Fanconi-Bickel syndrome presenting in neonatal screening for galactosaemia

Glycogen storage diseases

Glycogen storage diseases (GSDs) are a group of rare, monogenic disorders that share a defect in the synthesis or breakdown of glycogen. This Primer describes the multi-organ clinical features of hepatic GSDs and muscle GSDs, in addition to their epidemiology, biochemistry and mechanisms of disease, diagnosis, management, quality of life and future research directions. Some GSDs have available guidelines for diagnosis and management. Diagnostic considerations include phenotypic characterization, biomarkers, imaging, genetic testing, enzyme activity analysis and histology. Management includes surveillance for development of characteristic disease sequelae, avoidance of fasting in several hepatic GSDs, medically prescribed diets, appropriate exercise regimens and emergency letters. Specific therapeutic interventions are available for some diseases, such as enzyme replacement therapy to correct enzyme deficiency in Pompe disease and SGLT2 inhibitors for neutropenia and neutrophil dysfunction in GSD Ib. Progress in diagnosis, management and definitive therapies affects the natural course and hence morbidity and mortality. The natural history of GSDs is still being described. The quality of life of patients with these conditions varies, and standard sets of patient-centred outcomes have not yet been developed. The landscape of novel therapeutics and GSD clinical trials is vast, and emerging research is discussed herein.

Glycogen storage diseases: An update

Glycogen storage diseases (GSDs), also referred to as glycogenoses, are inherited metabolic disorders of glycogen metabolism caused by deficiency of enzymes or transporters involved in the synthesis or degradation of glycogen leading to aberrant storage and/or utilization. The overall estimated GSD incidence is 1 case per 20000-43000 live births. There are over 20 types of GSD including the subtypes. This heterogeneous group of rare diseases represents inborn errors of carbohydrate metabolism and are classified based on the deficient enzyme and affected tissues. GSDs primarily affect liver or muscle or both as glycogen is particularly abundant in these tissues. However, besides liver and skeletal muscle, depending on the affected enzyme and its expression in various tissues, multiorgan involvement including heart, kidney and/or brain may be seen. Although GSDs share similar clinical features to some extent, there is a wide spectrum of clinical phenotypes. Currently, the goal of treatment is to maintain glucose homeostasis by dietary management and the use of uncooked cornstarch. In addition to nutritional interventions, pharmacological treatment, physical and supportive therapies, enzyme replacement therapy (ERT) and organ transplantation are other treatment approaches for both disease manifestations and long-term complications. The lack of a specific therapy for GSDs has prompted efforts to develop new treatment strategies like gene therapy. Since early diagnosis and aggressive treatment are related to better prognosis, physicians should be aware of these conditions and include GSDs in the differential diagnosis of patients with relevant manifestations including fasting hypoglycemia, hepatomegaly, hypertransaminasemia, hyperlipidemia, exercise intolerance, muscle cramps/pain, rhabdomyolysis, and muscle weakness. Here, we aim to provide a comprehensive review of GSDs. This review provides general characteristics of all types of GSDs with a focus on those with liver involvement.

Fanconi-Bickel Syndrome: A Review of the Mechanisms That Lead to Dysglycaemia

Accumulation of glycogen in the kidney and liver is the main feature of Fanconi-Bickel Syndrome (FBS), a rare disorder of carbohydrate metabolism inherited in an autosomal recessive manner due to SLC2A2 gene mutations. Missense, nonsense, frame-shift (fs), in-frame indels, splice site, and compound heterozygous variants have all been identified in SLC2A2 gene of FBS cases. Approximately 144 FBS cases with 70 different SLC2A2 gene variants have been reported so far. SLC2A2 encodes for glucose transporter 2 (GLUT2) a low affinity facilitative transporter of glucose mainly expressed in tissues playing important roles in glucose homeostasis, such as renal tubular cells, enterocytes, pancreatic β-cells, hepatocytes and discrete regions of the brain. Dysfunctional mutations and decreased GLUT2 expression leads to dysglycaemia (fasting hypoglycemia, postprandial hyperglycemia, glucose intolerance, and rarely diabetes mellitus), hepatomegaly, galactose intolerance, rickets, and poor growth. The molecular mechanisms of dysglycaemia in FBS are still not clearly understood. In this review, we discuss the physiological roles of GLUT2 and the pathophysiology of mutants, highlight all of the previously reported SLC2A2 mutations associated with dysglycaemia, and review the potential molecular mechanisms leading to dysglycaemia and diabetes mellitus in FBS patients.

Fanconi Bickel Syndrome with Hypercalciuria due to GLUT 2 Mutation

BACKGROUND

Fanconi Bickel Syndrome is a rare, autosomal recessive, disorder of carbohydrate metabolism. Presence of hypercalciuria is rare.

CASE CHARACTERISTICS

4.5-years-old boy presented with growth failure, hepatomegaly, rickets, fasting hypoglycemia with postprandial hyperglycemia, fanconi syndrome and hypercalciuria.

OUTCOME

A rare mutation in GLUT-2 gene suggestive of Fanconi Bickel Syndrome.

MESSAGE

Fanconi Bickel Syndrome may present with hypercalciuria with proximal renal tubulopathy along with fasting hypoglycemia and postprandial hyperglycemia.

Fanconi-Bickel syndrome as an example of marked allelic heterogeneity

Renal tubular acidosis (RTA) encompasses many renal tubular disorders characterized by hyperchloremic metabolic acidosis with a normal anion gap. Untreated patients usually complain of growth failure, osteoporosis, rickets, nephrolithiasis and eventually renal insufficiency. Fanconi-Bickel syndrome (FBS) is an example of proximal RTA due to a single gene disorder; it is caused by defects in the facilitative glucose transporter 2 gene that codes for the glucose transporter protein 2 expressed in hepatocytes, pancreatic β-cells, enterocytes and renal tubular cells. It is a rare inherited disorder of carbohydrate metabolism manifested by huge hepatomegaly [hence it is classified as glycogen storage disease (GSD) type XI; GSD XI], severe hypophosphatemic rickets and failure to thrive due to proximal renal tubular dysfunction leading to glucosuria, phosphaturia, generalized aminoaciduria, bicarbonate wasting and hypophosphatemia. The disorder has been reported from all parts of Europe, Turkey, Israel, Arabian countries, Japan and North America. Many mutant alleles have been described, its exact frequency is unknown and there is no single mutation found more frequently than the others. The presence of consanguinity in affected families suggests an autosomal recessive pattern of inheritance. New cases of FBS have been recently reported in the Middle and Far East in collaboration with specialized centers. Two novel mutations have been discovered in two unrelated Egyptian families. The first was two bases deletion, guanine and adenine, (c.253_254delGA) causing a frameshift mutation (p. Glu85fs) and the second is mutation in exon6 in splicing acceptor site with intron5 (c.776-1G>C or IVS5-1G>A). Moreover, a new different mutation was described in a 3 year old Indian boy.

Neonatal diabetes: an expanding list of genes allows for improved diagnosis and treatment

There has been major progress in recent years uncovering the genetic causes of diabetes presenting in the first year of life. Twenty genes have been identified to date. The most common causes accounting for the majority of cases are mutations in the genes encoding the two subunits of the ATP-sensitive potassium channel (K(ATP)), KCNJ11 and ABCC8, and the insulin gene (INS), as well as abnormalities in chromosome 6q24. Patients with activating mutations in KCNJ11 and ABCC8 can be treated with oral sulfonylureas in lieu of insulin injections. This compelling example of personalized genetic medicine leading to improved glucose regulation and quality of life may-with continued research-be repeated for other forms of neonatal diabetes in the future.

Nutritional therapy for glycogen storage diseases

Glycogen storage diseases (GSDs) are a group of inherited disorders characterized by enzyme defects that affect the glycogen synthesis and degradation cycle, classified according to the enzyme deficiency and the affected tissue. The understanding of GSD has increased in recent decades, and nutritional management of some GSDs has allowed better control of hypoglycemia and metabolic complications. However, growth failure and liver, renal, and other complications are frequent problems in the long-term outcome. Hypoglycemia is the main biochemical consequence of GSD type I and some of the other GSDs. The basis of dietary therapy is nutritional manipulation to prevent hypoglycemia and improve metabolic dysfunction, with the use of continuous nocturnal intragastric feeding or cornstarch therapy at night and foods rich in starches with low concentrations of galactose and fructose during the day and to prevent hypoglycemia during the night.

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.

Fanconi's syndrome in HIV+ adults: report of three cases and literature review

We diagnosed Fanconi's syndrome (phosphate depletion and dysfunction of the renal tubules) in three HIV(+) patients. This was temporally related to their HIV treatment. Physicians caring for patients with HIV should recognize the association of this rare syndrome with antiretroviral medications and monitor their patients carefully.

INTRODUCTION

Fanconi's syndrome is caused by increased excretion of phosphate, glucose, amino acids, and other intermediary metabolites, and can result in osteomalacia.

MATERIALS AND METHODS

We diagnosed this syndrome in three HIV(+) patients.

RESULTS

The first was a 43-year-old woman referred for multiple painful stress fractures. She demonstrated hypophosphatemia, metabolic acidosis, phosphaturia, glucosuria, and generalized aminoaciduria. These abnormalities resolved with oral phosphate replacement and discontinuation of the antiretroviral medication tenofovir. The second patient was a 39-year-old man with hypophosphatemia and bone pain. His symptoms improved with discontinuation of adefovir and supplementation of phosphate, potassium, and calcitriol. The third patient was a 48-year-old man who presented with symptomatic tetany caused by hypocalcemia (total serum calcium of 6.5 mg/dl [8.5-10.5 mg/dl]). Nine months before presentation, he had been treated with cidofovir for retinitis caused by cytomegalovirus. With calcium, phosphate, potassium, and calcitriol therapy, his laboratory abnormalities improved substantially, although he continues to require daily electrolyte replacement.

CONCLUSIONS

Each patient demonstrated generalized renal tubular dysfunction temporally related to treatment with antiretroviral drugs. The mechanism responsible for these abnormalities is not known; however, physicians caring for patients with HIV disease should recognize the association of Fanconi's syndrome with antiretroviral medications and monitor susceptible patients to prevent potential skeletal and neuromuscular complications.

The Fanconi-Bickel syndrome: a case of neonatal onset

A male newborn infant was recognized having Fanconi-Bickel syndrome (FBS) in the neonatal period. The presenting clinical findings were hyperglycemia and polyuria detected during an episode of acute enteritis. Physical examination was normal, biochemical analyses were suggestive of FBS: glycosuria, proteinuria, phosphaturia, generalized aminoaciduria, and increased levels of urinary beta 2-microglobulin, serum glucose and serum alkaline phosphatase. The molecular genetic analysis showed homozygosity for mutations within the gene of the glucose transporter 2 (Glut 2), 1213 C>T. The patient demonstrated improved clinical and metabolic status following institution of diet with frequent small meals and galactose-free-milk as well as pharmacological treatment with phosphate and vitamin alpha-OH-D3. In conclusion, infants showing hyperglycemia and polyuria may be considered having FBS also in the neonatal period. Early institution of adequate caloric intake and replacement of electrolytes and vitamin D may avoid or reduce metabolic complications.

Intestinal glucose transport: evidence for a membrane traffic-based pathway in humans

BACKGROUND & AIMS

The presence of glucose transporter 2 (GLUT2) molecules in the basolateral membrane of enterocytes has long been considered to be of major importance for intestinal glucose absorption. The aim of this study was to reevaluate the role of GLUT2 in a patient with congenital GLUT2 deficiency (Fanconi-Bickel syndrome, FBS).

METHODS

Oral mono- and disaccharide tolerance tests including gaschromatographic determination of breath hydrogen concentrations were performed in an FBS patient. For comparison, a patient with a microsomal carbohydrate transport defect, glucose-6-phosphate translocase 1 (G6PT1) deficiency, and a control individual were investigated.

RESULTS

No increase in breath hydrogen concentration was found in the GLUT2-deficient patient after a glucose load. In G6PT1 deficiency, basal hydrogen concentrations were repeatedly found to be elevated.

CONCLUSIONS

From the fact that a GLUT2-deficient patient does not show any impairment of intestinal monosaccharide transport measurable by the hydrogen breath test, we conclude that mechanisms other than facilitative glucose transport by GLUT2 must be involved in the transport of monosaccharides at the basolateral membrane of enterocytes. When relating this observation to the high intestinal expression of human hexokinase, G6PT1, and glucose-6-phosphatase and to our results of oral carbohydrate tolerance tests in a G6PT1-deficient patient, there is evidence that a microsomal membrane traffic-based transport pathway, as recently suggested for GLUT2-deficient animals, also plays a major role in transcellular monosaccharide transport of the human intestine.

Identification of a novel mutation in the GLUT2 gene in a patient with Fanconi-Bickel syndrome presenting with neonatal diabetes mellitus and galactosaemia

We describe a patient with Fanconi-Bickel syndrome diagnosed by clinical manifestations and the identification of a novel mutation in the GLUT 2 gene. She was initially diagnosed with neonatal diabetes mellitus due to hyperglycaemia and glycosuria at 3 days of life. In addition, newborn screening for galactosaemia revealed hypergalactosaemia. Thereafter, she was managed with lactose-free milk and insulin therapy. However, she failed to grow and her liver became progressively enlarged. Her liver function deteriorated with increased prothrombin time. A liver biopsy done at age 9 months showed micronodular cirrhosis with marked fatty changes and she succumbed to hepatic failure with pneumonia at 10 months of age. DNA sequencing analysis of the GLUT 2 gene using her genomic DNA revealed a novel mutation in codon 5, lysine5 stop(K5X).

Mutation analysis of the GLUT2 gene in patients with Fanconi-Bickel syndrome

Fanconi-Bickel syndrome (FBS) is an autosomal recessive disorder manifesting hepatorenal glycogen accumulation, Fanconi nephropathy, and impaired utilization of glucose and galactose. Several mutations in a gene encoding a glucose transporter, GLUT2, have recently been reported in patients with FBS. We performed molecular analysis on three Japanese patients and found four novel mutations: a splice-site mutation (IVS2-2A>G), a nonsense mutation (Q287X), and two missense mutations (L389P and V423E). Heterozygotes of L389P or V423E mutation from the patients' families showed renal glucosuria. These data suggested that GLUT2 gene defects may be a cause of renal glucosuria.

Mutations in GLUT2, the gene for the liver-type glucose transporter, in patients with Fanconi-Bickel syndrome

Fanconi-Bickel syndrome (FBS) is a rare autosomal-recessive inborn error of metabolism characterized by hepatorenal glycogen accumulation, Fanconi nephropathy and impaired utilization of glucose and galactose. To date, no underlying enzymatic defect in carbohydrate metabolism has been identified. Therefore, and because of the impairment of both glucose and galactose metabolism, a primary defect of monosaccharide transport across membranes has been suggested. Here we report mutations in the gene encoding the facilitative glucose transporter 2 (GLUT2) in three FBS families, including the original patient described in 1949 by Fanconi and Bickel. Homozygous mutations were found in affected individuals, whereas all parents tested were heterozygous for the respective mutation. Because all detected mutations (delta T446-449, C1251T and C1405T) predict truncated translation products that cannot be expected to have functional monosaccharide transport activity, GLUT2 mutations are probably the cause of FBS.