Monoallelic Expression of Imprinted Genes

Primary imprinting marks established in the gametes lead to stable monoal-lelic expression of imprinted genes in the embryo. This monoallelic expression is the consequence of distinct cascades of epigenetic events occurring on the parental alleles during development. The active allele undergoes all the required chromatin reorganization leading to gene expression, a process that could be named 'genome formatting (Paro 2000). In contrast, the imprinted allele acquires a functional imprint that impairs the 'formatting' for gene expression on this allele and maintains a stable silent state.

The acquisition of functional imprints, which is often called 'imprint reading', involves the establishment of' secondary imprinting marks', i.e. epigenetic modifications established in cis after fertilization from the primary imprinting marks. These secondary marks include additional DMRs that are possibly cell-type specific and this hierarchy in the establishment of DMRs is crucial for the mechanisms of genomic imprinting (Weber et al. 2001; Lopes et al. 2003).

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