The mouse Snrpn minimal promoter and its human orthologue: activity and imprinting

Background: Microdeletions in chromosome 15q13-15 of Prader-Willi (PWS) and Angelman Syndrome (AS) patients suggested that SNRPN promoter/exon 1, toge ther with a short sequence located approximately 35 kb upstream, constitute an imprinting control centre that regulates the entire 2 Mb PWS/AS imprint ed domain. We have recently shown that a minitransgene composed of the huma n upstream sequence and mouse Snrpn promoter/exon 1 barbours all the elemen ts necessary for establishing and maintaining an imprinted state. Results: Here we describe, using transfection experiments, the Snrpn minima l promoter (SMP), being composed of the entire 76 bp exon 1 and 84 bp of up stream sequence. A 7 bp element (SBE) within SMP that, in its unmethylated state binds a specific protein, is absolutely required for promoter activit y. The orthologous human sequence, in spite of the fact that it possesses a n identical SBE, failed to display promoter activity in transfection experi ments and failed to create a methylated state of the maternal allele. Trans genic experiments reveal that a mutation in SBE of the mouse sequence did n ot completely abolish methylation of the maternal allele, indicating that s equences outside SBE participate in this process. Replacement of human exon 1 with the mouse orthologue replenished promoter activity, but left the ma ternal allele in the transgenic experiment unmethylated. The reciprocal chi mera, in which mouse exon 1 was replaced by the human orthologue resulted i n loss of promoter activity and did not support differential methylation. Conclusions: The observations made by in vitro and in vivo experiments sugg est that several cis elements which are involved in Snrpn promoter activity and the imprinting process are present in the mouse promoter and absent in the human orthologous sequence.