Transcription Is Required to Establish Maternal Imprinting at the Prader-Willi Syndrome and Angelman Syndrome Locus

Abstract

The Prader-Willi syndrome (PWS [MIM 17620]) and Angelman syndrome (AS [MIM 105830]) locus is controlled by a bipartite imprinting center (IC) consisting of the PWS-IC and the AS-IC. The most widely accepted model of IC function proposes that the PWS-IC activates gene expression from the paternal allele, while the AS-IC acts to epigenetically inactivate the PWS-IC on the maternal allele, thus silencing the paternally expressed genes. Gene order and imprinting patterns at the PWS/AS locus are well conserved from human to mouse; however, a murine AS-IC has yet to be identified. We investigated a potential regulatory role for transcription from the Snrpn alternative upstream exons in silencing the maternal allele using a murine transgene containing Snrpn and three upstream exons. This transgene displayed appropriate imprinted expression and epigenetic marks, demonstrating the presence of a functional AS-IC. Transcription of the upstream exons from the endogenous locus correlates with imprint establishment in oocytes, and this upstream exon expression pattern was conserved on the transgene. A transgene bearing targeted deletions of each of the three upstream exons exhibited loss of imprinting upon maternal transmission. These results support a model in which transcription from the Snrpn upstream exons directs the maternal imprint at the PWS-IC. Prader-Willi and Angelman syndromes are neurobehavioral disorders arising from dysregulation of a cluster of imprinted genes located at chromosome 15q11–q13. PWS results from the absence of paternally expressed genes and AS from the absence of maternally expressed genes. Two elements, termed the PWS-IC and the AS-IC, are responsible for allele-specific gene expression. The PWS-IC activates expression of paternally expressed genes, while the AS-IC is thought to silence the PWS-IC in the female germ line, rendering it inactive on the future maternal allele. Mouse models have been effective for studying the IC-directed regulation of this locus; however, the murine AS-IC has yet to be characterized. In this study, we have determined the identity of the AS-IC and investigated how it functions to inactivate the PWS-IC. Our results suggest that the murine AS-IC consists of several promoters that direct expression of transcripts through the PWS-IC in oocytes. Thus, faulty transcription in oocytes may lead to AS imprinting defects.