Transcriptional (TGS) and post-transcriptional gene silencing (PTGS) can be transmitted to the progeny. They are both reversible. Thus, the progeny do not show genetic but epigenetic changes. In mammals and plants, these changes are generally associated with hypermethylation of the corresponding genes. In plants, RNA-directed DNA methylation (RdDM) is involved in both TGS and PTGS. Interestingly, not only TGS and PTGS but also RdDM are initiated by double stranded RNA (dsRN). Gene imprinting and X-chromosome inactivation of mammals seem to be correlated with production of sense/antisense RNA and with occurrence of de novo DNA methylation. Thus, it is reasonable to assume that RdDM also exists in mammals and that it is involved in epigenetic processes. In order to find evidence for the hypothesis, RdDM which is still barely characterised should be investigated in a plant system. With the expected data it could be possible to analyse whether de novo methylation can be directed by RNA in mammals, as well. In addition, it will be examined whether de novo DNA methylation can be initiated upon intruduction of dsRNA into mammalian cells. In contrast to previous experiments, the dsRNA will be provided by transcription of inverted repeat (IR) constructs. Moreover, it will be interesting to see whether the IR transcripts are capable to activate the RNA interference (RNAi) mechanism.
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