Summary auto-generated
This 1993 study investigates how DNA methylation regulates human papillomavirus (HPV) gene expression, focusing on the upstream regulatory region (URR) that controls the E6 and E7 oncogenes. Using in vitro methylation of HPV-18 reporter constructs in cervical carcinoma cell lines, researchers found that selective methylation of the viral URR reduces transcriptional activity approximately 10-fold, with methylation sites clustered in the enhancer region. Importantly, competitive titration experiments revealed that suppression occurs indirectly through methyl-CpG-binding proteins rather than direct interference with transcription factor binding. Analysis of methylated HPV DNA in chromatin using restriction enzyme accessibility assays showed that extensively methylated viral DNA adopts a nucleosomal organization characteristic of transcriptionally inactive heterochromatin. The authors conclude that DNA methylation functions as a critical epigenetic regulatory mechanism controlling HPV expression and potentially the proliferation of infected cells during cervical carcinogenesis.
Key findings
- DNA methylation of the HPV-18 upstream regulatory region reduces transcriptional activity ~10-fold, with methylation sites concentrated in the viral enhancer region
- Transcriptional suppression by methylation is mediated indirectly through methyl-CpG-binding proteins, not through direct steric blocking of transcription factors
- Extensively methylated viral DNA in cervical carcinoma cells adopts nucleosomal organization characteristic of transcriptionally inactive chromatin
- DNA methylation represents an important epigenetic mechanism regulating HPV expression during cervical carcinogenesis
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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 carried 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.