Gene silencing and expression during mammalian development : The special case of the imprinted H19 gene

Sammanfattning: Embryonic development consists of a vast number of fine-tuned, complex processesthat organise the body plan and individual organs. To enable developmental decisions,genes pivotal for normal development must be stably activated and silenced, asappropriate. Imprinted genes are exclusively expressed from the paternal or the maternalallele, such that one allele is activated and the opposite allele silenced in the same cell,illustrating an excellent example of epigenetic silencing.We investigated H19, a putative tumour suppresser gene, which is highly expressedduring embryonic and foetal development, exclusively from the maternal allele. A novelmethod, allele-specific in situ hybridisation (ASISH), was developed to examine allelespecific activation/silencing events during early mouse development and in experimentalsystems. In order to understand the function of the H19 gene, we also investigated post-transcriptional regulation processes, as well as potential roles for the H19 transcript.We were able to show that the H19 gene can be expressed from the same chromosome as Igf2 in the choroid plexus of the mouse, contrary to previous suggestions.We also found that the silencing of the paternal H19 allele is stepwise, leading to avariegated expression pattern during early mouse development. The manifestation of theimprinting status of H19 involves deacetylation of histones, since the silencing of thepaternal HI9 allele could be prevented by trichostatin A, an inhibitor of histonedeacetylases. Collectively, the results suggest an analogy between genomic imprintingand position-effect variegation in Drosophila.We have also discovered a novel splice variant of the H19 transcript which isgenotype-specific. Moreover, we document that the genotype and epigenotype synergiseto control the expression of the splice variant. It was also shown that H19 transcript isassociated with polysomes and may control expression of the IGF2 gene in trans.The first experimental link between genomic imprinting and the fetal alcohol syndrome was also documented, since the ethanol metabolite, acetaldehyde perturbs the manifestation of the H19 impritning status during early mouse development.