Structural characterization of Rasgrf1 and a novel linked imprinted locus.

Imprinted genes in mammals are expressed either from the maternally or the paternally inherited allele. Previously, a genome wide scan identified novel imprinted genes based on their association with differentially methylated regions (DMRs). One of the identified genes, Rasgrf1, showed paternal expression in neonatal brain and was located on mouse chromosome 9. This gene is associated with a DMR, located about 30 kb upstream of Rasgrf1 exon 1. In order to better understand and identify novel elements involved in the regulation of this gene we have isolated and characterized genomic clones coding for mouse and human Rasgrf1 and RASGRF1, respectively. The mouse gene consists of 26 exons spanning approximately 140 kb of genomic DNA while the human gene has 28 exons. The human gene has an additional 39 bp exon inserted between exons 13 and 14 and exon 18 is split in two separate exons in human. The major transcription start site of Rasgrf1, as identified by primer extension, is 1324 bp upstream of the ATG translation start codon. Finally, a genomic region upstream of exon 1, spanning 489 bp, was determined to possess the essential promoter activity for Rasgrf1 gene. A second gene, A19, located 10 kb upstream of the DMR has been characterized. A19 is mainly expressed in testis and at lower levels in neonatal and adult brain tissue. The A19 transcript is non-coding and expressed in mouse testis and brain. A19 is imprinted with expression occurring from just the paternal allele in brain.