Defining the protein complement of CpG islands

Abstract

In higher eukaryotes, the DNA base Cytosine can exist in a variety of modified forms when in the dinucleotide CpG. Although a methylated form tends to dominate within the genome, approximately 1% of all CpG dinucleotides are found unmodified at high densities spanning around 1Kb and tend to co-localise to the 5’ ends of around 60% of annotated gene promoters. These unique DNA sequences are known as CpG islands (CGIs) and their role within the genome to date is largely unknown. Methylation of CGIs in cancers however has been linked to silencing of associated genes implying a role in gene regulation. Furthermore these sites are also interesting as they remain specifically nonmodified within a genome rich in methylated CpG. We set out to better understand the roles for CGIs through the characterisation of any specific CGI binding proteins. Digestion of nuclei with methyl sensitive restriction enzymes facilitates the purification of CGI fragments. Subsequent immunohistochemistry on the CGI chromatin fragments along with ChIP-PCR over several CGIs revealed an enrichment of the “active” histone modifications including H3K4me3, a depletion of the “silencing” marks such as H3K27me3, as well as a group of CGI specific binding factors. These latter proteins contained a domain previously shown to bind to non-methylated CpG dinucleotides (the CXXC domain) and as such were ideal candidates for CGI specific factors, in particular a protein called Cfp1. Genome wide sequencing revealed a striking correlation between Cfp1 and H3K4me3 which were both seen at around 80% of islands. Furthermore, the presence of Cfp1/H3K4me3 at islands tended to have a negative correlation with the presence of chromatin rich in the silencing histone modification H3K27me3. Closer investigation of the Cfp1 protein reveals it to be a true non-methyl CGI binding factor in vivo and shRNA reduction of Cfp1 levels to around 10% of wild type resulted in a precipitous drop in H3K4me3 levels over CGIs without a dramatic reduction in global H3K4me3 levels. As Cfp1 has been shown to be part of the Set1 histone H3K4 methyltransferase complex responsible for this modification, this CXXC protein may be attracting this histone modifying complex and as such represents a method whereby the underlying DNA sequence (CpG) can drive the overlying epigenetic state. This study may go some way to understanding the functional significance of CGIs within the genome.