Janice Y. Chou

Glycogen storage disease type I and G6Pase-β deficiency: etiology and therapy

Glycogen storage disease type I (GSD-I) consists of two subtypes: GSD-Ia, a deficiency in glucose-6-phosphatase-α (G6Pase-α) and GSD-Ib, which is characterized by an absence of a glucose-6-phosphate (G6P) transporter (G6PT). A third disorder, G6Pase-β deficiency, shares similarities with this group of diseases. G6Pase-α and G6Pase-β are G6P hydrolases in the membrane of the endoplasmic reticulum, which depend on G6PT to transport G6P from the cytoplasm into the lumen. A functional complex of G6PT and G6Pase-α maintains interprandial glucose homeostasis, whereas G6PT and G6Pase-β act in conjunction to maintain neutrophil function and homeostasis. Patients with GSD-Ia and those with GSD-Ib exhibit a common metabolic phenotype of disturbed glucose homeostasis that is not evident in patients with G6Pase-β deficiency. Patients with a deficiency in G6PT and those lacking G6Pase-β display a common myeloid phenotype that is not shared by patients with GSD-Ia. Previous studies have shown that neutrophils express the complex of G6PT and G6Pase-β to produce endogenous glucose. Inactivation of either G6PT or G6Pase-β increases neutrophil apoptosis, which underlies, at least in part, neutrophil loss (neutropenia) and dysfunction in GSD-Ib and G6Pase-β deficiency. Dietary and/or granulocyte colony-stimulating factor therapies are available; however, many aspects of the diseases are still poorly understood. This Review will address the etiology of GSD-Ia, GSD-Ib and G6Pase-β deficiency and highlight advances in diagnosis and new treatment approaches, including gene therapy.

Impaired neutrophil activity and increased susceptibility to bacterial infection in mice lacking glucose-6-phosphatase–β

Neutropenia and neutrophil dysfunction are common in many diseases, although their etiology is often unclear. Previous views held that there was a single ER enzyme, glucose-6-phosphatase-alpha (G6Pase-alpha), whose activity--limited to the liver, kidney, and intestine--was solely responsible for the final stages of gluconeogenesis and glycogenolysis, in which glucose-6-phosphate (G6P) is hydrolyzed to glucose for release to the blood. Recently, we characterized a second G6Pase activity, that of G6Pase-beta (also known as G6PC), which is also capable of hydrolyzing G6P to glucose but is ubiquitously expressed and not implicated in interprandial blood glucose homeostasis. We now report that the absence of G6Pase-beta led to neutropenia; defects in neutrophil respiratory burst, chemotaxis, and calcium flux; and increased susceptibility to bacterial infection. Consistent with this, G6Pase-beta-deficient (G6pc3-/-) mice with experimental peritonitis exhibited increased expression of the glucose-regulated proteins upregulated during ER stress in their neutrophils and bone marrow, and the G6pc3-/- neutrophils exhibited an enhanced rate of apoptosis. Our results define a molecular pathway to neutropenia and neutrophil dysfunction of previously unknown etiology, providing a potential model for the treatment of these conditions.

Mutations in the glucose‐6‐phosphatase‐α (G6PC) gene that cause type Ia glycogen storage disease

Glucose‐6‐phosphatase‐α (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. Hum Mutat 29(7), 921–930, 2008. Published 2008 Wiley‐Liss, Inc.

Long-term correction of murine glycogen storage disease type Ia by recombinant adeno-associated virus-1-mediated gene transfer

Glycogen storage disease type Ia (GSD-Ia) is caused by a deficiency in glucose-6-phosphatase-α (G6Pase-α), a nine-transmembrane domain, endoplasmic reticulum-associated protein expressed primarily in the liver and kidney. Previously, we showed that infusion of an adeno-associated virus (AAV) serotype 2 vector carrying murine G6Pase-α (AAV2-G6Pase-α) into neonatal GSD-Ia mice failed to sustain their life beyond weaning. We now show that neonatal infusion of GSD-Ia mice with an AAV serotype 1-G6Pase-α (AAV1-G6Pase-α) or AAV serotype 8-G6Pase-α (AAV8-G6Pase-α) results in hepatic expression of the G6Pase-α transgene and markedly improves the survival of the mice. However, only AAV1-G6Pase-α can achieve significant renal transgene expression. A more effective strategy, in which a neonatal AAV1-G6Pase-α infusion is followed by a second infusion at age one week, provides sustained expression of a complete, functional, G6Pase-α system in both the liver and kidney and corrects the metabolic abnormalities in GSD-Ia mice for the 57 week length of the study. This effective use of gene therapy to correct metabolic imbalances and disease progression in GSD-Ia mice holds promise for the future of gene therapy in humans.

Severe congenital neutropenia resulting from G6PC3 deficiency with increased neutrophil CXCR4 expression and myelokathexis

Mutations in more than 15 genes are now known to cause severe congenital neutropenia (SCN); however, the pathologic mechanisms of most genetic defects are not fully defined. Deficiency of G6PC3, a glucose-6-phosphatase, causes a rare multisystem syndrome with SCN first described in 2009. We identified a family with 2 children with homozygous G6PC3 G260R mutations, a loss of enzymatic function, and typical syndrome features with the exception that their bone marrow biopsy pathology revealed abundant neutrophils consistent with myelokathexis. This pathologic finding is a hallmark of another type of SCN, WHIM syndrome, which is caused by gain-of-function mutations in CXCR4, a chemokine receptor and known neutrophil bone marrow retention factor. We found markedly increased CXCR4 expression on neutrophils from both our G6PC3-deficient patients and G6pc3(-/-) mice. In both patients, granulocyte colony-stimulating factor treatment normalized CXCR4 expression and neutrophil counts. In G6pc3(-/-) mice, the specific CXCR4 antagonist AMD3100 rapidly reversed neutropenia. Thus, myelokathexis associated with abnormally high neutrophil CXCR4 expression may contribute to neutropenia in G6PC3 deficiency and responds well to granulocyte colony-stimulating factor.

Neutrophil stress and apoptosis underlie myeloid dysfunction in glycogen storage disease type Ib

Glycogen storage disease type Ib (GSD-Ib) is caused by a deficiency in the glucose-6-phosphate (G6P) transporter (G6PT) that works with a liver/kidney/intestine-restricted glucose-6-phosphatase-alpha (G6Pase-alpha) to maintain glucose homeostasis between meals. Clinically, GSD-Ib patients manifest disturbed glucose homeostasis and neutrophil dysfunctions but the cause of the latter is unclear. Neutrophils express the ubiquitously expressed G6PT and G6Pase-beta that together transport G6P into the endoplasmic reticulum (ER) lumen and hydrolyze it to glucose. Because we expected G6PT-deficient neutrophils to be unable to produce endogenous glucose, we hypothesized this would lead to ER stress and increased apoptosis. Using GSD-Ib mice, we showed that GSD-Ib neutrophils exhibited increased production of ER chaperones and oxidative stress, consistent with ER stress, increased annexin V binding and caspase-3 activation, consistent with an increased rate of apoptosis. Bax activation, mitochondrial release of proapoptotic effectors, and caspase-9 activation demonstrated the involvement of the intrinsic mitochondrial pathway in these processes. The results demonstrate that G6P translocation and hydrolysis are required for normal neutrophil functions and support the hypothesis that neutrophil dysfunction in GSD-Ib is due, at least in part, to ER stress and increased apoptosis.

Complete Normalization of Hepatic G6PC Deficiency in Murine Glycogen Storage Disease Type Ia Using Gene Therapy

Glycogen storage disease type Ia (GSD-Ia) patients deficient in glucose-6-phosphatase-alpha (G6Pase-alpha or G6PC) manifest disturbed glucose homeostasis. We examined the efficacy of liver G6Pase-alpha delivery mediated by AAV-GPE, an adeno-associated virus (AAV) serotype 8 vector expressing human G6Pase-alpha directed by the human G6PC promoter/enhancer (GPE), and compared it to AAV-CBA, that directed murine G6Pase-alpha expression using a hybrid chicken beta-actin (CBA) promoter/cytomegalovirus (CMV) enhancer. The AAV-GPE directed hepatic G6Pase-alpha expression in the infused G6pc(-/-) mice declined 12-fold from age 2 to 6 weeks but stabilized at wild-type levels from age 6 to 24 weeks. In contrast, the expression directed by AAV-CBA declined 95-fold over 24 weeks, demonstrating that the GPE is more effective in directing persistent in vivo hepatic transgene expression. We further show that the rapid decline in transgene expression directed by AAV-CBA results from an inflammatory immune response elicited by the AAV-CBA vector. The AAV-GPE-treated G6pc(-/-) mice exhibit normal levels of blood glucose, blood metabolites, hepatic glycogen, and hepatic fat. Moreover, the mice maintained normal blood glucose levels even after 6 hours of fasting. The complete normalization of hepatic G6Pase-alpha deficiency by the G6PC promoter/enhancer holds promise for the future of gene therapy in human GSD-Ia patients.

Neutropenia in type Ib glycogen storage disease

Purpose of reviewGlycogen storage disease type Ib, characterized by disturbed glucose homeostasis, neutropenia, and neutrophil dysfunction, is caused by a deficiency in a ubiquitously expressed glucose-6-phosphate transporter (G6PT). G6PT translocates glucose-6-phosphate (G6P) from the cytoplasm into the lumen of the endoplasmic reticulum, in which it is hydrolyzed to glucose either by a liver/kidney/intestine-restricted glucose-6-phosphatase-α (G6Pase-α) or by a ubiquitously expressed G6Pase-β. The role of the G6PT/G6Pase-α complex is well established and readily explains why G6PT disruptions disturb interprandial blood glucose homeostasis. However, the basis for neutropenia and neutrophil dysfunction in glycogen storage disease type Ib is poorly understood. Recent studies that are now starting to unveil the mechanisms are presented in this review. Recent findingsCharacterization of G6Pase-β and generation of mice lacking either G6PT or G6Pase-β have shown that neutrophils express the G6PT/G6Pase-β complex capable of producing endogenous glucose. Loss of G6PT activity leads to enhanced endoplasmic reticulum stress, oxidative stress, and apoptosis that underlie neutropenia and neutrophil dysfunction in glycogen storage disease type Ib. SummaryNeutrophil function is intimately linked to the regulation of glucose and G6P metabolism by the G6PT/G6Pase-β complex. Understanding the molecular mechanisms that govern energy homeostasis in neutrophils has revealed a previously unrecognized pathway of intracellular G6P metabolism in neutrophils.

The glucose-6-phosphate transporter is a phosphate-linked antiporter deficient in glycogen storage disease type Ib and Ic

Glycogen storage disease type Ib (GSD‐Ib) is caused by deficiencies in the glucose‐6‐phosphate (G6P) transporter (G6PT) that have been well characterized. Interestingly, deleterious mutations in the G6PT gene were identified in clinical cases of GSD type Ic (GSD‐Ic) proposed to be deficient in an inorganic phosphate (Pi) transporter. We hypothesized that G6PT is both the G6P and Pi transporter. Using reconstituted proteoliposomes we show that both G6P and Pi are efficiently taken up into Pi‐loaded G6PT‐proteoliposomes. The G6P uptake activity decreases as the internal:external Pi ratio decreases and the Pi uptake activity decreases in the presence of external G6P. Moreover, G6P or Pi uptake activity is not detectable in Pi‐loaded proteoliposomes containing the p.R28H G6PT null mutant. The G6PT‐proteoliposome‐ mediated G6P or Pi uptake is inhibited by cholorgenic acid and vanadate, both specific G6PT inhibitors. Glucose‐6‐phosphatase‐α (G6Pase‐α), which facilitates microsomal G6P uptake by G6PT, fails to stimulate G6P uptake in Pi‐loaded G6PT‐proteoliposomes, suggesting that the G6Pase‐α‐mediated stimulation is caused by decreasing G6P and increasing Pi concentrations in microsomes. Taken together, our results suggest that G6PT has a dual role as a G6P and a Pi transporter and that GSD‐Ib and GSD‐Ic are deficient in the same G6PT gene.—Chen, S.‐Y., Pan, C.‐J., Nandigama, K., Mansfield, B., Ambudkar, S., Chou, J. The glucose‐6‐phosphate transporter is a phosphate‐linked antiporter deficient in glycogen storage disease type Ib and Ic. FASEB J. 22, 2206–2213 (2008)

Prevention of hepatocellular adenoma and correction of metabolic abnormalities in murine glycogen storage disease type Ia by gene therapy

Glycogen storage disease type Ia (GSD‐Ia), which is characterized by impaired glucose homeostasis and chronic risk of hepatocellular adenoma (HCA), is caused by deficiencies in the endoplasmic reticulum (ER)‐associated glucose‐6‐phosphatase‐α (G6Pase‐α or G6PC) that hydrolyzes glucose‐6‐phosphate (G6P) to glucose. G6Pase‐α activity depends on the G6P transporter (G6PT) that translocates G6P from the cytoplasm into the ER lumen. The functional coupling of G6Pase‐α and G6PT maintains interprandial glucose homeostasis. We have shown previously that gene therapy mediated by AAV‐GPE, an adeno‐associated virus (AAV) vector expressing G6Pase‐α directed by the human G6PC promoter/enhancer (GPE), completely normalizes hepatic G6Pase‐α deficiency in GSD‐Ia (G6pc−/−) mice for at least 24 weeks. However, a recent study showed that within 78 weeks of gene deletion, all mice lacking G6Pase‐α in the liver develop HCA. We now show that gene therapy mediated by AAV‐GPE maintains efficacy for at least 70‐90 weeks for mice expressing more than 3% of wild‐type hepatic G6Pase‐α activity. The treated mice displayed normal hepatic fat storage, had normal blood metabolite and glucose tolerance profiles, had reduced fasting blood insulin levels, maintained normoglycemia over a 24‐hour fast, and had no evidence of hepatic abnormalities. After a 24‐hour fast, hepatic G6PT messenger RNA levels in G6pc−/− mice receiving gene therapy were markedly increased. Because G6PT transport is the rate‐limiting step in microsomal G6P metabolism, this may explain why the treated G6pc−/− mice could sustain prolonged fasts. The low fasting blood insulin levels and lack of hepatic steatosis may explain the absence of HCA. Conclusion: These results confirm that AAV‐GPE–mediated gene transfer corrects hepatic G6Pase‐α deficiency in murine GSD‐Ia and prevents chronic HCA formation. (HEPATOLOGY 2012;56:1719–1729)