Nearly all cells in the human body share the same primary genome sequence consisting of four nucleotide bases. One of the bases, cytosine, is commonly modified by methylation of its 5 position in CpG dinucleotides (mCpG). Most CpG dinucleotides in the human genome are methylated, but the level of CpG methylation varies with genetic location (promoter versus gene body), whether genes are active versus silenced, and cell type. Research has shown that the maintenance of a particular cellular state after cell division is dependent on faithful transmission of methylated CpGs, as well as inheritance of the mother cells’ repertoire of transcription factors by the daughter cells. These two mechanisms of epigenetic inheritance are linked to each other; the binding of transcription factors can be affected by cytosine methylation, and cytosine methylation can, in turn, be added or removed by proteins that associate with transcription factors.

The genetic and epigenetic language, which imparts when and where genes are expressed, is understood at a conceptual level. However, a more detailed understanding is needed of the genomic regulatory mechanism by which methylated cytosines affect transcription factor binding. Because cytosine methylation changes DNA structure, it has the potential to affect binding of all transcription factors. However, a systematic analysis of binding of a large collection of transcription factors to all possible DNA sequences has not previously been conducted.

To globally characterize the effect of cytosine methylation on transcription factor binding, we systematically analyzed binding specificities of full-length transcription factors and extended DNA binding domains to unmethylated and CpG-methylated DNA by using methylation-sensitive SELEX (systematic evolution of ligands by exponential enrichment). We evaluated binding of 542 transcription factors and identified a large number of previously uncharacterized transcription factor recognition motifs. Binding of most major classes of transcription factors, including bHLH, bZIP, and ETS, was inhibited by mCpG. In contrast, transcription factors such as homeodomain, POU, and NFAT proteins preferred to bind methylated DNA. This class of binding was enriched in factors with central roles in embryonic and organismal development.

The observed binding preferences were validated using several orthogonal methods, including bisulfite-SELEX and protein-binding microarrays. In addition, the preference of the pluripotency factor OCT4 to bind to a mCpG-containing motif was confirmed by chromatin immunoprecipitation analysis in mouse embryonic stem cells with low or high levels of CpG methylation (due to deficiency in all enzymes that methylate cytosines or contribute to their removal, respectively). Crystal structure analysis of the homeodomain proteins HOXB13, CDX1, CDX2, and LHX4 revealed three key residues that contribute to the preference of this developmentally important family of transcription factors for mCpG. The preference for binding to mCpG was due to direct hydrophobic interactions with the 5-methyl group of methylcytosine. In contrast, inhibition of binding of other transcription factors to methylated sequences was found to be caused by steric hindrance.

Our work constitutes a global analysis of the effect of cytosine methylation on DNA binding specificities of human transcription factors. CpG methylation can influence binding of most transcription factors to DNA—in some cases negatively and in others positively. Our finding that many developmentally important transcription factors prefer to bind to mCpG sites …