Glycogen storage disease type Ia: recent experience with mutation analysis, a summary of mutations reported in the literature and a newly developed diagnostic flow chart

High childhood serum triglyceride concentrations associate with hepatocellular adenoma development in patients with glycogen storage disease type Ia

Background & Aims

Glycogen storage disease type Ia (GSDIa) is an inborn error of carbohydrate metabolism caused by pathogenic variants in the glucose-6-phosphatase catalytic subunit 1 (G6PC1) gene and is associated with hepatocellular adenoma (HCA) formation. Data on risk factors for HCA occurrence in GSDIa are scarce. We investigated HCA development in relation to sex, G6PC1 genotype, and serum triglyceride concentration (TG).

Methods

An observational study of patients with genetically confirmed GSDIa ≥12 years was performed. Patients were categorised for sex; presence of 2, 1, or 0 predicted severe G6PC1 variant (PSV); and median TG during childhood (<12 years; stratified for above/below 5.65 mmol/L, i.e. 500 mg/dl).

Results

Fifty-three patients (23 females) were included, of which 26 patients developed HCA at a median (IQR) age of 21 (17–25) years. At the age of 25 years, 48% of females and 30% of males had developed HCA (log-rank p = 0.045). Two-thirds of patients with GSDIa carried 2 PSVs, 20% carried 1, and 13% carried none. Neither the number of PSVs nor any specific G6PC1 variants were associated with HCA occurrence. Childhood TG was 3.4 (3.0–4.2) mmol/L in males vs. 5.6 (4.0–7.9) mmol/L in females (p = 0.026). Childhood TG >5.65 mmol/L was associated with HCA development at younger age, compared with patients with childhood TG <5.65 mmol/L (18 vs. 33 years; log-rank p = 0.001). Cox regression analysis including TG, sex, and TG–sex interaction correction revealed childhood TG >5.65 mmol/L as an independent risk factor for HCA development (hazard ratio [HR] 6.0; 95% CI 1.2–29.8; p = 0.028).

Conclusions

In patients with GSDIa, high childhood TG was associated with an increased risk of HCA, and earlier onset of HCA development, independent of sex-associated hypertriglyceridaemia, and G6PC1 genotype.

Lay summary

Glycogen storage disease type Ia (GSDIa) is a rare, inherited metabolic disease that can be complicated by liver tumours (hepatocellular adenomas), which in turn may cause bleeding or progress to liver cancer. Risk factors associated with hepatocellular adenoma formation in patients with GSDIa are largely unknown. In our study, we found that high serum triglyceride concentrations during childhood, but not specific genetic variants, were associated with increased risk of hepatocellular adenoma diagnosis later in life.

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Glycogen storage disease Ia (GSDIa) is a metabolic disease caused by mutations in glucose-6-phosphatase catalytic subunit 1 (G6PC1).

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Patients with GSDIa often develop hepatocellular adenoma (HCA), with unclear risk factors.

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Metabolic control in GSDIa is commonly evaluated through serum triglyceride concentration (TG).

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Patients with GSDIa with high childhood TG had increased risk and earlier onset of HCA.

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Sex-associated hypertriglyceridaemia and G6PC1 genotype were not associated with HCA.

Classifying molecular phenotypes of G6PC variants for pathogenic properties and to guide therapeutic development

Due to advances in sequencing technologies, identification of genetic variants is rapid. However, the functional consequences of most genomic variants remain unknown. Consequently, variants of uncertain significance (VUS) that appear in clinical DNA diagnostic reports lack sufficient data for interpretation. Algorithms exist to aid prediction of a variant's likelihood of pathogenicity, but these predictions usually lack empiric evidence. To examine the feasibility of generating functional evidence in vitro for a given variant's role in disease, a panel of 29 coding sequence variants in the G6PC gene was assessed. G6PC encodes glucose-6 phosphatase enzyme, and reduction in its function causes the rare metabolic disease glycogen storage disease type 1a (GSD1a). Variants were heterologously expressed as fusion proteins in a hepatocyte-derived cell line and examined for effects on steady-state protein levels, biosynthetic processing, and intracellular distribution. The screen revealed variant effects on protein levels, N-linked glycosylation status, and cellular distribution. Of the eight VUS tested, seven behaved similar to wild-type protein while one VUS, p.Cys109Tyr, exhibited features consistent with pathogenicity for all molecular phenotypes assayed, including significantly reduced protein levels, alteration in protein glycosylation status, and abnormally diffuse protein localization pattern, and has recently been reported in a patient with GSD1a. Our results show that such a screen can add empiric evidence to existing databases to aid in diagnostics, and also provides further classification for molecular phenotypes that could be used in future therapeutic screening approaches for small molecule or gene editing strategies directed at specific variants.

Neurological Characteristics of Pediatric Glycogen Storage Disease

Glycogen storage diseases (GSD) encompass a group of rare inherited diseases due dysfunction of glycogen metabolism. Hypoglycemia is the most common primary manifestation of GSD, and disturbances in glucose metabolism can cause neurological damage. The aims of this study were to first investigate the metabolic, genetic, and neurological profiles of children with GSD, and to test the hypothesis whether GSD type I would have greater neurological impact than GSD type IX. A cross-sectional study was conducted with 12 children diagnosed with GSD [Types: Ia (n=5); 1, Ib (n=1); 4, IXa (n=5); and 1, IXb (n=1)]. Genetic testing was conducted for the following genes using multigene panel analysis. The biochemical data and magnetic resonance imaging of the brain presented by the patients were evaluated. The criteria of adequate metabolic control were adopted based on the European Study on Glycogen Storage Disease type I consensus. Pathogenic mutations were identified using multigene panel analyses. The mutations and clinical chronology were related to the disease course and neuroimaging findings. Adequate metabolic control was achieved in 67% of patients (GSD I, 43%; GSD IX, 100%). Fourteen different mutations were detected, and only two co-occurring mutations were observed across families (G6PC c.247C>T and c.1039C>T). Six previously unreported variants were identified (5 PHKA2; 1 PHKB). The proportion of GSD IX was higher in our cohort compared to other studies. Brain imaging abnormalities were more frequent among patients with GSD I, early-symptom onset, longer hospitalization, and inadequate metabolic control. The frequency of mutations was similar to that observed among the North American and European populations. None of the mutations observed in PHKA2 have been described previously. Therefore, current study reports six GSD variants previously unknown, and neurological consequences of GSD I. The principal neurological impact of GSD appeared to be related to inadequate metabolic control, especially hypoglycemia.

Novel variants in Turkish patients with glycogen storage disease

BACKGROUND

Glycogen storage diseases (GSD) are disorders of autosomal recessive carbohydrate metabolism, characterized by glycogen accumulation. The liver and muscle tissue are commonly affected but patients may present with different clinical manifestations. The presence of glycogen can be demonstrated in biopsies and definitive diagnosis can be made by enzymatic or molecular analysis. The aim of this study was to determine specific gene mutations in our cases with GSD.

METHODS

Thirty-eight patients with clinical and laboratory diagnoses of GSD were studied. Thirty-two patients had undergone genetic analysis. In our study, a next-generation sequencing panel was used.

RESULTS

Five novel variants of uncertain significance (VUS), which were likely to be pathogenic, were detected in seven patients. Two new pathogenic variations of c.927delT (p.Phe309LeufsTer4) homozygous and c.44C>G (p.Ser15Ter) homozygous in the G6PC gene were detected in two GSD type Ia patients. In our two non-sibling GSD type III patients, c.1439T>G (p.Leu480Arg) homozygous novel-VUS was detected in the AGL gene. In our GSD type IV patient, c.1054G>C (p.Asp352His) homozygous novel-VUS was detected in the GBE1 gene. In GSD type VI, two sibling patients had a c.1454A>G (p.Asn485Ser) homozygous novel-VUS change in the PYGL gene.

CONCLUSIONS

We determined the gene mutations specific to cohorts in our cases with GSD. The novel pathogenic, likely pathogenic, and VUS changes identified will contribute to the relationship between the patients' clinical and laboratory findings.

Glycogen storage diseases: Twenty-seven new variants in a cohort of 125 patients

BACKGROUND

Hepatic glycogen storage diseases (GSDs) are a group of rare genetic disorders in which glycogen cannot be metabolized to glucose in the liver because of enzyme deficiencies along the glycogenolytic pathway. GSDs are well-recognized diseases that can occur without the full spectrum, and with overlapping in symptoms.

METHODS

We analyzed a cohort of 125 patients with suspected hepatic GSD through a next-generation sequencing (NGS) gene panel in Ion Torrent platform. New variants were analyzed by pathogenicity prediction tools.

RESULTS

Twenty-seven new variants predicted as pathogenic were found between 63 variants identified. The most frequent GSD was type Ia (n = 53), followed by Ib (n = 23). The most frequent variants were p.Arg83Cys (39 alleles) and p.Gln347* (14 alleles) in G6PC gene, and p.Leu348Valfs (21 alleles) in SLC37A4 gene.

CONCLUSIONS

The study presents the largest cohort ever analyzed in Brazilian patients with hepatic glycogenosis. We determined the clinical utility of NGS for diagnosis. The molecular diagnosis of hepatic GSDs enables the characterization of diseases with similar clinical symptoms, avoiding hepatic biopsy and having faster results.

3'-UTR SNP rs2229611 in G6PC1 affects mRNA stability, expression and Glycogen Storage Disease type-Ia risk

The frequency of rs2229611, previously reported in Chinese, Caucasians, Japanese and Hispanics, was investigated for the first time in Indian ethnicity. We analyzed its role in the progression of Glycogen Storage Disease type-Ia (GSD-Ia) and breast cancer. Genotype data on rs2229611 revealed that the risk of GSD-Ia was higher (P=0.0195) with CC compared to TT/TC genotypes, whereas no such correlation was observed with breast cancer cases. We observed a strong linkage disequilibrium (LD) among rs2229611 and other disease causing G6PC1 variants (|D'|=1, r²=1). Functional validation performed in HepG2 cells using luciferase constructs showed significant (P<0.05) decrease in expression than wild-type 3'-UTR due to curtailed mRNA stability. Furthermore, AU-rich elements (AREs) mediated regulation of G6PC1 expression characterized using 3'-UTR deletion constructs showed a prominent decrease in mRNA stability. We then examined whether miRNAs are involved in controlling G6PC1 expression using pmirGLO-UTR constructs, with evidence of more distinct inhibition in the reporter function with rs2229611. These data suggests that rs2229611 is a crucial regulatory SNP which in homozygous state leads to a more aggressive disease phenotype in GSD-Ia patients. The implication of this result is significant in predicting disease onset, progression and response to disease modifying treatments in patients with GSD-Ia.

Molecular analysis of glycogen storage disease type Ia in Iranian Azeri Turks: identification of a novel mutation

Glycogen storage diseases (GSDs) are caused by abnormalities in enzymes that are involved in the regulation of gluconeogenesis and glycogenolysis. GSD I, an autosomal recessive metabolic disorder, is the most common GSD and has four subtypes. Here, we examined GSD Ia caused by the defective glucose-6-phosphatase catalytic (G6PC) gene. We investigated the frequency of GSD Ia and clarified its molecular aspect in patients with the main clinical and biochemical characteristics of GSD, including 37 unrelated patients with a mean age of three years at the time of diagnosis. All patients belonged to the Azeri Turkish population. Hypoglycaemia and hypertriglyceridaemia were the most frequent laboratory findings. Mutations were detected by performing direct sequencing. Mutation analysis of the G6PC gene revealed that GSD Ia accounted for 11% in GSD patients with involvement of liver. Three patients were homozygous for R83C mutation. In addition, a novel stop mutation, Y85X, was identified in a patient with the typical features of GSD Ia.

A novel homozygous no-stop mutation in G6PC gene from a Chinese patient with glycogen storage disease type Ia

Glycogen storage disease type Ia (GSD-Ia) is an autosomal recessive genetic disorder resulting in hypoglycemia, hepatomegaly and growth retardation. It is caused by mutations in the G6PC gene encoding Glucose-6-phosphatase. To date, over 80 mutations have been identified in the G6PC gene. Here we reported a novel mutation found in a Chinese patient with abnormal transaminases, hypoglycemia, hepatomegaly and short stature. Direct sequencing of the coding region and splicing-sites in the G6PC gene revealed a novel no-stop mutation, p.*358Yext*43, leading to a 43 amino-acid extension of G6Pase. The expression level of mutant G6Pase transcripts was only 7.8% relative to wild-type transcripts. This mutation was not found in 120 chromosomes from 60 unrelated healthy control subjects using direct sequencing, and was further confirmed by digestion with Rsa I restriction endonuclease. In conclusion, we revealed a novel no-stop mutation in this study which expands the spectrum of mutations in the G6PC gene. The molecular genetic analysis was indispensable to the diagnosis of GSD-Ia for the patient.

Determining mutations in G6PC and SLC37A4 genes in a sample of Brazilian patients with glycogen storage disease types Ia and Ib

Glycogen storage disease (GSD) comprises a group of autosomal recessive disorders characterized by deficiency of the enzymes that regulate the synthesis or degradation of glycogen. Types Ia and Ib are the most prevalent; while the former is caused by deficiency of glucose-6-phosphatase (G6Pase), the latter is associated with impaired glucose-6-phosphate transporter, where the catalytic unit of G6Pase is located. Over 85 mutations have been reported since the cloning of G6PC and SLC37A4 genes. In this study, twelve unrelated patients with clinical symptoms suggestive of GSDIa and Ib were investigated by using genetic sequencing of G6PC and SLC37A4 genes, being three confirmed as having GSD Ia, and two with GSD Ib. In seven of these patients no mutations were detected in any of the genes. Five changes were detected in G6PC, including three known point mutations (p.G68R, p.R83C and p.Q347X) and two neutral mutations (c.432G > A and c.1176T > C). Four changes were found in SLC37A4: a known point mutation (p.G149E), a novel frameshift insertion (c.1338_1339insT), and two neutral mutations (c.1287G > A and c.1076-28C > T). The frequency of mutations in our population was similar to that observed in the literature, in which the mutation p.R83C is also the most frequent one. Analysis of both genes should be considered in the investigation of this condition. An alternative explanation to the negative results in this molecular study is the possibility of a misdiagnosis. Even with a careful evaluation based on laboratory and clinical findings, overlap with other types of GSD is possible, and further molecular studies should be indicated.

Glycogen Storage Disease type 1a - a secondary cause for hyperlipidemia: report of five cases

BACKGROUND AND AIMS

Glycogen storage disease type Ia (GSD Ia) is a rare metabolic disorder, caused by deficient activity of glucose-6-phosphatase-α. It produces fasting induced hypoglycemia and hepatomegaly, usually manifested in the first semester of life. Besides, it is also associated with growth delay, anemia, platelet dysfunction, osteopenia and sometimes osteoporosis. Hyperlipidemia and hyperuricemia are almost always present and hepatocellular adenomas and renal dysfunction frequent late complications.

METHODS

The authors present a report of five adult patients with GSD Ia followed in internal medicine appointments and subspecialties.

RESULTS

Four out of five patients were diagnosed in the first 6 months of life, while the other one was diagnosed in adult life after the discovery of hepatocellular adenomas. In two cases genetic tests were performed, being identified the missense mutation R83C in one, and the mutation IVS4-3C > G in the intron 4 of glucose-6-phosphatase gene, not previously described, in the other. Growth retardation was present in 3 patients, and all of them had anemia, increased bleeding tendency and hepatocellular adenomas; osteopenia/osteoporosis was present in three cases. All but one patient had marked hyperlipidemia and hyperuricemia, with evidence of endothelial dysfunction in one case and of brain damage with refractory epilepsy in another case. Proteinuria was present in two cases and end-stage renal disease in another case. There was a great variability in the dietary measures; in one case, liver transplantation was performed, with correction of the metabolic derangements.

CONCLUSIONS

Hyperlipidemia is almost always present and only partially responds to dietary and drug therapy; liver transplantation is the only definitive solution. Although its association with premature atherosclerosis is rare, there have been reports of endothelial dysfunction, raising the possibility for increased cardiovascular risk in this group of patients. Being a rare disease, no single metabolic center has experience with large numbers of patients and the recommendations are based on clinical experience more than large scale studies.

Glycogen storage disease type Ia in canines: a model for human metabolic and genetic liver disease

A canine model of Glycogen storage disease type Ia (GSDIa) is described. Affected dogs are homozygous for a previously described M121I mutation resulting in a deficiency of glucose-6-phosphatase-α. Metabolic, clinicopathologic, pathologic, and clinical manifestations of GSDIa observed in this model are described and compared to those observed in humans. The canine model shows more complete recapitulation of the clinical manifestations seen in humans including “lactic acidosis”, larger size, and longer lifespan compared to other animal models. Use of this model in preclinical trials of gene therapy is described and briefly compared to the murine model. Although the canine model offers a number of advantages for evaluating potential therapies for GSDIa, there are also some significant challenges involved in its use. Despite these challenges, the canine model of GSDIa should continue to provide valuable information about the potential for generating curative therapies for GSDIa as well as other genetic hepatic diseases.

Glycogen storage disease type I and G6Pase-β deficiency: etiology and therapy

Glycogen storage disease type I (GSD-I) consists of two subtypes: GSD-Ia, a deficiency in glucose-6-phosphatase-α (G6Pase-α) and GSD-Ib, which is characterized by an absence of a glucose-6-phosphate (G6P) transporter (G6PT). A third disorder, G6Pase-β deficiency, shares similarities with this group of diseases. G6Pase-α and G6Pase-β are G6P hydrolases in the membrane of the endoplasmic reticulum, which depend on G6PT to transport G6P from the cytoplasm into the lumen. A functional complex of G6PT and G6Pase-α maintains interprandial glucose homeostasis, whereas G6PT and G6Pase-β act in conjunction to maintain neutrophil function and homeostasis. Patients with GSD-Ia and those with GSD-Ib exhibit a common metabolic phenotype of disturbed glucose homeostasis that is not evident in patients with G6Pase-β deficiency. Patients with a deficiency in G6PT and those lacking G6Pase-β display a common myeloid phenotype that is not shared by patients with GSD-Ia. Previous studies have shown that neutrophils express the complex of G6PT and G6Pase-β to produce endogenous glucose. Inactivation of either G6PT or G6Pase-β increases neutrophil apoptosis, which underlies, at least in part, neutrophil loss (neutropenia) and dysfunction in GSD-Ib and G6Pase-β deficiency. Dietary and/or granulocyte colony-stimulating factor therapies are available; however, many aspects of the diseases are still poorly understood. This Review will address the etiology of GSD-Ia, GSD-Ib and G6Pase-β deficiency and highlight advances in diagnosis and new treatment approaches, including gene therapy.

Inborn errors of carbohydrate metabolism

Glycogen storage diseases (GSD) and inborn errors of galactose and fructose metabolism are the most common representatives of inborn errors of carbohydrate metabolism. In this review the focus is set on the current knowledge about clinical symptoms, diagnosis and treatment. Hepatomegaly and hypoglycaemia are the main findings in liver-affecting GSD like type I, III and IX. Diagnosis is usually made by non invasive investigations, e.g. mutation analysis. In GSD I, a carbohydrate balanced diet with frequent meals and nocturnal continuous tube feeding or addition of uncooked corn starch are the mainstays of treatment to prevent hypoglycaemia. Liver transplantation has been performed in different types of GSD. It should only be considered in high risk patients e.g. with substantial cirrhosis. Many countries have included classical galactosaemia in their newborn screening programs. A lactose-free infant formula can be life-saving in affected neonates whereas a strict fructose-restricted diet is indicated in hereditary fructose intolerance.

A potential role for muscle in glucose homeostasis: in vivo kinetic studies in glycogen storage disease type 1a and fructose-1,6-bisphosphatase deficiency

BACKGROUND

A potential role for muscle in glucose homeostasis was recently suggested based on characterization of extrahepatic and extrarenal glucose-6-phosphatase (glucose-6-phosphatase-beta). To study the role of extrahepatic tissue in glucose homeostasis during fasting glucose kinetics were studied in two patients with a deficient hepatic and renal glycogenolysis and/or gluconeogenesis.

DESIGN

Endogenous glucose production (EGP), glycogenolysis (GGL), and gluconeogenesis (GNG) were quantified with stable isotopes in a patient with glycogen storage disease type 1a (GSD-1a) and a patient with fructose-1,6-bisphosphatase (FBPase) deficiency. The [6,6-(2)H(2)]glucose dilution method in combination with the deuterated water method was used during individualized fasting tests.

RESULTS

Both patients became hypoglycemic after 2.5 and 14.5 h fasting, respectively. At that time, the patient with GSD-1a had EGP 3.84 micromol/kg per min (30% of normal EGP after an overnight fast), GGL 3.09 micromol/kg per min, and GNG 0.75 micromol/kg per min. The patient with FBPase deficiency had EGP 8.53 micromol/kg per min (62% of normal EGP after an overnight fast), GGL 6.89 micromol/kg per min GGL, and GNG 1.64 micromol/kg per min.

CONCLUSION

EGP was severely hampered in both patients, resulting in hypoglycemia. However, despite defective hepatic and renal GNG in both disorders and defective hepatic GGL in GSD-1a, both patients were still able to produce glucose via both pathways. As all necessary enzymes of these pathways have now been functionally detected in muscle, a contribution of muscle to EGP during fasting via both GGL as well as GNG is suggested.

Glycogen storage disease type I in Tunisia: an epidemiological analysis

OBJECTIVE

Analysis of epidemiological data concerning GSD I in Tunisia.

SUBJECTS AND METHODS

All the cases diagnosed as GSD I between 1992 and 2005 in a paediatric department recruiting all the metabolic diseases referred from the North of Tunisia were reviewed. Individual data (sex, socioeconomic and educational background, geographic origins, insurance coverage) were collected and pedigrees were reconstituted.

RESULTS

Twenty-two cases (9 boys and 13 girls from 20 homes) were identified. Fourteen belonged to 11 families originating from the North of Tunisia; ten of them are still alive. Both parents in 4 homes (21%) and one parent in 9 homes (47%) were illiterate. Most of the homes (60%) had a low income and 45% comprised at least 3 children. Only 7 homes (35%) had health insurance. Pedigrees indicated 44 infant deaths and at least 10 other cases fulfilling the clinical features of GSD I but not diagnosed.

CONCLUSION

The paediatric prevalence of GSD I in the North of Tunisia can be estimated to 7.93 cases per one million inhabitants and its incidence to 1/100,000 births. However, it is likely to be more frequent because of underreporting or underdiagnosis leading to precocious deaths.

Mutations in the glucose-6-phosphatase-alpha (G6PC) gene that cause type Ia glycogen storage disease

Glucose-6-phosphatase-alpha (G6PC) is a key enzyme in glucose homeostasis that catalyzes the hydrolysis of glucose-6-phosphate to glucose and phosphate in the terminal step of gluconeogenesis and glycogenolysis. Mutations in the G6PC gene, located on chromosome 17q21, result in glycogen storage disease type Ia (GSD-Ia), an autosomal recessive metabolic disorder. GSD-Ia patients manifest a disturbed glucose homeostasis, characterized by fasting hypoglycemia, hepatomegaly, nephromegaly, hyperlipidemia, hyperuricemia, lactic acidemia, and growth retardation. G6PC is a highly hydrophobic glycoprotein, anchored in the membrane of the endoplasmic reticulum with the active center facing into the lumen. To date, 54 missense, 10 nonsense, 17 insertion/deletion, and three splicing mutations in the G6PC gene have been identified in more than 550 patients. Of these, 50 missense, two nonsense, and two insertion/deletion mutations have been functionally characterized for their effects on enzymatic activity and stability. While GSD-Ia is not more prevalent in any ethnic group, mutations unique to Caucasian, Oriental, and Jewish populations have been described. Despite this, GSD-Ia patients exhibit phenotypic heterogeneity and a stringent genotype-phenotype relationship does not exist.

Evidence for high-capacity bidirectional glucose transport across the endoplasmic reticulum membrane by genetically encoded fluorescence resonance energy transfer nanosensors

Glucose release from hepatocytes is important for maintenance of blood glucose levels. Glucose-6-phosphate phosphatase, catalyzing the final metabolic step of gluconeogenesis, faces the endoplasmic reticulum (ER) lumen. Thus, glucose produced in the ER has to be either exported from the ER into the cytosol before release into circulation or exported directly by a vesicular pathway. To measure ER transport of glucose, fluorescence resonance energy transfer-based nanosensors were targeted to the cytosol or the ER lumen of HepG2 cells. During perfusion with 5 mM glucose, cytosolic levels were maintained at approximately 80% of the external supply, indicating that plasma membrane transport exceeded the rate of glucose phosphorylation. Glucose levels and kinetics inside the ER were indistinguishable from cytosolic levels, suggesting rapid bidirectional glucose transport across the ER membrane. A dynamic model incorporating rapid bidirectional ER transport yields a very good fit with the observed kinetics. Plasma membrane and ER membrane glucose transport differed regarding sensitivity to cytochalasin B and showed different relative kinetics for galactose uptake and release, suggesting catalysis by distinct activities at the two membranes. The presence of a high-capacity glucose transport system on the ER membrane is consistent with the hypothesis that glucose export from hepatocytes occurs via the cytosol by a yet-to-be-identified set of proteins.

Mutation frequencies for glycogen storage disease Ia in the Ashkenazi Jewish population

Glycogen storage disease type Ia (GSDIa) is a severe autosomal recessive disorder caused by deficiency of the enzyme D-glucose-6-phosphatase (G6Pase). While numerous mutations have been found in cosmopolitan European populations, Ashkenazi Jewish (AJ) patients appear to primarily carry the R83C mutation, but possibly also the Q347X mutation found generally in Caucasians. To determine the frequency for both these mutations in the AJ population, we tested 20,719 AJ subjects for the R83C mutation and 4,290 subjects for the Q347X mutation. We also evaluated the mutation status of 30 AJ GSDIa affected subjects. From the carrier screening, we found 290 subjects with R83C, for a carrier frequency for this mutation of 1.4%. This carrier frequency translates into a predicted disease prevalence of 1 in 20,000, five times higher than for the general Caucasian population, confirming a founder effect and elevated frequency of GSDIa in the AJ population. We observed no carriers of the Q347X mutation. Among the 30 GSDIa affected AJ subjects, all were homozygous for R83C. These results indicate that R83C is the only prevalent mutation for GSDIa in the Ashkenazi population.

Mutation spectrum of type I glycogen storage disease in Hungary

We performed mutation analysis in 12 Hungarian type I glycogen storage disease (GSD I) patients in order to determine the mutation spectrum. All patients were clinically classified as GSD Ia. Nine patients carried biallelic G6PC mutations (p.Q27fsX35, p.D38V, p.W70X, p.K76N, p.W77R, p.R83C, p.E110Q, p.G222R), with E110Q reported only in Hungary. However, three patients displayed two common G6PT1 (SLC37A4) mutations (p.L348fsX400, p.C183R) which were originally described in association with GSD Inon-a. Review of the literature and our data show that G6PT1 mutations are not associated with neutropenia and related clinical findings in approximately 10% of these cases. Homozygosity for the truncating G6PT1 mutation p.L348fsX400 can be observed with and without neutropenia, indicating that one or more modifiers of the action of G6PT1 exist. Our data are suitable to provide DNA-based and thus noninvasive confirmation of diagnosis in Hungarian patients with this disorder.

Mutation spectrum of the glucose-6-phosphatase gene and its implication in molecular diagnosis of Korean patients with glycogen storage disease type Ia

Glycogen storage disease type Ia (GSD Ia; MIM 232200) is an autosomal recessive inherited metabolic disorder resulting from a deficiency of the microsomal glucose-6-phosphatase (G6Pase), the enzyme that catalyzes the terminal step in gluconeogenesis and glycogenolysis. Various mutations in the G6Pase gene (G6PC) have been found in patients with GSD Ia. To elucidate the spectrum of the G6PC gene mutations, 13 unrelated Korean patients with GSD Ia were analyzed. We were able to identify mutant alleles in all patients, including three known mutations (727G > T, G122D, and T255I) and two novel mutations (P178A and Y128X). The frequency of the 727G > T mutation in Korean patients with GSD Ia was 81% (21/26), which was slightly lower than that (86-92%) in Japanese but much higher than that (44.4%) in Taiwan Chinese. Except one, all patients were either homozygous (9/13) or compound heterozygous (3/13) for the 727G > T mutation; the only patient without the 727G > T mutation was a compound heterozygote for the G122D and Y128X mutations. Our findings suggest that a DNA-based test can be used as the initial diagnostic approach in Korean patients clinically suspected to have GSD Ia, thereby avoiding invasive liver biopsy.

Bone mineral density in children, adolescents and adults with glycogen storage disease type Ia: a cross-sectional and longitudinal study

The occurrence of (symptoms related to) osteopenia is a known complication in glycogen storage disease type Ia (GSD Ia) patients. However, only limited information is available about bone mineral density (BMD). Using dual energy x-ray absorptiometry, we studied both cross-sectional and longitudinal lumbar spine areal BMD (BMD(areal) in g/cm2), areal BMD corrected for delayed bone maturation (BMD(bone age) in g/cm2), and volumetric BMD (BMD(vol) in g/cm3). Prepubertal GSD Ia patients (n = 8) had normal BMD (median z-scores BMD(areal) -0.6, BMD(bone age) -0.5 and BMD(vol) -0.5), whereas adolescent patients (n = 12) and adult patients (n = 9) had significantly reduced BMD (BMD(areal) -2.3, BMD(bone age) -1.6, BMD(vol) -2.0, and BMD(areal) -1.9, BMD(vol) -1.5, respectively). Our longitudinal study, showing a stable BMD(areal) but a trend to a decrease in BMD(vol) in prepubertal patients during follow-up, did not clarify whether the difference in BMD between prepubertal and adolescent/adult patients reflects a diminished accretion of BMD during childhood or reflects historical differences in treatment. In adolescent and adult GSD Ia patients, BMD(areal) and BMD(vol) were reduced but stable during follow-up. Especially patients with delayed bone maturation were at risk for reduced BMD. No correlation between parameters of metabolic control and BMD could be detected. Daily calcium intake was within recommended allowances ranges. Abnormal biochemical results included hypomagnesaemia (29%), hypercalciuria (34%) and reduced tubular resorption of phosphate (21%). Although the underlying pathophysiology of reduced BMD in GSD Ia remains unsolved, metabolic control should be optimized to correct as much as possible metabolic and endocrine abnormalities that may influence both bone matrix formation and bone mineral accretion.

The biochemistry and molecular biology of the glucose-6-phosphatase system

Progress has continued to be made over the past 4 years in our understanding of the glucose-6-phosphatase (G6Pase) system. The gene for a second component of the system, the putative glucose-6-P transporter (G6PT), was cloned, and mutations in this gene were found in patients diagnosed with glycogen storage disease type 1b. The functional characterization of this putative G6PT has been initiated, and the relationship between substrate transport via the G6PT and catalysis by the system's catalytic subunit continues to be explored. A lively debate over the feasibility of various aspects of the two proposed models of the G6Pase system persists, and the functional/structural relationships of the individual components of the system remain a hot topic of interest in G6Pase research. New evidence supportive of physiologic roles for the biosynthetic functions of the G6Pase system in vivo also has emerged over the past 4 years.

Guidelines for management of glycogen storage disease type I - European Study on Glycogen Storage Disease Type I (ESGSD I)

Life-expectancy in glycogen storage disease type I (GSD I) has improved considerably. Its relative rarity implies that no metabolic centre has experience of large series of patients and experience with long-term management and follow-up at each centre is limited. There is wide variation in methods of dietary and pharmacological treatment. Based on the data of the European Study on Glycogen Storage Disease Type I, discussions within this study group, discussions with the participants of the international SHS-symposium 'Glycogen Storage Disease Type I and II: Recent Developments, Management and Outcome' (Fulda, Germany; 22-25th November 2000) and on data from the literature, guidelines are presented concerning: (1). diagnosis, prenatal diagnosis and carrier detection; (2). (biomedical) targets; (3). recommendations for dietary treatment; (4). recommendations for pharmacological treatment; (5). metabolic derangement/intercurrent infections/emergency treatment/preparation elective surgery; and (6). management of complications (directly) related to metabolic disturbances and complications which may develop with ageing and their follow-up.

CONCLUSION

In this paper guidelines for the management of GSD I are presented.

Glycogen storage disease type I: diagnosis, management, clinical course and outcome. Results of the European Study on Glycogen Storage Disease Type I (ESGSD I)

Glycogen storage disease type I (GSD I) is a relatively rare metabolic disease and therefore, no metabolic centre has experience of large numbers of patients. To document outcome, to develop guidelines about (long-term) management and follow-up, and to develop therapeutic strategies, the collaborative European Study on GSD I (ESGSD I) was initiated. This paper is a descriptive analysis of data obtained from the retrospective part of the ESGSD I. Included were 231 GSD Ia and 57 GSD Ib patients. Median age of data collection was 10.4 years (range 0.4-45.4 years) for Ia and 7.1 years (0.4-30.6 years) for Ib patients. Data on dietary treatment, pharmacological treatment, and outcome including mental development, hyperlipidaemia and its complications, hyperuricaemia and its complications, bleeding tendency, anaemia, osteopenia, hepatomegaly, liver adenomas and carcinomas, progressive renal disease, height and adult height, pubertal development and bone maturation, school type, employment, and pregnancies are presented. Data on neutropenia, neutrophil dysfunction, infections, inflammatory bowel disease, and the use of granulocyte colony-stimulating factor are presented elsewhere (Visser et al. 2000, J Pediatr 137:187-191; Visser et al. 2002, Eur J Pediatr DOI 10.1007/s00431-002-1010-0).

CONCLUSION

there is still wide variation in methods of dietary and pharmacological treatment of glycogen storage disease type I. Intensive dietary treatment will improve, but not correct completely, clinical and biochemical status and fewer patients will die as a direct consequence of acute metabolic derangement. With ageing, more and more complications will develop of which progressive renal disease and the complications related to liver adenomas are likely to be two major causes of morbidity and mortality.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

Molecular genetics of type 1 glycogen storage disease

Glycogen storage disease type 1 (GSD 1) comprises a group of autosomal recessive inherited metabolic disorders caused by deficiency of the microsomal multicomponent glucose-6-phosphatase system. Of the two known transmembrane proteins of the system, malfunction of the catalytic subunit (G6Pase) characterizes GSD 1a. GSD 1 non-a is characterized by defective microsomal glucose-6-phosphate or pyrophosphate/phosphate transport due to mutations in G6PT (glucose-6-phosphate translocase gene) encoding a microsomal transporter protein. Mutations in G6Pase and G6PT account for approximately 80 and approximately 20% of GSD 1 cases, respectively. G6Pase and G6PT work in concert to maintain glucose homeostasis in gluconeogenic organs. Whereas G6Pase is exclusively expressed in gluconeogenic cells, G6PT is ubiquitously expressed and its deficiency generally causes a more severe phenotype. Rapid confirmation of clinically suspected diagnosis of GSD 1, reliable carrier testing, and prenatal diagnosis are facilitated by mutation analyses of the chromosome 11-bound G6PT gene as well as the chromosome 17-bound G6Pase gene.