Glycogen storage disease type 1a in Israel: biochemical, clinical, and mutational studies

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.

A c.3216_3217delGA mutation in AGL gene in Tunisian patients with a glycogen storage disease type III: evidence of a founder effect

Glycogen storage disease type III (GSD III) is an autosomal recessive disorder characterized by excessive accumulation of abnormal glycogen in the liver and muscles and caused by deficiency in the glycogen debranching enzyme, the amylo-1,6-glucosidase (AGL). In this study, we report the clinical, biochemical and genotyping features of five unrelated GSD III patients coming from the same region in Tunisia. The concentration of erythrocyte glycogen and AGL activity were measured by colorimetric and fluorimetric methods, respectively. Four CA/TG microsatellite markers flanking the AGL gene in chromosome 1 were amplified with fluoresceinated primers. The full coding exons and their relevant exon-intron boundaries of the AGL gene were directly sequenced for the patients and their parents. All patients showed a striking increase of erythrocytes glycogen content. No AGL activity was detected in peripheral leukocytes. Sequencing of the AGL gene identified a c.3216_3217delGA (p.Glu1072AspfsX36) mutation in the five patients which leads to a premature termination, abolishing the AGL activity. Haplotype analysis showed that the mutation was associated with a common homozygote haplotype. Our results suggested the existence of a founder effect responsible for GSD III in this region of Tunisia.

Ashkenazi Jewish screening in the twenty-first century

Ashkenazi Jewish genetic screening has expanded significantly in the past 4 decades. Individuals of Eastern European (Ashkenazi) Jewish (AJ) descent are at increased risk of having offspring with particular genetic diseases that have significant morbidity and mortality. In addition, there are some disorders, such as cystic fibrosis, for which northern European Caucasians are at comparable risk with those of an AJ background. Carrier screening for many of these Jewish genetic disorders has become standard of care. As technology advances, so does the number of disorders for which screening is available. Thus, we need to continue to be cognizant of informed consent, test sensitivity, confidentiality, prenatal diagnosis, preimplantation genetic screening, and public health concerns regarding testing.

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.

Comprehensive arrayed primer extension array for the detection of 59 sequence variants in 15 conditions prevalent among the (Ashkenazi) Jewish population

In the Ashkenazi Jewish population, serious and lethal genetic conditions occur with relatively high frequency. A single test that encompasses the majority of population-specific mutations is not currently available. For comprehensive carrier screening and molecular diagnostic purposes, we developed a population-specific and inclusive microarray. The arrayed primer extension genotyping microarray carries 59 sequence variant detection sites, of which 53 are detectable bi-directionally. These sites represent the most common variants in Tay-Sachs disease, Bloom syndrome, Canavan disease, Niemann-Pick A, familial dysautonomia, torsion dystonia, mucolipidosis type IV, Fanconi anemia, Gaucher disease, factor XI deficiency, glycogen storage disease type 1a, maple syrup urine disease, nonsyndromic sensorineural hearing loss, familial Mediterranean fever, and glycogen storage disease type III. Several mutations in the selected disorders that are not prevalent per se in the Ashkenazi Jewish populations, as well pseudodeficiency alleles, are also included in the array. The initial technical evaluation of this microarray demonstrates that it is comprehensive, robust, sensitive, specific, and easily modifiable. This cost-effective array is based on a diversely applied platform technology and is suitable for both carrier screening and disease detection in Ashkenazi and Sephardic Jewish populations.

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.

Molecular basis of autosomal recessive diseases among the Palestinian Arabs

In the review of the literature, 71 different autosomal recessive diseases have been delineated that are relatively frequent among Palestinian Arabs. Among those, in 40 the mutation(s) responsible for the diseases are known. Fourteen of these disorders were caused by a single mutation, while the other 26 were due to multiple mutations. Most of the mutations were found in homozygosity among the affected patients. It is probable that the high frequency of most of the genetic diseases among the Palestinian Arabs is due to a founder effect as the result of the high consanguinity rates in this population. However, in some cases the high frequency was demonstrated to be secondary to the presence of multiple mutations, either allelic or in different genes in a small geographic region. This phenomenon remains unexplained but may be secondary to a selective advantage to the carriers, either specific to the region or to the population.

The molecular basis of glycogen storage disease type 1a: structure and function analysis of mutations in glucose-6-phosphatase

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.

Continuous glucose monitoring in children with glycogen storage disease type I

Glycogen storage disease type I (GSD I) is characterized by impaired production of glucose from glycogenolysis and gluconeogenesis resulting in severe fasting hypoglycaemia. The aim of the present study was to examine the efficacy of a continuous subcutaneous glucose monitoring system (CGMS MiniMed), to determine the magnitude and significance of hypoglycaemia in GSD I and to evaluate the efficacy of its dietary treatment. Four children with GSD I were studied over a 72-h period. Results indicated that the values recorded with continuous subcutaneous glucose monitoring were highly correlated with paired blood glucose values measured by glucometer. Significant periods of asymptomatic hypoglycaemia were noted, especially during night-time. The study suggests that repeated continuous subcutaneous glucose monitoring may serve as a useful tool for the assessment of the long-term management of GSD I patients.

A genetic profile of contemporary Jewish populations

The Jews are an ancient people with a history spanning several millennia. Genetic studies over the past 50 years have shed light on Jewish origins, the relatedness of Jewish communities and the genetic basis of Mendelian disorders among Jewish peoples. In turn, these observations have been used to develop genetic testing programmes and, more recently, to attempt to discover new genes for susceptibility to common diseases.

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.

New lessons in the regulation of glucose metabolism taught by the glucose 6-phosphatase system

The operation of glucose 6-phosphatase (EC 3.1.3.9) (Glc6Pase) stems from the interaction of at least two highly hydrophobic proteins embedded in the ER membrane, a heavily glycosylated catalytic subunit of m 36 kDa (P36) and a 46-kDa putative glucose 6-phosphate (Glc6P) translocase (P46). Topology studies of P36 and P46 predict, respectively, nine and ten transmembrane domains with the N-terminal end of P36 oriented towards the lumen of the ER and both termini of P46 oriented towards the cytoplasm. P36 gene expression is increased by glucose, fructose 2,6-bisphosphate (Fru-2,6-P2) and free fatty acids, as well as by glucocorticoids and cyclic AMP; the latter are counteracted by insulin. P46 gene expression is affected by glucose, insulin and cyclic AMP in a manner similar to P36. Accordingly, several response elements for glucocorticoids, cyclic AMP and insulin regulated by hepatocyte nuclear factors were found in the Glc6Pase promoter. Mutations in P36 and P46 lead to glycogen storage disease (GSD) type-1a and type-1 non a (formerly 1b and 1c), respectively. Adenovirus-mediated overexpression of P36 in hepatocytes and in vivo impairs glycogen metabolism and glycolysis and increases glucose production; P36 overexpression in INS-1 cells results in decreased glycolysis and glucose-induced insulin secretion. The nature of the interaction between P36 and P46 in controling Glc6Pase activity remains to be defined. The latter might also have functions other than Glc6P transport that are related to Glc6P metabolism.

Glycogen storage diseases. Phenotypic, genetic, and biochemical characteristics, and therapy

The glycogen storage diseases are caused by inherited deficiencies of enzymes that regulate the synthesis or degradation of glycogen. In the past decade, considerable progress has been made in identifying the precise genetic abnormalities that cause the specific impairments of enzyme function. Likewise, improved understanding of the pathophysiologic derangements resulting from individual enzyme defects has led to the development of effective nutritional therapies for each of these disorders. Meticulous adherence to dietary therapy prevents hypoglycemia, ameliorates the biochemical abnormalities, decreases the size of the liver, and results in normal or nearly normal physical growth and development. Nevertheless, serious long-term complications, including nephropathy that can cause renal failure and hepatic adenomata that can become malignant, are a major concern in GSD-I. In GSD-III, the risk for hypoglycemia diminishes with age, and the liver decreases in size during puberty. Cirrhosis develops in some adult patients, and progressive myopathy and cardiomyopathy occur in patients with absent GDE activity in muscle. It remains unclear whether these complications of glycogen storage disease can be prevented by dietary therapy. Glycogen storage diseases caused by lack of phosphorylase activity are milder disorders with a good prognosis. The liver decreases in size, and biochemical abnormalities disappear by puberty.

A convenient diagnostic function test of peripheral blood neutrophils in glycogen storage disease type Ib

Neutrophils from patients suffering from glycogen storage disease type Ib (GSD-Ib) show several defects. one of which is a decreased rate of glucose utilization. In this study, we established experimental conditions to show the stimulation of the neutrophil respiratory burst by extracellular glucose. With phorbol-myristate-acetate as stimulus of the burst, the activity of the NADPH oxidase in GSD-Ib neutrophils hardly increased on addition of glucose. In control and GSD-type Ia neutrophils, a clear increase was observed. The lack of response to extracellular glucose in GSD-Ib neutrophils is correlated with the inability to raise intracellular glucose-6-P levels on glucose addition, thereby limiting the activity of the generation of NADPH in the hexose-monophosphate shunt. Our study shows the usefulness of this test for the diagnosis of neutrophil function abnormality in GSD-Ib patients.

The molecular background of glycogen metabolism disorders

The molecular pathology of classical glycogen storage disorders, glycogen synthase deficiency and Fanconi-Bickel syndrome is reviewed. The isolation of the respective cDNAs, the chromosomal localization of the genes and the elucidation of the genomic organization enabled mutation analysis in most disorders. The findings have shed light on the multi-protein structure of the glucose-6-phosphatase system, the phosphorylase kinase enzymatic complex and the molecular background of the differential tissue expression in debranching enzyme deficiency. The immediate practical benefit of these studies is our extending ability to predict the outcome of clinical variants and to offer genetic counseling to most families. The elucidation of the tertiary structure of these proteins and their structure-function relationship poses major challenges for the future.

Glycogen storage disease type 1a in three siblings with the G270V mutation

Glycogen storage disease type 1a (von Gierke disease, GSD1a) is caused by the deficiency of microsomal glucose-6-phosphatase (G6Pase) activity. The cloning of G6Pase cDNA and characterization of the human G6Pase gene enabled the identification of the mutations causing GSD1a. Here we report on the clinical and biochemical features of three GSD1a siblings of a Muslin Arab family with a G270V mutation. Two older patients presented with an unusually mild clinical and biochemical course.

Molecular Genetics of Type 1 Glycogen Storage Diseases

Glycogen storage disease type 1 (GSD-1), also known as von Gierke disease, is caused by a deficiency in the activity of the enzyme glucose-6-phosphatase (G6Pase). It is an autosomal recessive disorder characterized by hypoglycemia, hepatomegaly, kidney enlargement, growth retardation, lactic acidemia, hyperlipidemia and hyperuricemia. The disease presents with both clinical and biochemical heterogeneity consistent with the existence of two major subgroups, GSD-1a and GSD-1b, which have been confirmed at the molecular genetic level. GSD-1a, the most prevalent form, is caused by mutations in the G6Pase gene that abolish or greatly reduce enzymatic activity. The gene maps to chromosome 17q21 and encodes a microsomal transmembrane protein. Animal models of GSD-1a exist and are being exploited to delineate the disease more precisely. It has been proposed that GSD-1b is caused by a defect in the microsomal glucose-6-phosphate transporter. The gene responsible for GSD-1b has been mapped to chromosome 11q23 and a cDNA encoding a microsomal transmembrane protein has been identified. The function of this putative GSD-1b protein remains to be determined. These recent developments, along with newly characterized animal models of GSD-1a, are increasing our understanding of the interrelationship between the components of the G6Pase complex and type 1 glycogen storage diseases.

Mutations in the glucose-6-phosphatase gene of 53 Italian patients with glycogen storage disease type Ia

Type Ia glycogen storage disease (GSD1a) is an autosomal recessive metabolic disorder caused by a deficiency in glucose-6-phosphatase (G6Pase). Recent cloning of the G6Pase gene and the subsequent identification of several disease-causing mutations have shown an ethnic molecular heterogeneity. Using SSCP analysis and DNA sequencing, we characterized the G6Pase gene of 53 unrelated Italian patients. The two most common mutations, R83C and Q347X, accounted for 66.9% of the mutant alleles. Eight novel mutations and three rare mutations were identified in 15.7% of disease alleles. These results suggest that a DNA-based method can be used as an initial screening in Italian patients clinically suspected of having GSD1a, avoiding liver biopsy for enzymatic diagnosis. In particular, a noninvasive diagnosis is a suitable method for the Italian subpopulation coming from Sicily, where the R83C mutation is present in 80% of mutant alleles. Molecular carrier detection and prenatal diagnosis can be provided to GSD1a families with identified mutation in the propositus.

Asparagine-linked oligosaccharides are localized to a luminal hydrophilic loop in human glucose-6-phosphatase

Deficiency of glucose-6-phosphatase (G6Pase), an endoplasmic reticulum transmembrane glycoprotein, causes glycogen storage disease type 1a. We have recently shown that human G6Pase contains an odd number of transmembrane segments, supporting a nine-transmembrane helical model for this enzyme. Sequence analysis predicts the presence of three potential asparagine (N)-linked glycosylation sites, N96TS, N203AS, and N276SS, conserved among mammalian G6Pases. According to this model, Asn96, located in a 37-residue luminal loop, is a potential acceptor for oligosaccharides, whereas Asn203 and Asn276, located in a 12-residue cytoplasmic loop and helix 7, respectively, would not be utilized for this purpose. We therefore characterized mutant G6Pases lacking one, two, or all three potential N-linked glycosylation sites. Western blot and in vitro translation studies showed that G6Pase is glycosylated only at Asn96, further validating the nine-transmembrane topology model. Substituting Asn96 with an Ala (N96A) moderately reduced enzymatic activity and had no effect on G6Pase synthesis or degradation, suggesting that oligosaccharide chains do not play a major role in protecting the enzyme from proteolytic degradation. In contrast, mutation of Asn276 to an Ala (N276A) destabilized the enzyme and markedly reduced enzymatic activity. We present additional evidence suggesting that the integrity of transmembrane helices is essential for G6Pase stability and catalytic activity.

Glucose-6-phosphatase gene (727G-->T) splicing mutation is prevalent in Hong Kong Chinese patients with glycogen storage disease type 1a

Glycogen storage disease type la (GSD1a) is an autosomal recessive metabolic disorder caused by a deficiency in glucose-6-phosphatase (G6Pase). We analyzed the G6Pase genes of two unrelated Chinese families with GSD1a. DNA sequencing of all five exons and the exon-intron boundaries revealed a G T transversion at nucleotide 727 (727G-->T) in exon 5, which has previously been reported to cause abnormal splicing. In one family, the subject and her affected sister were confirmed to be homozygous for this mutation and their parents to be heterozygotes. In the other family, the proband was identified to be heterozygous for this mutation, and a novel mutation, the 341delG in exon 2, was identified. This mutation alters the reading frame and creates a stop codon TAA 15 codons downstream from the mutation, resulting in a truncated protein. Family studies revealed that the father was heterozygous for the 727G-->T mutation and that the mother was heterozygous for the 341delG mutation. This is the first time that the 727G T mutation has been found in Chinese patients or outside Japan. Since we only tested two GSD1a families and found 727G-->T in both, we believe that this mutation may also be prevalent in our local Chinese population. To investigate allele frequencies, we screened 385 Chinese healthy volunteers and found two asymptomatic carriers. Our findings suggest that the 727G-->T mutation is indeed prevalent in Hong Kong.