The Effect of DNA Methylation on Gene Regulation of Human Papillomaviruses

Abstract

Integration of human papillomaviruses (HPVs) into the host genome is considered to be an early and important event in HPV-linked cervical carcinogenesis. Consequently, the viral DNA potentially becomes a target for cellular control mechanisms normally acting on the corresponding integration site. Besides resulting position effects, host-specific DNA methylation may play a functional role in HPV gene regulation. To elucidate the influence of such a kind of epigenetic modification on viral transcription, in vitro methylation studies on HPV-18 upstream regulatory region (URR)-controlled reporter plasmids were carded out. Selective methylation of the viral URR results in a down-regulation of the transcriptional activity, which can be attributed to nonrandom distribution of methyl-acceptor sites clustered within the constitutive enhancer region. In vivo competition experiments show that suppression is not directly mediated by steric hindrance of methyl residues with transcription factors, but rather is due to the association with methyl-CpG DNA-binding proteins. Using a restriction enzyme accessibility assay on both the DNA and chromatin levels, it could be demonstrated that, in vivo, extensively methylated viral DNA is nucleosomally organized, characteristic of transcriptionally inactive chromatin. These data suggest that DNA methylation is an important regulatory pathway in the modulation of HPV expression and as a consequence the proliferation rate of virus-infected cells.

Clonal Selection for Transcriptionally Active Viral Oncogenes during Progression to Cancer

“…Our observations are in excellent agreement with a recent report (2) that HPV-16 DNA is targeted for CpG methylation in vivo and that changing methylation patterns were observed with different lesion grades. The data are also consistent with reports that transfected and integrated HPV DNA is heavily methylated and transcriptionally silent (5,29,45). However, reactivation was not demonstrated in those studies.…”

Nucleotide sequences and further characterization of human papillomavirus DNA present in the CaSki, SiHa and HeLa cervical carcinoma cell lines.

“…Still, because tandem head-to-tail nondisrupted full-length genomes are among the most prevalent of the CaSki HPV-16 copies (Baker et al, 1987), it is more likely that previously identified factors, e.g. methylation status of the LCR or condensed, transcriptionally inert chromatin in the region of integration (List et al, 1994 ;Rosl et al, 1989Rosl et al, , 1993 play a larger role in the paucity of mRNA transcripts from the full-length CaSki HPV-16 genomes.…”

Conserved Methylation Patterns of Human Papillomavirus Type 16 DNA in Asymptomatic Infection and Cervical Neoplasia

“…Ongoing pilot studies confirmed the occurrence of DNA methylation in HPV-6, -11, -18, and -31 (1A; our unpublished data). The consequence of CpG methylation is repression of HPV transcription as shown by transfection of in vitro methylated HPV DNA (34) and by in vivo studies with the cell lines CaSki and SiHa (2,45). Our research presently concentrates on HPV-16, the most prevalent HPV type in cervical carcinoma (6).…”