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. 2012 Nov 26;3:264. doi: 10.3389/fgene.2012.00264

Figure 1.

Figure 1

Simplified schemes of the Gnas and Snrpn-Ube3a imprinted loci. The features of the maternal and paternal alleles are indicated in red and blue, respectively. Genes or transcripts are named in the central part. Arrows mark transcription start sites and undulating lines ncRNAs. Open and filled boxes represent non-coding and coding exons, respectively, while black boxes indicate silenced genes. Differentially methylated regions (DMRs) of DNA are marked by MMM. (A) For the Gnas locus the alternatively spliced coding transcripts and proteins are named above and below the alleles. DMRs at Nespas and Exon 1A (EXON A/B in human) are established in the maternal germline, while methylation at Nesp occurs during early embryonic development. Gnas is expressed biallelically, but is silenced on the paternal allele in some tissues (hatched box), in which Exon 1A shows comparatively high expression levels (interrupted undulating line). Nesp represents a coding transcript (ORF limited to Nesp exon 2), but a regulatory RNA is initiated from a separate promoter in oocytes (red undulating line). Nespas is expressed from the unmethylated imprinting control region (ICR) of the locus. (B) The Snrpn-Ube3a imprinting cluster contains several genes (represented by single-exon boxes) that show monoallelic expression in brain. Ube3a is biallelically expressed in most tissues, but silenced on the paternal allele in neurons (hatched box). The Snrpn long ncRNA occurs in multiple, variably processed forms, including brain-specific variants that contain upstream promoters/exons (U-exons) and sequences overlapping with Ube3a (interrupted undulating lines). Snord115 and Snord116 represent clusters of C/D box snoRNAs, which are generated from the Snrpn ncRNA. The bipartite ICR of the human locus is indicated as AS-IC and PWS-IC, the latter being conserved in mouse around the main Snrpn start site. C15orf2 is not present in the murine locus. Somatic DMRs at Ndn and Mkrn3 are established during development. Transcription from U-exons occurs in the female germline in growing oocytes (red undulating line) and is involved in the establishment of methylation at the PWS-IC.