Molecular Genetics Of Heritable Human Disorders

Child Health And Human Development

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Abstract

Glycogen storage disease type 1 (GSD-1) is a group of autosomal recessive disorders caused by a deficiency in the endoplasmic reticulum (ER)-bound glucose-6-phosphatase (G6Pase) system that is comprised of two integral membrane proteins, glucose-6-phosphate transporter (G6PT) and G6Pase. G6PT translocates G6P, the product of gluconeogenesis and glycogenolysis, from cytoplasm to the lumen of the ER where the active site of G6Pase is positioned. Inside the ER, G6Pase catalyzes the conversion of G6P to glucose and phosphate. Therefore, G6PT and G6Pase work in concert to maintain glucose homeostasis. Deficiencies in G6Pase and G6PT cause GSD-1a and GSD-1b, respectively. Both display symptoms associated with functional G6Pase deficiency, including hypoglycemia, hepatomegaly, kidney enlargement, growth retardation, hyperlipidemia, hyperuricemia, and lactic acidemia. GSD-1b patients also suffer from infectious complications due to chronic neutropenia and functional deficiencies of neutrophils and monocytes. We observed that the clinical manifestations (eg. growth retardation, hepatomegaly, hyperlipidemia, and renal dysfunction) of GSD-1 patients are shared by Hnf1alpha-/- mice deficient of a transcriptional activator, hepatocyte nuclear factor 1 alpha (HNF1alpha). Hnf1alpha-/- mice also develop non-insulin-dependent diabetes mellitus (NIDDM) caused by defective insulin secretion. It has been speculated that over-expression of G6Pase might contribute to the pathophysiology of NIDDM. We therefore sought to determine whether there is a molecular link between HNF1alpha deficiency and function of the G6Pase system. Transactivation studies revealed that HNF1alpha is required for transcription of the G6PT gene. Hepatic G6PT mRNA levels and microsomal G6P transport activity are also markedly reduced in Hnf1alpha-/- mice as compared to Hnf1alpha+/+ and Hnf1alpha+/- littermates. On the other hand, hepatic G6Pase mRNA expression and activity are up-regulated in Hnf1alpha-/- mice, consistent with observations that G6Pase expression is increased in diabetic animals. Taken together, the results strongly suggest that metabolic abnormalities in Hnf1alpha-null mice are caused, in part, by G6PT deficiency and by perturbations of the G6Pase system, establishing, for the first time, a molecular link between the common phenotypes of GSD-1 and Hnf1alpha-/- mice. G6P plays a pivotal role in intermediate metabolism. Changes in G6P levels in cells affect not only glucose metabolism but also glycogen biosynthesis and lipid biosynthesis. Therefore, it is of vital importance to understand the regulation of G6PT gene expression. Here, we delineate the role of glucocorticoids in transcription of the G6PT gene. The glucocorticoids are involved in the regulation of a wide range of physiological process, including glucose metabolism. We show that the basal G6PT promoter is contained within nucleotides -369 to -1 upstream of the translation state site, which contains three activation elements. We demonstrate that glucocorticoids activate G6PT gene transcription and the glucocorticoid action is mediated through a glucocorticoid response element within activation element-2 of the promoter. Taken together, the results suggest that glucocorticoids play a pivotal role in regulating the G6PT gene. To date, 75 G6Pase mutations have been identified in GSD-1a patients on the basis of its absence from the normal population and/or on their co-segregation with the disease phenotype. These include 48 missense, 9 nonsense, 15 insertion/deletion, and 3 splicing mutations. Interestingly, 64% candidate mutations are missense mutations that result in single amino acid substitutions. However, only 19 missense mutations have been functionally characterized. Characterization of these mutations will provide valuable information on functionally important residues of the protein. Using site-directed mutagenesis and transient expression assays, we have characterized all 48 missense mutations. The database of residual activity retained by these mutants will serve as a reference in evaluating genotype-phenotype relationships and the minimal G6Pase activity required to correct the GSD-1a phenotype.