The GNAS Locus: Quintessential Complex Gene Encoding Gsalpha, XLalphas, and other Imprinted Transcripts.
- Authors
- Bastepe, Murat
- Year
- 2007
- Journal
- Current genomics
- PMID
- 19412439
- DOI
- 10.2174/138920207783406488
- PMCID
- PMC2671723
The currently estimated number of genes in the human genome is much smaller than previously predicted. As an explanation for this disparity, most individual genes have multiple transcriptional units that represent a variety of biologically important gene products. GNAS exemplifies a gene of such complexity. One of its products is the alpha-subunit of the stimulatory heterotrimeric G protein (Gsalpha), a ubiquitous signaling protein essential for numerous different cellular responses. Loss-of-function and gain-of-function mutations within Gsalpha-coding GNAS exons are found in various human disorders, including Albright's hereditary osteodystrophy, pseudohypoparathyroidism, fibrous dysplasia of bone, and some tumors of different origin. While Gsalpha expression in most tissues is biallelic, paternal Gsalpha expression is silenced in a small number of tissues, playing an important role in the development of phenotypes associated with GNAS mutations. Additional products derived exclusively from the paternal GNAS allele include XLalphas, a protein partially identical to Gsalpha, and two non-coding RNA molecules, the A/B transcript and the antisense transcript. The maternal GNAS allele leads to NESP55, a chromogranin-like neuroendocrine secretory protein. In vivo animal models have demonstrated the importance of each of the exclusively imprinted GNAS products in normal mammalian physiology. However, although one or more of these products are also disrupted by most naturally occurring GNAS mutations, their roles in disease pathogenesis remain unknown. To further our understanding of the significance of this gene in physiology and pathophysiology, it will be important to elucidate the cellular roles and the mechanisms regulating the expression of each GNAS product.
Multiple imprinted sense and antisense transcripts from the complex GNAS locus. Exons 1-13 encode Gsα , which is biallelic in most tissues; however, paternal Gsα allele is silenced in a small number of tissues, including the renal proximal tubule, thyroid, and pituitary. From differentially methylated promoters arise several other transcripts, including the maternally expressed NESP55 and the paternally expressed XLαs. Both of these transcripts use individual first exons that splice onto exons 2-13. In addition, the paternal GNAS allele gives rise to a transcript termed A/B (also referred to as 1A or 1’), which also shares exons 2-13 and is presumed to be non-coding. Note that the A/B transcript contains an ORF (colored grey) that could lead to a translational product, but the existence of endogenous A/B protein is not supported experimentally. Another non-coding transcript is also derived from the paternal GNAS allele, but this transcript is made from the antisense strand (AS transcript). Boxes and connecting lines depict exons and introns, respectively. Open rectangles and rectangles filled with CH3 show non-methylated and methylated DMRs, respectively. The distance between AS exon 5 and exon NESP55 is ~19 kb and the distance between exon A/B and exon XL ~35 kb. Maternal (mat) and paternal (pat) GNAS products are illustrated above and below the gene structure, respectively, with splicing patterns indicated by broken lines. Filled boxes indicate untranslated sequences.
Splice variants of Gsα. Alternative splicing of exon 3 leads to Gsα-L and Gsα-S, each of which has subvariants due to alternative splicing of a serine codon at the start of exon 4. Gsα-N1 is formed through the use of an exon that comprises an in-frame termination codon and is located between exons 3 and 4. Boxes and connecting lines depict exons and introns, respectively. The alternatively spliced serine codon is depicted as a circle. Splicing patterns are indicated by broken lines. Filled boxes indicate 5’ and 3’ untranslated regions.
XLαs and its multiple variants. XLαs is derived from a promoter upstream of that which drives the expression of Gsα . Panel A. Alternative splicing leads to different XLαs variants that are analogues to Gsα variants. In addition, an N-terminally extended XLαs variant, termed XXLαs, is made, representing the 5’ extension of the ORF. For simplicity, GNAS exons located upstream of exon XL are not shown. Panel B. XLαs and XXLαs mRNA include two ORFs each. The second ORF leads to ALEX or ALEXX, respectively.
Regulation of gene expression from the GNAS locus. The maternal GNAS allele comprises two germ-line imprint marks, one at the AS promoter and the other at exon A/B. Ablation of the non-methylated, paternal AS promoter causes derepression of NESP55, and ablation of the non-methylated, paternal exon A/B leads to derepression of Gsα; the latter occurs in tissues where the latter is normally silenced from the paternal allele (*). Most familial PHP-Ib cases exhibit isolated loss of GNAS exon A/B imprinting. Deletions identified in those cases point to a region within the STX16 locus, probably around exon 4, as comprising a cis-acting long-range regulatory element that is necessary for the establishment of the maternal exon A/B imprint. All GNAS maternal imprints are lost in most sporadic and some familial PHP-Ib cases. Deletions identified in the latter suggest that the NESP55 DMR, which contains not only exon NESP55 but also exons 3 and 4 of the AS transcript, comprises a cis-acting element that controls the imprinting of the entire maternal GNAS allele. Arrows indicate the regulatory effects revealed by mutations in PHP-Ib and the study of knockout mouse models. mat, maternal; pat, paternal.
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