Free Article Review On Human Papillomavirus-Associated Cancers: Viral Characteristics, Epidemiology, Clinical Manifestation And Current Advances In Treatment
HPV-associated cancers are caused by HPV, which is a non-enveloped double stranded DNA virus that is ubiquitous in nature. It attacks the squamous epithelial cells present in the mucous membranes of the body. The virus manifests itself as benign warts or as malignant cancers. Some of the cancers caused by HPV are HPV-associated squamous cell cancer of the head and neck (HNSCC), cervical intraepithelial neoplasia (CIN), penile intraepithelial neoplasia (PIN), anal intraepithelial neoplasia (AIN) and oropharyngeal cancer. HPV+ HNSCC is the sixth most common cancer in the world. Cervical cancer is the second most common cancer in women and the 5-year survival rate of oropharyngeal cancer due to HPV is less than 50% due to its apparent asymptomatic manifestation in case of oral cancer. Benign lesions and warts clear up on their own within 12-24 months. The virus is transmitted sexually and occurs equally in both the sexes, regardless of the sexual orientation. HPV strains 6 and 11 are dominant in benign warts while HPV 16 and 18 are dominant in the oncogenic manifestations. The virus replicates using its early transcription protein E1 to E5 and manifest carcinogenic effects using its protein E6 and E7. The capsid protein, L1 is used in vaccines to invoke immunity in susceptible individuals. Currently, the only treatment available is the HPV quadrivalent vaccine against the strain 6, 11, 16 and 18 and the bivalent vaccine against HPV 16 and 18. These vaccines have been found to be effective when used as a preventive measure while curative treatment using these vaccines have been found to be about 50% effective.
Human papillomavirus (HPV) associated cancers includes the invasive squamous cell carcinoma of the head and neck (HNSCC) (especially oropharyngeal cancer), cervical cancer and anogenital cancer. HPV positive (HPV +) HNSCC is the sixth most prevalent cancer, globally with over 35,000 new cases per year in the US (Lajer et al., 2012). Cervical cancer is the second most occurring cancer in women, worldwide (Stanley, 2012). Anogenital cancers (penis and anus in men) and oropharynx cancer are common cancers occurring in gay men infected by HPV. The causative agent, HPV, is transmitted sexually in men and women of various sexual orientations and accounts for approximately 6.2 million new cases, annually in the US (Moreira et al., 2014).
HPV affects a range of cutaneous and mucosal tissues of the human body and manifests in the form of common warts to malignant tumors. Body parts that HPV invades are the oral cavity, nasal cavity, anogenital regions, skin and the pulmonary tract (Cubie, 2013). Until 1970, it was believed that HPV infections were benign warts and required no treatment. However, recombinant DNA technology proved that HPV not only causes trivial warts but also has potential to cause carcinogenesis, especially cervical cancer and anogenital condyloma acuminata (Moreira et al., 2014). There are more than 180 types of HPV divided so based on the DNA sequence. These types are sub-divided into low-risk (that cause benign warts such HPV 6 and 11) and high-risk (that cause cancer such as HPV 16 and 18) (Stanley, 2012). Table 1 (Cubie, 2013) describes the various HPV infections associated with the several subtypes of HPV. 70% of the HPV infections are known to resolve on their own within 12 months while 90% resolve within 24 months (Cubie, 2013). Current treatment regime includes use of two types of HPV vaccines that are directed at the HPV 6, 11, 16 and 18 (quadrivalent) and those directed only at HPV 16 and 18 (bivalent). The emphasis on HPV associated diseases is due to the fact that HPV infection can be easily controlled and even prevent the occurrence of cancer with regular screening and vaccination (Markowitz et al., 2013).
HPV is the most common asymptomatic sexually transmitted disease (STD) that clears up on its own without any medication. There are currently 20 million cases of HPV infections in the US. Women have the highest risk of infection between ages of 20 and 24 years and during menopause. Studies indicate that any sexually active person has more than 50% chance of contracting HPV infection during their lifetime and by 50 years of age, 80% of those women would have contracted HPV. The lifetime risk of being diagnosed with cervical cancer is 1 in 145 (Broomall, Reynolds & Jacobson, 2010). Women with cervical cancer were found to be susceptible to developing oropharyngeal cancer as a secondary manifestation of HPV infection, indicating that the sexual habits play a key role in the transmission of the virus. Prevalence of HPV+ HNSCC, especially tonsillar cancer, was estimated at 93% in 2007 (Lajer & Buchwald, 2010). Oropharyngeal cancer is asymptomatic until the cancer is at a late stage. This fact along with the doubling in the number of cases in the past 15 years indicates a need for preventive measures (Lajer et al., 2012).
Molecular characteristics of HPV
HPV is a small virus that is roughly 50-55 mm in diameter. HPV belongs to the family Papillomaviridae and contains eight kilobase of circular non-enveloped double stranded DNA with seventy two capsomere capside. Two structural proteins make up the capsomere, namely, the 43-53kD minor capside protein called L2 and the 57kD late protein called L1 (Stanley, 2012). The genome consists of 8 to 10 partially overlapping open reading frames (ORFs). The viral DNA can be divided into a non-coding long control region (LCR) or an upper regulatory region (URR), early protein region (E1 to E7) and late protein coding regions (L1 and L2). E6 and E7 proteins confer HPV its malignancy (Stanley, 2012). E1 is a 68kDa monomeric protein that contains ATPase and 3’-5’ helicase activities (Miller, Puricelli & Stack, 2012). It binds to the adenine-thymine abundant regions of the early transcription region of the HPV DNA. This recognition site of E1 is quite close to the recognition site of E2 protein, which helps in increasing the affinity of the E1 protein to its binding site. This affinity helps in fixing E1 at the origin. E2 protein is a transcription factor of 50kDa (Miller et al., 2012). E1 and E2 proteins together are required for viral DNA replication. E4 protein helps in keratinization of the cells and acts as a biomarker for the disease and E5, a 83 amino acid hydrophobic protein, cloaks the HPV from the host’s immune system and is thought to activate epidermal growth factor receptor (EGFR) (Miller et al., 2012). E6, an 18kDa 150 amino acid protein containing two zinc-binding domains with two Cys-X-X-Cys motifs (Miller et al., 2012). E6 inhibits tumor suppressor activity go the p53 gene by inhibiting its transcriptional activity via direct binding. E7 is a 13 kDa protein containing around hundred amino acids that could be called as jack of all trades due to the fact that it converts every host cellular activity to the virus’s advantage. There are two viral promoters called p97/p105 and p742 that transcribe the early and the late proteins, respectively. The p97 or the p105 promoter is situated upstream of the E6 ORF (Miller et al., 2012). Fig. 1 (Stanley, 2012) elaborates on the HPV genome structure.
Transmission and pathogenesis
HPV attacks the cells in the mucous membranes (Stanley, 2012). The tissues of the oral cavity, cervix and the genital area resemble each other, which explains the occurrence of warts and lesions caused by HPV. Such cells have the ability to differentiate into keranocyte. The epidermis is composed of many layers commencing with the deepest layer called the basal layer that consists of the basal keratinocytes (Stanley, 2012). These cells are the stem cell of the epidermis. The virus requires fully differentiated epithelial cells for capsid formation and requires differentiating squamous epithelial cells for completion of the replication cycle (Stanley, 2012). The differentiating keratinocytes in the stratum spinosum and granulosum of squamous epithelium containing the fully replicated viral particles move towards the surface, resulting in the accumulation and deposition of the virons onto the surface of the mucous membrane. Accumulated HPV on the horny layer (outermost layer) makes it easy for further transmission through contact and further spread within the same host through cuts and wounds (Stanley, 2012).
HPV uses the heparan sulfate proteoglycans (HSPGs) such as syndecan-1 and glypicans present in the basal membrane for initial attachment, induce capsid conformational changes required for viral replication and viral internalization (Miller et al., 2012). HPV capsids bind with laminin 5 and intergrins alpha-6, beta-4 and beta-1, though the reason behind this process is not understood. There are also three proposed mechanisms for viral entry, namely, clathrin-mediated endocytosis, caveolar endocytosis and caveolae-clathrin independent pathway (Miller et al., 2012). Once internalized, HPV is thought to infiltrate the microfilaments and use dynein for transportation within the cell (Miller et al., 2012). High-risk serotypes of HPV contains sections of CpG in their genome, which upon methylation by the host immune system transforms them into invasive carcinoma causing HPV (Miller et al., 2012). Integration of the high risk-HPV into the host’s genome leads to disruption of expression of E2 protein and simultaneous increase in expression of E6 and E7 proteins. The overexpression of E6/E7 disrupts the normal functioning of the tumor suppressor genes called retinoblastoma (Rb) and p53 (Miller et al., 2012). This process results in immortalization of the cells. Integration of the virus into the host genome causes gene instability by causing defects in mitotic spindle. Mitotic spindles are required for proper separation of chromosomes, failure to do which would result in chromosomal damage. Viral genome integration is necessary for the occurrence of cervical and anogenital cancer but not required for oropharyngeal oncogenesis (Miller et al., 2012). The immune system of the host responds to HPV infection by the secretion of interferons and cytokines. The adaptive immune system generates CD4+ T helper cells and the corresponding CD8+ cytotoxic T cells specific to HPV (Uyar & Rader, 2014). Individuals develop immune tolerance and resist virus clearance from the body when the number of HPV specific CD4+ and CD8+ cells are low. In case of invasive carcinoma, a low ratio of CD8+ cytotoxic cells to the T regulatory cells could result in a poor prognosis (Uyar & Rader, 2014).
Clinical presentation and diagnosis of HPV-associated cancer
HPV-associated cancers present as intraepithelial neoplasia. That is, under microscopic examination, the invaded tissue would exhibit structural instability and growth. Cervical interepithelial neoplasia (CIN), penile intraepithelial neoplasia (PIN) and anal intraepithelial neoplasia (AIN) are the different forms of HPV-associated cancers that are named according to the site of carcinogenesis (Cubie, 2013).
Cervival interepithelial neoplasia (CIN)
Precancerous cervical lesions could be identified as white whorls containing dense nuclear material by applying 5% acetic acid to the epithelium (Cubie, 2013). The white whorls are called as flat warts, which can be observed under magnification using a colposcope. The infected cells can also be fixed on a slide and stained for microscopic examination, a process called Pap smear test developed by Papanicolau in 1940 (Cubie, 2013). Microscopic examination would reveal productive HPV infection in the form of hyperchromatic nuclei and vacuolated cells. Liquid based cytology (LBC) is another method used for the creation and study of uniform monolayer of epithelial cells when the samples obtained are inadequate. LBC helps in identification of HPV-specific changes such as keratinization, basal layer hypertrophy and epidermal disorganization (Cubie, 2013). Recent studies suggest that detection of HPV DNA would be more effective in preventing invasive cervical cancer when compared to detection using traditional histological analysis alone (Rijkaart et al., 2011). However, HPV based screening could also over-diagnose regressive CIN (Ronco et al., 2010). HPV-associated cervical cancer commences development at the junctions present between the squamous cell of the ectocervix and the glandular columnar cells of the endocervix (Cubie, 2013). Fluorodeoxyglucose positron emission tomography (FDG-PET)/computed tomography (CT) is used to identify metabolic activity stemming from glucose uptake, which is directly correlated to carcinogenic activity. Studies have indicated that a FDG-PET scan showing infiltration of the lymph node spells poor prognosis (Uyar & Rader, 2014).
Penile intraepithelial neoplasia (PIN) and anal intraepithelial neoplasia (AIN)
The occurrence of AIN in general population is 2% (Palefsky et al., 2011). A junction between the squamous epithelial cells and columnar epithelial cells occur in the anus, which offers the perfect location for the growth of HPV (Cubie, 2013). Harmless anal lesions (anal condyloma) are caused by HPV-6 or 11. The treatment is painful and costly (Palefsky et al., 2011). Typically, CIN, PIN and AIN can take anywhere between 5 to 10 years to progress. In all types of HPV associated cancers, the tissue contains DNA from multiple strains of HPV, suggesting that HPV infection could be polyclonal in nature. PIN is relatively rare owing to a lack of transformation junction as required by the virus for proliferation (Cubie, 2013).
HPV-associated oral cancers
An apparent lack of visible lesions during the early stages of oropharyngeal cancer caused by HPV is one of the reasons of reduced 5-year survival rate. However, HPV associated oral cancers have a good prognosis when treated early (Cubie, 2013). However, unlike Pap smear tests, no oral cell screening method has been proven to be effective in catching the oral cancer early in susceptible individuals. No definitive treatment regime has been established for advanced oropharyngeal cancer (Bossi et al., 2014). Studies suggest that the number sexual partners (greater than six lifetime oral sex partners and greater than 2 lifetime vaginal sex partners) along with early onset of sexual activity seem to mediate the risk of occurrence of HPV-associated oropharyngeal cancer (Miller et al., 2012).
Treatment for HPV-associated cancers
The widely used treatments of HPV infections are quadrivalent HPV vaccine and bivalent HPV vaccine. In the US, they are sold under the brand name Gardasil and Cervarix, respectively. Study by Moreira et al. (2014) suggests that vaccination would help quicken the clearance of HPV infection. The quadrivalent vaccine contains viral-like particle (VLP) of the four HPV strains, namely, HPV 6, 11, 16 and 18. These four strains have been found to be the most predominant ones in both benign warts and malignant tumors (Broomall et al., 2010). Therefore, scientists believe that vaccinating susceptible groups with this vaccine would not only prevent the occurrence of HPV infection but also curb the infection in individuals with pre-existing HPV infection (Moreira et al., 2014). The VLP is produced using recombinant DNA technology that employs the help of a genetically modified yeast (Saccharomyces cerevisiae). The VLPs are mixed with amorphous aluminum hydroxyphosphate sulfate, an adjuvant used in vaccines (Broomall et al., 2010).
In a study conducted on boys and men aged 16 to 26 years from 18 different countries using a randomized controlled double-blinded experimental design, it was found that the quadrivalent HPV vaccine prevented infection by high grade-HPV strains and cleared benign genital warts in persistent HPV infection causes (Giuliano et al., 2011). In a study with intent-to-treat men who have sex with men, the vaccine was found to be 50.3% effective in treating and 77.5% in preventing infection in naïve individuals (Palefsky et al., 2011). In another study, women who underwent cervical surgery for removal of cancerous cervical tissue were vaccinated with three doses of HPV vaccine. The researchers concluded that the vaccination prevented the recurrence of HPV related infection by reducing the occurrence up to 46.2% and 86.3% in the case of high risk-HPV (Joura et al., 2012).
The bivalent HPV vaccine contains VLPs mixed with an aluminum salt and monophospholipid, an adjuvant (Broomall et al., 2010). Adjuvants help in invoking the immune response quickly and efficiently. Both the quadri- and bivalent vaccines contain L1 capsid like particles. The bivalent vaccine has been found to be effective against CIN. The vaccine has been found to be 53% effective in treating and 93% effective in preventing HPV infection. No similar studies have been conducted in men using these vaccines (Broomall et al., 2010). Cryotherapy is another approach that has a clearance rate of 80% in low-grade squamous intraepithelial lesions (LSIL). When the same approach is coupled with visual inspection using acetic acid for high-grade squamous intraepithelial lesions (HSIL), the therapy has been found to be promising (Chumworathayi et al., 2010).
HPV-associated cancers and lesions are preventable and treatable. High risk-HPV infections are asymptomatic until much later and have the ability to turn cancerous. This situation demands the need for improved screening and diagnostic techniques that could help detect HPV infections early. As prevention is better than cure, data suggest that HPV vaccination in naïve individuals could prevent development of genital warts and the various location specific INs up to ~ 90%. Sexually active adolescents should be educated on the importance of protection and self-screening methods for early treatment.
HPV types associated with particular diseases
Note. Table showing list of infections and cancer caused by HPV and the corresponding strains. Adapted from “Diseases associated with human papillomavirus infection”, by H.A. Cubie, 2013, Virology, 445(1), pp. 22. © 2013 Elsevier Inc.
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