Mutation spectrum in patients with fructose-1,6-bisphosphatase deficiency

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.761A  >  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.

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

OBJECTIVES

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.

METHODS

In this study, we describe the clinical, biochemical, and molecular genetic features of six unrelated Turkish patients from six different families who were genetically diagnosed with FBPase deficiency in our clinic between 2008 and 2020. Their clinical and laboratory data were collected retrospectively. Next-generation sequencing (NGS) was performed for the molecular genetic analysis.

RESULTS

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 26 months. 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.

CONCLUSIONS

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.

Novel fructose bisphosphatase 1 gene mutation presenting as recurrent episodes of vomiting in an Indian child

Fructose-1, 6-bisphosphatase 1 (FBP1) deficiency is an autosomal recessive disorder of gluconeogenesis resulting in severe and recurrent life-threatening episodes of hypoglycemia and lactic acidosis in infancy. We report a 16 month-old girl who presented with recurrent episodes of vomiting, rapid breathing, lactic acidosis, hyperuricemia, and hypertriglyceridemia. Genetic analysis revealed a novel compound heterozygous mutation in FBP1 gene confirming the diagnosis of FBP1 deficiency. The patient was managed with treatment of acute episodes and preventive long-term dietary modifications. Long-term prognosis of FBP1 deficiency is excellent underlining the importance of early recognition of clinical signs, prompt diagnosis, and avoidance of fasting in this disease. FBP1 gene mutations have been described from various ethnic backgrounds, but there is limited data available from Indian population, hence the importance of this case.

Exon 2 deletion represents a common mutation in Turkish patients with fructose-1,6-bisphosphatase deficiency

Fructose-1,6-bisphosphatase (FBPase) deficiency is an autosomal recessive inborn error of gluconeogenesis. We aimed to investigate clinical and biochemical findings and molecular genetic data in ten Turkish patients with fructose-1,6-bisphosphatase deficiency. Ten Turkish patients who were diagnosed with fructose-1,6-biphosphatase deficiency in a single center from 2013 to 2019 were included in this study. Their clinical and laboratory data were collected retrospectively. All patients were hospitalised in intensive care unit mostly after catabolic stress conditions such as infections, starvation and rarely fructose consumption. Prognosis was good after correct diagnosis and treatment. Molecular analyses of FBP1 gene revealed a homozygous exon 2 deletion in eight patients, a novel homozygous c.910_911dupTT mutation in one patient and a homozygous IVS5 + 1G > A splicing mutation in one patient. Exon 2 deletion (previously termed exon 1) was found to be the most common mutation in Turkish fructose-1,6-biphosphatase deficiency patients.

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

INTRODUCTION

Fructose-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.

AIM

To characterize the genotype of Southern Brazilian FBPase-deficient patients.

METHODOLOGY

The 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.

RESULTS

Five 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.

CONCLUSIONS

Our 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.

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

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Clinical and Molecular Characterization of Patients with Fructose 1,6-Bisphosphatase Deficiency

Fructose-1,6-bisphosphatase (FBPase) deficiency is a rare, autosomal recessive inherited disease caused by the mutation of the FBP1 gene, the incidence is estimated to be between 1/350,000 and 1/900,000. The symptoms of affected individuals are non-specific and are easily confused with other metabolic disorders. The present study describes the clinical features of four Chinese pediatric patients who presented with hypoglycemia, hyperlactacidemia, metabolic acidosis, and hyperuricemia. Targeted-next generation sequencing using the Agilent SureSelect XT Inherited Disease Panel was used to screen for causal variants in the genome, and the clinically-relevant variants were subsequently verified using Sanger sequencing. Here, DNA sequencing identified six variations of the FBP1 gene (NM_000507.3) in the four patients. In Case 1, we found a compound heterozygous mutations of c.704delC (p.Pro235GlnfsX42) (novel) and c.960_961insG (p.Ser321Valfs) (known pathogenic). In Case 2, we found a compound heterozygous mutations of c.825 + 1G>A and c.960_961insG (both were known pathogenically). In Case 3, a homozygous missense mutation of c.355G>A (p.Asp119Asn) (reported in ClinVar database without functional study) was found. Case 4 had a compound heterozygous mutations c.720_729del (p.Tyr241GlyfsX33) (novel) and c.490G>A (p.Gly164Ser) (known pathogenically). Further in vitro studies in the COS-7cell line demonstrated that the mutation of ASP119ASN had no impact on protein expression, but decreased the enzyme activity, and with which the clinical significance of Asp119Asn can be determined to be likely pathogenic. This report not only expands upon the known spectrum of variation of the FBP1 gene, but also deepens our understanding of the clinical features of FBPase deficiency.

A summary of molecular genetic findings in fructose-1,6-bisphosphatase deficiency with a focus on a common long-range deletion and the role of MLPA analysis

Background

Fructose-1,6-bisphosphatase deficiency is a rare inborn error of metabolism affecting gluconeogenesis with only sporadic reports on its molecular genetic basis.

Results

We report our experience with mutation analysis in 14 patients (13 families) with fructose-1,6-bisphosphatase deficiency using conventional Sanger sequencing and multiplex ligation-dependent probe amplification analysis, and we provide a mutation update for the fructose bisphosphatase-1 gene (FBP1). Mutations were found on both chromosomes in all of our 14 patients including 5 novel mutations. Among the novel mutations is a 5412-bp deletion (c.-24-26_170 + 5192del) including the entire coding sequence of exon 2 of FBP1 that was repeatedly found in patients from Turkey and Armenia which may explain earlier poorly defined findings in patients from this area. This deletion can be detected with specific primers by generation of a junction fragment and by MLPA and SNP array assays. MLPA analysis was able to detect copy number variations in two further patients, one heterozygous for a deletion within exon 8, another heterozygous for a novel deletion of the entire FBP1 gene.

Conclusions

Based on our update for the FBP1 gene, currently listing 35 mutations worldwide, and knowledge of PCR conditions that allow simple detection of a common FBP1 deletion in the Armenian and Turkish population, molecular genetic diagnosis has become easier in FBP1 deficiency. Furthermore, MLPA analysis may plays a useful role in patients with this disorder.

Electronic supplementary material

The online version of this article (doi:10.1186/s13023-016-0415-1) contains supplementary material, which is available to authorized users.

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.

Transient pseudo-hypertriglyceridemia: a useful biochemical marker of fructose-1,6-bisphosphatase deficiency

Fructose-1,6-bisphosphatase (FBP) deficiency is an autosomal-recessive disorder of gluconeogenesis resulting from mutations within the FBP1 gene. During periods of trivial illness, individuals with FBP deficiency may develop ketotic hypoglycemia, metabolic acidosis, lactic acidemia, and an increased anion gap. Although detection of urinary excretion of glycerol by urine organic acid analysis has been previously described, the presence of transient pseudo-hypertriglyceridemia in serum during metabolic decompensation has not been reported before. This study describes four consanguineous Pakistani families, in which four patients were diagnosed with FBP deficiency. All showed transient pseudo-hypertriglyceridemia during the acute phase of metabolic decompensation, which resolved in a metabolically stable phase. Mutations in the FBP1 gene have been described from various ethnicities, but there is very limited literature available for the Pakistani population. This study also describes one novel mutation in the FBP1 gene which seems to be prevalent in Pakistani-Indian patients.

CONCLUSION

As a result of this study, transient pseudo-hypertriglyceridemia should be added to glyceroluria, ketotic hypoglycemia, metabolic acidosis, and lactic acidosis as a useful biochemical marker of FBP deficiency.

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.

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.

Novel FBP1 gene mutations in Arab patients with fructose-1,6-bisphosphatase deficiency

Deficiency of fructose-1,6-bisphosphatase (FBP) results in impaired gluconeogenesis, which is characterized by episodes of hyperventilation, apnea, hypoglycemia, and metabolic and lactic acidosis. This autosomal recessive disorder is caused by mutations in the FBP1 gene, which encodes for fructose-1,6-bisphosphatase 1 (FBP1). Although FBP1 gene mutations have been described in FBP-deficient individuals of various ethnicities, there has been limited investigation into the genetics of this disorder in Arab patients. This study employed five consanguineous Arab families, in which 17 patients were clinically diagnosed with FBP deficiency. Seven patients and six carrier parents were analyzed for mutations in the FBP1 gene. DNA sequencing of the FBP1 gene identified two novel mutations in these families. A novel six nucleotide repetitive insertion, c114_119dupCTGCAC, was identified in patients from three families. This mutation encodes for a duplication of two amino acids (p.Cys39_Thr40dup) in the N-terminal domain of FBP1. A novel nonsense c.841G>T mutation encoding for a p.Glu281X truncation in the active site of FBP1 was discovered in patients from two families. The newly identified mutations in the FBP1 gene are predicted to produce FBP1 deficiency. These mutations are the only known genetic causes of FBP deficiency in Arab patients. The p.Cys39_Thr40dup is the first reported amino acid duplication in FBP deficiency patients.

CONCLUSION

This study provides a strong rationale for genetic testing of FBP deficient patients of Arab ethnicity for recurrent or novel mutations in the FBP1 gene.

Two newly identified genomic mutations in a Japanese female patient with fructose-1,6-bisphosphatase (FBPase) deficiency

Fructose-1,6-bisphosphatase (FBPase) (EC 3.1.3.11) catalyzes the splitting of fructose-1,6-bisphosphate into fructose 6-phosphate and inorganic phosphate. FBPase deficiency is an autosomal recessive inherited disorder caused by distraction of the fructose-1,6-bisphosphatase 1 gene (FBP1) and features severely impaired gluconeogenesis. We studied a female patient with typical FBPase deficiency symptoms. The FBPase activity of her peripheral white blood cells was undetectable. Genetic analyses of FBP1 revealed her to be a compound-heterozygote of two new mutations F194S and P284R. Gene tracking in the family revealed the mother to be a heterozygote of F194S, and the father and a sister to be heterozygotes of P284R. As both Phe194 and Pro284 of FBPase are highly conserved in many species and close to crucial amino acid residues to FBPase functions, these mutations could be responsible for the loss of FBPase activities.

Diagnosis of fructose-1,6-bisphosphatase deficiency using cultured lymphocyte fraction: a secure and noninvasive alternative to liver biopsy

We report the result of enzymatic and molecular analyses, using cultured lymphocyte fractions (cultivated monocytes), of six Japanese patients (from five families) and one Italian patient with fructose-1,6-bisphosphatase (FBPase) deficiency. Enzymatic analysis demonstrated FBPase deficiency in all seven patients, including the Italian patient whose fructose-1,6-bisphosphatase activity has been reported to be normal in leukocytes but deficient in liver. Molecular analysis of the FBPase gene identified pathogenic mutations in only 8 among the total 12 alleles of six families. We have thus demonstrated the validity of using cultured monocytes as a secure and noninvasive alternative to liver biopsy for accurate diagnosis of FBPase deficiency.