Types of Human Papillomavirus (HPV)

HUMAN PAPILLOMAVIRUS ( HPV)  is a small, nonenvelope, double - stranded DNA virus of the Papillomaviridae family. Different HPVs are defined by their genotypes, with more than 100 described to date. HPV genotypes are clustered into genera based on genetic relatedness. Within each genus, HPVs are categorized into species based on distinct genotypes; members of a species have similar biological properties or phenotypes. Differences of 10% or more in the viral capsid gene are noted as different HPV types, differences of 2% – 10% as subtypes, and differences of less than 2% are variants. There are two (2) main genera/types of HPV.

1. Alpha papillomaviruses
2. Beta papillomaviruses

Nearly ninety percent (90%) of typed HPVs falling within these groups. The Genus alpha papillomaviruses are associated with genital and mucosal infections, although a few infect cutaneous epithelium. Genus alpha HPVs are further defined as high-risk or low-risk based on their association with anogenital cancers or benign genital warts, respectively. The genus beta papillomaviruses are associated with benign skin infections; however, in immunocompromised patients or patients with epidermodysplasia verruciformis (EV), a rare genetic disorder caused by mutations in the EVER1 or EVER2 genes, squamous cell carcinomas (SCC) can develop.


HPV requires actively dividing and differentiating epithelium, typically the basal layer of stratified squamous epithelium, for its DNA replication, gene expression, and protein coat production. HPV reaches basal cells through epithelial microabrasions or at sites where the epithelium transitions from a monolayer to stratifi ed squamous cells, such as at the cervical transformation zone or the anal verge. Although these areas are more accessible to the virus for initial infection, they are poorer sites for viral  production. This makes these anatomical transition areas sites where abnormal and inadequate viral gene expression, and ultimately HPV genome integration, are likely to occur. With HPV genome integration, regulatory viral genes may be lost, and epithelial cells are driven to immortalization, which can progress to cancer.
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