DNA methylation is an essential enzymatic modification in mammals. This common epigenetic mark occurs predominantly at the fifth carbon of cytosines within the palindromic dinucleotide 5′-CpG-3′. The majority of methylated CpGs are located within repetitive elements including centromeric repeats, satellite sequences and gene repeats encoding ribosomal RNAs. CpG islands, frequently located at the 5′ end of genes, are typically unmethylated. DNA methylation also occurs at imprinted genes which exhibit parent-of-origin-specific patterns of methylation and expression. Imprinted methylation at differentially methylated domains (DMDs) is one of the regulatory mechanisms controlling the allele-specific expression of imprinted genes. Proper control of DNA methylation is needed for normal development and loss of methylation control can contribute to initiation and progression of tumorigenesis (reviewed in Plass and Soloway, 2002). Because patterns of imprinted DNA methylation are highly reproducible, imprinted loci make useful models for studying regulation of DNA methylation and may provide insights into how this regulation goes awry in cancer. Here, we review what is currently known about the mechanisms regulating imprinted DNA methylation. We will focus on cis-acting DNA sequences, trans-acting protein factors and the possible involvement of RNAs in control of imprinted DNA methylation.
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