Integration of human papillomavirus type 16 DNA into the human genome leads to increased stability of E6 and E7 mRNAs: implications for cervical carcinogenesis.

Abstract

In many cervical cancers, human papillomavirus type 16 (HPV-16) DNA genomes are found to be integrated into the host chromosome. In this study, we demonstrate that integration of HPV-16 DNA leads to increased steady-state levels of mRNAs encoding the viral oncogenes E6 and E7. This increase is shown to result, at least in part, from an increased stability of E6 and E7 mRNAs that arise specifically from those integrated viral genomes disrupted in the 3' untranslated region of the viral early region. Further, we demonstrate that the A+U-rich element within this viral early 3' untranslated region confers instability on a heterologous mRNA. We conclude that integration of HPV-16 DNA, as occurs in cervical cancers, can result in the increased expression of the viral E6 and E7 oncogenes through altered mRNA stability.Human papillomaviruses (HPVs) are small DNA viruses that infect epithelial cells. About 70 different genotypes have been identified to date, among which only a subset are associated with cervical cancers. More than 90% of cervical cancers contain these "high-risk" HPVs . In these cancers, two viral transforming genes, E6 and E7, are consistently expressed (1). The E6 and E7 proteins have been shown to interact with and inactivate the tumor-suppressor gene products p53 and pRb, respectively (34), and to immortalize human epithelial and fibroblastic cells as well as rodent fibroblasts (2-4). In transgenic mouse systems, expression of these genes leads to tumor formation (5)(6)(7)(8). These studies have demonstrated the likely importance of E6 and E7 in cervical carcinogenesis.The viral DNA genome of HPV-16 or -18 is often found integrated into the host chromosomes in cervical cancers (9)(10)(11). This viral DNA integration has been hypothesized to result in increased expression of E6 and E7 (12). To test this hypothesis, we have isolated a series of human cervical epithelial cell populations that harbor either extrachromosomal or integrated HPV-16 DNA (35). These cell populations were derived from a parental cell population, W12, that had been established from an HPV-16-positive cervical biopsy (13).Using these reagents, we have demonstrated that HPV-16 DNA integration correlates with increased expression of the viral E7 protein and with a selective growth advantage over cells harboring extrachromosomal HPV-16 DNA (35).In the current study, we have sought to define a mechanism by which integration leads to increased expression of papillomaviral transforming genes. We demonstrate that the high levels of E7 protein seen in the integrated clones correlate with increased steady-state levels of E6-and E7-specific mRNAs, at least in part as a result of changes in their stability. This increased stability appears to be the result of the integrative The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. S1 Nuclease Mapping Analysis. The E6/E7-specific cDN...

Human papillomavirus in high- and low-risk areas of oesophageal squamous cell carcinoma in China

“…Since HPV integration is considered to result in deletion of the E2 gene (Jeon and Lambert, 1995), we determined the status of HPV in the host cells on the basis of the E2/E6 ratio (Peitsaro et al, 2002). When the ratio was equal to or higher than unity, all the HPV genome was considered to be in an episomal form and not integrated.…”

Human papillomavirus-16 is integrated in lung carcinomas: a study in Chile

“…Since E2 is degraded when HPV is integrated (Jeon and Lambert, 1995) into cellular genome, E2/E6 ratio nearly equal to 1 indicates the presence of the episomal form only. Values less than 1 indicate the presence of both integrated and episomal forms, while a ratio of 0 indicates that the fragment was not detected after 45 cycles of amplification and that HPV exists in the integrated form only.…”

Human papillomavirus type 16 in head and neck carcinogenesis

“…During early stages of viral infection, the E2 protein represses transcription of the oncogenes E6 and E7 (Thierry and Howley 1991). Integration of the viral genome, which often takes place during carcinogenic progressing, disrupts the E2 and E1 ORFs and results in elevated expression of E6 and E7 (Jeon and Lambert 1995).…”