Inserm U257; ICGM
Asynchronous patterns of DNA replication are one of the properties of imprinted genes. This can be observed by fluorescent in situ hybridisation (FISH). The H19 and Igf2 genes belong to a cluster of imprinted genes which are localised in a 1 Mb region at the distal end of mouse chromosome 7. These two genes were previously shown to display asynchronous patterns of DNA replication. Analysis of the H19 3 null mutation (which deletes not only the transcription unit but also 10 kb of the H19 upstream region) suggested that the H19 locus mediates a transition between imprinted and non-imprinted DNA replication patterns (1). We are currently investigating the DNA replication patterns of the H19 3 null mutation which deletes only the transcription unit (2). Our first results suggest that in these mutants, the replication pattern is fully asynchronous, but further investigation should allow us to define a possible role for the 10 kb upstream region in this pattern. We have also studied the replication patterns of transgenic lines in order to determine if imprinted genes still retain the asynchronous pattern of replication in an ectopic location. We have analysed a transgenic line carrying a single copy of a 130 kb YAC, containing the Igf2 and H19 genes (3). This YAC is localized in the proximal region of chromosome 15, which does not contain any imprinted genes. Both H19 and Igf2 show an imprinted pattern of expression at this ectopic locus. FISH results of DNA replication patterns from this transgenic line will be presented. Although it is not clear if the FISH data reflect true asynchronous replication or rather a particular chromatin conformation specific to imprinted regions, our aim is to obtain a better insight in the mechanisms involved in imprinting.
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