Fanconi-Bickel syndrome: GLUT2 mutations associated with a mild phenotype

Exploring genetic variants in congenital monosaccharide-disaccharide metabolism: Carrier ratios and phenotypic insights

OBJECTIVES

Adverse food reactions, often underestimated, encompass congenital monosaccharide-disaccharide metabolism disorders, yielding diverse outcomes such as abdominal pain, diarrhea, bleeding disorders, and even death. This study retrospectively scrutinized genetic variants linked to these disorders in a cohort subjected to whole-exome sequence analysis (WES), determining carrier frequencies and genotype-phenotype correlations.

METHODS

Data from 484 patients, were retrospectively analyzed using a gene panel (ALDOB, FBP1, GALE, GALK1, GALM, GALT, LCT, SLC2A2, SLC5A1, SI) for congenital monosaccharide-disaccharide metabolism disorders. WES was performed on patients using the xGen Exome Research Panel v2 kit, utilizing Next Generation Sequence Analysis (NGS). The study encompassed pathogenic, likely pathogenic, and variant of uncertain significance (VUS) variants.

RESULTS

Among 484 patients (244 female, 240 male), 17.35% carried 99 variants (67 distinct) in the analyzed genes. Pathogenic/likely pathogenic allele frequency stood at 0.013, while VUS allele frequency was 0.088. Notably, 44% (37/84) of patients harboring mutations manifested at least one relevant phenotype. Carriage frequencies ranged from 1:25 (SI gene) to 1:968 (GALE gene), with the estimated disease frequency spanning from 1:2500 to 1:3748000.

CONCLUSIONS

Our study underscores clinical manifestations in heterozygous carriers of recessive genetic disorders, addressing gaps in carrier frequencies and phenotypic effects for congenital monosaccharide-disaccharide metabolism disorders. This knowledge can improve these conditions' diagnosis and management, potentially preventing adverse food reactions and their associated complications.

Importance about use of high-throughput sequencing in pediatric: case report of a patient with Fanconi-Bickel syndrome

BACKGROUND

Fanconi-Bickel syndrome is characterized by hepatorenal disease caused by anomalous glycogen storage. It occurs due to variants in the SLC2A2 gene. We present a male patient of 2 years 7 months old, with failure to thrive, hepatomegaly, metabolic acidosis, hypophosphatemia, hypokalemia, hyperlactatemia.

RESULTS

Exome sequencing identified the homozygous pathogenic variant NM_000340.2(SLC2A2):c.1093 C > T (p.Arg365Ter), related with Fanconi-Bickel syndrome. He received treatment with bicarbonate, amlodipine, sodium citrate and citric acid solution, enalapril, alendronate and zolendronate, and nutritional management with uncooked cornstarch, resulting in an improvement of one standard deviation in weight and height.

CONCLUSIONS

The importance of knowing the etiology in rare genetic disease is essential, not only to determine individual and familial recurrence risk, but also to establish the treatment and prognosis; in this sense, access to a new genomic technology in low- and middle-income countries is essential to shorten the diagnostic odyssey.

Glycogen storage diseases with liver involvement: a literature review of GSD type 0, IV, VI, IX and XI

Background

Glycogen storage diseases (GSDs) with liver involvement are classified into types 0, I, III, IV, VI, IX and XI, depending on the affected enzyme. Hypoglycemia and hepatomegaly are hallmarks of disease, but muscular and renal tubular involvement, dyslipidemia and osteopenia can develop. Considering the paucity of literature available, herein we provide a narrative review of these latter forms of GSDs.

Main body

Diagnosis is based on clinical manifestations and laboratory test results, but molecular analysis is often necessary to distinguish the various forms, whose presentation can be similar. Compared to GSD type I and III, which are characterized by a more severe impact on metabolic and glycemic homeostasis, GSD type 0, VI, IX and XI are usually known to be responsive to the nutritional treatment for achieving a balanced metabolic homeostasis in the pediatric age. However, some patients can exhibit a more severe phenotype and an important progression of the liver and muscular disease. The effects of dietary adjustments in GSD type IV are encouraging, but data are limited.

Conclusions

Early diagnosis allows a good metabolic control, with improvement of quality of life and prognosis, therefore we underline the importance of building a proper knowledge among physicians about these rare conditions. Regular monitoring is necessary to restrain disease progression and complications.

Opposite physiological and pathological mTORC1-mediated roles of the CB1 receptor in regulating renal tubular function

Activation of the cannabinoid-1 receptor (CB₁R) and the mammalian target of rapamycin complex 1 (mTORC1) in the renal proximal tubular cells (RPTCs) contributes to the development of diabetic kidney disease (DKD). However, the CB₁R/mTORC1 signaling axis in the kidney has not been described yet. We show here that hyperglycemia-induced endocannabinoid/CB₁R stimulation increased mTORC1 activity, enhancing the transcription of the facilitative glucose transporter 2 (GLUT2) and leading to the development of DKD in mice; this effect was ameliorated by specific RPTCs ablation of GLUT2. Conversely, CB₁R maintained the normal activity of mTORC1 by preventing the cellular excess of amino acids during normoglycemia. Our findings highlight a novel molecular mechanism by which the activation of mTORC1 in RPTCs is tightly controlled by CB₁R, either by enhancing the reabsorption of glucose and inducing kidney dysfunction in diabetes or by preventing amino acid uptake and maintaining normal kidney function in healthy conditions.

Renal proximal tubules modulate whole-body homeostasis by sensing various nutrients. Here the authors describe the existence and importance of a unique CB1/mTORC1/GLUT2 signaling axis in regulating nutrient homeostasis in healthy and diseased kidney.

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.

Hypothalamic MC4R regulates glucose homeostasis through adrenaline-mediated control of glucose reabsorption via renal GLUT2 in mice

AIMS/HYPOTHESIS

Melanocortin 4 receptor (MC4R) mutation is the most common cause of known monogenic obesity in humans. Unexpectedly, humans and rodents with MC4R deficiency do not develop hyperglycaemia despite chronic obesity and insulin resistance. To explain the underlying mechanisms for this phenotype, we determined the role of MC4R in glucose homeostasis in the presence and absence of obesity in mice.

METHODS

We used global and hypothalamus-specific MC4R-deficient mice to investigate the brain regions that contribute to glucose homeostasis via MC4R. We performed oral, intraperitoneal and intravenous glucose tolerance tests in MC4R-deficient mice that were either obese or weight-matched to their littermate controls to define the role of MC4R in glucose regulation independently of changes in body weight. To identify the integrative pathways through which MC4R regulates glucose homeostasis, we measured renal and adrenal sympathetic nerve activity. We also evaluated glucose homeostasis in adrenaline (epinephrine)-deficient mice to investigate the role of adrenaline in mediating the effects of MC4R in glucose homeostasis. We employed a graded [13C6]glucose infusion procedure to quantify renal glucose reabsorption in MC4R-deficient mice. Finally, we measured the levels of renal glucose transporters in hypothalamus-specific MC4R-deficient mice and adrenaline-deficient mice using western blotting to ascertain the molecular mechanisms underlying MC4R control of glucose homeostasis.

RESULTS

We found that obese and weight-matched MC4R-deficient mice exhibited improved glucose tolerance due to elevated glucosuria, not enhanced beta cell function. Moreover, MC4R deficiency selectively in the paraventricular nucleus of the hypothalamus (PVH) is responsible for reducing the renal threshold for glucose as measured by graded [13C6]glucose infusion technique. The MC4R deficiency suppressed renal sympathetic nerve activity by 50% in addition to decreasing circulating adrenaline and renal GLUT2 levels in mice, which contributed to the elevated glucosuria. We further report that adrenaline-deficient mice recapitulated the increased excretion of glucose in urine observed in the MC4R-deficient mice. Restoration of circulating adrenaline in both the MC4R- and adrenaline-deficient mice reversed their phenotype of improved glucose tolerance and elevated glucosuria, demonstrating the role of adrenaline in mediating the effects of MC4R on glucose reabsorption.

CONCLUSIONS/INTERPRETATION

These findings define a previously unrecognised function of hypothalamic MC4R in glucose reabsorption mediated by adrenaline and renal GLUT2. Taken together, our findings indicate that elevated glucosuria due to low sympathetic tone explains why MC4R deficiency does not cause hyperglycaemia despite inducing obesity and insulin resistance. Graphical abstract.

Fanconi Bickel syndrome: clinical phenotypes and genetics in a cohort of Sudanese children

Background

Fanconi-Bickel syndrome (FBS) is a rare condition of carbohydrate metabolism, caused by a recessive defect in the facilitative glucose transporter GLUT2 encoded by the SLC2A2 gene and characterized by a wide spectrum of phenotypical features. There is a paucity of reported data on FBS from Sub-Saharan Africa. Here, we describe the clinical, biochemical and genetic characteristics of our patients with FBS from Sudan, a country with a high consanguinity rate.

Patients & methods

Eleven patients from ten unrelated Sudanese families were included. Clinical & biochemical data were documented and imaging studies done including bone survey and abdominal ultrasound. Liver biopsy was done to confirm the pathological diagnosis in 45% of cases and molecular genetics was performed through contribution with the Exeter genomics laboratory for ten patients.

Results

Reported consanguinity was 70% among our patients. Growth was significantly impaired at presentation with mean weights of (-5.3 ± 1.8) SD and heights (-5.4 ± 2.5) SD. Severe chest deformity was present in (27%) and all patients showed features of rickets at presentation. Three patients had neonatal diabetes requiring insulin therapy of which one has been reported before. Six families lost undiagnosed siblings with similar clinical presentations. We identified a total of four homozygous pathogenic SLC2A2 variants in our patients, one of whom had a novel mutation.

Conclusions

FBS is not uncommon in Sudan where there is a high rate of consanguinity. Many cases are likely missed because of variable presentation and lack of public and professionals’ awareness. This is the first series to describe this condition from Sub-Saharan Africa.

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.

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.

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.

Hepatocellular Carcinoma in Fanconi-Bickel Syndrome

Fanconi-Bickel syndrome is a rare autosomal recessive disorder due to mutations in the facilitative glucose transporter 2 ( GLUT2 or SLC2A2) gene resulting in excessive glycogen storage predominantly in the liver and kidney. Previous case reports of this condition have described liver biopsies with glycogen storage and variable steatosis and/or fibrosis. Unlike in other types of glycogen storage disease, hepatocellular adenomas and carcinomas have not been described to date in this syndrome. A 6-year-old boy with consanguineous parents had short stature, poorly controlled rickets, hepatosplenomegaly, and renal tubular dysfunction clinically consistent with Fanconi-Bickel Syndrome. Sequencing of the SLC2A2 gene showed a homozygous variant of unknown significance [c.474A > C (p.Arg158Ser)] causing a missense mutation in an evolutionarily conserved residue. An incidental single hepatic lesion was discovered on imaging, and subsequent resection showed a 2.6 cm well-differentiated hepatocellular carcinoma with moderate atypia, diffuse immunoreactivity for glypican-3, and nuclear b-catenin, and with focal complete loss of the reticulin framework. The non-neoplastic liver showed marked glycogen accumulation with mild periportal fibrosis, rare bridging fibrosis, and no regenerative or adenomatous nodules. By electron microscopy, tumor cells had pleomorphic nuclei, prominent nucleoli, and scant cytoplasm with numerous mitochondria. Well-developed canaliculi were occasionally seen. The non-neoplastic liver showed glycogenosis with abundant cytoplasmic free (non-membrane bound) glycogen. Hepatocellular carcinoma should be considered as a possible complication of Fanconi-Bickel syndrome. This well differentiated carcinoma did not appear to be associated with hepatic adenomatosis as has been described in some hepatocellular carcinomas associated with other hepatic glycogen storage disorders. The nuclear beta-catenin immunoreactivity indicates a role for the Wnt signaling pathway in the pathogenesis of this tumor.

Reduced renal sympathetic nerve activity contributes to elevated glycosuria and improved glucose tolerance in hypothalamus-specific Pomc knockout mice

Objective

Hypothalamic arcuate nucleus-specific pro-opiomelanocortin deficient (ArcPomc^−/−) mice exhibit improved glucose tolerance despite massive obesity and insulin resistance. We demonstrated previously that their improved glucose tolerance is due to elevated glycosuria. However, the underlying mechanisms that link glucose reabsorption in the kidney with ArcPomc remain unclear. Given the function of the hypothalamic melanocortin system in controlling sympathetic outflow, we hypothesized that reduced renal sympathetic nerve activity (RSNA) in ArcPomc^−/− mice could explain their elevated glycosuria and consequent enhanced glucose tolerance.

Methods

We measured RSNA by multifiber recording directly from the nerves innervating the kidneys in ArcPomc^−/− mice. To further validate the function of RSNA in glucose reabsorption, we denervated the kidneys of WT and diabetic db/db mice before measuring their glucose tolerance and urine glucose levels. Moreover, we performed western blot and immunohistochemistry to determine kidney GLUT2 and SGLT2 levels in either ArcPomc^−/− mice or the renal-denervated mice.

Results

Consistent with our hypothesis, we found that basal RSNA was decreased in ArcPomc^−/− mice relative to their wild type (WT) littermates. Remarkably, both WT and db/db mice exhibited elevated glycosuria and improved glucose tolerance after renal denervation. The elevated glycosuria in obese ArcPomc^−/−, WT and db/db mice was due to reduced renal GLUT2 levels in the proximal tubules. Overall, we show that renal-denervated WT and diabetic mice recapitulate the phenotype of improved glucose tolerance and elevated glycosuria associated with reduced renal GLUT2 levels observed in obese ArcPomc^−/− mice.

Conclusion

Hence, we conclude that ArcPomc is essential in maintaining basal RSNA and that elevated glycosuria is a possible mechanism to explain improved glucose tolerance after renal denervation in drug resistant hypertensive patients.

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Hypothalamic POMC is essential in maintaining basal renal sympathetic nerve activity.

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Renal denervation improves glucose tolerance in wild-type and db/db mice by elevating their glycosuria.

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Decreased renal GLUT2 is responsible for elevated glycosuria in mice with suppressed renal sympathetic nerve activity.

Saving the sweetness: renal glucose handling in health and disease

Glucose homeostasis is highly controlled, and the function of the kidney plays an integral role in this process. The exquisite control of blood glucose relies, in part, on renal glucose filtration, renal glucose reabsorption, and renal gluconeogenesis. Particularly critical to maintaining glucose homeostasis is the renal reabsorption of glucose; with ~162 g of glucose filtered by the kidney per day, it is imperative that the kidney have the ability to efficiently reabsorb nearly 100% of this glucose back in the bloodstream. In this review, we focus on this central process, highlighting the renal transporters and regulators involved in both the physiology and pathophysiology of glucose reabsorption.

Molecular spectrum and differential diagnosis in patients referred with sporadic or autosomal recessive osteogenesis imperfecta

Background

Osteogenesis imperfecta (OI) is a heterogeneous bone disorder characterized by recurrent fractures. Although most cases of OI have heterozygous mutations in COL1A1 or COL1A2 and show autosomal dominant inheritance, during the last years there has been an explosion in the number of genes responsible for both recessive and dominant forms of this condition. Herein, we have analyzed a cohort of patients with OI, all offspring of unaffected parents, to determine the spectrum of variants accounting for these cases. Twenty patients had nonrelated parents and were sporadic, and 21 were born to consanguineous relationships.

Methods

Mutation analysis was performed using a next‐generation sequencing gene panel, homozygosity mapping, and whole exome sequencing (WES).

Results

Patients offspring of nonconsanguineous parents were mostly identified with COL1A1 or COL1A2 heterozygous changes, although there were also a few cases with IFITM5 and WNT1 heterozygous mutations. Only one sporadic patient was a compound heterozygote for two recessive mutations. Patients offspring of consanguineous parents showed homozygous changes in a variety of genes including CRTAP,FKBP10,LEPRE1,PLOD2,PPIB,SERPINF1,TMEM38B, and WNT1. In addition, two patients born to consanguineous parents were found to have de novo COL1A1 heterozygous mutations demonstrating that causative variants in the collagen I structural genes cannot be overlooked in affected children from consanguineous couples. Further to this, WES analysis in probands lacking mutations in OI genes revealed deleterious variants in SCN9A,NTRK1, and SLC2A2, which are associated with congenital indifference to pain (CIP) and Fanconi–Bickel syndrome (FBS).

Conclusion

This work provides useful information for clinical and genetic diagnosis of OI patients with no positive family history of this disease. Our data also indicate that CIP and FBS are conditions to be considered in the differential diagnosis of OI and suggest a positive role of SCN9A and NTRK1 in bone development.

Fanconi-Bickel syndrome in two Palestinian children: marked phenotypic variability with identical mutation

BACKGROUND

Fanconi-Bickel syndrome (FBS, OMIM 227810) is a rare autosomal recessive disease caused by a deficiency of glucose transporter 2 (GLUT2), a member of the facilitative glucose transporter family (Santer et al. J Inherit Metab Dis 21:191-194, 1998). The typical clinical picture is characterized by hepatorenal glycogen accumulation resulting in hepato- and nephromegaly, impaired utilization of glucose and galactose, proximal renal tubular dysfunction, rickets and severe short stature.

CASE PRESENTATION

We report 2 Palestinian patients from 2 families who were homozygous for the mutation p.R301X (C>T) in exon 7of GLUT2 gene. Patient 1 showed clinical and laboratory improvement with age characterized by normal growth and resolution of rickets. Patient 2 had severe phenotype characterized by progressive weight loss, persistent metabolic acidosis, marked polyuria and clinical and laboratory findings of rickets progressing to death at age 10 months.

CONCLUSION

This report further expands the clinical spectrum of FBS even with identical mutations. Other yet unknown genetic, environmental or stochastic factors may be responsible for phenotypic variability.

Monogenic Diabetes: What It Teaches Us on the Common Forms of Type 1 and Type 2 Diabetes

To date, more than 30 genes have been linked to monogenic diabetes. Candidate gene and genome-wide association studies have identified > 50 susceptibility loci for common type 1 diabetes (T1D) and approximately 100 susceptibility loci for type 2 diabetes (T2D). About 1-5% of all cases of diabetes result from single-gene mutations and are called monogenic diabetes. Here, we review the pathophysiological basis of the role of monogenic diabetes genes that have also been found to be associated with common T1D and/or T2D. Variants of approximately one-third of monogenic diabetes genes are associated with T2D, but not T1D. Two of the T2D-associated monogenic diabetes genes-potassium inward-rectifying channel, subfamily J, member 11 (KCNJ11), which controls glucose-stimulated insulin secretion in the β-cell; and peroxisome proliferator-activated receptor γ (PPARG), which impacts multiple tissue targets in relation to inflammation and insulin sensitivity-have been developed as major antidiabetic drug targets. Another monogenic diabetes gene, the preproinsulin gene (INS), is unique in that INS mutations can cause hyperinsulinemia, hyperproinsulinemia, neonatal diabetes mellitus, one type of maturity-onset diabetes of the young (MODY10), and autoantibody-negative T1D. Dominant heterozygous INS mutations are the second most common cause of permanent neonatal diabetes. Moreover, INS gene variants are strongly associated with common T1D (type 1a), but inconsistently with T2D. Variants of the monogenic diabetes gene Gli-similar 3 (GLIS3) are associated with both T1D and T2D. GLIS3 is a key transcription factor in insulin production and β-cell differentiation during embryonic development, which perturbation forms the basis of monogenic diabetes as well as its association with T1D. GLIS3 is also required for compensatory β-cell proliferation in adults; impairment of this function predisposes to T2D. Thus, monogenic forms of diabetes are invaluable "human models" that have contributed to our understanding of the pathophysiological basis of common T1D and T2D.

Hypothalamic POMC Deficiency Improves Glucose Tolerance Despite Insulin Resistance by Increasing Glycosuria

Hypothalamic proopiomelanocortin (POMC) is essential for the physiological regulation of energy balance; however, its role in glucose homeostasis remains less clear. We show that hypothalamic arcuate nucleus (Arc)POMC-deficient mice, which develop severe obesity and insulin resistance, unexpectedly exhibit improved glucose tolerance and remain protected from hyperglycemia. To explain these paradoxical phenotypes, we hypothesized that an insulin-independent pathway is responsible for the enhanced glucose tolerance. Indeed, the mutant mice demonstrated increased glucose effectiveness and exaggerated glycosuria relative to wild-type littermate controls at comparable blood glucose concentrations. Central administration of the melanocortin receptor agonist melanotan II in mutant mice reversed alterations in glucose tolerance and glycosuria, whereas, conversely, administration of the antagonist Agouti-related peptide (Agrp) to wild-type mice enhanced glucose tolerance. The glycosuria of ArcPOMC-deficient mice was due to decreased levels of renal GLUT 2 (rGLUT2) but not sodium-glucose cotransporter 2 and was associated with reduced renal catecholamine content. Epinephrine treatment abolished the genotype differences in glucose tolerance and rGLUT2 levels, suggesting that reduced renal sympathetic nervous system (SNS) activity is the underlying mechanism for the observed glycosuria and improved glucose tolerance in ArcPOMC-deficient mice. Therefore, the ArcPOMC-SNS-rGLUT2 axis is potentially an insulin-independent therapeutic target to control diabetes.

Segregation of a novel homozygous 6 nucleotide deletion in GLUT2 gene in a Fanconi-Bickel syndrome family

Fanconi-Bickel syndrome (FBS) is a rare autosomal recessive disorder characterized by hepatorenal glycogen accumulation, proximal renal tubular dysfunction, impaired utilization of glucose and galactose, rickets, and severe short stature. It has been shown to be caused by mutations in GLUT2 gene, a member of the facilitative glucose transporter family. Here, we report an Iranian family with 2 affected siblings. The clinical findings in the patients include developmental delay, failure to thrive, hepatomegaly, enlarged kidneys and rickets. A novel 6 nucleotide deletion (c.1061_1066del6, p.V355_S356del2) is shown to be segregated with the disease in this family.

Phenotypic variability in patients with fanconi-bickel syndrome with identical mutations

OBJECTIVE

To describe the phenotypic features of an ethnically homogenous group of patients with Fanconi-Bickel syndrome harboring the p.R310X mutation.

METHODS

The study group consisted of eight patients from a single Bedouin family with clinically and molecularly diagnosed Fanconi-Bickel syndrome who had been followed at the same tertiary medical center for 8 years or more. All were homozygous for the p.R310X mutation. The medical files were reviewed for presenting signs and symptoms, laboratory and imaging findings, treatment regimens, and disease severity over time.

RESULTS

Seven patients were diagnosed at our center before age 1 year, and one transferred from another center at age 16 years. Most patients presented with failure to thrive and/or hepatomegaly. All had short stature and doll-like facies. Most had biochemical abnormalities. Evaluation of the long-term findings revealed a wide spectrum of disease severity according to the following parameters: growth patterns, maximal electrolyte replacement therapy, skeletal and renal complications, frequency of follow-up visits, and hospitalizations for disease exacerbations. There was no apparent association of the clinical picture at presentation and later disease severity.

CONCLUSION

Fanconi-Bickel syndrome has a broad phenotypic variability in patients harboring the same homozygous p.R301X mutation. This finding might be explained by genetic elements such as modifier genes and epigenetic factors, as well as the effects of still-undetermined environmental and nutritional factors.

Mutations in SLC2A2 gene reveal hGLUT2 function in pancreatic β cell development

The structure-function relationships of sugar transporter-receptor hGLUT2 coded by SLC2A2 and their impact on insulin secretion and β cell differentiation were investigated through the detailed characterization of a panel of mutations along the protein. We studied naturally occurring SLC2A2 variants or mutants: two single-nucleotide polymorphisms and four proposed inactivating mutations associated to Fanconi-Bickel syndrome. We also engineered mutations based on sequence alignment and conserved amino acids in selected domains. The single-nucleotide polymorphisms P68L and T110I did not impact on sugar transport as assayed in Xenopus oocytes. All the Fanconi-Bickel syndrome-associated mutations invalidated glucose transport by hGLUT2 either through absence of protein at the plasma membrane (G20D and S242R) or through loss of transport capacity despite membrane targeting (P417L and W444R), pointing out crucial amino acids for hGLUT2 transport function. In contrast, engineered mutants were located at the plasma membrane and able to transport sugar, albeit with modified kinetic parameters. Notably, these mutations resulted in gain of function. G20S and L368P mutations increased insulin secretion in the absence of glucose. In addition, these mutants increased insulin-positive cell differentiation when expressed in cultured rat embryonic pancreas. F295Y mutation induced β cell differentiation even in the absence of glucose, suggesting that mutated GLUT2, as a sugar receptor, triggers a signaling pathway independently of glucose transport and metabolism. Our results describe the first gain of function mutations for hGLUT2, revealing the importance of its receptor versus transporter function in pancreatic β cell development and insulin secretion.

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.