Expression of the Snurf-Snrpn IC transcript in the oocyte and its putative role in the imprinting establishment of the mouse 7C imprinting domain.

The human chromosome 15q11-q13, or mouse chromosome 7C, is an imprinting domain controlled by bipartite imprinting centers (ICs): Prader-Willi syndrome (PWS)-IC and Angelman syndrome (AS)-IC. PWS-IC functions to maintain the paternal epigenotype on the paternal chromosome in somatic cells, while AS-IC plays a role in the establishment of the maternal epigenetic mark at PWS-IC in the female germline or early embryos. Several alternative exons and promoters of Snurf-Snrpn (SNRPN upstream reading frame-small nuclear ribonucleoprotein polypeptide N) are expressed as "IC transcripts". Previous studies have shown that IC-transcript expression is restricted to the brain. We studied expression of the mouse IC-transcript in tissues including brain and oocytes as well as in cultured neurons and glia cells by RT-PCR and by in situ hybridization (ISH) in oocytes. The IC transcript was strongly expressed in brain (especially in neurons) and ovary (especially in oocytes and granulosa cells), while no expression was found in other tissues. This was confirmed by quantitative analysis and ISH. Expression levels in the brain were 7-fold higher compared to those in ovaries. ISH signals were observed in oocytes and granulosa cells of the secondary and developing follicles. These findings, together with previous data, suggest that the IC transcript may be associated with the establishment of PWS-IC methylation on the maternal chromosome as an AS-IC cis-acting element.