The role of epigenetics in rare diseases.

Epigenetic control systems are based on chromatin modifications (DNA methylation, histone modifications and nucleosome positioning), which affect the local kinetics of gene expression. They play an important role in maintaining cell fate decisions, X inactivation and genomic imprinting. Aberrant chromatin states that are associated with a deleterious change in gene expression are called epimutations. An epimutation can be a primary epimutation that has occurred in the absence of any genetic change or a secondary epimutation that results from a mutation of a cis -acting regulatory element or trans -acting factor. Epimutations may play a causative role in disease, for example in imprinting disorders, or may be part of the pathogenetic mechanism as in the fragile X syndrome and in syndromes caused by a mutation affecting a chromatin modifier. For several diseases, DNA methylation testing is an important tool in the diagnostic work-up of patients.