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.

Three successful pregnancies through dietary management of fructose-1,6-bisphosphatase deficiency

Fructose-1,6-bisphosphatase (FBPase) deficiency (OMIM 229700) has been characterized as the cause of life-threatening hypoglycaemia and lactic acidaemia following prolonged fasting. The patient, an adult African-American woman, presented during the second trimester of her first pregnancy with recurrent episodes of lactic acidaemia and hypoglycaemia. She had recently been admitted to a nearby intensive care unit after presentation with profound hypoglycaemia and lactic acidosis, and was found to be pregnant. The history was remarkable for approximately 30 hospitalizations for hypoglycaemia and acidosis. She had previously undergone liver biopsy at another centre and was diagnosed with a 'glycogen storage disease', although no enzyme testing had been done for confirmation. Based on clinical symptoms, a diagnosis of FBPase deficiency was accomplished through gene sequencing, which revealed homozygosity for a panethnic, common mutation, 960/961insG in exon 7. The availability of mutation testing facilitated the confirmation of FBPase deficiency in this patient, obviating liver biopsy for enzyme activity confirmation. The patient underwent three successful pregnancies by strict compliance with dietary management, including nocturnal uncooked cornstarch to manage hypoglycaemia. The pregnancies were complicated by mild gestational diabetes, increased cornstarch requirements, and hypoglycaemia at the time of discharge from the hospital. The three infants had normal birth weights and experienced no complications during the neonatal period. The patient subsequently developed sensorineural hearing loss and early-onset cognitive impairment, despite compliance with the monitoring and treatment of hypoglycaemia. The experience with multiple pregnancies in this FBPase-deficient patient provides insight into the management of hypoglycaemia in inherited disorders of gluconeogenesis.

Human fructose-1,6-bisphosphatase gene (FBP1): exon-intron organization, localization to chromosome bands 9q22.2-q22.3, and mutation screening in subjects with fructose-1,6-bisphosphatase deficiency

Fructose-1,6-bisphosphatase (EC 3.1.3.11) is a key regulatory enzyme of gluconeogenesis that catalyzes the hydrolysis of fructose-1,6-bisphosphate to generate fructose-6-phosphate and inorganic phosphate. Deficiency of fructose-1,6-bisphosphatase is associated with fasting hypoglycemia and metabolic acidosis because of impaired gluconeogenesis. We have cloned and characterized the human liver fructose-1,6-bisphosphatase gene (FBP1). FBP1, localized to chromosome bands 9q22.2-q22.3 by fluorescence in situ hybridization, consists of seven exons that span > 31 kb, and the six introns are in the same position as in the rat gene. FBP1 was screened for mutations in two subjects with fructose-1,6-bisphosphatase deficiency. Four nucleotide substitutions were identified, two of which were silent mutations in the codons for Ala-216 (GCT-->GCC) and Gly-319 (GGG-->GGA). The other substitutions were in intron 3, a C-->T substitution 7 nucleotides downstream from the splice donor site, and in the promoter region, an A-->T substitution 188 nucleotides upstream from the start of transcription. These nucleotide substitutions were also found in normal unaffected subjects and thus are not the cause of fructose-1,6-bisphosphatase deficiency in the two subjects studied. The molecular basis of hepatic fructose-1,6-bisphosphatase deficiency in these subjects remains undetermined but could result from unidentified mutations in the promoter that decrease expression or from mutations in another gene that indirectly lead to decreased fructose-1,6-bisphosphatase activity.

Identification of a genetic mutation in a family with fructose-1,6- bisphosphatase deficiency

Fructose-1,6-bisphosphatase deficiency is an inheritable disorder of gluconeogenesis. Sequence analysis of the cDNA of the fructose-1,6-bisphosphatase mRNA isolated from monocytes from a girl with this disease and her consanguineous parents revealed that the patient and her parents were a homozygote and heterozygotes for an insertion of one G residue at G957GGGG961, respectively. This mutation resulted in translation of a truncated enzyme protein, and the mutant protein showed no fructose-1,6- bisphosphatase activity in an overexpression experiment in Escherichia coli. However, this mutation is located in a region of the amino acid sequence which is not well conserved among mammals. A mutagenized clone was prepared from the normal clone. The extents of substitutions and deletions of the amino acid sequence were predicted to be less in the mutagenized protein than in the mutant protein. This mutagenized clone also expressed no fructose-1,6-bisphosphatase activity, although both of two normal clones from control monocytes and a control liver sample expressed an apparently normal level of fructose-1,6-bisphosphatase activity. Thus, this mutation is concluded to be responsible for fructose-1,6-bisphosphatase deficiency in this patient.

Diagnosis of fructose-1,6-bisphosphatase deficiency using leukocytes: normal leukocyte enzyme activity in three female patients

In the past 5 years we have discovered 8 boys and 3 girls who suffer from fructose-1,6-bisphosphatase deficiency. Although they all showed the typical symptoms of the deficiency such as frequent vomiting, hypoglycemia, lactic acidosis, and hepatomegaly, the age at diagnosis varied from 2 months to 4 years. All the boys revealed the deficient enzyme activity in leukocytes but none of the girls. The liver biopsy was investigated in six patients to confirm the diagnosis. These results suggest the existence of tissue-specific isoenzymes for fructose-1,6-bisphosphatase possibly with a different gene origin.

Fructose and glucagon loading in siblings with fructose-1,6-diphosphatase deficiency in fed state

Hypoglycaemia induced by fructose administration is one of the diagnostic clues to fructose-1,6-diphosphatase (FDPase) deficiency (McKusick 229700). However, the pathological mechanism of this reactive hypoglycaemia is not fully known. This paper describes two siblings with FDPase deficiency, diagnosed enzymatically in leukocytes, who failed to correct reactive hypoglycaemia after glucagon administration even in the fed state, supporting a possibility that disturbed hepatic phosphorylase activity may be a main cause of reactive hypoglycaemia.

Characterization of human fructose-1,6-bisphosphatase in control and deficient tissues

The regulatory properties of human liver and muscle fructose-1,6-bisphosphatases (FBPase) have been studied in control tissues obtained at autopsy and in tissues from a neonate with FBPase deficiency who died as a result of an overwhelming acidosis. Evidence is presented which suggests that the alkaline isoenzyme of FBPase, which is widely regarded as a laboratory artefact, may have an important role in vivo in the regulation and control of glycolysis and gluconeogenesis. FBPase exhibits the hysteretic and dissociative properties associated with regulatory enzymes, and many of the factors which effect FBPase have inverse effects on phosphofructokinase activity, thus providing an integrated regulatory cycle for the control of the direction and rate of flux through the glycolytic pathway.