Human papillomaviruses (HPVs) are described as "types" based on their genome sequences and identified by a number. For example, HPV-6 is associated with genital warts, and HPV-16 with anogenital cancers. The genomes of many HPV types have been reisolated, sequenced and compared to reference "prototypes" countless times by laboratories throughout the world. It was found that each HPV type occurs in the form of "variants", identified by about 2% nucleotide differences in most genes and 5% in less conserved regions. Less than 100 variants of any HPV type have been detected, a scenario that is very different from the quasi-species formed by many RNA viruses. The variants of each HPV type form phylogenetic trees, and variants from specific branches are often unique to specific ethnic groups. Immigrant populations contain, depending on their respective ethnic origins, mixtures of variants. The absence of HPV genomes intermediate to specific types show that all HPV types existed already when humans became a species. Consequently, humans had always suffered from lesions like anogenital cancer, genital warts and common warts. A growing number of epidemiological, etiological and molecular data suggest that variants of the same HPV type are biologically distinct and may confer differential pathogenic risks. Since the distribution of some variants of HPV-16 and 18 correlates with the distribution of human populations that have an increased risk to develop anogenital cancer, the study of HPV type variation may point to one of the reasons for the higher incidence rates of these lesions in specific cohorts.
(c) 2005 Wiley-Liss, Inc.