORFs E1, E2, E3, E4, E5, and E6 are expressed during viral gene expression. These ORFs are found in undifferentiated or intermediately differentiated keratinocytes and are found in the early region of the viral genome.

The function of viral proteins

Core Proteins

• E1—ATP dependent helicase. Role in papillomavirus genome replication.
• E2—Coactivator of viral genome replication through the recruitment of E1 to the viral replication origin. Transcription facto or E6 & E7, also important for viral genome segregation.
• L1—Major capsid protein. Assembles into pentomeric capsomeres, which are the primary components of the icosahedral virion shell.
• L2—Minor capsid protein. Involved in encapsidation of viral DNA. Facilitates virus entry and trafficking to the nucleus.
• E4—E4 gene is embedded within the E2 gene and is expressed abundantly as an E1^E4 fusion protein during the late stages of the viral life cycle. Binds to cytokeratin filaments and disrupts their structure. E4 is thought also to contribute to virus release and transmission.

Accessory Proteins

• E5—Small transmembrane protein. In alpha-PVs, E5 interacts with the EGF receptor and activates mitogenic signaling pathways. Has a rolein evading the immune response and apoptosis. Beta-, gamma-, and mu-PVs lack E5 gene.
• E6—Drives cell cycle entry to allow genome amplification in the upper epithelial layers. E6 of high-risk alpha-PV types binds and degrades p53 and can also activate telomerase and contribute to transformation. Also involved in immune evasion. Some gamma-PVs lack the E6 gene.
• E7—Drives cell cycle entry to allow genome amplification in the upper cell layers. E7 of high-risk alpha-PV types binds and degrades pRb and can induce chromosomal instability. E7 is necessary for cell transformation. Both E6 and E7 have a number of cellular substrates, with the identity of these substrates differing between types.

The Function of Viral Proteins

All recognized papillomaviruses encode a set of “core” proteins that were present early in the evolution of papillomaviruses and are sequence and function conserved across PV types. E1, E2, L2, and L1 are examples. E4 may also be a core protein that has evolved to fulfill the epithelial specialization of papillomaviruses. During adaptation to different epithelial niches, accessory proteins evolved in each papillomavirus type. These genes have different sequences and functions depending on the type you are. These proteins contribute to virulence and pathogenicity by altering the cellular environment to aid virus life cycle completion.  

Pathogenesis

HPVs cause changes in epithelial cells that are both benign and malignant. Certain HPV forms are the most important component of the etiology of genital cancers, and cervical cancer is one of the most common causes of cancer-related death in women worldwide.

Oncogenic Potential of Papillomaviruses

In humans, one-third of epidermodysplasia verruciformis (EV) patients develop squamous cell carcinoma, which usually occurs in sun-exposed areas. Over 30% of these lesions contain HPV DNA, most typically forms 5 or 8. In the presence of favorable environmental or genetic factors, benign HPV-associated lesions may progress to carcinoma..

There has been a spike in records of SCC at several different sites as the number of allograft recipients has grown. The viruses isolated from similar lesions in epidermodysplasia verruciformis (EV) patients are linked to cutaneous cancers, while the viruses present in similar cancers in immunocompetent people are linked to genital cancers. The fact that disease is more prevalent in immunocompromised persons, such as transplant recipients, indicates that the immune system plays a role in regulating infectious and tumor growth.

Genital Cancers Associated with HPV

Although it is now known that the connection between genital cancers, especially cervical cancer, is causal rather than coincidental, not everyone infected with an oncogenic HPV type will develop cancer. In reality, it appears that people may be infected and have disease that regresses spontaneously, or have no visible lesions as a result of infection. It’s unclear why infection can have such a wide range of outcomes, but the immune response, as well as other less well-known factors like genetic history and epithelial cell response to infection, can play a role.

Other Cancers

Squamous cell carcinoma of the penis, vulva, vagina and some laryngeal carcinomas are also associated with HPV infection. In contrast to cervical cancer, only 50% of the vulvar squamous cell carcinomas are HPV positive.

Diagnosis

Apart from the well-known hand and verruca (plantar) warts, papillomavirus infections have a wide range of clinical manifestations, ranging from scaly flat lesions on the cutaneous epithelium in people with EV to aceto-white flat lesions on the cervix. Since HPV is the most common cause of cervical cancer, the Pap smear has long been the test of choice for screening women for cervical cancer. A liquid based cytology (LBC) media is used in the majority of diagnostic assays used today for the detection of HPV from cervical specimens. This enables HPV and Pap smear testing to be performed in the same vial.

Antibodies can now be detected in the serum of HPV-infected patients using an enzyme-linked immunosorbent assay (ELISA) technique. The mature viral particles are only formed in the epithelium’s outer layers, where the immune response is weakest. As a result, serological assays for diagnostic or screening purposes may not be very sensitive.

PCR is the most sensitive tool for detecting HPV infection. This is an effective technique that uses a small amount of template to amplify a particular piece of DNA. Several collections of partially or degenerative primers guided to the L1 ORF have been used to detect HPV types in DNA extracted from lesions.

Treatment and Prevention

HPV infections aren’t often treated; instead, the care is focused on HPV-related disorders that occur inside the host. Intraepithelial neoplasias and anogenital warts may be treated in a variety of ways, depending on the nature of the condition.

However, since premalignant lesions, particularly those on the cervix, can lead to malignancy, treatment to eradicate the disease is critical.

References:

1. Viruses 2020, 12, 1437; doi:10.3390/v12121437
2. Frontiers in cellular and infection microbiology 2020, 10:234 doi: 10.3389/fcimb.2020.00234
3. Principles & Practice of Clinical Virology 6^th Edition, Zuckerman et al.,
4. Clinical Virology Manual 5^th Edition, Loeffelholz M et al.,
5. Clinical and Diagnostic Virology, Cambridge