Abstract
BackgroundThe Human Papillomavirus (HPV) genome is divided into early and late coding sequences, including 8 open reading frames (ORFs) and a regulatory region (LCR). Viral gene expression may be regulated through epigenetic mechanisms, including cytosine methylation at CpG dinucleotides. We have analyzed the distribution of CpG sites and CpG islands/clusters (CGI) among 92 different HPV genomes grouped in function of their preferential tropism: cutaneous or mucosal. We calculated the proportion of CpG sites (PCS) for each ORF and calculated the expected CpG values for each viral type.ResultsCpGs are underrepresented in viral genomes. We found a positive correlation between CpG observed and expected values, with mucosal high-risk (HR) virus types showing the smallest O/E ratios. The ranges of the PCS were similar for most genomic regions except E4, where the majority of CpGs are found within islands/clusters. At least one CGI belongs to each E2/E4 region. We found positive correlations between PCS for each viral ORF when compared with the others, except for the LCR against four ORFs and E6 against three other ORFs. The distribution of CpG islands/clusters among HPV groups is heterogeneous and mucosal HR-HPV types exhibit both lower number and shorter island sizes compared to cutaneous and mucosal Low-risk (LR) HPVs (all of them significantly different).ConclusionsThere is a difference between viral and cellular CpG underrepresentation. There are significant correlations between complete genome PCS and a lack of correlations between several genomic region pairs, especially those involving LCR and E6. L2 and L1 ORF behavior is opposite to that of oncogenes E6 and E7. The first pair possesses relatively low numbers of CpG sites clustered in CGIs while the oncogenes possess a relatively high number of CpG sites not associated to CGIs. In all HPVs, E2/E4 is the only region with at least one CGI and shows a higher content of CpG sites in every HPV type with an identified E4. The mucosal HR-HPVs show either the shortest CGI size, followed by the mucosal LR-HPVs and lastly by the cutaneous viral subgroup, and a trend to the lowest CGI number, followed by the cutaneous viral subgroup and lastly by the mucosal LR-HPVs.
Highlights
The Human Papillomavirus (HPV) genome is divided into early and late coding sequences, including 8 open reading frames (ORFs) and a regulatory region (LCR)
CpG sites are underrepresented among HPV genomes, to a lesser extent than in their hosts After identifying all CpG sites among 92 available HPV DNA sequences, we calculated the expected CpG value for each viral genome
We found that there are both significant differences and correlations between CpG observed and expected values among all sequenced HPV genomes, and that viral genomes have an underrepresentation of only around 50% compared to eukaryotic genomes where underrepresentation is from 30 down to 5%
Summary
The Human Papillomavirus (HPV) genome is divided into early and late coding sequences, including 8 open reading frames (ORFs) and a regulatory region (LCR). We have analyzed the distribution of CpG sites and CpG islands/clusters (CGI) among 92 different HPV genomes grouped in function of their preferential tropism: cutaneous or mucosal. Viral types that preferentially infect mucosal epithelia are grouped into either a low-risk group (LR-HPV) not associated with cancer, or into a high-risk group (HR-HPV) whose members are found in almost all cases of cervical cancer [1]. Most of the work on HPV methylation has been carried out on the two main viral types involved in cervical cancer (types 16 and 18). In both genomes there is a progressive increase in methylation from asymptomatic carriers, through benign lesions and pre-malignant disease, to cancer tumors. There is heterogeneity of CpG methylation in viral genomes derived from clinical specimens [4,5,6,7], and the exact role of viral DNA methylation remains unclear
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