FBPase Deficiency Research Articles

Metabolic management of a successful pregnancy and postpartum complications in fructose-1,6-bisphosphatase deficiency.

Fructose-1,6-bisphosphatase (FBPase) deficiency is a rare, inborn error of metabolism, that causes hypoglycemia and lactic acidosis in response to inadequate glucose intake and/or high intakes of fructose, sucrose, or sorbitol. Pregnancy in women with FBPase deficiency puts them at high risk for metabolic decompensation due to increased glucose demands from the growing fetus. Here we report a 31-year-old primipara who was treated starting at 14 weeks gestation with a diet high in complex carbohydrates and low in fructose, sucrose, and sorbitol and close monitoring of glucose levels throughout her pregnancy. She delivered a healthy 2860 g baby at 37 weeks via vaginal delivery with no complications or hypoglycemia. At 5 months postpartum and 5 months of life, the patient and baby are doing well, although the patient experienced an episode of hypoglycemia and lactic acidosis at 4 months postpartum due to the increased metabolic demands of breastfeeding. This report adds to the limited case reports that discuss outcomes and proposed interventions during pregnancy in individuals with FBPase deficiency.

Intrafamilial phenotypic variability due to a missense pathogenic variant in FBP1 gene

BackgroundFBPase deficiency as an autosomal recessive disorder is due pathogenic variants in the FBP1 gene. It usually presents with hyperlactic acidemia and hypoglycaemia starting from early childhood. Here, genotypes and phenotypes of all reported patients and their distributions are presented. In addition, we present an Iranian family with two affected children presenting with unusual symptoms due to pathogenic variants in the FBP1 gene.Clinical evaluations and laboratory assessments were performed for the affected members. Whole exome sequencing (WES) was applied in order to find the causal variant. In addition to segregation analysis within the family, variant pathogenicity analyses and predictions were done via bioinformatics tools and according to ACMG guidelines. The genotypes and detailed clinical features were documented for all patients. ResultsThe study included a population of 104 patients with different variants of the FBP1 gene; 75 were homozygotes. The average age of onset was 14.97 months. The most frequent clinical features were metabolic acidosis (71 cases), hypoglycemia (70 cases), vomiting (46 cases), hyperuricemia (37 cases), and respiratory distress (25 cases). 74 families were from Asia. The most common genotypes were c.841G > A/c.841G > A and c.472C > T/c.472C > T. WES test showed a pathogenic homozygous variant, c.472C > T in two cases of a family: a six-and-a-half-year-old girl with an older brother with different symptoms. All laboratory evaluations in the patient were normal except for the blood sugar. The patient experienced her first hypoglycemic episode at age 3. ConclusionsThis is an unusual presentation of FBPase deficiency with intrafamilial phenotypic variability.

Identification of genotype–biochemical phenotype correlations associated with fructose 1,6-bisphosphatase deficiency

Fructose-1,6-bisphosphatase (FBPase) deficiency, caused by an FBP1 mutation, is an autosomal recessive disorder characterized by hypoglycemic lactic acidosis. Due to the rarity of FBPase deficiency, the mechanism by which the mutations cause enzyme activity loss still remains unclear. Here we identify compound heterozygous missense mutations of FBP1, c.491G>A (p.G164D) and c.581T>C (p.F194S), in an adult patient with hypoglycemic lactic acidosis. The G164D and F194S FBP1 mutants exhibit decreased FBP1 protein expression and a loss of FBPase enzyme activity. The biochemical phenotypes of all previously reported FBP1 missense mutations in addition to G164D and F194S are classified into three functional categories. Type 1 mutations are located at pivotal residues in enzyme activity motifs and have no effects on protein expression. Type 2 mutations structurally cluster around the substrate binding pocket and are associated with decreased protein expression due to protein misfolding. Type 3 mutations are likely nonpathogenic. These findings demonstrate a key role of protein misfolding in mediating the pathogenesis of FBPase deficiency, particularly for Type 2 mutations. This study provides important insights that certain patients with Type 2 mutations may respond to chaperone molecules.

A novel variant in the FBP1 gene causes fructose-1,6-bisphosphatase deficiency through increased ubiquitination

Fructose-1,6-bisphosphatase (FBPase) deficiency is an autosomal recessive disorder characterized by impaired gluconeogenesis caused by mutations in the fructose-1,6-bisphosphatase 1 (FBP1) gene. The molecular mechanisms underlying FBPase deficiency caused by FBP1 mutations require investigation. Herein, we report the case of a Chinese boy with FBPase deficiency who presented with hypoglycemia, ketonuria, metabolic acidosis, and repeated episodes of generalized seizures that progressed to epileptic encephalopathy. Whole-exome sequencing revealed compound heterozygous variants, c.761 A > G (H254R) and c.962C > T (S321F), in FBP1. The variants, especially the novel H254R, reduced protein stability and enzymatic activity in patient-derived leukocytes and transfected HepG2 and U251 cells. Mutant FBP1 undergoes enhanced ubiquitination and proteasomal degradation. NEDD4-2 was identified as an E3 ligase for FBP1 ubiquitination in transfected cells and the liver and brain of Nedd4-2 knockout mice. The H254R mutant FBP1 interacted with NEDD4-2 at significantly higher levels than the wild-type control. Our study identified a novel H254R variant of FBP1 underlying FBPase deficiency and elucidated the molecular mechanism underlying the enhanced NEDD4-2-mediated ubiquitination and proteasomal degradation of mutant FBP1.

An Iranian Patient with Fructose 1,6 Bisphosphatase Deficiency: A Case Report

Background: Deficiency of hepatic fructose 1,6 bisphosphatase (FBPase), a key enzyme in gluconeogenesis, impairs the formation of glucose from all gluconeogenic precursors including dietary fructose. Patients present with life threatening metabolic acidosis, fasting hypoglycemia, hepatomegaly, hyperketosis, elevated lactate and uric acid level. Glycerol and glycerol-3 phosphate have been found in the urine. The diagnosis of FBPase deficiency is confirmed via DNA molecular analysis from peripheral leukocytes. The acute life threatening episodes are treated with IV glucose at high rate and bicarbonate to control hypoglycemia and acidosis.
 Case Report: Here we report a girl referred with anorexia, lethargy, recurrent vomiting, progressive respiratory distress, and hepatomegaly following respiratory viral infection. She also had a history of twice similar attacks but milder than previous episodes. The test results showed hypoglycemia and severe metabolic acidosis. Despite proper treatment, the patient died of pulmonary edema following a respiratory viral infection.
 Conclusion: Once FBPase deficiency has been diagnosed and adequate management introduced, its course is usually benign. Growth both psychomotor and intellectual development are unimpaired and tolerance to fasting improves with age.

Fructose 1,6 bisphosphatase deficiency: outcomes of patients in a single center in Turkey and identification of novel splice site and indel mutations in FBP1.

Fructose 1,6 bisphosphatase (FBPase) deficiency is a rare autosomal recessively inherited metabolic disease. It is encoded by FBP1, and the enzyme catalyzes the hydrolysis of fructose-1,6-bisphosphate to fructose 6-phosphate. Patients with recurrent episodes of metabolic acidosis, hypoglycemia, hypertriglyceridemia, and hyperketonemia are present. In this study, we describe the clinical, biochemical, and molecular genetic features of six unrelated Turkish patients from six different families who weregenetically diagnosed with FBPase deficiency in ourclinic between 2008 and 2020. Their clinical and laboratory data were collected retrospectively. Next-generation sequencing (NGS) was performed for the molecular genetic analysis. All patients were hospitalized with recurrent hypoglycemia and metabolic acidosis episodes. Three out of six patients were presented in the neonatal period. The mean age at diagnosis was 26months. NGS revealed a known homozygous gross deletion including exon 2 in three patients (50%), a known homozygous c.910_911dupTT pathogenic variant in one patient (16%), a novel homozygous c.651_653delCAGinsTAA likely pathogenic variant, and another novel homozygous c.705+5G>A splice site variant. Leukocyte FBPase analysis detected no enzyme activity in the patient with homozygous c.705+5G>A splice site variant. We identified two novel mutations in this study. One of them is a splice site mutation which is five bases downstream of the exon, and the other one is an indel mutation. Both of the splice site and indel mutations are exceedingly rare in FBP1, and to the best of our knowledge, there are second splice site and indel variants reported in the literature. Exon 2 deletion is the most common mutation consistent with the previous reports in Turkish patients. FBPase is a frequent cause of hypoglycemia and metabolic acidosis, and the widespread use of molecular genetic analysis would contribute to the enlightenment of advanced genetic factors and possible genotype/phenotype correlation.

Fructose 1,6 Bisphosphatase Deficiency Mimicking Glycogen Storage Disease as Recurrent Hypoglycemia

Background: Fructose 1,6 Bisphosphatase (FBPase) deficiency is a rare and treatable cause of ketotic hypoglycemia in children. Affected children present in the postneonatal period with recurrent episodes of early morning hypoglycemia typically triggered by an infection. We present a child with recurrent hypoglycemic seizures who was initially considered as glycogen storage disease (GSD) type 1, but on further evaluation, was diagnosed with FBPase deficiency. Clinical Description: A 2.5-year-old developmentally normal boy presented with the second episode of hypoglycemic seizure. He had a similar episode following a fasting time of 10 h at 2 years of age. Critical sample analysis revealed ketosis, lactic acidosis, hyperuricemia, and raised triglycerides. He was diagnosed with probable GSD type 1. At 2.5 years of age, he had another episode of hypoglycemic seizures following a similar fasting spell, and critical sample evaluation revealed similar findings. However, he did not have the classical cherubic facies associated with GSD type 1, and a repeat ultrasound abdomen showed normal-sized liver. Management: The clinical presentation and critical sample evaluation were suggestive of gluconeogenesis defect. However, the child did not have any other end-organ involvement. Hence, a possibility of FBPase deficiency was considered. The genetic testing confirmed compound heterozygous mutations involving the FBP1 gene. Conclusion: Fructose 1,6 bisphosphonate deficiency is a close mimicker of GSD 1.

A rare case of fructose-1,6-bisphosphatase deficiency: a delayed diagnosis story

Abstract Objectives Fructose-1,6-bisphosphatase deficiency (FBPase deficiency, OMIM 229700) is an early-onset rare genetic disorder caused by mutations in the FBP1 gene. Case presentation Our patient was 17-years-old when she was diagnosed with the disease. Initial sequencing analysis with Ion Torrent technology failed to detect the gross deletion that covered complete exon 2 (c.-24-26_170 + 5192del) of FBP1 gene and caused the delay in diagnosis. Deletion was then detected when sequencing was performed in an Illumina MiSeq platform. Conclusions This case emphasizes the importance of sequencing data analysis for precise diagnosis of rare diseases and therapy planning.

MON-188 Characterization and Categorization Based on Genotype-Biochemical Phenotype Association in Fructose-1,6-Bisphosphatase Deficiency

Fructose-1,6-bisphosphatase (FBPase) deficiency, caused by an FBP1 mutation, is an autosomal recessive disorder characterized by hypoglycemia and metabolic acidosis. Due to the rarity of FBPase deficiency, elucidating the mechanism by which the mutations cause enzyme activity loss is challenging. We performed whole-exome sequencing in an adult patient with severe hypoglycemic lactic acidosis and identified that the patient carried compound heterozygous missense mutations of FBP1 (NM_000507.3) with c.491G>A (p.G164D) and c.581T>C (p.F194S). Biochemical analysis using FBP1-KO HepG2 cells generated by CRISPR/Cas9 system revealed that FBP1 mutant (G164D or F194S) overexpression decreased protein expression and enzyme activity loss. The interactome analysis based on Liquid chromatography-tandem MS data for binding partners demonstrated that FBP1, particularly its mutant, interacts with the proteins involved in the molecular chaperone related to unfolded protein response including heat shock protein (HSP). Indeed, G164D and F194S mutants exhibited increased HSPs interaction and aggregated in the endoplasmic reticulum, suggesting involvement of protein misfolding in its pathogenesis. The biochemical phenotypes of all FBP1 missense mutations previously reported were examined and categorized into three functional phenotypes: Type 1 mutations, located at pivotal residues in enzyme activity motifs with no effects on protein expression and intracellular localization; Type 2 mutations, which mediate changes in amino acid hydrophobicity and structurally cluster around the substrate binding pocket, are associated with high HSP-binding, aggregation in the endoplasmic reticulum, and decreased protein expression; and Type 3 mutations, which are likely non-pathogenic mutations without biochemical phenotype. Thus, our findings demonstrate that protein misfolding contributes to FBPase deficiency pathogenesis, particularly in Type 2 mutations; therefore, its missense mutations can be classified into three categories based on genotype functional phenotype association.

Genetic analysis of patients with fructose-1,6-bisphosphatase deficiency

IntroductionFructose-1,6-bisphosphatase deficiency (FBPase deficiency) is a rare inborn error of metabolism that affects gluconeogenesis. Ketotic hypoglycemia is the main symptom and can occur at any age, usually after long periods of fasting or during illness. The diagnosis may be achieved by measurement of the enzyme activity in a liver sample, but FBP1 analysis has become the most common approach. AimTo characterize the genotype of Southern Brazilian FBPase-deficient patients. MethodologyThe FBP1 gene of six unrelated patients (one had consanguineous parents) with previous diagnoses of FBPase deficiency (enzymatic, pts A, B, D, E; genetic through Next-Generation Sequencing-NGS, pt F; enzymatic and Sanger sequencing, pt C) was first analyzed through NGS. Pathogenic variants found in NGS were confirmed by Sanger sequencing. The pathogenicity of novel missense variants was evaluated through in silico analysis. ResultsFive patients (pt A, B, D, E, F) had their genotype identified by NGS, all of them being homozygous. In Pt C, NGS detected only one pathogenic variant. Among the 11 alleles analyzed, only three variants were found, two being novel: c.958G > A and c.986T > C. In silico analysis indicated the pathogenicity of both variants. Interestingly, the three variants seem to be linked to specific haplotypes, indicating that an endogamy effect may be acting on these alleles in the population of Southern Brazil. ConclusionsOur data suggest that NGS is a good tool for the diagnosis of FBPase deficiency. Variants c.958G > A and c.986T > C are the most prevalent variants in the country.

International practices in the dietary management of fructose 1-6 biphosphatase deficiency

BackgroundIn fructose 1,6 bisphosphatase (FBPase) deficiency, management aims to prevent hypoglycaemia and lactic acidosis by avoiding prolonged fasting, particularly during febrile illness. Although the need for an emergency regimen to avoid metabolic decompensation is well established at times of illness, there is uncertainty about the need for other dietary management strategies such as sucrose or fructose restriction. We assessed international differences in the dietary management of FBPase deficiency.MethodsA cross-sectional questionnaire (13 questions) was emailed to all members of the Society for the Study of Inborn Errors of Metabolism (SSIEM) and a wide database of inherited metabolic disorder dietitians.ResultsThirty-six centres reported the dietary prescriptions of 126 patients with FBPase deficiency. Patients’ age at questionnaire completion was: 1-10y, 46% (n = 58), 11-16y, 21% (n = 27), and >16y, 33% (n = 41). Diagnostic age was: <1y, 36% (n = 46); 1-10y, 59% (n = 74); 11-16y, 3% (n = 4); and >16y, 2% (n = 2). Seventy-five per cent of centres advocated dietary restrictions. This included restriction of: high sucrose foods only (n = 7 centres, 19%); fruit and sugary foods (n = 4, 11%); fruit, vegetables and sugary foods (n = 13, 36%). Twenty-five per cent of centres (n = 9), advised no dietary restrictions when patients were well. A higher percentage of patients aged >16y rather than ≤16y were prescribed dietary restrictions: patients aged 1-10y, 67% (n = 39/58), 11-16y, 63% (n = 17/27) and >16y, 85% (n = 35/41). Patients classified as having a normal fasting tolerance increased with age from 30% in 1-10y, to 36% in 11-16y, and 58% in >16y, but it was unclear if fasting tolerance was biochemically proven. Twenty centres (56%) routinely prescribed uncooked cornstarch (UCCS) to limit overnight fasting in 47 patients regardless of their actual fasting tolerance (37%). All centres advocated an emergency regimen mainly based on glucose polymer for illness management.ConclusionsAlthough all patients were prescribed an emergency regimen for illness, use of sucrose and fructose restricted diets with UCCS supplementation varied widely. Restrictions did not relax with age. International guidelines are necessary to help direct future dietary management of FBPase deficiency.

Fructose-1,6-bisphosphatase deficiency as a cause of recurrent hypoglycemia and metabolic acidosis: Clinical and molecular findings in Malaysian patients

BackgroundFructose-1,6-bisphosphatase (FBPase) deficiency is a rare autosomal recessive inborn error of gluconeogenesis. We reported the clinical findings and molecular genetic data in seven Malaysian patients with FBPase deficiency. MethodsAll patients diagnosed with FBPase deficiency from 2010 to 2015 were included in this study. Their clinical and laboratory data were collected retrospectively. ResultsAll the patients presented with recurrent episodes of hypoglycemia, metabolic acidosis, hyperlactacidemia and hepatomegaly. All of them had the first metabolic decompensation prior to 2 years old. The common triggering factors were vomiting and infection. Biallelic mutations in FBP1 gene (MIM*611570) were identified in all seven patients confirming the diagnosis of FBPase deficiency. In four patients, genetic study was prompted by detection of glycerol or glycerol-3-phosphate in urine organic acids analysis. One patient also had pseudo-hypertriglyceridemia. Seven different mutations were identified in FBP1, among them four mutations were new: three point deletions (c.392delT, c.603delG and c.704delC) and one splice site mutation (c.568-2A > C). All four new mutations were predicted to be damaging by in silico analysis. One patient presented in the neonatal period and succumbed due to sepsis and multi-organ failure. Among six survivors (current age ranged from 4 to 27 years), four have normal growth and cognitive development. One patient had short stature and another had neurological deficit following status epilepticus due to profound hypoglycemia. ConclusionFBPase deficiency needs to be considered in any children with recurrent hypoglycemia and metabolic acidosis. Our study expands the spectrum of FBP1 gene mutations.

Mutation analysis of an adult patient with fructose-1, 6-bisphosphatase deficiency in a Chinese family

The gene mutations of a patient with fructose-1, 6-bisphosphatase (FBPase) deficiency and her parents were analyzed and her clinical manifestations, laboratory tests, and genetic characteristics were reviewed. The molecular analysis of FBP1 gene showed a G residue duplication at base 960 in exon 7(c.960dupG) in this patient while her parents carried the heterozygous c. 960dupG mutation. The prominent clinical feature of this patient was the benign course of the disease with age. However, acute attack could be triggered by stress, long-time fasting, a large amounts of fructose intake, etc. The typical clinical manifestations were severe lactic acidosis, hypoglycemia, and elevated liver enzymes. (Chin J Endocrinol Metab, 2017, 33: 752-754) Key words: Fructose-1, 6-bisphosphatase deficiency; Lactic acidosis; Fructose-1, 6-bisphosphatase 1 gene

Genetic analysis of fructose-1,6-bisphosphatase (FBPase) deficiency in nine consanguineous Pakistani families.

Fructose-1,6-bisphosphatase (FBPase) deficiency is a rare inherited metabolic disorder characterized by recurrent episodes of hypoglycemia, ketosis and lactic acidosis. FBPase is encoded by FBP1 gene and catalyzes the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate in the last step of gluconeogenesis. We report here FBP1 mutations in nine consanguineous Pakistani families affected with FBPase deficiency. Nine families having one or two individuals affected with FBPase deficiency were enrolled over a period of 3 years. All FBP1 exonic regions including splicing sites were PCR-amplified and sequenced bidirectionally. Familial cosegregation of mutations with disease was confirmed by direct sequencing and PCR-RFLP analysis. Three different FBP1 mutations were identified. Each of two previously reported mutations (c.472C>T (p.Arg158Trp) and c.841G>A (p.Glu281Lys)) was carried by four different families. The ninth family carried a novel 4-bp deletion (c.609_612delAAAA), which is predicted to result in frameshift (p.Lys204Argfs*72) and loss of FBPase function. The novel variant was not detected in any of 120 chromosomes from normal ethnically matched individuals. FBPase deficiency is often fatal in the infancy and early childhood. Early diagnosis and prompt treatment is therefore crucial to preventing early mortality. We recommend the use of c.472C>T and c.841G>A mutations as first choice genetic markers for molecular diagnosis of FBPase deficiency in Pakistan.

Fructose 1,6-bisphosphatase deficiency: clinical, biochemical and genetic features in French patients.

Fructose-1,6-bisphosphatase (FBPase) deficiency is a very rare autosomal recessive disorder caused by a mutation of the fructose-1,6-bisphosphatase gene(FBP1). Disease is mainly revealed by hypoglycemia and lactic acidosis, both symptoms being characteristic for an enzymatic block in the last steps of the gluconeogenesis. Twelve patients with FBPase deficiency were diagnosed in France in the 2001-2013 period, using a diagnostic system based on a single blood sample which allows simultaneous enzyme activity measurement on mononuclear white blood cells and molecular analysis. Sequencing of exons and intron-exon junctions of FBP1 gene was completed in unsolved cases by a gene dosage assay developed for each exon. For most patients, first metabolic decompensation occurred before two years of age with a similar sequence: the triggering factors were fever, fasting, or decrease of food intake. However, diagnosis was made late at a mean age of 3 years, as mitochondrial defects or glycogen storage diseases were firstly suspected. Enzyme activity in leukocytes was dramatically decreased (<10%). Twelve different mutations were identified in 22 alleles among them seven were novels: one missense mutation c.472C > T, one point deletion c.48del, one point duplication c.865dupA, one deletion-insertion, and two splice mutations (c.427-1del and c.825 + 1G > A). We described the first intragenic deletion in FBP1 (g.97,364,754_97,382,011del) in homozygous state. Our report also confirms that this very rare disease is misdiagnosed, as other energetic defects are firstly suspected.

Pitfall in the Diagnosis of Fructose-1,6-Bisphosphatase Deficiency: Difficulty in Detecting Glycerol-3-Phosphate with Solvent Extraction in Urinary GC/MS Analysis.

Fructose-1,6-bisphosphatase (FBPase), an enzyme involved in gluconeogenesis, catalyzes the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate and inorganic phosphate. FBPase deficiency is an autosomal recessive inherited disorder, characterized by episodic attacks of hypoglycemia, ketosis, and lactic acidosis during fasting. In general, urinary organic acid analysis using gas chromatography-mass spectrometry (GC/MS) is very useful for the diagnosis of FBPase deficiency, because the appearance of glycerol or glycerol-3-phosphate in the urine is characteristic of this disease. Here, we report a case of FBPase deficiency in a girl with a history of several severe lactic acidosis events, both as a neonate and after the age of 12 months. The patient was identified as a compound heterozygote with two mutations in the FBPase 1 gene: c.841G>A (p.Glu281Lys) and c.960_961insG (p.Ser321fs). The c.841G>A is a newly identified pathogenic mutation. An abnormal level of glycerol-3-phosphate was not detected in the conventional urinary organic acid analysis using GC/MS after solvent extraction. This method, which is a widely used diagnostic standard, could not detect increased levels of glycerol or glycerol-3-phosphate in the patient's urine, which was sampled during the episode. However, glycerol and glycerol-3-phosphate were detected in the same sample, when it was analyzed using GC/MS with the urease pretreatment non-extraction method. Patients with FBPase deficiency have good glycemic control after correct treatment. Therefore, accurate and early diagnosis is essential for a good prognosis. Accordingly, when a patient presents with hypoglycemia and lactic acidosis, it is important to select the appropriate method of urinalysis for organic acids by GC/MS.

Novel Fructose-1,6-bisphosphatase Gene Mutation in Two Siblings

Fructose-1,6-bisphosphatase (FBPase) deficiency is an autosomal, recessively inherited disease that progresses with severe hypoglycemia, and metabolic attacks result in a defect in gluconeogenesis. If not appropriately treated, and if fructose is not excluded from the diet, the outcome could be fatal. Two Turkish children with FBPase deficiency were diagnosed based on mutation of the FBP1 gene. The first, a 2-year-old girl, was referred to our clinic because of lactic acidosis, uncorrectable hypoglycemia, and increased transaminases. FBPase deficiency was suspected in the patient, who recovered dramatically after a high-dose glucose infusion and adequate bicarbonate replacement. The second patient, a five-and-a-half-year-old male sibling of the patient, was also hospitalized, twice, because of hypoglycemic attacks and metabolic acidosis. Different from previous analyses, a homozygous c.658delT mutation was detected at exon 5 of the FBP1 gene in the two siblings. As a result of this mutation, there was a TGA (stop codon) at exon 6. There was first-degree consanguinity between the parents. These two cases were the first FBP1 gene mutations reported in our country.

Fructose 1,6‐bisphosphatase deficiency: enzyme and mutation analysis performed on calcitriol‐stimulated monocytes with a note on long‐term prognosis

Fructose 1,6-bisphosphatase (FBPase) deficiency is an inborn error of metabolism in the gluconeogenetic pathway. During periods of low food intake or infections, a defect in FBPase can result in hypoglycemia, ketonuria and metabolic acidosis. We established a diagnostic system for FBPase deficiency consisting of enzyme activity measurement and mutation detection in calcitriol-stimulated monocytes. In healthy individuals, we showed that FBPase activity is present in monocytes but not in other leukocytes. We describe the clinical course of four individuals from two Swedish families with FBPase deficiency. Family 1: patient 1 died at the age of 6 months after a severe episode with hypoglycemia and acidosis; patients 2 and 3 were followed for >30 years and were found to have a very favorable long-term prognosis. Their FBPase activity from jejunum (residual activity 15-25% of healthy controls), mixed leukocytes (low or normal levels), and calcitriol-stimulated monocytes (no detectable activity) was compared. Mutation analysis showed they were heterozygous for two genetic alterations (c.778G>A; c.881G>A), predicting amino acid exchanges at position p.G260R and p.G294E, originating from their parents. Family 2: patient 4 had no detectable levels of FBPase in stimulated monocytes. A mutation (c.648C>G) predicting a premature stop codon at position p.Y216X was found in one allele and a large deletion of about 300 kb, where the genes FBP2, FBP1 and a part of ONPEP are located, in the other. In conclusion, we present a reliable diagnostic system to verify an FBPase deficiency and find the genetic aberration.

A potential role for muscle in glucose homeostasis: in vivo kinetic studies in glycogen storage disease type 1a and fructose‐1,6‐bisphosphatase deficiency

BackgroundA potential role for muscle in glucose homeostasis was recently suggested based on characterization of extrahepatic and extrarenal glucose-6-phosphatase (glucose-6-phosphatase-β). To study the role of extrahepatic tissue in glucose homeostasis during fasting glucose kinetics were studied in two patients with a deficient hepatic and renal glycogenolysis and/or gluconeogenesis.DesignEndogenous glucose production (EGP), glycogenolysis (GGL), and gluconeogenesis (GNG) were quantified with stable isotopes in a patient with glycogen storage disease type 1a (GSD-1a) and a patient with fructose-1,6-bisphosphatase (FBPase) deficiency. The [6,6-2H2]glucose dilution method in combination with the deuterated water method was used during individualized fasting tests.ResultsBoth patients became hypoglycemic after 2.5 and 14.5 h fasting, respectively. At that time, the patient with GSD-1a had EGP 3.84 μmol/kg per min (30% of normal EGP after an overnight fast), GGL 3.09 μmol/kg per min, and GNG 0.75 μmol/kg per min. The patient with FBPase deficiency had EGP 8.53 μmol/kg per min (62% of normal EGP after an overnight fast), GGL 6.89 μmol/kg per min GGL, and GNG 1.64 μmol/kg per min.ConclusionEGP was severely hampered in both patients, resulting in hypoglycemia. However, despite defective hepatic and renal GNG in both disorders and defective hepatic GGL in GSD-1a, both patients were still able to produce glucose via both pathways. As all necessary enzymes of these pathways have now been functionally detected in muscle, a contribution of muscle to EGP during fasting via both GGL as well as GNG is suggested.

Novel compound heterozygous mutations in the fructose-1,6-bisphosphatase gene cause hypoglycemia and lactic acidosis

Fructose-1,6-bisphosphatase (FBPase) deficiency is an autosomal recessive disorder caused by a mutation of the fructose-1,6-bisphosphatase 1 (FBP1) gene and results in impaired gluconeogenesis. We describe a male patient with typical FBPase deficiency who presented with hypoglycemia and lactic acidosis. The FBPase activity in his peripheral leukocytes and liver was very low. We amplified and sequenced the entire FBP1 coding region of the patient and his family members. Direct and allele-specific sequence analysis of the FBP1 gene revealed that the proband had a compound heterozygote for the G164S and 838delT, which he inherited from his carrier parents. His father and mother had heterozygous 838delT and G164S mutations, respectively, without any symptoms of hypoglycemia. Gene tracking within the family revealed that his elder sister had a heterozygous G164S mutation without symptoms of hypoglycemia. A G164S mutation of FBP1 in a heterozygous pattern (G164S and InsG960_961) has been reported previously, but the heterozygous 838delT mutation is novel. Transient transfection studies using COS-7 cells demonstrated that FBPase proteins with G164S or 838delT mutations were enzymatically inactive. In conclusion, we report a new case of molecular diagnosis of FBPase deficiency and provide evidence that impaired FBPase activity may be caused by novel compound heterozygous mutations in the FBP1 gene.