Role of imprinting in abnormal human development.

Parental-specific differences in the expression of certain genes (imprinting), may be implicated in the pathogenesis of anomalous gestations, but only a minority manifest themselves as malformation syndromes. Delayed or lost gestations are much more frequent sequelae, as are those disorganized to such an extent that they are usually classified as neoplastic rather than developmental processes. Expression levels from imprinted loci are dependent not only on the number of genomic alleles present and their structural integrity, but also on their specific parental origin. Anomalous expression of imprinted genes during development is sometimes caused by imbalanced representation of maternal and paternal contributions, 'uniparental disomy'. Uniparental parthenogenetic or androgenetic gestations form ovarian teratomas or complete hydatidiform moles, respectively--examples of an arrested developmental program. Uniparental disomy of individual chromosomes or portions thereof has been associated with developmental delay or gestational loss. The phenotype of hemizygous mutation or deletion of imprinted genes is modified by the parental origin of the mutant copy, with dichotomous syndromes defined by parental inheritance, as in the Prader-Willi and Angelman syndromes. Lastly, failure of the imprinting process itself, 'loss of imprinting', may quantitatively alter expression levels of normally imprinted transforming or tumor-suppressing genes, thereby increasing risk for developmental tumors such as Wilms' tumor or choriocarcinoma.