A developmental window of opportunity for imprinted gene silencing mediated by DNA methylation and the Kcnq1ot1 noncoding RNA.

The Kcnq1 imprinted domain encodes a paternally expressed noncoding RNA Kcnq1ot1 and several paternally repressed protein-coding genes. Transcriptional regulation is controlled by the Kcnq1ot1 gene whose maternal germline methylation imprint overlaps with the Kcnq1ot1 promoter. The domain can be divided into two groups of genes. One group is imprinted in all lineages and is reliant on DNA methylation for its imprinting. The other group contains genes that are imprinted specifically in the placenta and retain their imprinting in the absence of Dnmt1, the primary DNA maintenance methylase. In the placenta paternal Kcnq1ot1 expression is associated with the acquisition of repressive histone modifications throughout the domain. Using the Dnmt1o knockout, we have analyzed the effect of removing DNA maintenance methylation at the eight-cell stage on the Kcnq1 imprinted domain. In the placenta the expression of the normally silent maternal Kcnq1ot1 allele leads to reduced expression of the surrounding maternally expressed genes. This repression is seen in both the placental-specific imprinted genes and the ubiquitously imprinted genes. Conversely, reduction of functional Dnmt1 results solely in reduced expression of the ubiquitously imprinted genes in the placenta. This suggests that Kcnq1ot1 expression can epigenetically silence placentally imprinted genes in the cluster only during a specific developmental window. This highlights the possibility that Kcnq1ot1-mediated repression is temporally regulated leading to epigenetic silencing of placental-specific genes. We show that allele-specific histone modifications are still present in the Dnmt1 ( -/- ) trophoblast at placental-specific imprinted loci and are likely responsible for maintaining the imprinting of these genes in the absence of DNA methylation.

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