Expression of HPV16 E6 or E7 increases integration of foreign DNA.

In most invasive cervical carcinomas, high-risk human papillomavirus (HPV) DNA is integrated into the host genome, while in pre-invasive cervical lesions the viral genome is typically maintained exclusively as an episome. In contrast, integration of low-risk HPV DNA is rare, as is the association of low-risk HPVs with carcinomas. High-risk HPV integration is associated with a selective growth advantage of affected cells, and hence, integration is likely to be an important genetic alteration contributing to cervical tumor progression. Expression of high-risk, but not low-risk, HPV E6 or E7 proteins disrupts the p53-dependent G1 arrest that cells normally display in response to DNA damage. Absence of this cell cycle checkpoint may predispose cells containing high-risk HPVs to genetic instability and to the accumulation of the genetic alterations that appear to be required for HPV-associated cervical tumor progression. We hypothesized that integration of high-risk HPV DNA into the host cell genome may be facilitated by E6- and/or E7-mediated disruption of the normal DNA damage response pathway. To test this hypothesis, we assessed the integration frequency of a reporter plasmid (pHyGal) in RKO cells expressing individual E6 or E7 genes of either high-risk (HPV16) or low-risk (HPV6, HPV11) type viruses. Cells expressing HPV16 E6 or HPV16 E7 exhibited a significantly increased frequency of pHyGal integration in comparison to RKO control cells or cells expressing low-risk HPV E6 or E7. Thus, expression of high-risk, but not low-risk, E6 and E7 proteins increases the frequency of foreign DNA integration into the host genome. These findings suggest that at least some of the difference in oncogenic potential observed between high-risk and low-risk HPV types may be determined by the increased ability of high-risk HPVs to integrate into host DNA.