Organization of the human glucokinase regulator gene GCKR

SIRT2 ablation inhibits glucose-stimulated insulin secretion through decreasing glycolytic flux

Rationale: Sirtuins are NAD⁺-dependent protein deacylases known to have protective effects against age-related diseases such as diabetes, cancer, and neurodegenerative disease. SIRT2 is the only primarily cytoplasmic isoform and its overall role in glucose homeostasis remains uncertain.

Methods: SIRT2-knockout (KO) rats were constructed to evaluate the role of SIRT2 in glucose homeostasis. The effect of SIRT2 on β-cell function was detected by investigating the morphology, insulin secretion, and metabolomic state of islets. The deacetylation and stabilization of GKRP in β-cells by SIRT2 were determined by western blot, adenoviral infection, and immunoprecipitation.

Results: SIRT2-KO rats exhibited impaired glucose tolerance and glucose-stimulated insulin secretion (GSIS), without change in insulin sensitivity. SIRT2 deficiency or inhibition by AGK2 decreased GSIS in isolated rat islets, with lowered oxygen consumption rate. Adenovirus-mediated overexpression of SIRT2 enhanced insulin secretion from rat islets. Metabolomics analysis revealed a decrease in metabolites of glycolysis and tricarboxylic acid cycle in SIRT2-KO islets compared with control islets. Our study further demonstrated that glucokinase regulatory protein (GKRP), an endogenous inhibitor of glucokinase (GCK), was expressed in rat islets. SIRT2 overexpression deacetylated GKRP in INS-1 β-cells. SIRT2 knockout or inhibition elevated GKRP protein stability in islet β-cells, leading to an increase in the interaction of GKRP and GCK. On the contrary, SIRT2 inhibition promoted the protein degradation of ALDOA, a glycolytic enzyme.

Conclusions: SIRT2 ablation inhibits GSIS through blocking GKRP protein degradation and promoting ALDOA protein degradation, resulting in a decrease in glycolytic flux.

Association of GCKR Gene Polymorphisms with the Risk of Nonalcoholic Fatty Liver Disease and Coronary Artery Disease in a Chinese Northern Han Population

Background and Aims: Accumulated studies have evaluated the effects of glucokinase regulatory protein (GCKR) gene polymorphisms on the risk of nonalcoholic fatty liver disease (NAFLD) and coronary artery disease (CAD), but the association of GCKR polymorphisms with the risk of NAFLD and CAD in the Chinese Han population have remained unclear. The aim of this study was to investigate the association between GCKR gene polymorphisms (rs780094 and rs1260326) and the risk of NAFLD and CAD in NAFLD patients in a Chinese Northern Han population. Methods: GCKR rs780094 and rs1260326 gene polymorphisms were genotyped by polymerase chain reaction sequencing for B-type ultrasonography-proven NAFLD patients with (n = 82) or without (n = 142) CAD, and in healthy controls (n = 152). Serum lipid profiles' levels were determined using biochemical methods. Statistical analyses were conducted using SPSS 22.0 statistical software. Results: As the results showed, significant differences in the serum lipid profiles existed between each group. No significant differences were observed in the distributions of genotypes and alleles of GCKR rs780094 and rs1260326 in each group. The GCKR rs780094 T and rs1260326 T allele carriers possessed decreased body mass index value, and serum fasting plasma glucose and TG levels in the overall subjects, respectively. In addition, the GCKR rs780094 T allele carriers possessed decreased serum fasting plasma glucose level in the controls and NAFLD + CAD patients. Conclusions: GCKR rs780094 and rs1260326 polymorphisms were found to be not associated with the risk of NAFLD nor of CAD in NAFLD patients in this Chinese Northern Han population. GCKR rs780094 T and rs1260326 T alleles could affect the body mass index value and serum fasting plasma glucose and triglyceride levels.

Targeted sequencing of candidate genes of dyslipidemia in Punjabi Sikhs: Population-specific rare variants in GCKR promote ectopic fat deposition

Dyslipidemia is a well-established risk factor for cardiovascular diseases. Although, advances in genome-wide technologies have enabled the discovery of hundreds of genes associated with blood lipid phenotypes, most of the heritability remains unexplained. Here we performed targeted resequencing of 13 bona fide candidate genes of dyslipidemia to identify the underlying biological functions. We sequenced 940 Sikh subjects with extreme serum levels of hypertriglyceridemia (HTG) and 2,355 subjects were used for replication studies; all 3,295 participants were part of the Asian Indians Diabetic Heart Study. Gene-centric analysis revealed burden of variants for increasing HTG risk in GCKR (p = 2.1x10-5), LPL (p = 1.6x10-3) and MLXIPL (p = 1.6x10-2) genes. Of these, three missense and damaging variants within GCKR were further examined for functional consequences in vivo using a transgenic zebrafish model. All three mutations were South Asian population-specific and were largely absent in other multiethnic populations of Exome Aggregation Consortium. We built different transgenic models of human GCKR with and without mutations and analyzed the effects of dietary changes in vivo. Despite the short-term of feeding, profound phenotypic changes were apparent in hepatocyte histology and fat deposition associated with increased expression of GCKR in response to a high fat diet (HFD). Liver histology of the GCKRmut showed severe fatty metamorphosis which correlated with ~7 fold increase in the mRNA expression in the GCKRmut fish even in the absence of a high fat diet. These findings suggest that functionally disruptive GCKR variants not only increase the risk of HTG but may enhance ectopic lipid/fat storage defects in absence of obesity and HFD. To our knowledge, this is the first transgenic zebrafish model of a putative human disease gene built to accurately assess the influence of genetic changes and their phenotypic consequences in vivo.

Insights from Genome-Wide Association Analyses of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is caused by hepatic steatosis, which can progress to nonalcoholic steatohepatitis, fibrosis/cirrhosis, and hepatocellular carcinoma in the absence of excessive alcohol consumption. Nonalcoholic fatty liver disease will become the number one cause of liver disease worldwide by 2020. Nonalcoholic fatty liver disease is correlated albeit imperfectly with obesity and other metabolic diseases such as diabetes, hyperlipidemia, and cardiovascular disease, but exactly how having one of these diseases contributes to the development of other metabolic diseases is only now being elucidated. Development of NAFLD and related metabolic diseases is genetically influenced in the population, and recent genome-wide association studies (GWASs) have discovered genetic variants that associate with these diseases. These GWAS-associated variants cannot only help us to identify individuals at high risk of developing NAFLD, but also to better understand its pathophysiology so that we can develop more effective treatments for this disease and related metabolic diseases in the future.

Differential Genetic Effects on Statin-Induced Changes Across Low-Density Lipoprotein-Related Measures

BACKGROUND

Statin therapy influences not only low-density lipoprotein (LDL) cholesterol levels but also LDL-related biomarkers, including non-high-density lipoprotein cholesterol (non-HDL-C), apolipoprotein B, total number of LDL particles, and mean LDL particle size. Recent studies have identified many genetic loci influencing circulating lipid levels and statin-induced LDL cholesterol reduction. However, it is unknown how these genetic variants influence statin-induced changes in LDL subfractions and non-HDL-C.

METHODS AND RESULTS

One hundred sixty candidate single-nucleotide polymorphisms for effects on circulating lipid levels or statin-induced LDL-cholesterol lowering were tested for association with response of LDL subfractions and non-HDL-C to rosuvastatin or placebo for 1 year among 7046 participants from the Justification for Use of Statins in Prevention: an Intervention Trial Evaluating Rosuvastatin (JUPITER) trial. Of the 51 single-nucleotide polymorphisms associated with statin response for ≥ 1 of the LDL subfractions or non-HDL-C, 20 single-nucleotide polymorphisms could be clustered according to effects predominantly on LDL particle size, predominantly on LDL particle number, and on apolipoprotein B but not on LDL cholesterol or non-HDL-C.

CONCLUSIONS

These differential associations point to pathways of LDL response to statin therapy and possibly to mechanisms of statin-dependent cardiovascular disease risk reduction.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00239681.

Functional variants of lipid level modifier MLXIPL, GCKR, GALNT2, CILP2, ANGPTL3 and TRIB1 genes in healthy Roma and Hungarian populations

The role of triglyceride metabolism in different diseases, such as cardiovascular or cerebrovascular diseases is still under extensive investigations. In genome-wide studies several polymorphisms have been reported, which are highly associated with plasma lipid level changes. Our goal was to examine eight variants: rs12130333 at the ANGPTL3, rs16996148 at the CILP2, rs17321515 at the TRIB1, rs17145738 and rs3812316 of the MLXIPL, rs4846914 at GALNT2, rs1260326 and rs780094 residing at the GCKR loci. A total of 399 Roma (Gypsy) and 404 Hungarian population samples were genotyped using PCR-RFLP method. Significant differences were found between Roma and Hungarian population samples in both MLXIPL variants (C allele frequency of rs17145738: 94.1% vs. 85.6%, C allele frequency of rs3812316: 94.2% vs. 86.8% in Romas vs. in Hungarians, p < 0.05), in ANGPTL3 (T allele frequency of rs1213033: 12.2% vs. 18.5% in Romas vs. Hungarians, p < 0.05) and GALNT2 (G allele frequency of rs4846914: 46.6% vs. 54.5% Romas vs. in Hungarians, p < 0.05), while no differences over SNPs could be verified and the known minor alleles showed no correlation with triglyceride levels in any population samples. The current study revealed fundamental differences of known triglyceride modifying SNPs in Roma population. Failure of finding evidence for affected triglyceride metabolism shows that these susceptibility genes are much less effective compared for example to the apolipoprotein A5 gene.

Association of glucokinase regulatory gene polymorphisms with risk and severity of non-alcoholic fatty liver disease: an interaction study with adiponutrin gene

BACKGROUND

Recent genome-wide association studies demonstrated an association between single nucleotide polymorphisms (SNPs) on the glucokinase regulatory gene (GCKR) with hepatic steatosis. This study attempted to investigate the association of GCKR rs780094 and rs1260326 with susceptibility to non-alcoholic fatty liver disease (NAFLD) and its severity.

METHODS

The genotypes were assessed on 144 histologically confirmed NAFLD patients and 198 controls using a Sequenom MassARRAY platform.

RESULTS

The GCKR rs1260326 and rs780094 allele T were associated with susceptibility to NAFLD (OR 1.49, 95 % CI 1.09-2.05, p = 0.012; and OR 1.51, 95 % CI 1.09-2.09, p = 0.013, respectively), non-alcoholic steatohepatitis (NASH) (OR 1.55, 95 % CI 1.10-2.17, p = 0.013; and OR 1.56, 95 % CI 1.10-2.20, p = 0.012, respectively) and NASH with significant fibrosis (OR 1.50, 95 % CI 1.01-2.21, p = 0.044; and OR 1.52, 95 % CI 1.03-2.26, p = 0.038, respectively). Following stratification by ethnicity, significant association was seen in Indian patients between the two SNPs and susceptibility to NAFLD (OR 2.64, 95 % CI 1.28-5.43, p = 0.009; and OR 4.35, 95 % CI 1.93-9.81, p < 0.0001, respectively). The joint effect of GCKR with adiponutrin rs738409 indicated greatly increased the risk of NAFLD (OR 4.14, 95 % CI 1.41-12.18, p = 0.010). Histological data showed significant association of GCKR rs1260326 with high steatosis grade (OR 1.76, 95 % CI 1.08-2.85, p = 0.04).

CONCLUSION

This study suggests that risk allele T of the GCKR rs780094 and rs1260326 is associated with predisposition to NAFLD and NASH with significant fibrosis. The GCKR and PNPLA3 genes interact to result in increased susceptibility to NAFLD.

Evolution of hepatic glucose metabolism: liver-specific glucokinase deficiency explained by parallel loss of the gene for glucokinase regulatory protein (GCKR)

BACKGROUND

Glucokinase (GCK) plays an important role in the regulation of carbohydrate metabolism. In the liver, phosphorylation of glucose to glucose-6-phosphate by GCK is the first step for both glycolysis and glycogen synthesis. However, some vertebrate species are deficient in GCK activity in the liver, despite containing GCK genes that appear to be compatible with function in their genomes. Glucokinase regulatory protein (GCKR) is the most important post-transcriptional regulator of GCK in the liver; it participates in the modulation of GCK activity and location depending upon changes in glucose levels. In experimental models, loss of GCKR has been shown to associate with reduced hepatic GCK protein levels and activity.

METHODOLOGY/PRINCIPAL FINDINGS

GCKR genes and GCKR-like sequences were identified in the genomes of all vertebrate species with available genome sequences. The coding sequences of GCKR and GCKR-like genes were identified and aligned; base changes likely to disrupt coding potential or splicing were also identified.

CONCLUSIONS/SIGNIFICANCE

GCKR genes could not be found in the genomes of 9 vertebrate species, including all birds. In addition, in multiple mammalian genomes, whereas GCKR-like gene sequences could be identified, these genes could not predict a functional protein. Vertebrate species that were previously reported to be deficient in hepatic GCK activity were found to have deleted (birds and lizard) or mutated (mammals) GCKR genes. Our results suggest that mutation of the GCKR gene leads to hepatic GCK deficiency due to the loss of the stabilizing effect of GCKR.

Common functional variants of APOA5 and GCKR accumulate gradually in association with triglyceride increase in metabolic syndrome patients

The common functional variants of the apolipoprotein A5 (APOA5) and the glucokinase regulatory protein genes (GCKR) have been shown to associate with increased fasting triglyceride (TG) levels. Albeit the basic association has been extensively investigated in several populations of different origin, less is known about quantitative traits of them. In our study accumulation rates of four APOA5 (T-1131, IVS3 + G476A, T1259C and C56G) and two GCKR (C1337T and rs780094) functional SNPs were analyzed in patients stratified into four TG quartile groups. Randomly selected 325 metabolic syndrome patients were separated into four quartile (q) groups based on the TG levels as follows q1: TG <1.38 mmol/l; q2: 1.38-1.93 mmol/l; q3: 1.94-2.83 mmol/l; and q4: TG >2.83 mmol/l. We observed significant stepwise increase of prevalence rates of minor allele frequencies in the four plasma TG quartiles for three APOA5 SNPs: -1131C (q1: 4.94%; q2: 8.64%; q3: 11.6%; q4: 12.3%), IVS3 + 476A (q1: 4.32%; q2: 7.4%; q3: 10.36%; q4: 11.1%), and 1259C (q1: 4.94%; q2: 7.41%; q3: 10.4%; q4: 11.7%). The haplotype analysis revealed, that the frequency of APOA5*2 haplotype gradually increased in q2, q3 and q4 (q1: 9.87%; q2: 14.8%; q3: 18.3%; q4: 21%). The distribution of the homozygotes of the two analyzed GCKR variants resembled to the APOA5 pattern. Contrary to the hypothetically predictable linear association coming from the current knowledge about the APOA5 and GCKR functions, the findings presented here revealed a unique, TG raise dependent gradual accumulation of the functional variants of in MS patients. Thus, the findings of the current study serve indirect evidence for the existence of rare APOA5 and GCKR haplotypes in metabolic syndrome patients with higher TG levels, which contribute to the complex lipid metabolism alteration in this disease.

GCKR mutations in Japanese families with clustered type 2 diabetes

OBJECTIVE

The aim was to investigate the genetic background of familial clustering of type 2 diabetes.

SUBJECTS AND METHODS

We recruited Japanese families with a 3-generation history of diabetes. Genome-wide linkage analysis was performed assuming an autosomal dominant model. Genes in the linkage region were computationally prioritized using Endeavour. We sequenced the candidate genes, and the frequencies of detected nucleotide changes were then examined in normoglycemic controls.

RESULTS

To exclude known genetic factors, we sequenced 6 maturity onset diabetes of the young (MODY) genes in 10 familial cases. Because we detected a MODY3 mutation HNF1A R583G in one case, we excluded this case from further investigation. Linkage analysis revealed a significant linkage region on 2p25-22 (LOD score=3.47) for 4 families. The 23.6-Mb linkage region contained 106 genes. Those genes were scored by computational prioritization. Eleven genes, i.e., top 10% of 106 genes, were selected and considered primary candidates. Considering their functions, we eliminated 3 well characterized genes and finally sequenced 8 genes. GCKR ranked highly in the computational prioritization. Mutations (minor allele frequency less than 1%) in exons and the promoter of GCKR were found in index cases of the families (3 of 18 alleles) more frequently than in controls (0 of 36 alleles, P=0.033). In one pedigree with 9 affected members, the mutation GCKR g.6859C>G was concordant with affection status. No mutation in other 7 genes that ranked highly in the prioritization was concordant with affection status in families.

CONCLUSIONS

We propose that GCKR is a susceptibility gene in Japanese families with clustered diabetes. The family based approach seems to be complementary with a large population study.

Functional variants of glucokinase regulatory protein and apolipoprotein A5 genes in ischemic stroke

Both the natural variants of the apolipoprotein A5 (APOA5) and the glucokinase regulatory protein gene (GCKR) have been shown to associate with increased fasting triglyceride levels. Here, we investigated the possible association of the functional variants of these two genes with non-fasting triglyceride levels and their susceptibility nature in ischemic stroke. A total of 513 stroke patients and 172 healthy controls were genotyped. All the APOA5 variants (T-1131C, IVS3 + G476A, C56G, and T1259C) were associated with increased triglyceride levels in all stroke patients and controls; except for T1259C, they all conferred risk for the disease. No such association was found for the examined GCKR rs1260326 (C1337T) variant. Furthermore, we examined the effects of specific combinations of the GCKR rs1260326 and APOA5 polymorphisms. Our findings confirmed the previous results regarding the association of APOA5 variants with triglyceride-level increase and stroke susceptibility of these alleles. By contrast, we could not detect any association of the studied GCKR allele with triglyceride levels or with the susceptibility of stroke in the same cohort of patients. In addition, the effect of APOA5 did not change significantly when specific combinations of the two genes were present.

Novel polymorphisms in the GCKR gene and their influence on glucose and insulin levels in a Danish twin population

AIM

The glucokinase regulatory protein gene is a candidate gene for Type 2 diabetes. This study reveals three new polymorphisms and examines the impact of one new and one known polymorphism on insulin secretion and parameters associated with the insulin resistance syndrome in Danish twins with different degrees of glucose tolerance.

METHODS

Single nucleotide polymorphism detection was performed in 20 healthy subjects and in 20 subjects with Type 2 diabetes. The effect of the polymorphisms on lipid, glucose and insulin measures was studied in 566 same-sex twins aged 55-74 years.

RESULTS

The new nucleotide (nt) 363 polymorphism was found only in subjects with impaired glucose tolerance and Type 2 diabetes. The nt 11216 polymorphism influenced insulin measured at 120 min during an oral glucose tolerance test (OGTT). Subjects with genotype C11216C/T11216C had 21% higher insulin values (P<0.05) than subjects with genotype T11216T. In twins discordant for this genotype, the C-allele was associated with significantly higher plasma insulin levels at all time points during the OGTT, higher beta-cell function and lower plasma glucose levels during the OGTT.

CONCLUSION

The C-allele of nt 11216 polymorphism was associated with increased insulin secretion, and may therefore exert a potentially protective effect against Type 2 diabetes. This remains to be shown in a larger study population.

Differences in regulatory properties between human and rat glucokinase regulatory protein

The inhibition of glucokinase by rat and Xenopus GKRPs (glucokinase regulatory protein) is well documented. We report a comparison of the effects of human and rat GKRPs on glucokinase activity. Human GKRP is a more potent inhibitor of glucokinase than rat GKRP in the absence of fructose 6-phosphate or sorbitol 6-phosphate, and has a higher affinity for these ligands. However, human and rat GKRPs have similar affinities for fructose 1-phosphate and chloride. Residues that are not conserved between the rodent and human proteins affect both the affinity for fructose 6-phosphate and sorbitol 6-phosphate and the inhibitory potency of GKRP on glucokinase in the absence of these ligands.

Expression of glucose transporter isoform GLUT-2 and glucokinase genes in human brain

The glucose transporter isoform-2 (GLUT-2) and glucokinase are considered to be components of a glucose sensor system controlling several key processes, and hence may modulate feeding behaviour. We have found GLUT-2 and glucokinase mRNAs in several brain regions, including the ventromedial and arcuate nuclei of the hypothalamus. GLUT-2, glucokinase and glucokinase regulatory protein mRNAs and proteins were present in these areas as determined by biochemical approaches. In addition, glucose-phosphorylating activity with a high apparent Km for glucose that displayed no product inhibition by glucose-6-phosphate was observed. Increased glycaemia after meals may be recognized by specific hypothalamic neurones due to the high Km of GLUT-2 and glucokinase. This enzyme is considered to be the true glucose sensor because it catalyses the rate-limiting step of glucose catabolism its activity being regulated by interaction with glucokinase regulatory protein, that functions as a metabolic sensor.

Mutations in the glucokinase regulatory protein gene in 2p23 in obese French caucasians

AIMS/HYPOTHESIS

Glucokinase regulatory protein (GKRP) controls the activity of glucokinase in liver but possibly also in some areas of the central nervous system, suggesting that it could play a role in body mass control. Its gene is located in a region (2p21-23) linked to serum leptin levels. Our goal was to investigate whether mutations in the GKRP gene were associated with obesity.

METHODS

Mutations were sought in the GKRP gene of 57 patients from the families of the French genome-wide scan for obesity that contributed most to the positive LOD score with 2p21-23. The identified mutations were further sought in 720 unrelated obese individuals and 384 individuals of normal weight and their effect on the properties of recombinant GKRP were investigated.

RESULTS

The most frequent mutation (Pro446Leu) had a similar allele frequency in the obese (0.63) and normal weight (0.64) subjects and did not affect the properties of GKRP. Similarly, no effect on the properties of GKRP was observed with Arg590Tyr, found in 10 out of 720 obese subjects and in 2 out of 384 control subjects (p=0.18). Mutation Arg227Stop was found in one obese family and in 1 out of 384 control subjects and led to an insoluble protein. Mutation Arg518Gln, replacing a conserved residue, led to a marked decrease in the affinity of GKRP for both fructose 6-phosphate and fructose 1-phosphate and to a destabilization of GKRP. However, this mutation did not co-segregate with obesity in the single family in which it was found.

CONCLUSIONS/INTERPRETATION

Mutations that affect the properties of GKRP are found in the French population, but they do not seem to account for the linkage between the 2p23 locus and quantitative markers of obesity.

The influence of the polymorphism in apolipoprotein B codon 2488 on insulin and lipid levels in a Danish twin population

AIMS

The apolipoprotein B codon 2488 polymorphism has been associated with the metabolism of lipoproteins in subjects with Type 2 diabetes. However, no data are available on the influence of the polymorphism on insulin or glucose metabolism. This study examines the impact of the polymorphism on parameters associated with the insulin resistance syndrome in Danish twins.

METHODS

The effect of the polymorphism on lipid, glucose and insulin measures was studied in 548 same sex twins aged 55-74 years.

RESULTS

The codon 2488 polymorphism influenced fasting triglyceride levels, as well as insulin, as measured at 120 min in an oral glucose tolerance test. Subjects with the genotype T2488T had 14% higher triglyceride levels (P = 0.02) and 31% higher insulin levels (P = 0.004) than subjects with genotype C2488C. In twins discordant for genotype, the T-allele was associated with higher levels of triglyceride (P = 0.04) and insulin (P = 0.02) and lower levels of HDL-cholesterol (P = 0.04).

CONCLUSION

The T-allele of the codon 2488 polymorphism influenced parameters related to the insulin resistance syndrome, i.e. increased levels of insulin, increased levels of triglyceride and decreased levels of HDL. As the polymorphism is silent, these effects must be mediated through linkage to other polymorphisms in apolipoprotein B or other genes on chromosome 2.

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.