15.3.1.1.2: Human Papillomavirus (HPV)
Human papillomavirus (HPV) is a virus from the papillomavirus family that is capable of infecting humans.
Learning Objectives
- Discuss the relationship between the human papillomavirus (HPV) and the development of cancer
Key Points
- HPVs establish productive infections only in keratinocytes of the skin or mucous membranes.
- While the majority of the known types of HPV cause no symptoms in most people, some types can cause warts (verrucae), while others can – in a minority of cases – lead to cancers of the cervix, vulva, vagina, penis, oropharynx and anus.
- In more developed countries, cervical screening using a Papanicolaou (Pap) test or liquid-based cytology is used to detect abnormal cells that may develop into cancer. If abnormal cells are found, women are invited to have a colposcopy.
- HPV vaccines (Cervarix and Gardasil), which prevent infection with the HPV types (16 and 18) that cause 70% of cervical cancer, may lead to further decreases in cervical cancer.
Key Terms
- verrucae : warts
- Papanicolaou (Pap) test : screening of the cervical cells used to detect abnormal cells that may develop into cancer.
Virus
- Genetic material: double-stranded DNA
- Virion structure: naked icosahedral capsid
Replication
- about half of the more than 100 strains of HPV permanently integrate their DNA into the host cell's chromosomes
Disease and Treatment
Human papillomavirus (HPV) is a virus from the papillomavirus family that is capable of infecting humans. Like all papillomaviruses, HPVs establish productive infections only in keratinocytes of the skin or mucous membranes. While the majority of the known types of HPV cause no symptoms in most people, some types can cause warts (verrucae), while others can – in a minority of cases – lead to cancers of the cervix, vulva, vagina, penis, oropharynx and anus. Different strains of HPV target different tissues of the body. For instance, a HPV that causes plantar warts is a different strain from one which would cause genital warts.
Recently, HPV has been linked with an increased risk of cardiovascular disease. In addition, HPV 16 and 18 infections are strongly associated with an increased odds ratio of developing oropharyngeal (throat) cancer.
More than 30 to 40 types of HPV are typically transmitted through sexual contact and infect the anogenital region. Some sexually transmitted HPV types may cause genital warts. Persistent infection with “high-risk” HPV types — different from the ones that cause skin warts — may progress to precancerous lesions and invasive cancer. HPV infection is a cause of nearly all cases of cervical cancer . Oncogenic strains always integrate their DNA into the host cell's chromosomes. In addition, some strains of HPV carry genes for proteins that 1. inactivate the tumor suppressor p53 and 2. induce the host cell to replicate without the correct signals (a.k.a. oncogenes). This combination of DNA integration, oncogenes, and interference with tumor suppressors predisposes the infected cell to become cancerous. However, most infections with HPV do not cause disease.
Most HPV infections in young females are temporary and have little long-term significance. Seventy percent of infections are gone in 1 year and ninety percent in 2 years. However, when the infection persists — in 5% to 10% of infected women — there is high risk of developing precancerous lesions of the cervix, which can progress to invasive cervical cancer. This process usually takes 10–15 years, providing many opportunities for detection and treatment of the pre-cancerous lesion. Progression to invasive cancer can be almost always prevented when standard prevention strategies are applied, but the lesions still cause considerable burden necessitating preventive surgeries, which do in many cases involve loss of fertility.
In more developed countries, cervical screening using a Papanicolaou (Pap) test or liquid-based cytology is used to detect abnormal cells that may develop into cancer. If abnormal cells are found, women are invited to have a colposcopy. During a colposcopic inspection, biopsies can be taken and abnormal areas can be removed with a simple procedure, typically with a cauterizing loop or, more commonly in the developing world — by freezing (cryotherapy). Treating abnormal cells in this way can prevent them from developing into cervical cancer.
Pap smears have reduced the incidence and fatalities of cervical cancer in the developed world, but even so there were 11,000 cases and 3,900 deaths in the U.S. in 2008. Cervical cancer has substantial mortality in resource-poor areas; worldwide, there are an estimated 490,000 cases and 270,000 deaths each year.
HPV vaccines (Cervarix and Gardasil), which prevent infection with the HPV types (16 and 18) that cause 70% of cervical cancer, may lead to further decreases.
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