Clinical, genetic profile and therapy evaluation of 11 Chinese pediatric patients with Fanconi-Bickel syndrome

BACKGROUND

Fanconi-Bickel syndrome (FBS) is a rare autosomal recessive disorder characterized by impaired glucose and galactose utilization as well as proximal renal tubular dysfunction.

METHODS

Clinical, biochemical, genetic, treatment, and follow-up data for 11 pediatric patients with FBS were retrospectively analysed.

RESULTS

Hepatomegaly (10/11), short stature (10/11) and hypophosphataemic rickets (7/11) were the most common initial symptoms. At diagnosis, all patients had decreased fasting blood glucose (FBG), plasma bicarbonate (HCO3-) and serum phosphorus, as well as elevated liver transaminases, alkaline phosphatase (AKP) and proximal renal tubular dysfunction. Two infant patients were misdiagnosed with transient neonatal diabetes mellitus. After therapy with uncooked cornstarch and conventional rickets treatment, remission of hepatomegaly was observed in all patients, with significant improvements in pre-prandial blood glucose, liver transaminases, triglyceride, plasma HCO3- and AKP (p < 0.05). At the last follow-up, 5/7 patients with elevated AKP had nephrocalcinosis. The mean height standard deviation score (Ht SDS) of eight patients with regular treatment increased from - 4.1 to -3.5 (p = 0.02). Recombinant human growth hormone (rhGH) was administered to 4/9 patients, but their Ht SDS did not improve significantly (p = 0.13). Fourteen variants of the SLC2A2 gene were identified, with six being novel, among which one was recurrent: c.1217T > G (p.L406R) (allele frequency: 4/22, 18%). Patients with biallelic missense variants showed milder metabolic acidosis than those with null variants. Two of five patients from nonconsanguineous families with rare homozygous variations showed 5.3 Mb and 36.6 Mb of homozygosity surrounding the variants, respectively; a region of homozygosity (ROH) involving the entire chromosome 3 covering the SLC2A2 gene, suggesting uniparental disomy 3, was detected in one patient.

CONCLUSIONS

Early diagnosis of FBS is difficult due to the heterogeneity of initial symptoms. Although short stature is a major issue of treatment for FBS, rhGH is not recommended in FBS patients who have normal GH stimulation tests. Patients with biallelic null variants may require alkali supplementation since urine bicarbonate loss is genetically related. ROH is a mechanism for rare homozygous variants of FBS in nonconsanguineous families.

Craniosynostosis in a patient with Fanconi-Bickel syndrome: a case report

OBJECTIVES

Craniosynostosis may be a rare but severe complication of Fanconi-Bickel syndrome (FBS). Both conditions can be associated with feeding intolerance in young children. Prompt recognition and correction of increased intracranial pressure may lead to improved dietary tolerance in FBS patients and decrease morbidity.

CASE PRESENTATION

We present the case of a child with genetically confirmed FBS, severe feeding intolerance and evidence of metabolic bone disease. At two years of age, a diagnosis of multi-sutural craniosynostosis with increased intracranial pressure was made. The patient underwent cranial vault expansion using distraction osteogenesis, after which his feeding intolerance completely resolved.

CONCLUSIONS

This case highlights the importance of monitoring for secondary craniosynostosis in patients with FBS and frequent emesis. Objective markers of bone health may help identify children at highest risk, though the actual mechanism of development is likely multifactorial. Increased awareness of this potential association should prompt more routine screening and improve outcomes.

Case Report: Fanconi-Bickel Syndrome in a Chinese Girl With Diabetes and Severe Hypokalemia

Fanconi-Bickel syndrome (FBS) is a rare autosomal recessive carbohydrate metabolism disorder. The main symptoms of FBS are hepatomegaly, nephropathy, postprandial hyperglycemia, fasting hypoglycemia, and growth retardation. Hypokalemia is a rare clinical feature in patients with FBS. In this study, we present a neonate suffering from FBS. She presented with hypokalemia, dysglycaemia, glycosuria, hepatomegaly, abnormality of liver function, and brain MRI. Trio whole-exome sequencing (WES) and Sanger sequencing were performed to identify the causal gene variants. A compound heterozygous mutation (NM_000340.2; p. Trp420*) of SLC2A2 was identified. Here, we report a patient with FBS in a consanguineous family with diabetes, severe hypokalemia, and other typical FBS symptoms. Patients with common clinical features may be difficult to diagnose just by phenotypes in the early stage of life, but WES could be an important tool. We also discuss the use of insulin in patients with FBS and highlight the importance of a continuous glucose monitoring system (CGMS), not only in diagnosis but also to avoid hypoglycemic events.

Evidence for a Genotype-Phenotype Correlation in Patients with Pathogenic GLUT2 (SLC2A2) Variants

Fanconi-Bickel syndrome (FBS) is a very rare but distinct clinical entity with the combined features of hepatic glycogen storage disease, generalized proximal renal tubular dysfunction with disproportionately severe glucosuria, and impaired galactose tolerance. Here, we report five cases (out of 93 diagnosed in our lab) with pathogenic variants on both GLUT2 (SLC2A2) alleles. They come from 3 families and presented with an exceptionally mild clinical course. This course was correlated to data from old and most recent expression and transport studies in Xenopus oocytes. GLUT2 genotype in patients 1 and 2 was p.[153_4delLI];[P417R] with the first variant exhibiting normal membrane expression and partially retained transport activity (5.8%) for 2-deoxyglucose. In patient 3, the very first GLUT2 variant ever detected (p.V197I) was found, but for the first time it was present in a patient in the homozygous state. This variant had also shown unaffected membrane expression and remarkable residual activity (8%). The genotype in patient 4, p.[153_4delLI];[(E440A)], again included the 2-amino-acid deletion with residual transporter function, and patient 5 is the first found to be homozygous for this variant. Our results provide further evidence for a genotype-phenotype correlation in patients with GLUT2 variants; non-functional variants result in the full picture of FBS while dysfunctional variants may result in milder presentations, even glucosuria only, without other typical signs of FBS.

Nocturnal enteral nutrition is therapeutic for growth failure in Fanconi-Bickel syndrome

Fanconi-Bickel syndrome (FBS) is a rare autosomal recessive disorder characterised by impaired glucose liver homeostasis and proximal renal tubular dysfunction. It is caused by pathogenic variants in SLC2A2 coding for the glucose transporter GLUT2. Main clinical features include hepatomegaly, fasting hypoglycaemia, postprandial hyperglycaemia, Fanconi-type tubulopathy occasionally with rickets, and a severe growth disorder. While treatment for renal tubular dysfunction is well established, data regarding optimal nutritional therapy are scarce. Similarly, detailed clinical evaluation of treated FBS patients is lacking. These unmet needs were an incentive to conduct the present pilot study. We present clinical findings, laboratory parameters and molecular genetic data on 11 FBS patients with emphasis on clinical outcome under various nutritional interventions. At diagnosis, the patients' phenotypic severity could be classified into two categories: a first group with severe growth failure and rickets, and a second group with milder signs and symptoms. Three patients were diagnosed early and treated because of family history. All patients exhibited massive glucosuria at diagnosis and some in both groups had fasting hypoglycaemic episodes. Growth retardation improved drastically in all five patients treated by intensive nutritional intervention (nocturnal enteral nutrition) and uncooked cornstarch with final growth parameters in the normal range. The four severely affected patients who were treated with uncooked cornstarch alone did not catch up growth. All patients received electrolytes and l-carnitine supplementation to compensate for the tubulopathy. This is one of the largest series of FBS on therapeutic management with evidence that nocturnal enteral nutrition rescues growth failure.

Genetic testing of two Pakistani patients affected with rare autosomal recessive Fanconi-Bickel syndrome and identification of a novel SLC2A2 splice site variant

Fanconi-Bickel syndrome (FBS) is a rare autosomal recessive carbohydrate metabolism disorder caused by mutations in SLC2A2 encoding the glucose transporter 2 (GLUT2) protein. The clinical manifestations include hepatomegaly, conditional hypo/hyperglycemia, rickets, short stature and proximal renal tubular dysfunction. GLUT2 regulates monosaccharide homeostasis through sugar sensing and transmembrane transportation during high/low glucose levels. In the current study, we present two siblings suffering from FBS. The patients presented with doll-like facies, failure to gain weight and height, abdominal distension and firm hepatomegaly. The family had a history of deaths of twin male siblings in the neonatal period and twin female siblings at ages 10 months and 2.5 years, respectively. Clinical presentation and biochemical investigations including a complete blood count, electrolytes, liver and renal function tests suggested FBS. Mutation screening of SLC2A2 confirmed the diagnosis with identification of a novel homozygous splice site variant predicting an in-frame deletion [p.(Gly166-S169del)] in the GLUT2 protein. The in-silico analysis predicted the variant to affect the three-dimensional conformation of the fourth transmembrane helix of the encoded protein, rendering the non-functionality of GLUT2 in both patients of the family under study.

Functional and structural analysis of rare SLC2A2 variants associated with Fanconi-Bickel syndrome and metabolic traits

Deleterious variants in SLC2A2 cause Fanconi-Bickel Syndrome (FBS), a glycogen storage disorder, whereas less common variants in SLC2A2 associate with numerous metabolic diseases. Phenotypic heterogeneity in FBS has been observed, but its causes remain unknown. Our goal was to functionally characterize rare SLC2A2 variants found in FBS and metabolic disease-associated variants to understand the impact of these variants on GLUT2 activity and expression and establish genotype-phenotype correlations. Complementary RNA-injected Xenopus laevis oocytes were used to study mutant transporter activity and membrane expression. GLUT2 homology models were constructed for mutation analysis using GLUT1, GLUT3, and XylE as templates. Seventeen FBS variants were characterized. Only c.457_462delCTTATA (p.Leu153_Ile154del) exhibited residual glucose uptake. Functional characterization revealed that only half of the variants were expressed on the plasma membrane. Most less common variants (except c.593 C>A (p.Thr198Lys) and c.1087 G>T (p.Ala363Ser)) exhibited similar GLUT2 transport activity as the wild type. Structural analysis of GLUT2 revealed that variants affect substrate-binding, steric hindrance, or overall transporter structure. The mutant transporter that is associated with a milder FBS phenotype, p.Leu153_Ile154del, retained transport activity. These results improve our overall understanding of the underlying causes of FBS and impact of GLUT2 function on various clinical phenotypes ranging from rare to common disease.

Impaired glucose tolerance in Fanconi-Bickel syndrome: Eight patients with two novel mutations

Şeker-Yılmaz B, Kör D, Bulut FD, Yüksel B, Karabay-Bayazıt A, Topaloğlu AK, Ceylaner G, Önenli-Mungan N. Impaired glucose tolerance in Fanconi-Bickel syndrome: Eight patients with two novel mutations. Turk J Pediatr 2017; 59: 434-441. Fanconi-Bickel syndrome (FBS) is a rare, autosomal recessive disorder of carbohydrate metabolism caused by defects in the facilitative glucose transporter 2 (GLUT2 or SLC2A2) gene. Prominent findings are failure to thrive, renal tubular acidosis, hypoglycemia and postprandial hyperglycemia even mimicking diabetes mellitus. Eight patients from 6 families with FBS were included in this study. c.482_483insC homozygous mutation was detected in six patients from four different families. Mutation analysis of SLC2A2 gene revealed two novel homozygous mutations; c.1069delGinsAATAA and c.575A > G. Standard oral glucose tolerance test with 1.75 g/kg oral glucose was performed in six of the patients who were older than 3-years of age. Impaired glucose tolerance was found in all patients as expected and two of them had overt diabetes. None of the antidiabetic medications were given to them in order to avoid significant hypoglycemia. Beside the conservative treatment, follow up with frequent oral glucose tolerance tests are planned. We report these cases of FBS, as GSD XI is rare, two novel mutations were detected and also to highlight the risk of diabetes mellitus; although there is not a consensus about the treatment.

Distribution of glucose transporters in renal diseases

Kidneys play an important role in glucose homeostasis. Renal gluconeogenesis prevents hypoglycemia by releasing glucose into the blood stream. Glucose homeostasis is also due, in part, to reabsorption and excretion of hexose in the kidney.Lipid bilayer of plasma membrane is impermeable for glucose, which is hydrophilic and soluble in water. Therefore, transport of glucose across the plasma membrane depends on carrier proteins expressed in the plasma membrane. In humans, there are three families of glucose transporters: GLUT proteins, sodium-dependent glucose transporters (SGLTs) and SWEET. In kidney, only GLUTs and SGLTs protein are expressed. Mutations within genes that code these proteins lead to different renal disorders and diseases. However, diseases, not only renal, such as diabetes, may damage expression and function of renal glucose transporters.

A Fanconi-Bickel syndrome patient with a novel mutation and accompanying situs inversus totalis

Taştemel-Öztürk T, Bilginer-Gürbüz B, Tekşam Ö, Sivri S. A Fanconi-Bickel syndrome patient with a novel mutation and accompanying situs inversus totalis. Turk J Pediatr 2017; 59: 693-695. Fanconi-Bickel syndrome is a rare autosomal recessive disorder of carbohydrate metabolism, caused by mutations in the SLC2A2 gene, that codes for the glucose transporter protein 2 (GLUT2). The disease is characterized by proximal renal tubular dysfunction, impaired glucose and galactose utilization, and accumulation of glycogen in the liver and kidney. Signs and symptoms of Fanconi-Bickel syndrome begin in infancy and include failure to thrive, hepatomegaly, hypophosphatemic rickets, and short stature. Here in we report a Turkish Fanconi-Bickel syndrome case who also has situs inversus totalis and a novel mutation that has not been described before.

An Indian girl with Fanconi-Bickel syndrome without SLC2A2 gene mutation

Fanconi-Bickel syndrome is a rare autosomal-recessive disorder caused by defects in the facilitative glucose transporter 2 (GLUT2) gene. It is characterized by hepatorenal glycogen accumulation, tubular nephropathy and impaired utilization of glucose and galactose. In this communication, we present the case of a 5-year-old girl who presented with deforming rickets and massive hepatomegaly. Liver biopsy confirmed the diagnosis of glycogen storage disorder. However, the mutation of the SLC2A2 (GLUT2) gene was not found. Mutation negative patients with characteristic Fanconi-Bickel syndrome phenotype suggest additional underlying mechanisms that need exploration.

Fanconi-Bickel syndrome versus osteogenesis imperfeeta: An Iranian case with a novel mutation in glucose transporter 2 gene, and review of literature

Fanconi-Bickel syndrome is an extremely rare hereditary metabolic disease, characterized by hepatomegaly due to glycogen storage, refractory hypophosphatemic rickets, marked growth retardation and proximal renal tubular acidosis. Recurrent bone fractures are one of the hallmark findings. It is a single gene disorder; the responsible gene belongs to the facilitative glucose transporters 2 (GLUT2) family gene or (SLC2A2) mapped to the q26.1-26.3 locus on chromosome 3, and encodes the GLUT protein 2. This protein is expressed in pancreatic ί-cells, hepatocytes, renal tubules, and intestinal mucosa. Several mutations in the GLUT2 gene have been reported in different ethnicities. Herein we report an Iranian girl with a missed diagnosis of osteogenesis imperfecta. She was referred with the history of frequent fractures, and severe motor delay and was suspected to osteogenesis imperfecta. Following the case we detected refractory rickets instead of OI, sever growth failure, proximal renal tubulopathy and RTA, and enlarged kidneys, progressive hepatomegaly, and GSD on liver biopsy. Glucose and galactose tolerance tests confirmed abnormal carbohydrate metabolism. Molecular analysis on GLUT2 gene revealed a homozygous novel mutation in exon 5; it was 15 nucleotide deletion and 7 nucleotide insertion and caused a frame shift mutation, produced a premature truncated protein (P.A229QFsX19). This mutation has not been reported before in the relevant literature.

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.