Glucose-6-phosphatase gene mutations in Taiwan Chinese patients with glycogen storage disease type Ia

Glycogen storage disease type Ib: the first case in Taiwan

Glycogen storage disease (GSD) type Ib is caused by the deficiency of glucose-6-phosphate translocase activity. The elder brother of the proband died at age 20 months, and GSD Ia, a disease caused by the deficiency of glucose-6-phosphatase, was the diagnosis. Theproband developed hypoglycemia shortly after birth. Dietary therapy was instituted immediately, but his growth was poor and there were repeated episodes of pyogenic infection. Neutropenia had been observed since 6 months of age, but the diagnosis of GSD Ib was established only at 18 months of age two mutations (c.354_355insC (p. W118fsX12) and c.736T>C (p.W246R)) were detected on his SLC37A4 gene. Regular administration of G-CSF rapidly improved his health and decreased his hospital stay. Although GSD Ib is very rare in Taiwan, correct diagnosis is essential to save the lives of such patients.

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.

Glucose-6-phosphatase gene mutations in 20 adult Japanese patients with glycogen storage disease type 1a with reference to hepatic tumors

BACKGROUND AND AIMS

A few cases are reported of liver neoplasms observed in patients with glycogen storage disease type 1a (GSD1a). Genetic analysis was carried out in adult Japanese patients with GSD1a and their family members, and hepatic tumors were also investigated in these patients.

METHODS

DNA was extracted from the peripheral blood lymphocytes of 20 adult patients with GSD1a and 21 family members, and mutations were detected based on the differences in the polymerase chain reaction (PCR) products of the glucose-6-phosphatase (G6Pase) gene shown by single-strand conformation polymorphism (SSCP) analysis. Actual mutations were confirmed by direct sequencing. The relationship between the occurrence of liver tumors and the clinical characteristics of the patients was also investigated.

RESULTS

Nineteen of the 20 patients were homozygous for the G727T mutation and one was a compound heterozygote for G727T plus G327A mutations. All of the 19 homozygotes for G727T had hepatomegaly, three had hepatocellular carcinoma, one had cholangiocellular carcinoma, and seven had hepatic adenoma. There were no differences between the tumor and non-tumor groups with respect to laboratory biochemical data (P > 0.05). The mean age of G727T homozygotes with hepatocellular carcinoma was 48.3 years, and that of those with hepatic adenoma was approximately 20 years younger.

CONCLUSION

The G727T mutation seems to be common among Japanese patients with GSD1a, and the discovery of one heterozygote with a combination of G727T and G327A mutations (the latter mutation is common among Chinese) by the use of polymerase chain reaction-single strand conformation polymorphism analysis gave further insight into Japanese ancestry. This is the first study of liver tumors in a large group of adult GSD1a patients with the G727T mutation. As most of the patients in our series are free from other chronic liver diseases such as viral hepatitis, other genetic and/or acquired factors may have influence on the sequel to this metabolic disease.

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.

Molecular genetics of glycogen-storage disease type 1a in Chinese patients of Taiwan

The mutation spectrum of the glucose 6-phosphatase (G6Pase) gene in Chinese patients with type 1a glycogen-storage disease of Taiwan was studied by PCR/RFLP, temporal temperature gradient gel electrophoresis, and direct DNA sequencing methods. In addition to the two most prevalent mutations, 727G --> T (44.4%) and R83H (36.1%), that were detected by RFLP analysis, five other mutations, 341delG, 933insAA, Q104X, I341N, and H119L were identified. The frameshift mutations (341delG and 933insAA) and the nonsense mutation (Q104X) that produce truncated proteins are predicted to be disease-causing. The missense mutation, I341N, occurring in the last transmembrane domain of the ER-bound enzyme, retains a small amount of residual activity of approximately 10%. Except for R83H, the mutations have been described only in Asians. H119L, however, is of particular interest because of the essential role of the catalytic histidine of phosphohydrolase. This amino acid is believed to be involved in the formation of the phosphoryl-enzyme intermediate during catalysis. The patient who was compound heterozygous for 727G --> T and H119L mutations had essentially no G6Pase activity in her liver biopsy. This observation is consistent with the importance of H119L in catalysis.