Mechanisms of imprint dysregulation.

Genomic imprinting is an epigenetic process by which the male and the female germ line confer specific marks (imprints) onto certain gene regions, so that one allele of an imprinted gene is active and the other allele is silent. 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 a paternal chromosome carrying a maternal imprint or in a maternal chromosome carrying a paternal imprint. A wrong imprint leads to activation of an allele that should be silent or silencing of an allele that should be active. Since the dosage of imprinted genes is very important for development and growth, imprinting defects lead to 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 recurrence risk in affected families.