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.

Glycogen storage diseases (GSD) are inherited metabolic disorders of glycogen metabolism. Different hormones, including insulin, glucagon, and cortisol regulate the relationship of glycolysis, gluconeogenesis and glycogen synthesis. The overall GSD incidence is estimated 1 case per 20000-43000 live births. There are over 12 types and they are classified based on the enzyme deficiency and the affected tissue. Disorders of glycogen degradation may affect primarily the liver, the muscle, or both. Type Ia involves the liver, kidney and intestine (and Ib also leukocytes), and the clinical manifestations are hepatomegaly, failure to thrive, hypoglycemia, hyperlactatemia, hyperuricemia and hyperlipidemia. Type IIIa involves both the liver and muscle, and IIIb solely the liver. The liver symptoms generally improve with age. Type IV usually presents in the first year of life, with hepatomegaly and growth retardation. The disease in general is progressive to cirrhosis. Type VI and IX are a heterogeneous group of diseases caused by a deficiency of the liver phosphorylase and phosphorylase kinase system. There is no hyperuricemia or hyperlactatemia. Type XI is characterized by hepatic glycogenosis and renal Fanconi syndrome. Type II is a prototype of inborn lysosomal storage diseases and involves many organs but primarily the muscle. Types V and VII involve only the muscle.

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.

Glucose-6-phosphatase (G6Pase), a key enzyme in glucose homeostasis, is anchored to the endoplasmic reticulum by nine transmembrane helices. The amino acids comprising the catalytic center of G6Pase include Lys(76), Arg(83), His(119), Arg(170), and His(176). During catalysis, a His residue in G6Pase becomes phosphorylated generating an enzyme-phosphate intermediate. It was predicted that His(176) would be the amino acid that acts as a nucleophile forming a phosphohistidine-enzyme intermediate, and His(119) would be the amino acid that provides the proton needed to liberate the glucose moiety. However, the phosphate acceptor in G6Pase has eluded molecular characterization. To identify the His residue that covalently bound the phosphate moiety, we generated recombinant adenoviruses carrying G6Pase wild type and active site mutants. A 40-kDa [(32)P]phosphate-G6Pase intermediate was identified after incubating [(32)P]glucose 6-phosphate with microsomes expressing wild type but not with microsomes expressing either H119A or H176A mutant G6Pase. Human G6Pase contains five methionine residues at positions 1, 5, 121, 130, and 279. After cyanogen bromide cleavage, His(119) is predicted to be within a 116-amino acid peptide of 13.5 kDa with an isoelectric point of 5.3 (residues 6-121), and His(176) is predicted to be within a 149-amino acid peptide of 16.8 kDa with an isoelectric point of 9.3 (residues 131-279). We show that after digestion of a non-glycosylated [(32)P]phosphate-G6Pase intermediate by cyanogen bromide, the [(32)P]phosphate remains bound to a peptide of 17 kDa with an isoelectric point above 9, demonstrating that His(176) is the phosphate acceptor in G6Pase.

Glycogen storage disease type 1a is caused by a deficiency in glucose-6-phosphatase (G6Pase), a nine-helical endoplasmic reticulum transmembrane protein required for maintenance of glucose homeostasis. To date, 75 G6Pase mutations have been identified, including 48 mutations resulting in single-amino acid substitutions. However, only 19 missense mutations have been functionally characterized. Here, we report the results of structure and function studies of the 48 missense mutations and the DeltaF327 codon deletion mutation, grouped as active site, helical, and nonhelical mutations. The 5 active site mutations and 22 of the 31 helical mutations completely abolished G6Pase activity, but only 5 of the 13 nonhelical mutants were devoid of activity. Whereas the active site and nonhelical mutants supported the synthesis of G6Pase protein in a manner similar to that of the wild-type enzyme, immunoblot analysis showed that the majority (64.5%) of helical mutations destabilized G6Pase. Furthermore, we show that degradation of both wild-type and mutant G6Pase is inhibited by lactacystin, a potent proteasome inhibitor. Taken together, we have generated a data base of residual G6Pase activity retained by G6Pase mutants, established the critical roles of transmembrane helices in the stability and activity of this phosphatase, and shown that G6Pase is a substrate for proteasome-mediated degradation.

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.

The aim of this study was to determine the relative frequency of type Ia in glycogen storage disease (GSD) with prominent liver involvement and to determine its clinical and laboratory findings and prognosis in Turkish children. From 1980 to 1998, 45 out of 100 GSD patients (27 male) with liver involvement had been diagnosed for type Ia. The files were retrospectively evaluated and clinical and laboratory features were documented. In addition to routine laboratory evaluations, urine albumin, calcium excretions, and plasma biotinidase activity were measured. Breast-feeding was continued in all infants. After 6 months of age, uncooked cornstarch was administered to the patients. The relative frequency of type Ia in GSD with liver involvement was 45%. The diagnosis was made in 71% of patients before 2 years of age (median 1 year). Main complaint was abdominal protruding (57.8%), and main physical finding was hepatomegaly (100%). Forty percent of the patients had growth retardation at diagnosis. Among laboratory parameters, hypertriglyceridemia (97.8%) and hypertransaminasemia (95.6%) were the most frequent findings following plasma biotinidase activity, which was elevated in all patients. Microalbuminuria was determined in 52.8% of the patients and hypercalciuria in 23.8%. Histopathological findings of the liver included fibrosis (75.6%), steatosis (37.8%), mosaicism (24.4%) and nuclear hyperglycogenation (15.6%). During follow-up period, the ratio of patients with growth retardation did not change. Transaminases were decreased in 48.7% of the patients. Although triglyceride and cholesterol levels decreased in the majority of the patients, they did not normalise. The prevalence of type Ia in GSD with prominent liver involvement was found higher than the other reports. Microalbuminuria was also higher than the previous reports.