Allele specific chromatin states regulating expression of the imprinted <i>Gtl2</i> gene
Genomic imprinting is defined as the heritable repression of one allele of a gene in a diploid cell based upon parental origin. Epigenetic regulation of these genes leads to silencing of either the maternal or paternal copy.
A trend within imprinted domains is the abundance of genes that produce noncoding RNAs. Of 83 imprinted transcripts, 25 (30%) are noncoding RNAs. The function of many of these transcripts is unclear. One of these noncoding genes, H19, is transcribed by RNA polymerase II, and the RNA is spliced and polyadenylated. One experiment that suggested a functional role for H19 came from an experiment designed to study the effect of overexpressing H19 using transgenes in the mouse. To differentiate RNA originating from the transgene from that of the endogenous H19 gene, this study employed a modified H19 transgene into which a pair of XbaI restriction site linkers had been inserted (H19Xba). Very few transgenic animals were born, and they did not express this modified transcript, implying that the transgene was lethal to the developing embryo. We investigated further the H19Xba phenotype in order to elucidate a functional role of the H19 RNA. We were unable to recreate the original observation in mice that either overexpress or express at wild type levels the modified H19. While the cause of the previously observed H19Xba lethality cannot be explained at this time, the data presented here effectively rule out the most interesting possibility, that expression of the modified H19Xba RNA perturbs a normal function of the H19 RNA, causing embryonic lethality in the mouse.
The precise molecular mechanisms regulating imprinting remain elusive for most imprinted genes. However, differential DNA methylation and post translational modification of histone tails have been described at genomic regions surrounding many imprinted genes. In this study histone modifications upstream of the paternally imprinted Gtl2 gene were investigated. The data show that in midgestation embryos there is a differential pattern of historic acetylation between the maternal and paternal alleles within the Gtl2 differentially methylated region. Further work demonstrated that this differentially acetylation correlated with the differential DNA methylation.