Genetic Syndromes Determined by Alterations in Genomic Imprinting Pathways

Abstract

Genomic imprinting appears to be a mammal-specific phenomenon whereby differential gene expression according to parent of origin has evolved as a means to regulate many complex pathways related to growth, metabolism, and neurologic development. Imprinting is accomplished by an epigenetic mechanism involving, for example, methylation of specific sequences, without changing the underlying genetic code. This process occurs with very specific timing in relation to development of an embryo and is reset in the sex-specific gametes of each individual. Epigenetic deregulation of the imprinting process is the cause of specific genetic syndromes, many of which show alterations in growth, metabolism, and neurologic development. In fact, it has been demonstrated that alterations of a single imprinting control region (IGF2/H19) can lead to opposite disorders of congenital growth (Beckwith-Wiedemann syndrome and Silver-Russell syndrome). Recently, it has been suggested that the use of assisted reproductive techniques such as in vitro fertilization may heighten the susceptibility of embryos to epigenetic deregulation, leading to increased risk for certain imprinting disorders. Also, the knowledge that imprinted genes are involved in pathways of fetal and placental growth has prompted researchers to look into the role of imprinting in intrauterine growth restriction. This may have consequences well beyond the newborn period because epidemiologic studies have shown a strong link between poor fetal growth and the susceptibility to glucose intolerance and insulin resistance syndrome in adults.