Novel mutations 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.

Genetic Analysis of Tyrosinemia Type 1 and Fructose-1, 6 Bisphosphatase Deficiency Affected in Pakistani Cohorts

Background: Inborn errors of metabolism are inherited disorders that present in early childhood and are usually caused by monogenic recessive mutations in specific enzymes that metabolize dietary components. Distinct mutations are present in specific populations.Objective: To determine which genomic variants are present in Pakistani cohorts with hepatorenal tyrosinemia type 1 (HT1) and fructose 1,6-bisphosphatase deficiency (FBPD).Materials and Methods: We sequenced the fumaryl acetoacetate hydrolase encoding gene (FAH) including flanking regions in four unrelated HT1 cohorts and the fructose 1,6-bisphosphatase gene (FBP1) in eight FBPD cohorts.Results: We mapped two recessive mutations in FAH gene for HT1; c.1062 + 5G > A(IVS12 + 5G > A) in three families and c.974C > T(pT325M) in one. We identified three mutations in FBP1 gene; c.841G > A(p.E281K) in five FBPD families, c.472C > T(p.R158W) in two families and c.778G > A(p.G260R) in one.Conclusion: Knowledge of common variants for HTI and FBDP in our study population can be used in the future to build a diagnostic algorithm.

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.

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.

Fructose 1,6-bisphosphatase: getting the message across

Fructose 1,6-bisphosphatase (FBPase) is a key enzyme in gluconeogenesis. It is a potential drug target in the treatment of type II diabetes. The protein is also associated with a rare inherited metabolic disease and some cancer cells lack FBPase activity which promotes glycolysis facilitating the Warburg effect. Thus, there is interest in both inhibiting the enzyme (for diabetes treatment) and restoring its activity (in relevant cancers). The mammalian enzyme is tetrameric, competitively inhibited by Fructose 2,6-bisphosphate and negatively allosterically regulated by AMP. This allosteric regulation requires information transmission between the AMP binding site and the active site of the enzyme. A recent paper by Topaz et al. (Bioscience Reports (2019) 39, pii:BSR20180960) has added additional detail to our understanding of this information transmission process. Two residues in the AMP binding site (Lys¹¹² and Tyr¹¹³) were shown to be involved in initiating the message between the two sites. This tyrosine residue has recently be shown to be important with protein’s interaction with the antidiabetic drug metformin. A variant designed to increase metal ion affinity (M248D) resulted in a five-fold increase in enzymatic activity. Interestingly alterations of two residues at the subunit interfaces (Tyr¹⁶⁴ and Met¹⁷⁷) resulted in increased responsiveness to AMP. Overall, these findings may have implications in the design of novel FBPase inhibitors or activators.

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

BACKGROUND

Fructose-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.

METHODS

All patients diagnosed with FBPase deficiency from 2010 to 2015 were included in this study. Their clinical and laboratory data were collected retrospectively.

RESULTS

All 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.

CONCLUSION

FBPase deficiency needs to be considered in any children with recurrent hypoglycemia and metabolic acidosis. Our study expands the spectrum of FBP1 gene mutations.

Clinical and molecular characterization of Indian patients with fructose-1, 6-bisphosphatase deficiency: Identification of a frequent variant (E281K)

Fructose-1, 6-bisphosphatase deficiency is an autosomal recessive disorder of gluconeogenesis caused by genetic defect in the FBP1 gene. It is characterized by episodic, often life-threatening metabolic acidosis, liver dysfunction, and hyperlactatemia. Without a high index of suspicion, it may remain undiagnosed with devastating consequences. Accurate diagnosis can be achieved either by enzyme assay or gene studies. Enzyme assay requires a liver biopsy and is tedious, invasive, expensive, and not easily available. Therefore, genetic testing is the most appropriate method to confirm the diagnosis. Molecular studies were performed on 18 suspected cases presenting with episodic symptoms. Seven different pathogenic variants were identified. Two common variants were noted in two subpopulations from the Indian subcontinent; p.Glu281Lys (E281K) occurred most frequently (in 10 patients) followed by p.Arg158Trp (R158W, in 4 patients). Molecular analysis confirmed the diagnosis and helped in managing these patients by providing appropriate genetic counseling. In conclusion, genetic studies identified two common variants in the Indian subcontinent, thus simplifying the diagnostic algorithm in this treatable disorder.

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

BACKGROUND

In 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.

METHODS

A 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.

RESULTS

Thirty-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.

CONCLUSIONS

Although 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.

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.

Human Metabolic Enzymes Deficiency: A Genetic Mutation Based Approach

One of the extreme challenges in biology is to ameliorate the understanding of the mechanisms which emphasize metabolic enzyme deficiency (MED) and how these pretend to have influence on human health. However, it has been manifested that MED could be either inherited as inborn error of metabolism (IEM) or acquired, which carries a high risk of interrupted biochemical reactions. Enzyme deficiency results in accumulation of toxic compounds that may disrupt normal organ functions and cause failure in producing crucial biological compounds and other intermediates. The MED related disorders cover widespread clinical presentations and can involve almost any organ system. To sum up the causal factors of almost all the MED-associated disorders, we decided to embark on a less traveled but nonetheless relevant direction, by focusing our attention on associated gene family products, regulation of their expression, genetic mutation, and mutation types. In addition, the review also outlines the clinical presentations as well as diagnostic and therapeutic approaches.

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.

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.

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.

Characterization of the mouse liver fructose-1,6-bisphosphatase gene

A cDNA encoding fructose-1,6-bisphosphatase (FBPase) was isolated from mouse liver RNA. The cDNA encodes a polypeptide of 338 amino acids (36.9 kDa). The liver and muscle FBPase isoenzymes of the mouse show positional identities of 69% at the cDNA level and 72% at the protein primary structure level. Starting from genomic YAC libraries and based upon the cDNA sequence all functional parts of the mouse liver FBPase gene (including exon-intron boundaries) were PCR-amplified and sequenced. The 5'-flanking regions of the liver and muscle FBPase genes were compared and showed no sequence similarity. Both genes are co-localized at chromosome 13B3-C1. The transcriptional start site was assigned to a guanine 118 bases before the start codon in the liver FBPase gene. An analysis of the steady state mRNA levels of liver and muscle FBPase in various mouse tissues was performed by Northern blotting and RT/PCR.

Rapid, simplified and sensitive method for screening fructose-1,6-diphosphatase deficiency by analyzing urinary metabolites in urease/direct preparations and gas chromatography-mass spectrometry in the selected-ion monitoring mode

Children with fructose-1,6-diphosphatase (FDPase) deficiency often experience life threatening episodes such as ketotic hypoglycemia. We report here a rapid, simplified and sensitive method to analyze glycerol-3-phosphate (G3P) and glycerol in urine, that can be used to detect FDPase deficiency. We used the urease/direct preparation and gas chromatography-mass spectrometry in the selected-ion monitoring mode, enabling detection of G3P and glycerol level in normal controls. Using this approach, FDPase deficiency can be more easily diagnosed and differentiated from glycerol kinase deficiency or glycerol infusion patients. To date, diagnosis has been essentially based on the assay of enzymes in the liver. The proposed non-invasive method provides a clinically significant diagnostic tool that may help prevent episodic attacks.

Structure and chromosomal localization of the human and mouse muscle fructose-1,6-bisphosphatase genes

Mammalian skeletal muscle contains fructose-1,6-bisphosphatase (Fru-1,6-P(2)ase), a key enzyme of glyconeogenesis. We have shown previously that muscle Fru-1,6-P(2)ase is encoded by a gene different from that coding for the liver isoenzyme. Starting with genomic YAC libraries and based on the cDNA sequences of human and mouse muscle Fru-1,6-P(2)ases together with the known gene structures of two mammalian liver fructose-1,6-bisphosphatases, we have PCR-amplified and sequenced all functional parts of the human and mouse muscle fructose-1,6-bisphosphatase genes and determined their chromosomal localization. The human gene (FBP2), localized at chromosome 1p36.1-2, spans about 30 kb, while the mouse gene (Fbp2) at chromosome 13B3-C1 is more compact (about 21 kb). Intron lengths are only poorly conserved between the two genes, while intron number and positions are identical in all hitherto analyzed mammalian fructose-1,6-bisphosphatase isoenzyme genes. Transcriptional start sites were found to be located 97 and 95bp before the start codon in the human gene and 35 bp before the start codon in the mouse homolog. A comparison of the 5'-flanking sequences of the two genes revealed a 56% homology up to human bp -607 before the first transcriptional start point, while upstream of this region we found no similarity. The data presented in this paper provide a basis for further studies of the mechanism of expression regulation and the elucidation of the physiological role of the enzyme.