Among the human Alphapapillomaviruses
, a subset is considered oncogenic. The two major HR-types, HPV16 and HPV18 are responsible for about 70% of all cervical cancer cases 
. Recent work based on epidemiological and phylogenetic studies showed that all HR viruses cluster together in a HR-clade arguing that a common ancestor acquired an oncogenic potential () 
. However, the most recent report by the working group of the IARC found that only a limited number of HPV types within this HR-clade had sufficient evidence to conclude that they were cervix cancer carcinogens 
. Nevertheless, there is compelling evidence supporting an evolutionary aspect to HPV induced carcinogenesis. In this study, we set out to investigate a well-characterized phenotype associated with the molecular basis of HPV oncogenicity from a phylogenetic perspective.
E6-mediated p53 degradation has been considered a hallmark function of oncogenic HPVs, although E6 is a highly multi-functional protein 
. The attribution of cancer risk being associated with proteasomal degradation of p53 has been mainly based on studies comparing low risk types HPV6/11 to high-risk types HPV16/18. These viruses, although all members of the Alphapapillomavirus
genus have different tissue tropisms, cause different histological lesions (e.g., flat vs. verrucous) and are phylogenetically distant. These observations indicate that when extrapolating factors associated with cancer risk, evolution and biological niche adaptation (e.g., tissue tropism) need to be considered. In order to address the evolutionary importance of p53 degradation in the oncogenic process, we analyzed the ability of E6s from 27 HPV types (representing all alpha-HPV species) to degrade p53 in vivo
. Our results demonstrate that although all high-risk types did in fact degrade p53, several low-risk types not epidemiologically associated with cervix cancer (e.g., HPV53, HPV70 and HPV71) also decreased p53 levels. A recent in vitro
independently confirmed that E6 of HPV53, HPV56, HPV66 and HPV70 degraded p53 as efficiently as E6 of the highest-risk alpha-PV types. Interestingly, data from cutaneous HPV types (e.g., HPV5 and HPV8) associated with the development of non-melanoma skin cancer (NMSC) in the rare hereditary disease epidermodysplasia verruciformis (EV) 
indicated that these viruses neither bind to, nor degrade p53. Therefore, the phylogenetic position of the virus is a more relevant criterion than its oncogenic risk for predicting ability to degrade p53. Based on these observations, we hypothesize that p53 degradation was adapted by the most recent common ancestor (MRCA) of the current HR-HPV clade in order to successfully adapt to a biological niche within the genital mucosa. All extant members of the HR-clade have inherited this p53 degradation phenotype irrespective of oncogenic potential. Embedded within this hypothesis is the notion that p53 degradation might be necessary, but is not sufficient for HPV induced oncogenesis. Thus, p53 degradation appears to be important for these viruses to complete their life cycle in this particular niche of the human body.
Furthermore, these results suggest that there are additional biochemical activities distinguishing closely related non-oncogenic (e.g., HPV53 and HPV70) from oncogenic members of the high-risk clade consistent with the plethora of activities associated with E6 
. However, since E6 and E7 expression is needed for successful transformation of primary keratinocytes 
, it is possible that the differentiating functionality lies within the E7 protein and/or an interaction of functions between the E6 and E7 proteins, introducing additional complexity into the biological system. Identifying specific changes associated with epidemiological cancer risk should lead to a better understanding of the oncogenic mechanisms utilized by high-risk HPVs that distinguish them from their phylogenetically closely related nearest neighbors.
Of note, HPV71 was one of the three most prevalent HPV types detected in a population based study in Costa Rica 
. In addition, HPV71 is one of the types most likely to cause persistant cervicovaginal infection, but is not associated with neoplastic changes 
. It is feasible that E6 activity to degrade p53 provides HPV71 an evolutionary advantage accounting for in part, its high population prevalence and persistence.
In an attempt to understand the evolutionary adaptations that occurred in the MRCA of the HR-clade, we identified a single amino acid site (“position 31”) correlated with the ability to degrade p53. This position, situated within the putative helix 1 of the E6N domain, is predicted to be fully exposed to solvent and therefore ideally positioned for participating in protein-protein interactions possibly important for p53 degradation. Both sequence analysis and mutagenesis data suggest that at this position a polar (e.g., T or N) or negatively charged residue supports p53 degradation activity, whereas a positively charged residue (R or K) is generally unfavorable to p53 degradation activity. This position may be part of an interacting surface domain important for the p53 degradation process. However, this interacting surface probably comprises several other amino-acid chains. Depending on the HPV type, the importance of position 31 may vary.
A structure based analysis of E6 in the light of the proposed evolution of the p53 degrading potential should allow researchers to further tease apart the evolutionary importance of position 31 for p53 degradation. Finally, whether E6 induced p53 degradation is a critical hallmark of HPV induced cancer or a manifestation of evolution remains to be confirmed. These data further support Theodosius Dobzhansky comment that “Nothing in biology makes sense except in the light of evolution.”