Bsr, a nuclear-retained RNA with monoallelic expression

The imprinted Dlk1-Gtl2 and Prader-Willi syndrome (PWS) regions are characterized by a complex noncoding transcription unit spanning arrays of tandemly repeated C/D RNA genes. These noncoding RNAs (ncRNAs) are thought to play an essential but still poorly understood role. To better understand the intracellular fate of these large ncRNAs, fluorescence in situ hybridization was carried out at the rat Dlk1-Gtl2 domain. This locus contains a approximately 100-kb-long gene cluster comprising 86 homologous RBII-36 C/D RNA gene copies, all of them intron-encoded within the ncRNA gene Bsr. Here, we demonstrate that the Bsr gene is monoallelically expressed in primary rat embryonic fibroblasts as well as in hypothalamic neurons and yields a large amount of unspliced and spliced RNAs at the transcription site, mostly as elongated RNA signals. Surprisingly, spliced Bsr RNAs released from the transcription site mainly concentrate as numerous, stable nuclear foci that do not colocalize with any known subnuclear structures. On drug treatments, a fraction of Bsr RNA relocalizes to the cytoplasm and associates with stress granules (SGs), but not with P-bodies, pointing to a potential link between SGs and the metabolism of ncRNA. Thus, Bsr might represent a novel type of nuclear-retained transcript.