Genomic imprinting and imprinting defects

Genomic imprinting is an epigenetic process by which specific gene regions are marked by the male and the female germ lines by histone modifications and DNA methylation, so that only the paternal allele or only the maternal allele of a gene is active. Genomic imprints are erased in primordial germ cells, newly established during later stages of germ cell development and stably inherited through somatic cell divisions during postzygotic development. Defects in imprint erasure, establishment or maintenance result in aberrant epigenetic patterns and expression profiles and can cause specific diseases. Imprinting defects can occur spontaneously without any DNA sequence change (primary imprinting defect) or as the result of a mutation in a cis-regulatory element or a trans-acting factor (secondary imprinting defect). The distinction between primary and secondary imprinting defects is important for assessing the risk of recurrence in affected families.