GENE STRUCTURE, DNA METHYLATION, AND IMPRINTED EXPRESSION OF THE HUMAN SNRPN GENE

The human SNRPN(small nuclear ribonucleoprotein polypeptide N) gene is one of a gene family that encode proteins involved in pre-mRNA splici ng and maps to the smallest deletion region involved in the Prader-Wil li syndrome (PWS) within chromosome 15q11-q13. Paternal only expressio n of SNRPN has previously been demonstrated by use of cell lines from PWS patients (maternal allele only) and Angelman syndrome (AS) patient s (paternal allele only). We have characterized two previously unident ified 5' exons of the SNRPN gene and demonstrate that exons -1 and 0 a re included in the full-length transcript. This gene is expressed in a wide range of somatic tissues and at high, approximately equal levels in all regions of the brain. Both the first exon of SNRPN (exon -1) a nd the putative transcription start site are embedded within a CpG isl and. This CpG island is extensively methylated on the repressed matern al allele and is unmethylated on the expressed paternal allele, in a w ide range of fetal and adult somatic cells. This provides a quick and highly reliable diagnostic assay for PWS and AS, which is based on DNA -methylation analysis that has been tested on >100 patients in a varie ty of tissues. Conversely, several CpG sites similar to 22 kb down str eam of the transcription start site in intron 5 are preferentially met hylated on the expressed paternal allele in somatic tissues and male g erm cells, whereas these same sites are unmethylated in fetal oocytes. These findings are consistent with a key role for DNA methylation in the imprinted inheritance and subsequent gene expression of the human SNRPN gene.