Tandem repeats in the CpG islands of imprinted genes.

In contrast to most genes in mammalian genomes, imprinted genes are monoallelically expressed depending on the parental origin of the alleles. Imprinted gene expression is regulated by distinct DNA elements that exhibit allele-specific epigenetic modifications, such as DNA methylation. These so-called differentially methylated regions frequently overlap with CpG islands. Thus, CpG islands of imprinted genes may contain special DNA elements that distinguish them from CpG islands of biallelically expressed genes. Here, we present a detailed study of CpG islands of imprinted genes in mouse and in human. Our study shows that imprinted genes more frequently contain tandem repeat arrays in their CpG islands than randomly selected genes in both species. In addition, mouse imprinted genes more frequently possess intragenic CpG islands that may serve as promoters of allele-specific antisense transcripts. This feature is much less pronounced in human, indicating an interspecies variability in the evolution of imprinting control elements.