Genomic imprinting: parental influence on the genome

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Abstract
Genomic imprinting leads to allele-specific expression depending on the parent of origin of the allele. The most consistent difference between the alleles of an imprinted gene is in DNA methylation. But imprinted genes are also characterized by differences in chromatin conformation, histone modification, replication timing and recombination rate. The primary imprint causing these differences is unknown. The life cycle of imprints involves erasure during early germ cell development, establishment later in germ cell development or after fertilization, and maintenance during embryonic development, and begins again with erasure in the germ cells of the embryo. Genomic imprinting leads to imprinted gene expression. The mechanisms underlying this reading of the imprint can involve different aspects of gene expression: promoter methylation, the regulation of antisense transcripts, boundary elements, and silencers. Imprinted genes are often found in clusters. These can be regulated by imprinting centres. Dysregulation of imprinting has been found in many diseases: growth and behavioural defects (Beckwith–Wiedemann syndrome, Prader–Willi syndrome and Angelman syndrome) and cancer (Wilms tumour). Imprinted genes are implicated in fetal–maternal physiology, and one theory for the evolution of imprinting (the conflict theory) is that it reflects the competing interests of the maternal and paternal genomes in the developing embryo.