The GNAS1 gene encodes the ␣ subunit of the guanine nucleotide-binding protein G s , which couples signaling through peptide hormone receptors to cAMP generation. GNAS1 mutations underlie the hormone resistance syndrome pseudohypoparathyroidism type Ia (PHP-Ia), so the maternal inheritance displayed by PHP-Ia has raised suspicions that GNAS1 is imprinted. Despite this suggestion, in most tissues G s ␣ is biallelically encoded. In contrast, the large G protein XL␣s, also encoded by GNAS1, is paternally derived. Because the inheritance of PHP-Ia predicts the existence of maternally, rather than paternally, expressed transcripts, we have investigated the allelic origin of other mRNAs derived from GNAS1. We find this gene to be remarkable in the complexity of its allele-specific regulation. Two upstream promoters, each associated with a large coding exon, lie only 11 kb apart, yet show opposite patterns of allele-specific methylation and monoallelic transcription. The more 5 of these exons encodes the neuroendocrine secretory protein NESP55, which is expressed exclusively from the maternal allele. The NESP55 exon is 11 kb 5 to the paternally expressed XL␣s exon. The transcripts from these two promoters both splice onto GNAS1 exon 2, yet share no coding sequences. Despite their structural unrelatedness, the encoded proteins, of opposite allelic origin, both have been implicated in regulated secretion in neuroendocrine tissues. Remarkably, maternally (NESP55), paternally (XL␣s), and biallelically (G s ␣) derived proteins all are produced by different patterns of promoter use and alternative splicing of GNAS1, a gene showing simultaneous imprinting in both the paternal and maternal directions.
The GNAS1 gene encodes the ␣ subunit of the G protein G s , which couples receptor binding by several hormones to activation of adenylate cyclase. Null mutations of GNAS1 cause pseudohypoparathyroidism (PHP) type Ia, in which hormone resistance occurs in association with a characteristic osteodystrophy. The observation that PHP Ia almost always is inherited maternally has led to the suggestion that GNAS1 may be an imprinted gene. Here, we show that, although G s ␣ expression (directed by the promoter upstream of exon 1) is biallelic, GNAS1 is indeed imprinted in a promoter-specific fashion. We used parthenogenetic lymphocyte DNA to screen by restriction landmark genomic scanning for loci showing differential methylation between paternal and maternal alleles. This screen identified a region that was found to be methylated exclusively on a maternal allele and was located Ϸ35 kb upstream of GNAS1 exon 1. This region contains three novel exons that are spliced into alternative GNAS1 mRNA species, including one exon that encodes the human homologue of the large G protein XL␣s. Transcription of these novel mRNAs is exclusively from the paternal allele in all tissues examined. The differential imprinting of separate protein products of GNAS1 therefore may contribute to the anomalous inheritance of PHP Ia.
The clustering and coordinate regulation of many imprinted genes justifies positional searches for imprinted genes adjacent to known ones. We recently characterized a locus on 20q13, containing GNAS1, which has a highly complex imprinted expression pattern. In a search for neighbouring genes, we have now characterized a new gene, TH1, downstream of GNAS1. TH1 and GNAS1 are separated by more than 70 kb consisting largely of interspersed repetitive DNA. TH1 is the homologue of a gene that, in Drosophila, lies adjacent to the DNA repair gene mei-41. We have determined the full-length structures of human, mouse and Drosophila TH1. Though of unknown function, TH1 is highly conserved and widely expressed. Nonetheless, there is no similar Caenorhabditis elegans protein. We have also determined the complete genomic structures of human and Drosophila TH1. The Drosophila gene has five exons spanning 2.6 kb. The last three introns have precise equivalents in the human gene, which has 15 exons spanning 14 kb and is transcribed away from GNAS1. Using a single-nucleotide polymorphism in the 3' untranslated region, we have demonstrated biallelic TH1 expression in human fetal tissues, suggesting that, unlike GNAS1, TH1 is probably not imprinted. Immediately downstream of TH1 lies CTSZ, encoding the recently described cysteine protease, cathepsin Z. We have also elucidated the genomic structure of this gene; it has six exons spanning 12 kb and is oriented tail-to-tail with TH1, only 70 bp separating their polyadenylation sites. A polymorphism was again identified within the CTSZ 3' untranslated region and used to demonstrate biallelic expression in fetal tissues.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.