Molecular prenatal diagnosis of glycogen storage disease type Ia

Glycogen metabolism and glycogen storage disorders

Glucose is the main energy fuel for the human brain. Maintenance of glucose homeostasis is therefore, crucial to meet cellular energy demands in both - normal physiological states and during stress or increased demands. Glucose is stored as glycogen primarily in the liver and skeletal muscle with a small amount stored in the brain. Liver glycogen primarily maintains blood glucose levels, while skeletal muscle glycogen is utilized during high-intensity exertion, and brain glycogen is an emergency cerebral energy source. Glycogen and glucose transform into one another through glycogen synthesis and degradation pathways. Thus, enzymatic defects along these pathways are associated with altered glucose metabolism and breakdown leading to hypoglycemia ± hepatomegaly and or liver disease in hepatic forms of glycogen storage disorder (GSD) and skeletal ± cardiac myopathy, depending on the site of the enzyme defects. Overall, defects in glycogen metabolism mainly present as GSDs and are a heterogenous group of inborn errors of carbohydrate metabolism. In this article we review the genetics, epidemiology, clinical and metabolic findings of various types of GSD, and glycolysis defects emphasizing current treatment and implications for future directions.

Glucose-6-phosphatase deficiency

Glucose-6-phosphatase deficiency (G6P deficiency), or glycogen storage disease type I (GSDI), is a group of inherited metabolic diseases, including types Ia and Ib, characterized by poor tolerance to fasting, growth retardation and hepatomegaly resulting from accumulation of glycogen and fat in the liver. Prevalence is unknown and annual incidence is around 1/100,000 births. GSDIa is the more frequent type, representing about 80% of GSDI patients. The disease commonly manifests, between the ages of 3 to 4 months by symptoms of hypoglycemia (tremors, seizures, cyanosis, apnea). Patients have poor tolerance to fasting, marked hepatomegaly, growth retardation (small stature and delayed puberty), generally improved by an appropriate diet, osteopenia and sometimes osteoporosis, full-cheeked round face, enlarged kydneys and platelet dysfunctions leading to frequent epistaxis. In addition, in GSDIb, neutropenia and neutrophil dysfunction are responsible for tendency towards infections, relapsing aphtous gingivostomatitis, and inflammatory bowel disease. Late complications are hepatic (adenomas with rare but possible transformation into hepatocarcinoma) and renal (glomerular hyperfiltration leading to proteinuria and sometimes to renal insufficiency). GSDI is caused by a dysfunction in the G6P system, a key step in the regulation of glycemia. The deficit concerns the catalytic subunit G6P-alpha (type Ia) which is restricted to expression in the liver, kidney and intestine, or the ubiquitously expressed G6P transporter (type Ib). Mutations in the genes G6PC (17q21) and SLC37A4 (11q23) respectively cause GSDIa and Ib. Many mutations have been identified in both genes,. Transmission is autosomal recessive. Diagnosis is based on clinical presentation, on abnormal basal values and absence of hyperglycemic response to glucagon. It can be confirmed by demonstrating a deficient activity of a G6P system component in a liver biopsy. To date, the diagnosis is most commonly confirmed by G6PC (GSDIa) or SLC37A4 (GSDIb) gene analysis, and the indications of liver biopsy to measure G6P activity are getting rarer and rarer. Differential diagnoses include the other GSDs, in particular type III (see this term). However, in GSDIII, glycemia and lactacidemia are high after a meal and low after a fast period (often with a later occurrence than that of type I). Primary liver tumors and Pepper syndrome (hepatic metastases of neuroblastoma) may be evoked but are easily ruled out through clinical and ultrasound data. Antenatal diagnosis is possible through molecular analysis of amniocytes or chorionic villous cells. Pre-implantatory genetic diagnosis may also be discussed. Genetic counseling should be offered to patients and their families. The dietary treatment aims at avoiding hypoglycemia (frequent meals, nocturnal enteral feeding through a nasogastric tube, and later oral addition of uncooked starch) and acidosis (restricted fructose and galactose intake). Liver transplantation, performed on the basis of poor metabolic control and/or hepatocarcinoma, corrects hypoglycemia, but renal involvement may continue to progress and neutropenia is not always corrected in type Ib. Kidney transplantation can be performed in case of severe renal insufficiency. Combined liver-kidney grafts have been performed in a few cases. Prognosis is usually good: late hepatic and renal complications may occur, however, with adapted management, patients have almost normal life span. DISEASE NAME AND SYNONYMS: Glucose-6-phosphatase deficiency or G6P deficiency or glycogen storage disease type I or GSDI or type I glycogenosis or Von Gierke disease or Hepatorenal glycogenosis.

Glycogen storage diseases: New perspectives

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.

Unusual oral manifestations and evolution in glycogen storage disease type Ib

Glycogen storage disease type Ib is a rare inherited metabolic disorder that is caused by a deficiency of glucose-6-phosphate translocase with consequent accumulation of glycogen. The purpose of this study is to report a case affected by glycogen storage disease type Ib in which unusual oral findings were evident and to review the pertinent literature. The disease presents with failure to thrive, hepatomegaly, hypoglycemia, hyperlacticacidemia, neutropenia, and neutrophilic dysfunction causing increased susceptibility to recurrent infections. Common intraoral manifestations are dental caries, gingivitis, periodontal disease, delayed dental maturation and eruption, oral bleeding diathesis, and oral ulcers. Conversely, unusual oral lesions were observed in this case as hyperplastic-hypertrophic gingiva and giant cell granulomatous epulis. The treatment with granulocyte colony-stimulating factor markedly increased the neutrophil counts and reduced the frequency of infections and inflammations. Proper evaluation of the patient's oral condition, a program of preventive measures, and suitable medical consultation are important to minimize and avoid long-term complications.

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.

Metachromatic leukodystrophy: subtype genotype/phenotype correlations and identification of novel missense mutations (P148L and P191T) causing the juvenile-onset disease

Metachromatic leukodystrophy (MLD) is a lysosomal storage disease resulting from the deficient activity of arylsulfatase A (ASA) and the accumulation of sulfatides. The disease is characterized by several subtypes, designated by age at onset: the late-infantile-, juvenile-, and adult-onset variants. Mutation analysis of genomic DNA from a proband with each variant was performed to identify and characterize their causative ASA mutations. Two sisters with the infantile-onset disease were homoallelic for the missense mutation D335V, a juvenile-onset proband was heteroallelic for two novel missense mutations, P148L and P191T, and an adult-onset patient was heteroallelic for the H397Y and P426L mutations. The novel mutations were not identified in 108 normal alleles indicating that these base substitutions were not common polymorphisms. To further characterize the mutant gene products, the mutant enzymes were partially purified from cultured fibroblasts and their molecular weights and charges were compared by immunoblotting following SDS-PAGE or isoelectric focusing (IEF). Normal fibroblast ASA had a single, broad band at 54 kDa. The enzyme from the late-infantile-onset patient had distinct bands of 36 and 78 kDa, but lacked the normal 54-kDa species. The juvenile- and adult-onset patients each had a faint band of 54 kDa and several other bands ranging from 29 to 64 kDa. IEF revealed several bands for the partially purified normal enzyme with a relatively narrow pH range around 4.0, whereas numerous bands with a wider range of isoelectric points were observed with the enzymes from the juvenile- and adult-onset fibroblasts. In contrast, the enzyme from the late-infantile-onset proband had four bands with more acidic isoelectric points, none corresponding to those of the normal enzyme. These results document changes in both size and charge of the mutant enzymes from patients with different mutations and MLD subtypes.

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.

A case of glycogen storage disease type Ia with multiple hepatic adenomas and G727T mutation in the glucose-6-phosphatase gene, and a comparison with other mutations previously reported

We report a case of 23-yr-old man with glycogen storage disease (GSD) type Ia complicated by multiple hepatic adenomas. Analysis of the G-6-Pase gene using peripheral blood sample showed this patient to be homozygous for a G-to-T transversion at nucleotide 727 in exon 5. This mutation is prevalent among Japanese patients, suggesting that specific genotypes may correlate with different clinical courses or outcomes.