Methylation of cytosine

The paper does not answer the questions, unfortunately.

Methylation of cytosine is common throughout the human genome. This covalent modification most commonly occurs at cytosines within a 5′-CpG-3′ dinucleotide when a methyl group from S-adenosylmethionine (SAM)6 is enzymatically transferred to the 5 position of cytosine to generate 5-methylcytosine (5-MC) in genomic DNA. DNA methylation patterns are a product of the frequency of cytosine DNA methylation at specific sites along a strand of DNA. Recent genome-wide high-resolution DNA methylation analysis of a primary human fibroblast cell line demonstrated that 4.25% of total cytosines in genomic DNA are methylated, 67.7% of CpGs are methylated, and 99.98% of DNA methylation occurs in CpG dinucleotides (10). Similar analysis of a human embryonic stem cell line showed that 5.83% of cytosines are methylated, 82.7% of CpG dinucleotides are methylated, and 25% of all cytosine methylation occurs at non-CpG sites (10); an earlier study also reported high levels of non-CpG methylation in mouse embryonic stem cells in contrast to methylation patterns in mouse somatic cells (17). Although the importance of CpG methylation is established, the role of non-CpG methylation is a new active area of research.