Glomerular-specific imprinting of the mouse gsalpha gene: how does this relate to hormone resistance in albright hereditary osteodystrophy?

Activators of G protein signaling in the kidney

Heterotrimeric G proteins play a crucial role in regulating signal processing to maintain normal cellular homeostasis, and subtle perturbations in its activity can potentially lead to the pathogenesis of renal disorders or diseases. Cell-surface receptors and accessory proteins, which normally modify and organize the coupling of individual G protein subunits, contribute to the regulation of heterotrimeric G protein activity and their convergence and/or divergence of downstream signaling initiated by effector systems. Activators of G protein signaling (AGS) are a family of accessory proteins that intervene at multiple distinct points during the activation-inactivation cycle of G proteins, even in the absence of receptor stimulation. Perturbations in the expression of individual AGS proteins have been reported to modulate signal transduction pathways in a wide array of diseases and disorders within the brain, heart, immune system, and more recently, the kidney. This review will provide an overview of the expression profile, localization, and putative biologic role of the AGS family in the context of normal and diseased states of the kidney.

[Epigenetics and pseudohypoparathyroidism]

Parental imprinting and the type of the genetic alteration play a determinant role in the phenotype expression of GNAS locus associated to pseudohypoparathyroidism (PHP). This imprint is tissue-specific, mainly localized in the kidney and the thyroid. Only the maternal allele is expressed at this level. An alteration in the coding sequence of the gene leads to an haplo-insufficiency and a dysmorphic phenotype (Albright's syndrome). If the alteration is on the maternal allele, there is a hormonal resistance to the PTH at the kidney level and to the TSH at the thyroid level. The phenotype is known as a PHP1a. If the alteration is on the paternal allele, there are few clinical signs with no hormonal resistance and the phenotype is known as pseudo-pseudo-hypoparathyroidism (PPHP). Methylation anomalies of GNAS locus, in particular of exon 1A, are responsible for a lack of expression of Gαs at kidney and thyroid levels only. If these anomalies concern the maternal allele (the only one expressed) with a paternal pattern, there is no haplo-insufficiency and no dysmorphic syndrome. The hormonal resistance is yet again limited to PTH and TSH. The phenotype is known as PHP1b. In the familial forms, these methylation anomalies are associated with a deletion of the syntaxine 16 gene in the maternal allele. This gene contains probably the imprinting center of the locus.

Control of imprinting at the Gnas cluster

Genomic imprinting is a form of epigenetic regulation in mammals whereby a small subset of genes is silenced according to parental origin. Early work had indicated regions of the genome that were likely to contain imprinted genes. Distal mouse chromosome 2 is one such region and is associated with devastating but ostensibly opposite phenotypes when exclusively maternally or paternally derived. Misexpression of proteins encoded at the Gnas complex, which is located within the region, can largely account for the imprinting phenotypes. Gnas is a complex locus with extraordinary transcriptional and regulatory complexity. It gives rise to alternatively spliced isoforms that show maternal-, paternal- and biallelic expression as well as a noncoding antisense transcript. The objective of our work at Harwell is to unravel mechanisms controlling the expression of these transcripts. We have performed targeted deletion analysis to test candidate regulatory regions within the Gnas complex and, unlike other imprinted domains, two major control regions have been identified. One controls the imprinted expression of a single transcript and is subsidiary to and must interact with, a principal control region that affects the expression of all transcripts. This principal region contains the promoter for the antisense transcript, expression of which may have a major role in controlling imprinting at the Gnas cluster.

Studies of the regulation and function of the Gs alpha gene Gnas using gene targeting technology

The heterotrimeric G protein alpha-subunit G(s)alpha is ubiquitously expressed and mediates receptor-stimulated intracellular cAMP generation. Its gene Gnas is a complex imprinted gene which uses alternative promoters and first exons to generate other gene products, including the G(s)alpha isoform XL alpha s and the chromogranin-like protein NESP55, which are specifically expressed from the paternal and maternal alleles, respectively. G(s)alpha itself is imprinted in a tissue-specific manner, being biallelically expressed in most tissues but paternally silenced in a few tissues. Gene targeting of specific Gnas transcripts demonstrates that heterozygous mutation of G(s)alpha on the maternal (but not the paternal) allele leads to early lethality, perinatal subcutaneous edema, severe obesity, and multihormone resistance, while the paternal mutation leads to only mild obesity and insulin resistance. These parent-of-origin differences are the consequence of tissue-specific G(s)alpha imprinting. XL alpha s deficiency leads to a perinatal suckling defect and a lean phenotype with increased insulin sensitivity. The opposite metabolic effects of G(s)alpha and XL alpha s deficiency are associated with decreased and increased sympathetic nervous system activity, respectively. NESP55 deficiency has no metabolic consequences. Other gene targeting experiments have shown Gnas to have 2 independent imprinting domains controlled by 2 different imprinting control regions. Tissue-specific G(s)alpha knockout models have identified important roles for G(s)alpha signaling pathways in skeletal development, renal function, and glucose and lipid metabolism. Our present knowledge gleaned from various Gnas gene targeting models are discussed in relation to the pathogenesis of human disorders with mutation or abnormal imprinting of the human orthologue GNAS.

Imprinting the Gnas locus

Gnas is an enigmatic and rather complex imprinted gene locus. A single transcription unit encodes three, and possibly more, distinct proteins. These are determined by overlapping transcripts from alternative promoters with different patterns of imprinting. The canonical Gnas transcript codes for Gsalpha, a highly conserved signalling protein and an essential intermediate in growth, differentiation and homeostatic pathways. Monoallelic expression of Gnas is highly tissue-restricted. The alternative transcripts encode XLalphas, an unusual variant of Gsalpha, and the chromogranin-like protein Nesp55. These transcripts are expressed specifically from the paternal and maternal chromosomes, respectively. Their existence in the Gnas locus might imply functional connections amongst them or with Gsalpha. In this review, we consider how imprinting of Gnas was discovered, the phenotypic consequences of mutations in each of the gene products, both in the mouse and human, and provide some conjectures to explain why this elaborate imprinted locus has evolved in this manner in mammals.

Interactions between imprinting effects: summary and review

Mice with uniparental disomies (uniparental duplications) for defined regions of certain chromosomes, or certain disomies, show a range of developmental abnormalities most of which affect growth. These defects can be attributed to incorrect dosages of maternal or paternal copies of imprinted genes lying within the regions involved. Combinations of certain partial disomies result in interactions between the imprinting effects that seemingly independently affect foetal and/or placental growth in different ways or modify neonatal and postnatal development. The findings are generally in accord with the 'conflict hypothesis' for the evolution of genomic imprinting but do not demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that favours embryonic/foetal development, at consequent cost to the mother, will have been subject to evolutionary selection for only paternal allele expression. Reciprocally, any gene that reduces embryonic/foetal growth to limit disadvantage to the mother will have been selected for only maternal allele expression. It is concluded that survival of the placenta is core to the evolution of imprinting.

Imprinting control within the compact Gnas locus

Mouse distal chromosome 2 was one of the earliest described imprinting regions. Maternal and paternal inheritance of the region is associated with opposite phenotypes affecting growth, development and behaviour. Mis-expression of proteins determined by the imprinted Gnas locus can account for the phenotypes. The imprinting domain in mouse distal chromosome 2 is small, comprising the Gnas locus. This locus is unusually complex, containing biallelic, maternally and paternally expressed transcripts that share exons. It also contains two germline differentially methylated regions that have the characteristics of imprinting control regions. One of these specifically controls the tissue-specific imprinting of the Gnas exon 1 transcript but does not affect the imprinting of other transcripts. Imprinting of other transcripts may be controlled by the other germline differentially methylated region by a mechanism involving antisense RNA.

The influence of non-coding RNAs on allele-specific gene expression in mammals

Current research has revealed that the influence of RNA molecules on gene expression reaches beyond the realm of protein synthesis back into the nucleus, where it not only dictates the transcriptional activity of genes, but also shapes the chromatin architecture of extensive regions of DNA. Non-coding RNA, in the context of this review, refers to transcripts expressed and processed in the nucleus much like any protein coding gene, but lacking an open reading frame and often transcribed antisense to bona fide protein coding genes. In mammals, these types of transcripts are highly coincident with allele-specific silencing of imprinted genes and have a proven role in dosage compensation via X-inactivation. The biochemistry of how non-coding RNAs regulate transcription is the subject of intense research in both prokaryotic and eukaryotic models. Mechanisms such as RNA interference may have deep phylogenetic roots, but their relevance to imprinting and X-inactivation in mammals has not been proven. The remarkable diversity of non-coding transcription associated with parent-of-origin directed gene silencing hints at an equally diverse assortment of mechanisms.

Interactions between imprinting effects in the mouse

Mice with uniparental partial or complete disomies for any one of 11 identified chromosomes show abnormal phenotypes. The abnormalities, or imprinting effects, can be attributable to an incorrect dosage of maternal or paternal copies of imprinted gene(s) located within the regions involved. Here we show that combinations of partial disomies may result in interactions between imprinting effects that seemingly independently affect fetal and/or placental growth in different ways or modify neonatal and postnatal imprinting effects. Candidate genes within the regions have been identified. The findings are generally in accord with the "conflict hypothesis" for the evolution of genomic imprinting but do not clearly demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that represses or limits embryonic/fetal growth to the advantage of the mother--by any developmental means--will have been subject to evolutionary selection for paternal allele repression. Likewise, any gene that favors embryonic/fetal development at consequent cost to the mother--by any developmental means--will have faced selection for maternal allele repression. The classical Igf2-Igf2r axis may therefore be unique. The findings involve reinterpretation of older imprinting data and consequently revision of the mouse imprinting map.

The potential role of gene duplications in the evolution of imprinting mechanisms

Using the completed genomic sequences of mouse and human we performed a comparative analyses of imprinted genes and gene clusters. For many imprinted genes we could detect imprinted as well as non-imprinted paralogues. The inter- and intrachromosomal similarities between paralogues and their linkage to imprinting clusters suggests that imprinted genes were dispersed throughout the genome by gene duplications as well as translocation and transposition events. Our findings indicate that imprinting clusters may have been linked together on one (or a few) ancestral pre-imprinted chromosome(s), arguing for a common mechanistic origin of imprinting control. Imprinting may originally have evolved on a simple basis of dosage compensation required for some duplicated genes (chromosomes) followed by selection of sex-biased expression control.

High concentrations of long interspersed nuclear element sequence distinguish monoallelically expressed genes

Genes subject to monoallelic expression are expressed from only one of the two alleles either selected at random (random monoallelic genes) or in a parent-of-origin specific manner (imprinted genes). Because high densities of long interspersed nuclear element (LINE)-1 transposon sequence have been implicated in X-inactivation, we asked whether monoallelically expressed autosomal genes are also flanked by high densities of LINE-1 sequence. A statistical analysis of repeat content in the regions surrounding monoallelically and biallelically expressed genes revealed that random monoallelic genes were flanked by significantly higher densities of LINE-1 sequence, evolutionarily more recent and less truncated LINE-1 elements, fewer CpG islands, and fewer base-pairs of short interspersed nuclear elements (SINEs) sequence than biallelically expressed genes. Random monoallelic and imprinted genes were pooled and subjected to a clustering analysis algorithm, which found two clusters on the basis of aforementioned sequence characteristics. Interestingly, these clusters did not follow the random monoallelic vs. imprinted classifications. We infer that chromosomal sequence context plays a role in monoallelic gene expression and may involve the recognition of long repeats or other features. The sequence characteristics that distinguished the high-LINE-1 category were used to identify more than 1,000 additional genes from the human and mouse genomes as candidate genes for monoallelic expression.

Antisense transcripts with FANTOM2 clone set and their implications for gene regulation

We have used the FANTOM2 mouse cDNA set (60,770 clones), public mRNA data, and mouse genome sequence data to identify 2481 pairs of sense-antisense transcripts and 899 further pairs of nonantisense bidirectional transcription based upon genomic mapping. The analysis greatly expands the number of known examples of sense-antisense transcript and nonantisense bidirectional transcription pairs in mammals. The FANTOM2 cDNA set appears to contain substantially large numbers of noncoding transcripts suitable for antisense transcript analysis. The average proportion of loci encoding sense-antisense transcript and nonantisense bidirectional transcription pairs on autosomes was 15.1 and 5.4%, respectively. Those on the X chromosome were 6.3 and 4.2%, respectively. Sense-antisense transcript pairs, rather than nonantisense bidirectional transcription pairs, may be less prevalent on the X chromosome, possibly due to X chromosome inactivation. Sense and antisense transcripts tended to be isolated from the same libraries, where nonantisense bidirectional transcription pairs were not apparently coregulated. The existence of large numbers of natural antisense transcripts implies that the regulation of gene expression by antisense transcripts is more common that previously recognized. The viewer showing mapping patterns of sense-antisense transcript pairs and nonantisense bidirectional transcription pairs on the genome and other related statistical data is available on our Web site.

A comprehensive transcript map of the mouse Gnas imprinted complex

The recent publication of the FANTOM mouse transcriptome has provided a unique opportunity to study the diversity of transcripts arising from a single gene locus. We have focused on the Gnas complex, as imprinting loci themselves provide unique insights into transcriptional regulation. Thirteen full-length cDNAs from the FANTOM2 set were mapped to the Gnas locus. These represented one previously described transcript and 12 putative new transcripts. Of these, eight were found to be differentially expressed from either the maternal or paternal allele. Two clones extended Nespas in the 3' direction, providing evidence of antisense transcription spanning a 30-kb genomic region from a single allele. The transcripts were summarized into six transcriptional units, Nespas, Nesp, Gnasxl, F7, exon 1A, and Gnas. The resolution of the Gnas transcript map by the FANTOM2 clones revealed a pattern of alternate splicing. In addition to the transcripts described previously as splicing onto exon 2 of Gnas, each new sense transcript had an alternate short 3'UTR independent of Gnas. Both spliced and unspliced variants of the new imprinted sense transcripts were found. Whereas the functional significance of these alternate transcripts is not known, the availability of the FANTOM clones has provided remarkable insights into the repertoire of transcripts in the Gnas complex locus.

An Application of Molecular Genotyping in Mice

Microsatellite markers are simple sequence repeats within the mammalian genome that can be used for identifying disease loci, mapping genes of interest as well as studying segregation patterns related to meiotic nondisjunction. Different strains of mice have variable CA repeat lengths and PCR based methods can be used to identify them, thus allowing for specific genotypes to be assigned. Molecular genotyping offers such identification at any developmental stage, which allows for a broad range of anomalies to be studied. We studied chromosomal segregation in relation to nondisjunction in early-gestation mouse embryos using molecular genotyping. Information on the parental origin as well as the number of chromosomes a given progeny carried was obtained in our analysis.

Oral manifestations of Albright hereditary osteodystrophy: a case report

Albright hereditary osteodystrophy is a hereditary metabolic disorder of dominant autosomal etiology that is commonly characterized by short stature, round face, small metacarpus and metatarsus, mental retardation, osteoporosis, subcutaneous calcification, variable hypocalcemia, and hyperphosphatemia. In this study, we report a clinical case of a 17-year-old woman with Albright hereditary osteodystrophy, and we discuss her clinical, radiographic, and laboratory test characteristics together with the oral manifestations, and we correlate them with the characteristics found in the literature. We also discuss the odontological management of treatment of related periodontal disease and planning for corrections of related malocclusions.

The imprinted oedematous-small mutation on mouse chromosome 2 identifies new roles for Gnas and Gnasxl in development

The Gnas locus is highly complex and encodes several oppositely imprinted and alternatively spliced transcripts. Gnas itself encodes Gsalpha, which is involved in endocrine function and bone development, but the roles for the other transcripts have not been established. Here we describe a mouse mutation that provides further biological functions for the Gnas locus. The mutation Oed-Sml, induced by ethylnitrosourea (ENU), has been mapped to the distal chromosome 2 imprinting region that includes Gnas. The mutation displays two distinct phenotypes dependent on parental origin. When the mutation is maternally transmitted, a microcardia with gross edema (Oed) results. By contrast, when the mutation is paternally transmitted, a growth retardation (Sml) is seen that becomes evident within 5 days of birth. Here we show Oed-Sml to be a point mutation in Gnas exon 6, resulting in a valine to glutamate substitution at residue 159 (V159E). Both maternal- and paternal-specific transcripts derive from this missense mutation. The maternally expressed mutant Gnas transcript is the candidate for Oed and the paternally expressed mutant Gnasxl transcript is the candidate for Sml. We propose a new role for Gnas in heart growth and a role for Gnasxl in postnatal growth. These findings potentially have implications for human Albright hereditary osteodystrophy, a condition caused by mutations in GNAS.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metabolism.

Imprinted genes, cognition and behaviour

The idea that genes can influence behavioural predispositions and their underlying psychological determinants is becoming increasingly tractable. In this article, recent findings are reviewed on a special type of inheritance, related to the transmission of traits via what have been termed 'imprinted' genes. In imprinted genes one allele is silenced according to its parental origin. This results in the inheritance of traits down the maternal or paternal line, in contrast to the more frequent mode of inheritance that is indifferent to the parental origin of the allele. Drawing on the advances made possible by combining the approaches of cognitive neuropsychology, behavioural neuroscience and contemporary molecular genetics, the detailed evidence for imprinted effects on behavioural and cognitive phenotypes is considered, focusing on findings from mental disorders, Turner's syndrome and experimental work in animal models. As prevailing evolutionary theories stress an essential antagonistic role of imprinted effects, these data might link such apparently diverse issues as neurodevelopment and the vulnerability to mental disease with the 'battle of the sexes', as joined at the level of cognitive and behavioural functioning.

Variable imprinting of the heterotrimeric G protein G(s) alpha-subunit within different segments of the nephron

The heterotrimeric G protein G(s) is required for hormone-stimulated intracellular cAMP generation because it couples hormone receptors to the enzyme adenylyl cyclase. Hormones that activate G(s) in the kidney include parathyroid hormone, glucagon, calcitonin, and vasopressin. Recently, it has been demonstrated that the G(s)alpha gene is imprinted in a tissue-specific manner, leading to preferential expression of G(s)alpha from the maternal allele in some tissues. In the kidney, G(s)alpha is imprinted in the proximal tubule but not in more distal nephron segments, such as the thick ascending limb or collecting duct. This most likely explains why in both humans and mice heterozygous mutations in the maternal allele lead to parathyroid hormone resistance in the proximal tubule whereas mutations in the paternal allele do not. In contrast, heterozygous mutations have little effect on vasopressin action in the collecting ducts. In mice with heterozygous null G(s)alpha mutations (both those with mutations on the maternal or paternal allele), expression of the Na-K-2Cl cotransporter was decreased in the thick ascending limb, suggesting that its expression is regulated by cAMP. The G(s)alpha genes also generate alternative, oppositely imprinted transcripts encoding XLalphas, a G(s)alpha isoform with a long NH(2)-terminal extension, and NESP55, a chromogranin-like neurosecretory protein. The role, if any, of these proteins in renal physiology is unknown.

An imprinted transcript, antisense to Nesp, adds complexity to the cluster of imprinted genes at the mouse Gnas locus

The Gnas locus in distal mouse chromosome (Chr) 2 is emerging as a complex genomic region. It contains three imprinted genes in the order Nesp-Gnasxl-Gnas. Gnas encodes a G protein alpha-subunit, and Nesp and Gnasxl encode proteins of unknown function expressed in neuroendocrine tissue. Together, these genes form a single transcription unit because transcripts of Nesp and Gnasxl are alternatively spliced onto exon 2 of Gnas. Nesp and Gnasxl are expressed from opposite parental alleles, with Nesp encoding a maternal-specific transcript and Gnasxl encoding a paternal-specific transcript. We now identify a further imprinted transcript in this cluster. Reverse transcription-PCR analysis of Nesp expression in 15. 5-days-postcoitum embryos carrying only maternal or paternal copies of distal Chr 2 revealed an isoform that is exclusively paternally, rather than maternally, expressed. Strand-specific reverse transcription-PCR showed that this form is an antisense transcript. The existence of a paternally expressed antisense transcript was confirmed by Northern blot analysis. The sequence is contiguous with genomic sequence downstream of Nesp and encompasses Nesp exons 1 and 2 and an intervening intron. We propose that Nespas is an additional control element in the imprinting region of mouse distal Chr 2; it adds further complexity to the Gnas-imprinted gene cluster.

An imprinted transcript, antisense to Nesp, adds complexity to the cluster of imprinted genes at the mouse Gnas locus

The Gnas locus in distal mouse chromosome (Chr) 2 is emerging as a complex genomic region. It contains three imprinted genes in the order Nesp-Gnasxl-Gnas. Gnas encodes a G protein alpha-subunit, and Nesp and Gnasxl encode proteins of unknown function expressed in neuroendocrine tissue. Together, these genes form a single transcription unit because transcripts of Nesp and Gnasxl are alternatively spliced onto exon 2 of Gnas. Nesp and Gnasxl are expressed from opposite parental alleles, with Nesp encoding a maternal-specific transcript and Gnasxl encoding a paternal-specific transcript. We now identify a further imprinted transcript in this cluster. Reverse transcription-PCR analysis of Nesp expression in 15. 5-days-postcoitum embryos carrying only maternal or paternal copies of distal Chr 2 revealed an isoform that is exclusively paternally, rather than maternally, expressed. Strand-specific reverse transcription-PCR showed that this form is an antisense transcript. The existence of a paternally expressed antisense transcript was confirmed by Northern blot analysis. The sequence is contiguous with genomic sequence downstream of Nesp and encompasses Nesp exons 1 and 2 and an intervening intron. We propose that Nespas is an additional control element in the imprinting region of mouse distal Chr 2; it adds further complexity to the Gnas-imprinted gene cluster.

Identification of imprinted loci by methylation-sensitive representational difference analysis: application to mouse distal chromosome 2

Imprinted genes are distinguished by different patterns of methylation on their parental alleles, a property by which imprinted loci could be identified systematically. Here, representational difference analysis (RDA) is used to clone HpaII fragments with methylation differences on the maternal and paternal copies of distal chromosome (Chr) 2 in the mouse. Uniparental inheritance for this region causes imprinting phenotypes whose molecular basis is only partially understood. RDA led to the recovery of multiple differentially methylated HpaII fragments at two major sites of imprinted methylation: paternal-specific methylation at the Nesp locus and maternal-specific methylation at the Gnasxl locus. Nesp and Gnasxl represent oppositely imprinted promoters of the Gnas gene, which encodes the G-protein subunit, Gsalpha. The organization of the Nesp-Gnasxl-Gnas region was determined: Nesp and Gnasxl were found to be 15 kb apart, and Gnasxl was found to be 30 kb upstream of Gnas. Sites of imprinted methylation were also detected at the loci for neuronatin on Chr 2 and for M-cadherin on Chr 8. RDA was highly effective at identifying imprinted methylation, and its potential applications to imprinting studies are discussed.

The Role of Genomic Imprinting of Galpha in the Pathogenesis of Albright Hereditary Osteodystrophy

Albright hereditary osteodystrophy (AHO) is caused by heterozygous inactivating mutations of the gene encoding the alpha-subunit of the G protein Gs. The Gsalpha gene is a complex gene that uses various alternative promoters and produces various protein products. Recently, it has been shown that this gene is imprinted in a tissue-specific manner. The role of tissue-specific imprinting of Gsalpha in the pathogenesis of AHO is discussed.

A cluster of oppositely imprinted transcripts at the Gnas locus in the distal imprinting region of mouse chromosome 2

Imprinted genes tend to occur in clusters. We have identified a cluster in distal mouse chromosome (Chr) 2, known from early genetic studies to contain both maternally and paternally imprinted, but unspecified, genes. Subsequently, one was identified as Gnas, which encodes a G protein alpha subunit, and there is clinical and biochemical evidence that the human homologue GNAS1, mutated in patients with Albright hereditary osteodystrophy, is also imprinted. We have used representational difference analysis, based on parent-of-origin methylation differences, to isolate candidate imprinted genes in distal Chr 2 and found two oppositely imprinted genes, Gnasxl and Nesp. Gnasxl determines a variant G protein alpha subunit associated with the trans-Golgi network and Nesp encodes a secreted protein of neuroendocrine tissues. Gnasxl is maternally methylated in genomic DNA and encodes a paternal-specific transcript, whereas Nesp is paternally methylated with maternal-specific expression. Their reciprocal imprinting may offer insight into the distal Chr 2 imprinting phenotypes. Remarkably, Gnasxl, Nesp, and Gnas are all part of the same transcription unit; transcripts for Gnasxl and Nesp are alternatively spliced onto exon 2 of Gnas. This demonstrates an imprinting mechanism in which two oppositely imprinted genes share the same downstream exons.

Genomic imprinting: implications for human disease

Genomic imprinting refers to an epigenetic marking of genes that results in monoallelic expression. This parent-of-origin dependent phenomenon is a notable exception to the laws of Mendelian genetics. Imprinted genes are intricately involved in fetal and behavioral development. Consequently, abnormal expression of these genes results in numerous human genetic disorders including carcinogenesis. This paper reviews genomic imprinting and its role in human disease. Additional information about imprinted genes can be found on the Genomic Imprinting Website at http://www.geneimprint.com.

Bidirectional imprinting of a single gene: GNAS1 encodes maternally, paternally, and biallelically derived proteins

The GNAS1 gene encodes the alpha subunit of the guanine nucleotide-binding protein Gs, 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 Gsalpha is biallelically encoded. In contrast, the large G protein XLalphas, 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 XLalphas 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 (XLalphas), and biallelically (Gsalpha) 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.

Do we understand the evolution of genomic imprinting?

The conflict theory is the only hypothesis to have attracted any critical attention for the evolution of genomic imprinting. Although the earliest data appeared supportive, recent systematic analyses have not confirmed the model's predictions. The status of theory remains undecided, however, as post-hoc explanations can be provided as to why these predictions are not borne out.

Imprinting mechanisms

A number of recent studies have provided new insights into mechanisms that regulate genomic imprinting in the mammalian genome. Regions of allele-specific differential methylation (DMRs) are present in all imprinted genes examined. Differential methylation is erased in germ cells at an early stage of their development, and germ-line-specific methylation imprints in DMRs are reestablished around the time of birth. After fertilization, differential methylation is retained in core DMRs despite genome-wide demethylation and de novo methylation during preimplantation and early postimplantation stages. Direct repeats near CG-rich DMRs may be involved in the establishment and maintenance of allele-specific methylation patterns. Imprinted genes tend to be clustered; one important component of clustering is enhancer competition, whereby promoters of linked imprinted genes compete for access to enhancers. Regional organization and spreading of the epigenotype during development is also important and depends on DMRs and imprinting centers. The mechanism of cis spreading of DNA methylation is not known, but precedent is provided by the Xist RNA, which results in X chromosome inactivation in cis. Reading of the somatic imprints could be carried out by transcription factors that are sensitive to methylation, or by methyl-cytosine-binding proteins that are involved in transcriptional repression through chromatin remodeling.

The human GNAS1 gene is imprinted and encodes distinct paternally and biallelically expressed G proteins

The GNAS1 gene encodes the alpha subunit of the G protein Gs, 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 Gsalpha 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 approximately 35 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 XLalphas. 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.

G protein abnormalities in pituitary adenomas

It has been demonstrated that the majority of secreting and nonsecreting adenomas is monoclonal in origin suggesting that these neoplasia arise from the replication of a single mutated cell, in which growth advantage results from either activation of protooncogenes or inactivation of antioncogenes. Although a large number of genes has been screened for mutations, only few genetic abnormalities have been found in pituitary tumors such as allelic deletion of chromosome 11q13 where the MEN-1 gene has been localised, and mutations in the gene encoding the alpha subunit of the stimulatory Gs and Gi2 protein. These mutations constitutively activate the alpha subunit of the Gs and Gi2 protein by inhibiting their intrinsic GTPase activity. Both Gs alpha and Gi2alpha can be considered products of protooncogenes (gsp and gip2, respectively) since gain of function mutations that activate mitogenic signals have been recognized in human tumors. Gsp oncogene is found in 30-40% of GH-secreting adenomas, in a low percentage of nonfunctioning and ACTH-secreting pituitary adenomas, in toxic thyroid adenomas and differentiated thyroid carcinomas. The same mutations, occurred early in embriogenesis, have been also identified in tissues from patients affected with the McCune Albright syndrome. These mutations result in an increased cAMP production and in the subsequent overactivation of specific pathways involved in both cell growth and specific programmes of cell differentiation. By consequence, the endocrine tumors expressing gsp oncogene retain differentiated functions. The gip2 oncogene has been identified in about 10% of nonfunctioning pituitary adenomas, in tumors of the ovary and the adrenal cortex. However, it remains to be established whether Gi proteins activate mitogenic signals in pituitary cells. Since Gi proteins are involved in mediating the effect of inhibitory neurohormones on intracellular effectors, it has been proposed that in pituitary tumors the low expression of these proteins, particularly Gi1-3alpha, may contribute to uncontrolled pituitary cells growth by preventing the transduction of inhibitory signals. While by in vitro mutagenesis it has been demonstrated that activated mutant of Gq alpha, G12alpha, G13alpha and Gz alpha are fully oncogenic, it remains to be proved whether or not these abnormalities might naturally occur in human tumors and, in particular, in pituitary adenomas.

Variable and tissue-specific hormone resistance in heterotrimeric Gs protein alpha-subunit (Gsalpha) knockout mice is due to tissue-specific imprinting of the gsalpha gene

Albright hereditary osteodystrophy (AHO), an autosomal dominant disorder characterized by short stature, obesity, and skeletal defects, is associated with heterozygous inactivating mutations of GNAS1, the gene encoding the heterotrimeric G protein alpha-subunit (Gsalpha) that couples multiple receptors to the stimulation of adenylyl cyclase. It has remained unclear why only some AHO patients present with multihormone resistance and why AHO patients demonstrate resistance to some hormones [e.g., parathyroid hormone (PTH)] but not to others (e.g., vasopressin), even though all activate adenylyl cyclase. We generated mice with a null allele of the mouse homolog Gnas. Homozygous Gs deficiency is embryonically lethal. Heterozygotes with maternal (m-/+) and paternal (+/p-) inheritance of the Gnas null allele have distinct phenotypes, suggesting that Gnas is an imprinted gene. PTH resistance is present in m-/+, but not +/p-, mice. Gsalpha expression in the renal cortex (the site of PTH action) is markedly reduced in m-/+ but not in +/p- mice, demonstrating that the Gnas paternal allele is imprinted in this tissue. Gnas is also imprinted in brown and white adipose tissue. The maximal physiological response to vasopressin (urinary concentrating ability) is normal in both m-/+ and +/p- mice and Gnas is not imprinted in the renal inner medulla (the site of vasopressin action). Tissue-specific imprinting of Gnas is likely the mechanism for variable and tissue-specific hormone resistance in these mice and a similar mechanism might explain the variable phenotype in AHO.

Reproductive dysfunction in women with Albright's hereditary osteodystrophy

Most individuals with Albright's hereditary osteodystrophy (AHO) have deficient expression or function of G(s alpha), the alpha subunit of the guanine nucleotide binding protein that stimulates adenylyl cyclase, and are resistant to parathyroid hormone (PTH) and other hormones that act via stimulation of adenylyl cyclase. To determine the incidence and etiology of ovarian dysfunction in women with AHO, we examined the reproductive history and hypothalamic-pituitary-ovarian axis in 17 affected women aged 17-43 yr. All patients had typical PTH resistance and an approximately 50% reduction in erythrocyte G(s alpha) activity, (0.43 +/- 0.03 vs. 0.92 +/- 0.08 for normal control subjects, P < 0.001). Fourteen of the 17 patients (76%) were oligomenorrheic or amenorrheic, more than half had delayed or incomplete sexual development, and only two had a history of earlier pregnancy. Most women were mildly hypoestrogenic, with normal to slightly elevated serum gonadotropin levels. Computer analysis of 24 hour LH measurement showed that the frequency of LH peaks/24 h in AHO women varied widely, but as a group they were not statistically different from a group of normal women studied in the early follicular phase. Administration of 100 microg synthetic GnRH produced normal FSH and LH responses. We conclude that reproductive dysfunction is common in women with AHO and probably represents partial resistance to gonadotropins.

Analysis and identification of imprinted genes

Genomic imprinting in mammals results in the unequal expression of the two parental alleles of specific genes. The existence of imprinting in the mouse emerged from nuclear transplantation studies and from the abnormal phenotypes associated with uniparental inheritance of particular chromosome segments. Over the past 5 years, 20 or so imprinted genes have been identified. This has emphasized the important roles played by some imprinted genes in development, permitted a description of the epigenetic properties associated with imprinting, and provided the first insights into the regulation of imprinting. In this article, we discuss the generation of experimental material in which imprinting effects can be analyzed, review the properties of imprinted genes, and discuss how to examine them using state-of-the-art techniques. Finally, we consider the means by which new imprinted genes can be identified.

Genetic mapping of the mouse genome

This article provides an introduction to genetic mapping for scientists who wish to map specific genes or mutant phenotypes in the mouse. The basic principles of genetic mapping and the different types of genetic markers available are described in the first two sections of the chapter. The theoretical and empirical principles necessary to consider when designing mapping experiments are reviewed in the third section. Protocols for mapping phenotypic traits and cloned genes are detailed in the fourth and fifth sections.

Genomic imprinting in mammals

A handful of autosomal genes in the mammalian genome are inherited in a silent state from one of the two parents, and in a fully active form from the other, thereby rendering the organism functionally hemizygous for imprinted genes. To date 19 imprinted genes have been identified; 5 are expressed from the maternal chromosome while the rest are expressed from the paternal chromosome. Allele-specific methylation of CpG residues, established in one of the germlines and maintained throughout embryogenesis, has been clearly implicated in the maintenance of imprinting in somatic cells. Although the function of imprinting remains a subject of some debate, the process is thought to have an important role in regulating the rate of fetal growth.

Growth effects of uniparental disomies and the conflict theory of genomic imprinting

While numerous theories have been proposed for the evolution of genomic imprinting, few have been tested. The conflict theory proposes that imprinting is an intra-individual manifestation of classical parent-offspring conflict. This theory is unique in predicting that imprinted genes expressed from the paternally derived genome should be enhancers of pre- and post-natal growth, while those expressed from the maternally derived genome should be growth suppressors. We examine this prediction by reviewing the literature on growth of human and mouse progeny that have inherited both copies (or part thereof) of a particular chromosome from only one parent. Perhaps surprisingly, we find that much of the data do not support the hypothesis.

Targeted disruption of Gnas in embryonic stem cells

Mutations in the gene encoding the stimulatory G protein of adenylyl cyclase (G alpha(s)) are present in subjects with Albright hereditary osteodystrophy, a syndrome of characteristic developmental defects and, in some patients, resistance to multiple hormones that stimulate cAMP accumulation (pseudohypoparathyroidism type Ia). As the first step in generating a model of Albright hereditary osteodystrophy, the gene encoding G alpha(s) (Gnas) was disrupted in mouse embryonic stem (ES) cells by homologous recombination. Northern blot analysis and immunoblot analysis demonstrated that steady-state levels of G alpha(s) messenger RNA and G alpha(s) protein in targeted ES cells were approximately 50% of levels in untargeted ES cells. In response to 10 microM forskolin and to various concentrations of isoproterenol (0.1-3.0 microM), cAMP accumulation was reduced in the G alpha(s) knockout ES cell lines, relative to wild-type ES cells and to five of six ES cell lines with randomly integrated targeting vector. These results support the role of G alpha(s) haploinsufficiency in reducing the ability of hormones to generate cAMP in subjects with pseudohypoparathyroidism type Ia. The targeted disruption of Gnas in mouse ES cells establishes an in vitro system for further studies of the role of G alpha(s) and cAMP coupled signal transduction in differentiation and development.

Peg5/Neuronatin is an imprinted gene located on sub-distal chromosome 2 in the mouse

We have established a systematic screen for imprinted genes using a subtraction-hybridization method with day 8.5 fertilized and parthenogenetic embryos. Two novel imprinted genes, Peg1/Mest and Peg3, were identified previously by this method, along with the two known imprinted genes, Igf2 and Snrpn. Recently three additional candidate imprinted genes, Peg5-7 , were detected and Peg5 is analyzed further in this study. The cDNA sequence of Peg5 is identical to Neuronatin, a gene recently reported to be expressed mainly in the brain. Two novel spliced forms were detected with some additional sequence in the middle of the known Neuronatin sequences. All alternatively spliced forms of Peg5 were expressed only from the paternal allele, confirmed using DNA polymorphism in a subinterspecific cross. Peg5/Neuronatin maps to sub-distal Chr 2, proximal to the previously established imprinted region where imprinted genes cause abnormal shape and behavior in neonates.

Molecular evolution of imprinted genes: no evidence for antagonistic coevolution

Genomically imprinted genes are those for which expression is dependent on the sex of the parent from which they are derived. Numerous theories have been proposed for the evolution of genomic imprinting: one theory is that it is an intra-individual manifestation of classical parent -offspring conflict. This theory is unique in predicting that an arms race may develop between maternally and paternally derived genes for the control of foetal growth demands. Such antagonistic coevolution may be mediated through changes in the structure of the proteins concerned. Comparable coevolution is the most likely explanation for the rapid changes seen in antigenic components of parasites and antigen recognition components of immune systems. We have examined the evolution of insulin-like growth factor Igf2, and its antagonistic receptor Igf2r) and find that in contrast to immune genes, at the sites of mutual binding they are highly conserved. In addition, we have analysed the rate of molecular evolution of seven imprinted genes including Igf2 and Igf2r), sequenced in both mouse and rat, and had that this is the same as that of nonimprinted receptors and significantly lower than that of immune genes controlling for differences in mutation rates. Contrary to the expectations of the conflict hypothesis, we hence find no evidence for antagonistic coevolution of imprinted genes mediated by changes in sequence.